Fair Value of Financial Instruments (Schedule of Fair Value Measurement Using Significant Unobservable Inputs Level Three) (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Hedge Funds [Member]
Opening balance	$ 469,999	$ 242,135	$ 347,632	$ 184,725
Total realized and unrealized gains included in net income	5,435	1,742	21,884	6,584
Total realized and unrealized losses included in net income	(4,621)	(2,906)	(9,391)	(3,386)
Purchases	94,290	78,621	245,340	131,669
Sales	(2,836)		(43,198)
Transfers into Level 3
Transfers out of Level 3
Ending balance	562,267	319,592	562,267	319,592
Mortgage-Backed [Member]
Opening balance	234,087	233,667	172,558	253,979
Total realized and unrealized gains included in net income	4,251	9,703	5,495	18,084
Total realized and unrealized losses included in net income	(2,571)	(6,987)	(2,268)	(7,098)
Change in unrealized gains included in OCI		1,639		5,084
Change in unrealized losses included in OCI		(205)		(447)
Purchases	29,827	96,089	62,604	120,943
Sales	(34,329)	(47,560)	(43,287)	(119,228)
Transfers into Level 3	24,389	2,286	86,085	48,731
Transfers out of Level 3	(38,994)	(9,843)	(64,527)	(41,259)
Ending balance	216,660	278,789	216,660	278,789
Asset-Backed [Member]
Opening balance	143,829	36,532	48,707	104,871
Total realized and unrealized gains included in net income	596	79	660	634
Total realized and unrealized losses included in net income	(102)	(279)	(73)	(209)
Change in unrealized gains included in OCI		9		51
Change in unrealized losses included in OCI				(6)
Purchases	32,408	28,868	115,417	51,181
Sales	(2,800)	(2,058)	(3,226)	(5,246)
Transfers into Level 3	20,246	50,641	32,801	50,739
Transfers out of Level 3	(80,867)	(10,237)	(80,976)	(98,460)
Ending balance	$ 113,310	$ 103,555	$ 113,310	$ 103,555