Annual report pursuant to Section 13 and 15(d)

Postretirement Benefits, Schedule of Allocation of Plan Assets (Details)

v3.6.0.2
Postretirement Benefits, Schedule of Allocation of Plan Assets (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan Disclosure      
Defined Benefit Plan, Actual Plan Asset Allocations 100.00% 100.00%  
Fair value of plan assets at end of year $ 89,643 $ 86,828 $ 84,893
Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 82,371 79,761  
Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 7,272 7,067  
Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year $ 0 $ 0  
Equity securities      
Defined Benefit Plan Disclosure      
Defined Benefit Plan, Actual Plan Asset Allocations 78.10% 76.90%  
Debt securities      
Defined Benefit Plan Disclosure      
Defined Benefit Plan, Actual Plan Asset Allocations 12.60% 12.50%  
Real estate securities      
Defined Benefit Plan Disclosure      
Defined Benefit Plan, Actual Plan Asset Allocations 9.30% 10.60%  
Fair value of plan assets at end of year $ 8,367 $ 9,234  
Real estate securities | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 8,367 9,234  
Real estate securities | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 0 0  
Real estate securities | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 0 0  
U.S. companies      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 56,959 52,800  
U.S. companies | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 56,959 52,800  
U.S. companies | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 0 0  
U.S. companies | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 0 0  
International companies      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 13,045 13,957  
International companies | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 13,045 13,957  
International companies | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 0 0  
International companies | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 0 0  
Preferred securities      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 4,000 3,770  
Preferred securities | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 4,000 3,770  
Preferred securities | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 0 0  
Preferred securities | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 0 0  
Bond securities      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 7,272 7,067  
Bond securities | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 0 0  
Bond securities | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year 7,272 7,067  
Bond securities | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets at end of year $ 0 $ 0