Postretirement Benefits - Fair Value Measurement for Pension Plan Assets by Major Category and Level of Input (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair value measurements of pension plan assets:
Total	$ 767,569	$ 743,900	$ 592,544
Employee pension plan | Qualified Plan | Funded Plan
Fair value measurements of pension plan assets:
Total	767,569	743,900
Employee pension plan | Qualified Plan | Funded Plan | Quoted prices in active markets for identical assets Level 1
Fair value measurements of pension plan assets:
Total	9,519	6,470
Employee pension plan | Qualified Plan | Funded Plan | Observable inputs Level 2
Fair value measurements of pension plan assets:
Total	758,050	737,430
Employee pension plan | Qualified Plan | Funded Plan | Unobservable inputs Level 3
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Total equity securities
Fair value measurements of pension plan assets:
Total	293,437	434,099
Employee pension plan | Qualified Plan | Funded Plan | Total equity securities | Quoted prices in active markets for identical assets Level 1
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Total equity securities | Observable inputs Level 2
Fair value measurements of pension plan assets:
Total	293,437	434,099
Employee pension plan | Qualified Plan | Funded Plan | Total equity securities | Unobservable inputs Level 3
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | U.S. equity securities
Fair value measurements of pension plan assets:
Total	182,495	288,861
Employee pension plan | Qualified Plan | Funded Plan | U.S. equity securities | Quoted prices in active markets for identical assets Level 1
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | U.S. equity securities | Observable inputs Level 2
Fair value measurements of pension plan assets:
Total	182,495	288,861
Employee pension plan | Qualified Plan | Funded Plan | U.S. equity securities | Unobservable inputs Level 3
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Non-U.S. equity securities
Fair value measurements of pension plan assets:
Total	110,942	145,238
Employee pension plan | Qualified Plan | Funded Plan | Non-U.S. equity securities | Quoted prices in active markets for identical assets Level 1
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Non-U.S. equity securities | Observable inputs Level 2
Fair value measurements of pension plan assets:
Total	110,942	145,238
Employee pension plan | Qualified Plan | Funded Plan | Non-U.S. equity securities | Unobservable inputs Level 3
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Debt securities
Fair value measurements of pension plan assets:
Total	464,613	303,331
Employee pension plan | Qualified Plan | Funded Plan | Debt securities | Quoted prices in active markets for identical assets Level 1
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Debt securities | Observable inputs Level 2
Fair value measurements of pension plan assets:
Total	464,613	303,331
Employee pension plan | Qualified Plan | Funded Plan | Debt securities | Unobservable inputs Level 3
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Other
Fair value measurements of pension plan assets:
Total	9,519	6,470
Employee pension plan | Qualified Plan | Funded Plan | Other | Quoted prices in active markets for identical assets Level 1
Fair value measurements of pension plan assets:
Total	9,519	6,470
Employee pension plan | Qualified Plan | Funded Plan | Other | Observable inputs Level 2
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Other | Unobservable inputs Level 3
Fair value measurements of pension plan assets:
Total	$ 0	$ 0