Taxation - Summary of capital loss carryforwards (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Tax Credit Carryforward [Line Items]
Capital loss carryforwards available		$ 281,760
Net realized capital losses		63,757	$ 89,568
Capital loss valuation allowance, amount		(63,757)
Net Capital Loss Carryforwards Tax Effected		0
Ireland
Tax Credit Carryforward [Line Items]
Capital loss carryforwards available		122,142
Net realized capital losses		30,536
Capital loss valuation allowance, amount		(30,536)
Net Capital Loss Carryforwards Tax Effected		0
United Kingdom
Tax Credit Carryforward [Line Items]
Capital loss carryforwards available	[1]	14,927
Net realized capital losses	[1]	2,836
Capital loss valuation allowance, amount	[1]	(2,836)
Net Capital Loss Carryforwards Tax Effected		0
United States
Tax Credit Carryforward [Line Items]
Capital loss carryforwards available		144,691
Net realized capital losses		30,385
Capital loss valuation allowance, amount		(30,385)
Net Capital Loss Carryforwards Tax Effected		$ 0
Minimum | United States
Tax Credit Carryforward [Line Items]
Capital loss carryforwards, expiration date		Jan. 01, 2018
Maximum | United States
Tax Credit Carryforward [Line Items]
Capital loss carryforwards, expiration date		Dec. 31, 2021

[1] oss carryforwards of $41.2 million will expire at the end of 2018 with another $103.5 million expiring in future years through 2021.