Derivative Instruments - Earnings relating to derivative instruments not formally designated as hedging instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments Gain Loss [Line Items]
Net realized and unrealized gains (losses) on derivative instruments	$ (41,732)	$ 2,521	$ 53,123
Not Designated as Hedging Instrument | Investment Related Derivatives
Derivative Instruments Gain Loss [Line Items]
Interest rate exposure	(213)	781	(2,347)
Foreign exchange exposure	(4,384)	(592)	(1,542)
Credit exposure	(553)	(1,539)	(2,537)
Financial market exposure	264	(2,494)	(2,190)
Not Designated as Hedging Instrument | Other Non Investment Derivatives
Derivative Instruments Gain Loss [Line Items]
Foreign exchange exposure	(1,542)	(1,701)	60,319
Credit exposure	770	4,424	1,375
Modified coinsurance funds withheld contract	(34,133)	3,300	45
Gain Loss On Other Not Designated As Hedging Instruments	(1,941)	342	0
Life Funds Withheld Assets
Derivative Instruments Gain Loss [Line Items]
Net realized and unrealized gains (losses) on derivative instruments	$ (206,015)	$ (540,090)	$ (151,691)