Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 2,679,800	$ 2,223,426
Other comprehensive income before reclassifications	28,137	(5,154)
Amounts reclassified from accumulated other comprehensive (loss) income	(129)	(103)
Net decrease in other comprehensive loss	28,008	(5,257)
Ending Balance	3,581,956	2,679,800
Foreign currency [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(47,966)	(42,502)
Other comprehensive income before reclassifications	27,409	(5,464)
Amounts reclassified from accumulated other comprehensive (loss) income	0	0
Net decrease in other comprehensive loss	27,409	(5,464)
Ending Balance	(20,557)	(47,966)
Change in unrealized gains on available for sale securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(599)	(806)
Other comprehensive income before reclassifications	728	310
Amounts reclassified from accumulated other comprehensive (loss) income	(129)	(103)
Net decrease in other comprehensive loss	599	207
Ending Balance	0	(599)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(48,565)	(43,308)
Ending Balance	$ (20,557)	$ (48,565)