Postretirement Benefits (Details 10) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair value measurements of pension plan assets:
Total	$ 743,900	$ 592,544	$ 551,491
Employee pension plan | Qualified plan | Funded plan
Fair value measurements of pension plan assets:
Total	743,900	592,544
Employee pension plan | Qualified plan | Funded plan | Level 1
Fair value measurements of pension plan assets:
Total	6,470	5,948
Employee pension plan | Qualified plan | Funded plan | Level 2
Fair value measurements of pension plan assets:
Total	737,430	586,596
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	434,099	333,399
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	434,099	333,399
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	288,861	225,446
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	288,861	225,446
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	145,238	107,953
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	145,238	107,953
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	303,331	253,197
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	303,331	253,197
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	6,470	5,948
Employee pension plan | Qualified plan | Funded plan | Other | Level 1
Fair value measurements of pension plan assets:
Total	6,470	5,948
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