Supplemental Equity Information - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 2,553,005	$ 2,456,744
Amounts reclassified from AOCL	$ 23,375	$ 1,075	35,827	3,717
Other comprehensive income (loss), net of tax	26,213	(4,641)	40,060	(12,234)
Ending Balance	705,875	2,496,158	705,875	2,496,158
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(124,632)	(113,854)	(127,341)	(116,496)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCL	1,351	1,075	4,060	3,717
Other comprehensive income (loss), net of tax	1,351	1,075	4,060	3,717
Ending Balance	(123,281)	(112,779)	(123,281)	(112,779)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(23,608)	(23,282)	(28,560)	(20,129)
Other comprehensive income (loss) before reclassifications	1,428	(1,043)	6,380	(4,196)
Amounts reclassified from AOCL	22,024	0	22,024	0
Other comprehensive income (loss), net of tax	23,452	(1,043)	28,404	(4,196)
Ending Balance	(156)	(24,325)	(156)	(24,325)
Other
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	2,364	1,400	(5,672)	5,674
Other comprehensive income (loss) before reclassifications	0	(3,743)	(1,707)	(8,017)
Amounts reclassified from AOCL	0	0	9,743	0
Other comprehensive income (loss), net of tax	0	(3,743)	8,036	(8,017)
Ending Balance	2,364	(2,343)	2,364	(2,343)
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(145,876)	(135,736)	(161,573)	(130,951)
Other comprehensive income (loss) before reclassifications	1,428	(4,786)	4,673	(12,213)
Amounts reclassified from AOCL	23,375	1,075	35,827	3,717
Other comprehensive income (loss), net of tax	24,803	(3,711)	40,500	(8,496)
Ending Balance	$ (121,073)	$ (139,447)	$ (121,073)	$ (139,447)