Derivatives (Details) ([NondesignatedMember], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Derivative [Line Items]
Asset derivatives at fair value	$ 20,396		$ 31,269
Liability derivatives at fair value	26,583		32,025
Fair value	(6,187)		(756)
[ForeignExchangeForwardMember]
Derivative [Line Items]
Asset derivatives at fair value	11,229		7,865
Liability derivatives at fair value	3,744		5,816
Net notional exposure	2,411,819		2,555,230
Fair value	7,485		2,049
[ForeignExchangeOptionMember]
Derivative [Line Items]
Asset derivatives at fair value	1,061		1,074
Liability derivatives at fair value	0		321
Net notional exposure	116,893		110,079
Fair value	1,061		753
[FutureMember]
Derivative [Line Items]
Asset derivatives at fair value	0		13,572
Liability derivatives at fair value	11,462		14,173
Net notional exposure	1,995,581		2,534,995
Fair value	(11,462)		(601)
[CreditDefaultSwapBuyingProtectionMember]
Derivative [Line Items]
Asset derivatives at fair value	16		92
Liability derivatives at fair value	1,071		1,285
Net notional exposure	60,000		94,961
Fair value	(1,055)		(1,193)
[CreditDefaultSwapSellingProtectionMember]
Derivative [Line Items]
Asset derivatives at fair value	452		246
Liability derivatives at fair value	0		772
Net notional exposure	17,500		17,500
Fair value	452		(526)
[InsuranceLinkedSecuritiesMember]
Derivative [Line Items]
Asset derivatives at fair value	56	[1]	0	[1]
Liability derivatives at fair value	1,290	[1]	968	[1]
Net notional exposure	155,964	[1]	136,375	[1]
Fair value	(1,234)	[1]	(968)	[1]
[TotalReturnSwapMember]
Derivative [Line Items]
Asset derivatives at fair value	6,214		7,673
Liability derivatives at fair value	621		640
Net notional exposure	68,880		122,230
Fair value	5,593		7,033
[InterestRateSwapMember]
Derivative [Line Items]
Asset derivatives at fair value	0	[2]	0	[2]
Liability derivatives at fair value	8,395	[2]	7,992	[2]
Net notional exposure	0	[2]	0	[2]
Fair value	(8,395)	[2]	(7,992)	[2]
[ToBeAnnouncedSecuritiesMember]
Derivative [Line Items]
Asset derivatives at fair value	1,368		747
Liability derivatives at fair value	0		58
Net notional exposure	123,400		104,315
Fair value	$ 1,368		$ 689

[1] At September 30, 2012 and December 31, 2011, insurance-linked securities include a longevity swap for which the notional amount is not reflective of the overall potential exposure of the swap. As such, the Company has included the probable maximum loss under the swap within the net notional exposure as an approximation of the notional amount.

[2] (2) The Company enters into interest rate swaps to mitigate notional exposures on certain total return swaps. Accordingly, the notional value of interest rate swaps is not presented separately in the table.