Derivative Instruments - Net Realized and unrealized gains (losses) on derivative instruments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative [Line Items]
Net realized and unrealized gains (losses) on derivative instruments	$ 4,221	$ (7,069)
Derivatives not designated as hedging instruments | Investment related derivatives
Derivative [Line Items]
Interest rate exposure	997	(214)
Foreign currency exposure	(3,318)	(745)
Credit exposure	(41)	(854)
Financial market exposure	(1,348)	185
Derivatives not designated as hedging instruments | Other non-investment derivatives
Derivative [Line Items]
Foreign currency exposure	0	(1,210)
Credit exposure	615	632
Modified coinsurance and funds withheld contracts, including life retrocession embedded derivative	2,492	(3,925)
Other	4,824	(938)
Life Funds Withheld Assets
Derivative [Line Items]
Net realized and unrealized gains (losses) on derivative instruments	$ 22,921	$ (50,101)