Income tax expense - Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income tax (expense) benefit
Balance at beginning of year	$ (103,185)	$ (100,832)	$ (80,087)
Balance at end of year	(116,200)	(103,185)	(100,832)
Tax benefit
Income tax (expense) benefit
Change in valuation allowance	$ (13,015)	$ (2,353)	(7,301)
Acquired tax attributes
Income tax (expense) benefit
Change in valuation allowance			$ (13,444)