Derivative Instruments - Hedged items - amount of gain/(loss) recognized in income attributable to risk (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments Gain Loss [Line Items]
Ineffective portion of hedging relationship - gain/(loss)	$ (256)	$ 497	$ (4,585)
Designated as Hedging Instrument | Fair Value Hedging Member | Net realized and unrealized gains (losses) on derivative instruments [Member]
Derivative Instruments Gain Loss [Line Items]
Gain (loss) recognized in income on derivative	(15,663)	(22,568)	(22,370)
Ineffective portion of hedging relationship - gain/(loss)	(256)	497	(4,585)
Designated as Hedging Instrument | Fair Value Hedging Member | Net realized and unrealized gains (losses) on derivative instruments [Member] | Deposit Liabilities
Derivative Instruments Gain Loss [Line Items]
Gain (loss) recognized in income on derivative	0	0	(6,182)
Designated as Hedging Instrument | Fair Value Hedging Member | Net realized and unrealized gains (losses) on derivative instruments [Member] | Fixed Maturity Investments
Derivative Instruments Gain Loss [Line Items]
Gain (loss) recognized in income on derivative	15,407	23,065	23,967
Designated as Hedging Instrument | Fair Value Hedging Member | Net realized and unrealized gains (losses) on derivative instruments [Member] | Interest Rate Exposure
Derivative Instruments Gain Loss [Line Items]
Gain (loss) recognized in income on derivative	0	0	1,813
Designated as Hedging Instrument | Fair Value Hedging Member | Net realized and unrealized gains (losses) on derivative instruments [Member] | Foreign Exchange Exposure
Derivative Instruments Gain Loss [Line Items]
Gain (loss) recognized in income on derivative	$ (15,663)	$ (22,568)	$ (24,183)