Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of service cost	$ (131,792)	$ (130,235)	$ (262,265)	$ (250,225)
Reclassification adjustments	(11,304)	(11,205)	(22,856)	(21,694)
Total before tax	(29,613)	(27,886)	(58,141)	(52,568)
Reclassification out of Accumulated Other Comprehensive Income | Pension and other postretirement plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of service cost	1,874	912	3,550	1,905
Total before tax	1,874	912	3,550	1,905
Total before tax	(594)	(321)	(1,146)	(643)
Total net of tax	1,280	591	2,404	1,262
Reclassification out of Accumulated Other Comprehensive Income | Derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	1,646	1,719	3,323	3,443
Reclassification adjustments	1,646	1,719	3,323	3,443
Total before tax	(583)	(626)	(1,218)	(1,253)
Total net of tax	$ 1,063	$ 1,093	$ 2,105	$ 2,190