Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative Financial Instruments
Aggregate fair value of derivative instruments with credit-risk-related contingent features, in case of net liability position	$ 55,875	$ 0
Amount of unrestricted cash collateral to counterparties in the normal course of business	54,400	0
Unrestricted cash collateral from derivative counterparties held	0	11,985
Net derivative gains estimated to be reclassified from accumulated other comprehensive income (loss) into net income within the next twelve months	7,916
Derivative financial instruments
Notional amount	1,593,845	1,633,435
Net derivatives, Fair value	(55,740)	18,752
Asset derivatives, Fair value	32,418	32,766
Liability derivatives, Fair value	88,158	14,014
Total derivative gains (losses) in net investment income	12,567	(15,428)	1,366
Net investment gains (losses) on closed derivative positions	(10,221)	(38,794)	(17,076)
Swap
Derivative financial instruments
Number of derivative transactions	75	143
Average notional amount of derivative instruments	8,685	16,361
Futures
Derivative financial instruments
Number of derivative transactions	931	1,678
Average number of contracts per transaction	11	18
Swaption
Derivative financial instruments
Number of derivative transactions	46	44
Average notional amount of derivative instruments	5,528	5,986
Cross-currency swaps
Derivative financial instruments
Number of derivative transactions	23	1
Average notional amount of derivative instruments	12,710	39,030
Increase in derivatives notional amount due to additional swaps opened to hedge newly purchased assets denominated in British pounds and Euros	355,218
Interest rate swaps, interest rate futures and interest rate swaptions
Derivative financial instruments
Decrease in derivative notional amount as a result of a change in interest rate risk hedging strategy	394,808
Derivatives designated as hedges
Derivative financial instruments
Notional amount	792,224	289,030
Net derivatives, Fair value	(56,387)	26,124
Asset derivatives, Fair value	31,014	27,135
Liability derivatives, Fair value	87,401	1,011
Derivatives designated as hedges \| Cash flow hedges
Derivative financial instruments
Notional amount	608,448	253,230
Net derivatives, Fair value	(54,996)	27,135
Asset derivatives, Fair value	30,756	27,135
Liability derivatives, Fair value	85,752
Derivatives designated as hedges \| Cash flow hedges \| Interest rate swaps
Derivative financial instruments
Notional amount	184,200	184,200
Net derivatives, Fair value	26,113	20,894
Asset derivatives, Fair value	26,113	20,894
Derivatives designated as hedges \| Cash flow hedges \| Cross-currency swaps
Derivative financial instruments
Notional amount	424,248	69,030
Net derivatives, Fair value	(81,109)	6,241
Asset derivatives, Fair value	4,643	6,241
Liability derivatives, Fair value	85,752
Derivatives designated as hedges \| Fair value hedges
Derivative financial instruments
Notional amount	183,776	35,800
Net derivatives, Fair value	(1,391)	(1,011)
Asset derivatives, Fair value	258
Liability derivatives, Fair value	1,649	1,011
Derivatives designated as hedges \| Fair value hedges \| Interest rate swaps
Derivative financial instruments
Notional amount	183,776	35,800
Net derivatives, Fair value	(1,391)	(1,011)
Asset derivatives, Fair value	258
Liability derivatives, Fair value	1,649	1,011
Derivatives not designated as hedges
Derivative financial instruments
Notional amount	801,621	1,344,405
Net derivatives, Fair value	647	(7,372)
Asset derivatives, Fair value	1,404	5,631
Liability derivatives, Fair value	757	13,003
Derivatives not designated as hedges \| Interest rate swaps
Derivative financial instruments
Notional amount	29,264	392,235
Net derivatives, Fair value	305	(8,316)
Asset derivatives, Fair value	1,062	4,687
Liability derivatives, Fair value	757	13,003
Derivatives not designated as hedges \| Futures on equity indices
Derivative financial instruments
Notional amount	3,133	2,680
Derivatives not designated as hedges \| Interest rate futures
Derivative financial instruments
Notional amount	80,550	59,090
Derivatives not designated as hedges \| Interest rate swaptions
Derivative financial instruments
Notional amount	688,674	890,400
Net derivatives, Fair value	342	944
Asset derivatives, Fair value	$ 342	$ 944

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Represents the amount of decrease in the notional amount of derivative instruments of the entity.

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Average notional amount specified by the derivative(s). Expressed as an absolute value.

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This element represents the net increase or decrease in the fair value of the closed positions derivatives included in earnings.

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The estimated value of gains (losses), net anticipated to be transferred in the next twelve months from accumulated other comprehensive income into earnings.

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The number of derivative transactions entered into by the entity during the reporting period.

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Represents the amount of increase in the notional amount of derivative instruments of the entity.

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Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

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The amount of the obligation to return cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

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The amount of the right to reclaim cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

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Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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Net Increase or Decrease in the fair value of the derivative or group of derivatives included in earnings.

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The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

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