Derivative Instruments (Details) - Hedged items - amount of gain/(loss) recognized in income attributable to risk (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative Instruments Gain Loss [Line Items]
Ineffective portion of hedging relationship - gain/(loss)	$ (197)	$ (1,042)	$ (4,830)	$ (778)
Designated as Hedging Instrument [Member] | 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	(22,323)	38,733	(28,313)	22,282
Designated as Hedging Instrument [Member] | Fair Value Hedging Member | Net realized and unrealized gains (losses) on derivative instruments [Member] | Deposit Liabilities [Member]
Derivative Instruments Gain Loss [Line Items]
Gain/(loss) recognized in income on derivative	0	(21,659)	(6,182)	(25,950)
Designated as Hedging Instrument [Member] | Fair Value Hedging Member | Net realized and unrealized gains (losses) on derivative instruments [Member] | Fixed Maturity Investments [Member]
Derivative Instruments Gain Loss [Line Items]
Gain/(loss) recognized in income on derivative	22,126	(18,116)	29,665	2,890
Designated as Hedging Instrument [Member] | Fair Value Hedging Member | Net realized and unrealized gains (losses) on derivative instruments [Member] | Interest Rate Exposure [Member] | Gain Loss Recognized In Income On Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Gain/(loss) recognized in income on derivative	0	20,514	1,813	25,366
Designated as Hedging Instrument [Member] | Fair Value Hedging Member | Net realized and unrealized gains (losses) on derivative instruments [Member] | Foreign Exchange Exposure [Member] | Gain Loss Recognized In Income On Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Gain/(loss) recognized in income on derivative	$ (22,323)	$ 18,219	$ (30,126)	$ (3,084)