Derivatives (Details) ([NondesignatedMember], USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Asset derivatives at fair value	$ 12,880	$ 31,269
Liability derivatives at fair value	31,437	32,025
Fair value	(18,557)	(756)
[ForeignExchangeForwardMember]
Derivative [Line Items]
Asset derivatives at fair value	2,483	7,865
Liability derivatives at fair value	5,683	5,816
Notional value	2,199,422	2,555,230
Fair value	(3,200)	2,049
[ForeignExchangeOptionMember]
Derivative [Line Items]
Asset derivatives at fair value	182	1,074
Liability derivatives at fair value	2,021	321
Notional value	128,000	110,079
Fair value	(1,839)	753
[FutureMember]
Derivative [Line Items]
Asset derivatives at fair value	1,819	13,572
Liability derivatives at fair value	8,149	14,173
Notional value	2,285,316	2,534,995
Fair value	(6,330)	(601)
[CreditDefaultSwapBuyingProtectionMember]
Derivative [Line Items]
Asset derivatives at fair value	22	92
Liability derivatives at fair value	1,142	1,285
Notional value	77,459	94,961
Fair value	(1,120)	(1,193)
[CreditDefaultSwapSellingProtectionMember]
Derivative [Line Items]
Asset derivatives at fair value	324	246
Liability derivatives at fair value	79	772
Notional value	17,500	17,500
Fair value	245	(526)
[InsuranceLinkedSecuritiesMember]
Derivative [Line Items]
Asset derivatives at fair value	261	0
Liability derivatives at fair value	5,423	968
Notional value	155,964	136,375
Fair value	(5,162)	(968)
[TotalReturnSwapMember]
Derivative [Line Items]
Asset derivatives at fair value	7,115	7,673
Liability derivatives at fair value	746	640
Notional value	99,212	122,230
Fair value	6,369	7,033
[InterestRateSwapMember]
Derivative [Line Items]
Asset derivatives at fair value	0	0
Liability derivatives at fair value	8,194	7,992
Notional value	0	0
Fair value	(8,194)	(7,992)
[ToBeAnnouncedSecuritiesMember]
Derivative [Line Items]
Asset derivatives at fair value	674	747
Liability derivatives at fair value	0	58
Notional value	101,000	104,315
Fair value	$ 674	$ 689