Derivative Instruments (Details) - Hedged items - amount of gain/(loss) recognized in income attributable to risk (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Derivative Instruments Gain Loss [Line Items]
Ineffective portion of hedging relationship - gain/(loss)	$ (2,194)	$ (957)	$ (4,633)	$ 264
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	14,567	4,849	(5,990)	(16,451)
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	(9,789)	(6,672)	(6,182)	(4,291)
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	(6,972)	866	7,539	21,006
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	7,270	5,726	1,813	4,852
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	$ 7,297	$ (877)	$ (7,803)	$ (21,303)