Supplemental Equity Information - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 995,041	$ 2,553,005
Amounts reclassified from AOCL	5,625	1,278
Other comprehensive income, net of tax	5,775	1,247
Reclassification of stranded tax effects to retained earnings	24,800
Ending Balance	1,036,636	2,574,822
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(107,037)	(127,341)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from AOCL	5,625	1,278
Other comprehensive income, net of tax	5,625	1,278
Reclassification of stranded tax effects to retained earnings	(24,845)
Ending Balance	(126,257)	(126,063)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	114	(28,560)
Other comprehensive income (loss) before reclassifications	150	1,197
Amounts reclassified from AOCL	0	0
Other comprehensive income, net of tax	150	1,197
Reclassification of stranded tax effects to retained earnings	0
Ending Balance	264	(27,363)
Other
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	0	(5,672)
Other comprehensive income (loss) before reclassifications	0	(2,293)
Amounts reclassified from AOCL	0	0
Other comprehensive income, net of tax	0	(2,293)
Reclassification of stranded tax effects to retained earnings	0
Ending Balance	0	(7,965)
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(106,923)	(161,573)
Other comprehensive income (loss) before reclassifications	150	(1,096)
Amounts reclassified from AOCL	5,625	1,278
Other comprehensive income, net of tax	5,775	182
Reclassification of stranded tax effects to retained earnings	(24,845)
Ending Balance	$ (125,993)	$ (161,391)