Income taxes - Location (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Income (loss) before income tax (expense) benefit	$ 29,020	$ (15,312)	$ 52,991	$ 134,633	$ (306,429)	$ (13,388)	$ 24,201	$ (25,863)	$ 201,332	$ (321,479)	$ 293,747
Bermuda
Operating Loss Carryforwards [Line Items]
Income (loss) before income tax (expense) benefit									$ 197,973	(273,697)	266,497
Effective tax rate									0.00%
United States
Operating Loss Carryforwards [Line Items]
Income (loss) before income tax (expense) benefit									$ 3,245	(47,771)	27,172
Effective tax rate									21.00%
United Kingdom
Operating Loss Carryforwards [Line Items]
Income (loss) before income tax (expense) benefit									$ 114	$ (11)	$ 78
Effective tax rate									19.00%