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Benefit Plans - Change in Project Benefit/Accumulated Benefit Obligations (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension and Supplemental Executive Retirement Plans      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit Obligation at Beginning of Year $ 349,483 $ 379,324  
Company Service Cost 9,130 10,256 $ 8,565
Interest Cost 15,906 15,847 15,987
Plan Participants' Contributions 0 0  
Net Actuarial (Gain)/Loss due to Assumption Changes 14,450 (24,118)  
Net Actuarial (Gain)/Loss due to Plan Experience 5,428 7,155  
Benefit Payments from Fund (21,831) (32,646)  
Benefit Payments Directly by Company (2,669) (7,661)  
Plan Amendments 16 19  
Other Adjustment (105) 1,307  
Benefit Obligation at End of Year 369,808 349,483 379,324
Other Postretirement Benefit Plan      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit Obligation at Beginning of Year 16,423 18,225  
Company Service Cost 751 833 659
Interest Cost 704 697 653
Plan Participants' Contributions 408 361  
Net Actuarial (Gain)/Loss due to Assumption Changes 497 (2,083)  
Net Actuarial (Gain)/Loss due to Plan Experience 357 (397)  
Benefit Payments from Fund (1,678) (1,147)  
Benefit Payments Directly by Company 0 0  
Plan Amendments 0 0  
Other Adjustment (84) (66)  
Benefit Obligation at End of Year $ 17,378 $ 16,423 $ 18,225