Taxation - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, beginning of the year	$ 97,780	$ 80,590	$ 37,190
Increases for tax positions taken during the year	3,290	25,250	20,370
Increases for tax positions taken in prior years	15,940	11,700	38,650
Decreases for tax positions taken in prior years	(35,230)	(6,440)	(4,850)
Decreases for settlement with taxing authorities	(16,320)	0	0
Decreases for lapse of the applicable statute of limitations	(500)	(13,320)	(10,770)
Unrecognized tax benefits, end of the year	$ 64,960	$ 97,780	$ 80,590