Supplemental Equity Information - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance		$ 995,041		$ 995,041	$ 2,553,005
Amounts reclassified from AOCL	$ 1,851		$ 23,375	8,445	35,827
Other comprehensive income, net of tax	1,828		26,213	8,855	40,060
Reclassification of stranded tax effects to retained earnings		24,800
Ending Balance	1,205,583		705,875	1,205,583	705,875
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(125,288)	(107,037)	(124,632)	(107,037)	(127,341)
Other comprehensive income (loss) before reclassifications	0		0	0	0
Amounts reclassified from AOCL	1,851		1,351	8,445	4,060
Other comprehensive income, net of tax	1,851		1,351	8,445	4,060
Reclassification of stranded tax effects to retained earnings				(24,845)
Ending Balance	(123,437)		(123,281)	(123,437)	(123,281)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	547	114	(23,608)	114	(28,560)
Other comprehensive income (loss) before reclassifications	(23)		1,428	410	6,380
Amounts reclassified from AOCL	0		22,024	0	22,024
Other comprehensive income, net of tax	(23)		23,452	410	28,404
Reclassification of stranded tax effects to retained earnings				0
Ending Balance	524		(156)	524	(156)
Other
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	0	0	2,364	0	(5,672)
Other comprehensive income (loss) before reclassifications	0		0	0	(1,707)
Amounts reclassified from AOCL	0		0	0	9,743
Other comprehensive income, net of tax	0		0	0	8,036
Reclassification of stranded tax effects to retained earnings				0
Ending Balance	0		2,364	0	2,364
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(124,741)	$ (106,923)	(145,876)	(106,923)	(161,573)
Other comprehensive income (loss) before reclassifications	(23)		1,428	410	4,673
Amounts reclassified from AOCL	1,851		23,375	8,445	35,827
Other comprehensive income, net of tax	1,828		24,803	8,855	40,500
Reclassification of stranded tax effects to retained earnings				(24,845)
Ending Balance	$ (122,913)		$ (121,073)	$ (122,913)	$ (121,073)