Postretirement Benefits - Fair Value Measurement for Pension Plan Assets by Major Category and Level of Input (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair value measurements of pension plan assets:
Total	$ 1,081,061	$ 907,625	$ 767,569
Employee pension plan | Qualified Plan | Funded Plan
Fair value measurements of pension plan assets:
Total	1,081,061	907,625
Employee pension plan | Qualified Plan | Funded Plan | Level 1
Fair value measurements of pension plan assets:
Total	147,397	10,791
Employee pension plan | Qualified Plan | Funded Plan | Level 2
Fair value measurements of pension plan assets:
Total	933,664	896,834
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	493,595	414,337
Employee pension plan | Qualified Plan | Funded Plan | Total equity securities | Level 1
Fair value measurements of pension plan assets:
Total	134,841	0
Employee pension plan | Qualified Plan | Funded Plan | Total equity securities | Level 2
Fair value measurements of pension plan assets:
Total	358,754	414,337
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	296,624	248,585
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	296,624	248,585
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	196,971	165,752
Employee pension plan | Qualified Plan | Funded Plan | Non-U.S. equity securities | Level 1
Fair value measurements of pension plan assets:
Total	134,841	0
Employee pension plan | Qualified Plan | Funded Plan | Non-U.S. equity securities | Level 2
Fair value measurements of pension plan assets:
Total	62,130	165,752
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	574,910	482,497
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	574,910	482,497
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	12,556	10,791
Employee pension plan | Qualified Plan | Funded Plan | Other | Level 1
Fair value measurements of pension plan assets:
Total	12,556	10,791
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