Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustments	$ 11,064	$ 11,913	$ 33,920	$ 33,607
Total before tax	(113,787)	(99,308)	(331,671)	(295,765)
Provision for income taxes	30,019	29,435	88,160	82,003
Total net of tax	(83,768)	(69,873)	(243,511)	(213,762)
Pension and other postretirement plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(2,075)	(915)	(5,625)	(2,820)
Provision for income taxes	607	321	1,753	964
Total net of tax	1,468	594	3,872	1,856
Amortization of service cost
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	2,075	915	5,625	2,820
Reclassification out of Accumulated Other Comprehensive Income | Reclassification adjustments for derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustments	1,681	1,701	5,004	5,144
Total before tax	1,681	1,701	5,004	5,144
Provision for income taxes	(627)	(619)	(1,845)	(1,872)
Total net of tax	$ 1,054	$ 1,082	$ 3,159	$ 3,272