Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Dec. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance				$ 5,950,628	$ 6,467,445
Other comprehensive income (loss) before reclassifications				(2,104)	1,019
Amounts reclassified from AOCI				649	767
Stranded tax effect resulting from the Tax Cuts and Job Act	$ 0			(5,540)
Stockholders' equity, adjusted balance	5,799,202
Other comprehensive income (loss), net		$ (2,446)	$ 1,318	(1,455)	1,786
Ending Balance	5,950,628	4,773,050	6,072,417	4,773,050	6,072,417
Unrealized Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance				0	0
Other comprehensive income (loss) before reclassifications				0	739
Amounts reclassified from AOCI				0	0
Stranded tax effect resulting from the Tax Cuts and Job Act				0
Stockholders' equity, adjusted balance	0
Other comprehensive income (loss), net				0	739
Ending Balance	0	0	739	0	739
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance				(27,603)	(35,865)
Other comprehensive income (loss) before reclassifications				0	0
Amounts reclassified from AOCI				649	767
Stranded tax effect resulting from the Tax Cuts and Job Act				(5,540)
Stockholders' equity, adjusted balance	(33,143)
Other comprehensive income (loss), net				649	767
Ending Balance	(27,603)	(32,494)	(35,098)	(32,494)	(35,098)
Foreign Currency Items
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance				(15,285)	(16,275)
Other comprehensive income (loss) before reclassifications				(2,104)	280
Amounts reclassified from AOCI				0	0
Stranded tax effect resulting from the Tax Cuts and Job Act				0
Stockholders' equity, adjusted balance	(15,285)
Other comprehensive income (loss), net				(2,104)	280
Ending Balance	(15,285)	(17,389)	(15,995)	(17,389)	(15,995)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance				(42,888)	(52,140)
Stranded tax effect resulting from the Tax Cuts and Job Act	(5,540)
Stockholders' equity, adjusted balance	(48,428)
Other comprehensive income (loss), net				(1,455)	1,786
Ending Balance	$ (42,888)	$ (49,883)	$ (50,354)	$ (49,883)	$ (50,354)