Financial Instruments (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Assets at Fair Value
Financial instruments owned	$ 1,096,181,000	$ 1,554,660,000
Investments	20,478,000	18,310,000
Liabilities at Fair Value
Securities sold but not yet purchased	132,730,000	184,996,000
Level 1
Assets at Fair Value
Total financial assets at fair value	5,342,000	6,994,000
Liabilities at Fair Value
Total financial liabilities at fair value	130,210,000	160,216,000
Level 1 | Preferred stock
Assets at Fair Value
Financial instruments owned	2,439,000	316,000
Liabilities at Fair Value
Securities sold but not yet purchased		184,000
Level 1 | U.S. Government obligations
Assets at Fair Value
Financial instruments owned	1,996,000	5,789,000
Liabilities at Fair Value
Securities sold but not yet purchased	128,504,000	158,059,000
Level 1 | Equity securities
Assets at Fair Value
Financial instruments owned	675,000	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	232,000
Liabilities at Fair Value
Securities sold but not yet purchased	1,704,000	1,971,000
Level 2
Assets at Fair Value
Total financial assets at fair value	1,087,899,000	1,485,778,000
Liabilities at Fair Value
Total financial liabilities at fair value	2,520,000	24,050,000
Level 2 | Agency mortgage-backed securities
Assets at Fair Value
Financial instruments owned	903,928,000	1,084,254,000
Level 2 | Loans
Assets at Fair Value
Financial instruments owned	77,573,000	228,226,000
Level 2 | Federal agency obligations
Assets at Fair Value
Financial instruments owned	46,021,000	158,774,000
Liabilities at Fair Value
Securities sold but not yet purchased		11,796,000
Assets, net of liabilities, at fair value		147,000,000
Level 2 | Corporate debt securities
Assets at Fair Value
Financial instruments owned	30,246,000	14,524,000
Liabilities at Fair Value
Securities sold but not yet purchased	2,520,000	12,254,000
Level 2 | Residential mortgage-backed securities
Assets at Fair Value
Financial instruments owned	23,077,000
Level 2 | Commercial mortgage-backed securities
Assets at Fair Value
Financial instruments owned	4,880,000
Level 2 | U.S. Government obligations
Assets at Fair Value
Financial instruments owned	100,000
Level 2 | Other debt obligations
Assets at Fair Value
Financial instruments owned	2,074,000
Level 3
Assets at Fair Value
Total financial assets at fair value	23,418,000	80,198,000
Liabilities at Fair Value
Total financial liabilities at fair value		730,000
Level 3 | Agency mortgage-backed securities
Assets at Fair Value
Financial instruments owned	1,110,000	1,367,000
Level 3 | Residential mortgage-backed securities
Assets at Fair Value
Financial instruments owned	149,000	18,419,000
Level 3 | Commercial mortgage-backed securities
Assets at Fair Value
Financial instruments owned	18,000	38,154,000
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 | Other debt obligations
Assets at Fair Value
Financial instruments owned		192,000
Level 3 | Equity securities
Assets at Fair Value
Financial instruments owned	28,000	112,000
Level 3 | Collateralized debt obligations
Assets at Fair Value
Financial instruments owned	671,000	647,000
Level 3 | Derivatives
Assets at Fair Value
Financial instruments owned	964,000	1,696,000
Level 3 | Investments
Assets at Fair Value
Investments	20,478,000	18,310,000
Total
Assets at Fair Value
Total financial assets at fair value	1,116,659,000	1,572,970,000
Liabilities at Fair Value
Total financial liabilities at fair value	132,730,000	184,996,000
Total | Agency mortgage-backed securities
Assets at Fair Value
Financial instruments owned	905,038,000	1,085,621,000
Total | Loans
Assets at Fair Value
Financial instruments owned	77,573,000	228,226,000
Total | Federal agency obligations
Assets at Fair Value
Financial instruments owned	46,021,000	158,774,000
Liabilities at Fair Value
Securities sold but not yet purchased		11,796,000
Total | Corporate debt securities
Assets at Fair Value
Financial instruments owned	30,246,000	14,524,000
Liabilities at Fair Value
Securities sold but not yet purchased	2,520,000	12,254,000
Total | Residential mortgage-backed securities
Assets at Fair Value
Financial instruments owned	23,226,000	18,419,000
Total | Commercial mortgage-backed securities
Assets at Fair Value
Financial instruments owned	4,898,000	38,154,000
Total | Preferred stock
Assets at Fair Value
Financial instruments owned	2,439,000	1,617,000
Liabilities at Fair Value
Securities sold but not yet purchased		914,000
Total | U.S. Government obligations
Assets at Fair Value
Financial instruments owned	2,096,000	5,789,000
Liabilities at Fair Value
Securities sold but not yet purchased	128,504,000	158,059,000
Total | Other debt obligations
Assets at Fair Value
Financial instruments owned	2,074,000	192,000
Total | Equity securities
Assets at Fair Value
Financial instruments owned	703,000	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	671,000	647,000
Total | Derivatives
Assets at Fair Value
Financial instruments owned	1,196,000	1,696,000
Liabilities at Fair Value
Securities sold but not yet purchased	1,704,000	1,971,000
Total | Investments
Assets at Fair Value
Investments	$ 20,478,000	$ 18,310,000