Income Taxes - Components of pre-tax loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.K. loss	$ (112,355)	$ (199,032)	$ (222,043)
U.S. income (loss)	(38,817)	3,733	(11,880)
Other loss	42,063	(5,477)	(2,590)
Pre-tax loss	$ (109,109)	$ (200,776)	$ (236,513)