Postretirement Benefits - Fair Value Measurement for Pension Plan Assets by Major Category and Level of Input (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair value measurements of pension plan assets:
Total	$ 907,625	$ 767,569	$ 743,900
Employee pension plan | Qualified Plan | Funded Plan
Fair value measurements of pension plan assets:
Total	907,625	767,569
Employee pension plan | Qualified Plan | Funded Plan | Level 1
Fair value measurements of pension plan assets:
Total	10,791	9,519
Employee pension plan | Qualified Plan | Funded Plan | Level 2
Fair value measurements of pension plan assets:
Total	896,834	758,050
Employee pension plan | Qualified Plan | Funded Plan | 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	414,337	293,437
Employee pension plan | Qualified Plan | Funded Plan | Total equity securities | Level 1
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Total equity securities | Level 2
Fair value measurements of pension plan assets:
Total	414,337	293,437
Employee pension plan | Qualified Plan | Funded Plan | Total equity securities | 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	248,585	182,495
Employee pension plan | Qualified Plan | Funded Plan | U.S. equity securities | Level 1
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | U.S. equity securities | Level 2
Fair value measurements of pension plan assets:
Total	248,585	182,495
Employee pension plan | Qualified Plan | Funded Plan | U.S. equity securities | 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	165,752	110,942
Employee pension plan | Qualified Plan | Funded Plan | Non-U.S. equity securities | Level 1
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Non-U.S. equity securities | Level 2
Fair value measurements of pension plan assets:
Total	165,752	110,942
Employee pension plan | Qualified Plan | Funded Plan | Non-U.S. equity securities | 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	482,497	464,613
Employee pension plan | Qualified Plan | Funded Plan | Debt securities | Level 1
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Debt securities | Level 2
Fair value measurements of pension plan assets:
Total	482,497	464,613
Employee pension plan | Qualified Plan | Funded Plan | Debt securities | 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	10,791	9,519
Employee pension plan | Qualified Plan | Funded Plan | Other | Level 1
Fair value measurements of pension plan assets:
Total	10,791	9,519
Employee pension plan | Qualified Plan | Funded Plan | Other | Level 2
Fair value measurements of pension plan assets:
Total	0	0
Employee pension plan | Qualified Plan | Funded Plan | Other | Level 3
Fair value measurements of pension plan assets:
Total	$ 0	$ 0