Income tax (expense) benefit - Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income tax (expense) benefit
Balance at beginning of year	$ (80,087)	$ (54,114)	$ (40,487)
Balance at end of year	(100,832)	(80,087)	(54,114)
Tax benefit
Income tax (expense) benefit
Change in valuation allowance	(7,301)	$ (25,973)	$ (13,627)
Acquired tax attributes
Income tax (expense) benefit
Change in valuation allowance	$ (13,444)