Income tax (expense) benefit - Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in the deferred tax valuation allowance
Balance at beginning of year	$ (54,114)	$ (40,487)
Balance at ending of year	(80,087)	(54,114)	$ (40,487)
Deferred Tax Valuation Allowance
Changes in the deferred tax valuation allowance
Balance at beginning of year	(54,114)	(40,487)	(30,507)
Tax benefit	(25,973)	(13,627)	(9,980)
Balance at ending of year	$ (80,087)	$ (54,114)	$ (40,487)