Derivative Instruments - Net Realized and unrealized gains (losses) on derivative instruments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative [Line Items]
Net realized and unrealized gains (losses) on derivative instruments	$ 16,126	$ (906)	$ 20,347	$ (7,975)
Derivatives not designated as hedging instruments | Investment related derivatives
Derivative [Line Items]
Interest rate exposure	(1,317)	372	(320)	158
Foreign currency exposure	3,378	(988)	60	(1,733)
Credit exposure	(38)	(525)	(79)	(1,379)
Financial market exposure	7,492	0	6,144	185
Derivatives not designated as hedging instruments | Other non-investment derivatives
Derivative [Line Items]
Foreign currency exposure	0	34	0	(1,176)
Credit exposure	0	138	615	770
Modified coinsurance and funds withheld contracts, including life retrocession embedded derivative	6,611	238	9,103	(3,687)
Other	0	(175)	4,824	(1,113)
Life Funds Withheld Assets
Derivative [Line Items]
Net realized and unrealized gains (losses) on derivative instruments	$ (36,120)	$ (34,596)	$ (13,199)	$ (84,697)