Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Assets at Fair Value
Financial instruments owned	$ 1,100,731	$ 1,554,660
Investments	19,253	18,310
Liabilities at Fair Value
Securities sold but not yet purchased	262,274	184,996
Level 1
Assets at Fair Value
Total financial assets at fair value	23,106	6,994
Liabilities at Fair Value
Total financial liabilities at fair value	256,601	160,216
Level 1 | U.S. government obligations
Assets at Fair Value
Financial instruments owned	21,758	5,789
Liabilities at Fair Value
Securities sold but not yet purchased	249,516	158,059
Level 1 | Preferred stock
Assets at Fair Value
Financial instruments owned		316
Liabilities at Fair Value
Securities sold but not yet purchased	1,864	184
Level 1 | Equities
Assets at Fair Value
Financial instruments owned	664	889
Liabilities at Fair Value
Securities sold but not yet purchased		2
Level 1 | Derivatives
Assets at Fair Value
Financial instruments owned	684
Liabilities at Fair Value
Securities sold but not yet purchased	5,221	1,971
Level 2
Assets at Fair Value
Total financial assets at fair value	1,048,428	1,485,778
Liabilities at Fair Value
Total financial liabilities at fair value	5,673	24,050
Level 2 | Agency mortgage-backed securities
Assets at Fair Value
Financial instruments owned	863,143	1,084,254
Level 2 | Loans
Assets at Fair Value
Financial instruments owned	80,961	228,226
Level 2 | Federal agency obligations
Assets at Fair Value
Financial instruments owned	69,973	158,774
Liabilities at Fair Value
Securities sold but not yet purchased		11,796
Level 2 | Corporate debt securities
Assets at Fair Value
Financial instruments owned	22,725	14,524
Liabilities at Fair Value
Securities sold but not yet purchased	5,673	12,254
Level 2 | Residential mortgage-backed securities
Assets at Fair Value
Financial instruments owned	11,053
Level 2 | U.S. government obligations
Assets at Fair Value
Financial instruments owned	99
Level 2 | Other debt obligations
Assets at Fair Value
Financial instruments owned	377
Level 2 | Commercial mortgage-backed securities
Assets at Fair Value
Financial instruments owned	97
Level 3
Assets at Fair Value
Total financial assets at fair value	48,450	80,198
Liabilities at Fair Value
Total financial liabilities at fair value		730
Level 3 | Agency mortgage-backed securities
Assets at Fair Value
Financial instruments owned	8	1,367
Level 3 | Residential mortgage-backed securities
Assets at Fair Value
Financial instruments owned	11,281	18,419
Level 3 | Other debt obligations
Assets at Fair Value
Financial instruments owned	10,487	192
Level 3 | Commercial mortgage-backed securities
Assets at Fair Value
Financial instruments owned	3,819	38,154
Level 3 | Preferred stock
Assets at Fair Value
Financial instruments owned		1,301
Liabilities at Fair Value
Securities sold but not yet purchased		730
Level 3 | Equities
Assets at Fair Value
Financial instruments owned	98	112
Level 3 | Collateralized debt obligations
Assets at Fair Value
Financial instruments owned	551	647
Level 3 | Derivatives
Assets at Fair Value
Financial instruments owned	2,953	1,696
Level 3 | Investments
Assets at Fair Value
Investments	19,253	18,310
Total
Assets at Fair Value
Total financial assets at fair value	1,119,984	1,572,970
Liabilities at Fair Value
Total financial liabilities at fair value	262,274	184,996
Total | Agency mortgage-backed securities
Assets at Fair Value
Financial instruments owned	863,151	1,085,621
Total | Loans
Assets at Fair Value
Financial instruments owned	80,961	228,226
Total | Federal agency obligations
Assets at Fair Value
Financial instruments owned	69,973	158,774
Liabilities at Fair Value
Securities sold but not yet purchased		11,796
Total | Corporate debt securities
Assets at Fair Value
Financial instruments owned	22,725	14,524
Liabilities at Fair Value
Securities sold but not yet purchased	5,673	12,254
Total | Residential mortgage-backed securities
Assets at Fair Value
Financial instruments owned	22,334	18,419
Total | U.S. government obligations
Assets at Fair Value
Financial instruments owned	21,857	5,789
Liabilities at Fair Value
Securities sold but not yet purchased	249,516	158,059
Total | Other debt obligations
Assets at Fair Value
Financial instruments owned	10,864	192
Total | Commercial mortgage-backed securities
Assets at Fair Value
Financial instruments owned	3,916	38,154
Total | Preferred stock
Assets at Fair Value
Financial instruments owned		1,617
Liabilities at Fair Value
Securities sold but not yet purchased	1,864	914
Total | Equities
Assets at Fair Value
Financial instruments owned	762	1,001
Liabilities at Fair Value
Securities sold but not yet purchased		2
Total | Collateralized debt obligations
Assets at Fair Value
Financial instruments owned	551	647
Total | Derivatives
Assets at Fair Value
Financial instruments owned	3,637	1,696
Liabilities at Fair Value
Securities sold but not yet purchased	5,221	1,971
Total | Investments
Assets at Fair Value
Investments	$ 19,253	$ 18,310