Derivative Instruments - Net Realized and unrealized gains (losses) on derivative instruments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative [Line Items]
Net realized and unrealized gains (losses) on derivative instruments	$ (20,434)	$ 5,490	$ (28,409)	$ 2,774
Derivatives not designated as hedging instruments | Investment related derivatives
Derivative [Line Items]
Interest rate exposure	(109)	0	49	781
Foreign currency exposure	(1,963)	74	(3,696)	(592)
Credit exposure	(289)	(812)	(1,668)	(959)
Financial market exposure	0	(45)	185	(2,866)
Derivatives not designated as hedging instruments | Other non-investment derivatives
Derivative [Line Items]
Foreign currency exposure	26	(347)	(1,150)	(1,345)
Credit exposure	0	4,477	770	4,424
Modified coinsurance and funds withheld contracts, including life retrocession embedded derivative	(17,980)	2,143	(21,667)	3,331
Gain Loss On Other Not Designated As Hedging Instruments	(119)	0	(1,232)	0
Life Funds Withheld Assets
Derivative [Line Items]
Net realized and unrealized gains (losses) on derivative instruments	$ (31,662)	$ (225,610)	$ (116,359)	$ (691,432)