Derivatives (Details) ([NondesignatedMember], USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Asset derivatives at fair value	$ 33,655	$ 31,269
Liability derivatives at fair value	16,874	32,025
Fair value	16,781	(756)
[ForeignExchangeForwardMember]
Derivative [Line Items]
Asset derivatives at fair value	5,608	7,865
Liability derivatives at fair value	2,132	5,816
Notional value	2,955,928	2,555,230
Fair value	3,476	2,049
[ForeignExchangeOptionMember]
Derivative [Line Items]
Asset derivatives at fair value	3,338	1,074
Liability derivatives at fair value	0	321
Notional value	116,147	110,079
Fair value	3,338	753
[FutureMember]
Derivative [Line Items]
Asset derivatives at fair value	17,393	13,572
Liability derivatives at fair value	1,995	14,173
Notional value	2,526,540	2,534,995
Fair value	15,398	(601)
[CreditDefaultSwapBuyingProtectionMember]
Derivative [Line Items]
Asset derivatives at fair value	17	92
Liability derivatives at fair value	1,464	1,285
Notional value	95,301	94,961
Fair value	(1,447)	(1,193)
[CreditDefaultSwapSellingProtectionMember]
Derivative [Line Items]
Asset derivatives at fair value	356	246
Liability derivatives at fair value	102	772
Notional value	17,500	17,500
Fair value	254	(526)
[InsuranceLinkedSecuritiesMember]
Derivative [Line Items]
Asset derivatives at fair value	0	0
Liability derivatives at fair value	3,528	968
Notional value	160,964	136,375
Fair value	(3,528)	(968)
[TotalReturnSwapMember]
Derivative [Line Items]
Asset derivatives at fair value	6,943	7,673
Liability derivatives at fair value	418	640
Notional value	113,328	122,230
Fair value	6,525	7,033
[InterestRateSwapMember]
Derivative [Line Items]
Asset derivatives at fair value	0	0
Liability derivatives at fair value	7,029	7,992
Notional value	0	0
Fair value	(7,029)	(7,992)
[ToBeAnnouncedSecuritiesMember]
Derivative [Line Items]
Asset derivatives at fair value	0	747
Liability derivatives at fair value	206	58
Notional value	132,620	104,315
Fair value	$ (206)	$ 689