Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Movement in accumulated other comprehensive loss
Balance at the beginning of period	$ 684,231
Other comprehensive income, net of tax of nil	(510)
Balance at the end of period	1,983,431
Accumulated Other Comprehensive Income/(Loss)
Movement in accumulated other comprehensive loss
Balance at the beginning of period	(480)
Balance at January 1, 2018		$ (217)
Other comprehensive income before reclassifications	315
Amounts reclassified from accumulated other comprehensive income	(822)
Other comprehensive income, net of tax of nil	(507)
Balance at the end of period	(724)
Accumulated Other Comprehensive Income/(Loss) | Accounting Standards Update 2016-16
Movement in accumulated other comprehensive loss
Adjustment to opening balance of accumulated other comprehensive income		263
Foreign Currency Translation Adjustments
Movement in accumulated other comprehensive loss
Balance at the beginning of period	(85)
Balance at January 1, 2018		178
Other comprehensive income before reclassifications	(1,909)
Amounts reclassified from accumulated other comprehensive income	0
Other comprehensive income, net of tax of nil	(1,909)
Balance at the end of period	(1,731)
Foreign Currency Translation Adjustments | Accounting Standards Update 2016-16
Movement in accumulated other comprehensive loss
Adjustment to opening balance of accumulated other comprehensive income		263
Unrealized Losses on available-for-Sale Securities
Movement in accumulated other comprehensive loss
Balance at the beginning of period	(395)
Balance at January 1, 2018		(395)
Other comprehensive income before reclassifications	2,224
Amounts reclassified from accumulated other comprehensive income	(822)
Other comprehensive income, net of tax of nil	1,402
Balance at the end of period	$ 1,007
Unrealized Losses on available-for-Sale Securities | Accounting Standards Update 2016-16
Movement in accumulated other comprehensive loss
Adjustment to opening balance of accumulated other comprehensive income		$ 0