Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of service cost	$ 934	$ 1,653	$ (3,565)	$ 1,717
Reclassification adjustments	10,958	11,064	33,098	33,920
Income before income taxes	(133,241)	(113,787)	(400,114)	(331,671)
Provision for income taxes	26,889	30,019	87,678	88,160
Net income	(106,352)	(83,768)	(312,436)	(243,511)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustments	1,610	1,681	4,869	5,004
Income before income taxes	1,610	1,681	4,869	5,004
Provision for income taxes	(366)	(627)	(1,105)	(1,845)
Net income	1,244	1,054	3,764	3,159
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Including Portion Attributable to Noncontrolling Interest
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of service cost	1,725	2,075	5,396	5,625
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	1,725	2,075	5,396	5,625
Provision for income taxes	(455)	(607)	(1,417)	(1,753)
Net income	$ 1,270	$ 1,468	$ 3,979	$ 3,872