Financial Instruments (Details 4) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended					6 Months Ended	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Commercial mortgage-backed securities	Jun. 30, 2011 Commercial mortgage-backed securities	Jun. 30, 2012 Commercial mortgage-backed securities	Jun. 30, 2011 Commercial mortgage-backed securities	Jun. 30, 2012 Residential mortgage-backed securities	Jun. 30, 2011 Residential mortgage-backed securities	Jun. 30, 2012 Residential mortgage-backed securities	Jun. 30, 2011 Residential mortgage-backed securities	Jun. 30, 2012 Other debt obligations	Jun. 30, 2011 Other debt obligations	Jun. 30, 2012 Other debt obligations	Jun. 30, 2011 Other debt obligations	Jun. 30, 2012 Agency mortgage-backed securities	Jun. 30, 2011 Agency mortgage-backed securities	Jun. 30, 2012 Agency mortgage-backed securities	Jun. 30, 2011 Agency mortgage-backed securities	Jun. 30, 2012 Collateralized debt obligations	Jun. 30, 2011 Collateralized debt obligations	Jun. 30, 2012 Collateralized debt obligations	Jun. 30, 2011 Collateralized debt obligations	Jun. 30, 2012 Equity securities	Dec. 31, 2011 Equity securities	Jun. 30, 2011 Equity securities	Mar. 31, 2011 Equity securities	Dec. 31, 2010 Equity securities	Jun. 30, 2012 Preferred stock	Jun. 30, 2012 Investments	Jun. 30, 2011 Investments	Jun. 30, 2012 Investments	Jun. 30, 2011 Investments	Jun. 30, 2012 Derivatives	Jun. 30, 2011 Derivatives	Jun. 30, 2012 Derivatives	Jun. 30, 2011 Derivatives
Changes in the company's Level 3 financial instruments
Balance at the beginning of the period	$ 75,146	$ 137,380	$ 79,468	$ 128,203	$ 27,850	$ 68,247	$ 38,154	$ 46,571	$ 26,603	$ 32,385	$ 18,419	$ 33,604	$ 318	$ 9,555	$ 192	$ 5,843	$ 362	$ 2,430	$ 1,367	$ 806	$ 585	$ 7,135	$ 647	$ 23,235	$ 103	$ 112	$ 60	$ 60	$ 60	$ 571	$ 18,440	$ 16,883	$ 18,310	$ 18,084	$ 885	$ 685	$ 1,696
Total gains or (losses) (realized and unrealized)	(4,331)	1,938	(5,211)	21,899	(2,638)	(3,726)	(6,081)	10,966	(2,871)	(575)	(1,403)	(147)	(26)	52	(16)	57	(361)	(168)	(408)	(236)	(31)	6,566	(93)	11,471	9					188	(150)	(785)	(20)	(1,471)	1,737	574	2,622	1,259
Purchases	34,330	107,742	35,777	175,157	16,746	62,245	10,108	115,451	12,203	10,831	11,067	11,909	2,770	22,857	6,357	30,698	1,750	8,043	1,760	8,043	61	3,766	61	9,056						5,624	800		800
Sales	(59,878)	(111,409)	(63,215)	(187,932)	(34,171)	(66,737)	(34,339)	(112,904)	(24,878)	(13,731)	(17,224)	(15,943)	(504)	(16,186)	(3,976)	(20,241)	(264)	(2,292)	(1,232)	(600)	(61)	(12,463)	(61)	(38,244)						(6,383)
Settlements	(1,597)	(2,714)	(3,149)	(4,390)	(4)	(20)	(59)	(75)	(702)	(1,717)	(504)	(2,230)	(5)	(280)	(4)	(359)	(1)		(1)			(12)		(526)										(515)	(885)	(685)	(2,581)	(685)
Transfers in and/or out of Level 3	(2,101)		(2,101)	(7,218)	(2,101)		(2,101)	(3,650)																(3,568)
Balance at the end of the period	41,569	125,719	41,569	125,719	5,682	56,359	5,682	56,359	10,355	27,193	10,355	27,193	2,553	15,998	2,553	15,998	1,486	8,013	1,486	8,013	554	1,424	554	1,424	112	112	60	60	60		19,090	16,098	19,090	16,098	1,737	574	1,737	574
Changes in unrealized gains/(losses) on Level 3 assets still held at the reporting date	$ (873)	$ (4,593)	$ (844)	$ (4,664)	$ (1,036)	$ (4,923)	$ (1,253)	$ (4,687)	$ (882)	$ 101	$ (859)	$ 92	$ (15)	$ 13	$ (15)	$ 4	$ (177)	$ (146)	$ (208)	$ (180)	$ (31)	$ (63)	$ (93)	$ (42)	$ 9						$ (478)	$ (149)	$ (153)	$ (425)	$ 1,737	$ 574	$ 1,737	$ 574