Derivative Instruments (Details) - Hedged items - amount of gain/(loss) recognized in income attributable to risk (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative Instruments Gain Loss [Line Items]
Ineffective portion of hedging relationship - gain/(loss)	$ (4,585)	$ (1,875)	$ (13,675)
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]
Ineffective portion of hedging relationship - gain/(loss)	(4,585)	(1,875)	(13,675)
Designated as Hedging Instrument [Member] | Fair Value Hedging Member | Net realized and unrealized gains (losses) on derivative instruments [Member] | Gain Loss Recognized In Income On Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Gain/(loss) recognized in income on derivative	(22,370)	40,815	113,924
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	(6,182)	(27,391)	(84,393)
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	23,967	(15,299)	(27,266)
Designated as Hedging Instrument [Member] | Fair Value Hedging Member | Net realized and unrealized gains (losses) on derivative instruments [Member] | Notes Payable And Debt [Member]
Derivative Instruments Gain Loss [Line Items]
Gain/(loss) recognized in income on derivative	0	0	(15,940)
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	1,813	25,680	94,068
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	$ (24,183)	$ 15,135	$ 19,856