Financial Instruments (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Assets at Fair Value
Financial instruments owned	$ 763,550,000	$ 1,554,660,000
Investments	19,090,000	18,310,000
Liabilities at Fair Value
Securities sold but not yet purchased	110,333,000	184,996,000
Level 1
Assets at Fair Value
Total financial assets at fair value	37,110,000	6,994,000
Liabilities at Fair Value
Total financial liabilities at fair value	103,551,000	160,216,000
Level 1 | U.S. government obligations
Assets at Fair Value
Financial instruments owned	28,374,000	5,789,000
Liabilities at Fair Value
Securities sold but not yet purchased	100,204,000	158,059,000
Level 1 | Preferred stock
Assets at Fair Value
Financial instruments owned	7,887,000	316,000
Liabilities at Fair Value
Securities sold but not yet purchased	2,057,000	184,000
Level 1 | Equities
Assets at Fair Value
Financial instruments owned	573,000	889,000
Liabilities at Fair Value
Securities sold but not yet purchased	2,000	2,000
Level 1 | Derivatives
Assets at Fair Value
Financial instruments owned	276,000
Liabilities at Fair Value
Securities sold but not yet purchased	1,288,000	1,971,000
Level 2
Assets at Fair Value
Total financial assets at fair value	703,961,000	1,485,778,000
Liabilities at Fair Value
Securities sold but not yet purchased		24,050,000
Total financial liabilities at fair value	6,782,000
Level 2 | Agency mortgage-backed securities
Assets at Fair Value
Financial instruments owned	494,767,000	1,084,254,000
Level 2 | Federal agency obligations
Assets at Fair Value
Financial instruments owned	103,228,000	158,774,000
Liabilities at Fair Value
Securities sold but not yet purchased		11,796,000
Level 2 | Loans
Assets at Fair Value
Financial instruments owned	65,048,000	228,226,000
Level 2 | Corporate debt securities
Assets at Fair Value
Financial instruments owned	26,578,000	14,524,000
Liabilities at Fair Value
Securities sold but not yet purchased	6,782,000	12,254,000
Assets, net of liabilities, at fair value	19,800,000
Level 2 | Commercial mortgage-backed securities
Assets at Fair Value
Financial instruments owned	6,255,000
Level 2 | Other debt obligations
Assets at Fair Value
Financial instruments owned	8,060,000
Level 2 | Equities
Assets at Fair Value
Financial instruments owned	25,000
Level 3
Assets at Fair Value
Total financial assets at fair value	41,569,000	80,198,000
Liabilities at Fair Value
Securities sold but not yet purchased		730,000
Level 3 | Agency mortgage-backed securities
Assets at Fair Value
Financial instruments owned	1,486,000	1,367,000
Level 3 | Commercial mortgage-backed securities
Assets at Fair Value
Financial instruments owned	5,682,000	38,154,000
Level 3 | Other debt obligations
Assets at Fair Value
Financial instruments owned	2,553,000	192,000
Level 3 | Residential mortgage-backed securities
Assets at Fair Value
Financial instruments owned	10,355,000	18,419,000
Level 3 | Preferred stock
Assets at Fair Value
Financial instruments owned		1,301,000
Liabilities at Fair Value
Securities sold but not yet purchased		730,000
Level 3 | Equities
Assets at Fair Value
Financial instruments owned	112,000	112,000
Level 3 | Collateralized debt obligations
Assets at Fair Value
Financial instruments owned	554,000	647,000
Level 3 | Derivatives
Assets at Fair Value
Financial instruments owned	1,737,000	1,696,000
Level 3 | Investments
Assets at Fair Value
Investments	19,090,000	18,310,000
Total
Assets at Fair Value
Total financial assets at fair value	782,640,000	1,572,970,000
Liabilities at Fair Value
Total financial liabilities at fair value	110,333,000	184,996,000
Total | Agency mortgage-backed securities
Assets at Fair Value
Financial instruments owned	496,253,000	1,085,621,000
Total | Federal agency obligations
Assets at Fair Value
Financial instruments owned	103,228,000	158,774,000
Liabilities at Fair Value
Securities sold but not yet purchased		11,796,000
Total | Loans
Assets at Fair Value
Financial instruments owned	65,048,000	228,226,000
Total | U.S. government obligations
Assets at Fair Value
Financial instruments owned	28,374,000	5,789,000
Liabilities at Fair Value
Securities sold but not yet purchased	100,204,000	158,059,000
Total | Corporate debt securities
Assets at Fair Value
Financial instruments owned	26,578,000	14,524,000
Liabilities at Fair Value
Securities sold but not yet purchased	6,782,000	12,254,000
Total | Commercial mortgage-backed securities
Assets at Fair Value
Financial instruments owned	11,937,000	38,154,000
Total | Other debt obligations
Assets at Fair Value
Financial instruments owned	10,613,000	192,000
Total | Residential mortgage-backed securities
Assets at Fair Value
Financial instruments owned	10,355,000	18,419,000
Total | Preferred stock
Assets at Fair Value
Financial instruments owned	7,887,000	1,617,000
Liabilities at Fair Value
Securities sold but not yet purchased	2,057,000	914,000
Total | Equities
Assets at Fair Value
Financial instruments owned	710,000	1,001,000
Liabilities at Fair Value
Securities sold but not yet purchased	2,000	2,000
Total | Collateralized debt obligations
Assets at Fair Value
Financial instruments owned	554,000	647,000
Total | Derivatives
Assets at Fair Value
Financial instruments owned	2,013,000	1,696,000
Liabilities at Fair Value
Securities sold but not yet purchased	1,288,000	1,971,000
Total | Investments
Assets at Fair Value
Investments	$ 19,090,000	$ 18,310,000