Accumulated Other Comprehensive Income/(Loss) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Movement in accumulated other comprehensive loss
Balance at the beginning of period	$ 684,231
Net-current period other comprehensive income	3,353
Balance at the end of period	1,226,668
Accumulated Other Comprehensive Income/(Loss)
Movement in accumulated other comprehensive loss
Balance at the beginning of period	(480)
Balance as of January 1, 2018		$ (217)
Other comprehensive income before reclassifications	3,658
Amounts reclassified from accumulated other comprehensive income	(327)
Net-current period other comprehensive income	3,331
Balance at the end of period	3,114
Foreign Currency Translation Adjustments
Movement in accumulated other comprehensive loss
Balance at the beginning of period	(85)
Balance as of January 1, 2018		178
Other comprehensive income before reclassifications	2,283
Net-current period other comprehensive income	2,283
Balance at the end of period	2,461
Unrealized Losses on available-for-Sale Securities
Movement in accumulated other comprehensive loss
Balance at the beginning of period	(395)
Balance as of January 1, 2018		(395)
Other comprehensive income before reclassifications	1,375
Amounts reclassified from accumulated other comprehensive income	(327)
Net-current period other comprehensive income	1,048
Balance at the end of period	$ 653
Accounting Standards Update 2016-16 | Accumulated Other Comprehensive Income/(Loss)
Movement in accumulated other comprehensive loss
Adjustment to opening balance of accumulated other comprehensive income		263
Accounting Standards Update 2016-16 | Foreign Currency Translation Adjustments
Movement in accumulated other comprehensive loss
Adjustment to opening balance of accumulated other comprehensive income		$ 263