Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets at Fair Value
Total financial instruments owned, at fair value	$ 883	$ 1,096,181
Investments	17,884	20,478
Liabilities at Fair Value
Securities sold, but not yet purchased		132,730
Level 1
Assets at Fair Value
Total financial instruments owned, at fair value		5,342
Total		5,342
Liabilities at Fair Value
Total financial liabilities, at fair value		130,210
Level 1 | Preferred stock
Assets at Fair Value
Total financial instruments owned, at fair value		2,439
Level 1 | U.S. government obligations
Assets at Fair Value
Total financial instruments owned, at fair value		1,996
Liabilities at Fair Value
Securities sold, but not yet purchased		128,504
Level 1 | Equity securities
Assets at Fair Value
Total financial instruments owned, at fair value		675
Liabilities at Fair Value
Securities sold, but not yet purchased		2
Level 1 | Derivatives
Assets at Fair Value
Total financial instruments owned, at fair value		232
Liabilities at Fair Value
Securities sold, but not yet purchased		1,704
Level 2
Assets at Fair Value
Total financial instruments owned, at fair value		1,087,899
Total		1,087,899
Liabilities at Fair Value
Total financial liabilities, at fair value		2,520
Level 2 | Agency mortgage-backed securities
Assets at Fair Value
Total financial instruments owned, at fair value		903,928
Level 2 | Loans
Assets at Fair Value
Total financial instruments owned, at fair value		77,573
Level 2 | Federal agency obligations
Assets at Fair Value
Total financial instruments owned, at fair value		46,021
Level 2 | Corporate debt securities
Assets at Fair Value
Total financial instruments owned, at fair value		30,246
Liabilities at Fair Value
Securities sold, but not yet purchased		2,520
Level 2 | Residential mortgage-backed securities
Assets at Fair Value
Total financial instruments owned, at fair value		23,077
Level 2 | Commercial mortgage-backed securities
Assets at Fair Value
Total financial instruments owned, at fair value		4,880
Level 2 | U.S. government obligations
Assets at Fair Value
Total financial instruments owned, at fair value		100
Level 2 | Other debt obligations
Assets at Fair Value
Total financial instruments owned, at fair value		2,074
Level 3
Assets at Fair Value
Total financial instruments owned, at fair value		2,940
Total		23,418
Level 3 | Agency mortgage-backed securities
Assets at Fair Value
Total financial instruments owned, at fair value		1,110
Level 3 | Residential mortgage-backed securities
Assets at Fair Value
Total financial instruments owned, at fair value		149
Level 3 | Commercial mortgage-backed securities
Assets at Fair Value
Total financial instruments owned, at fair value		18
Level 3 | Equity securities
Assets at Fair Value
Total financial instruments owned, at fair value		28
Level 3 | Collateralized debt obligations
Assets at Fair Value
Total financial instruments owned, at fair value		671
Level 3 | Derivatives
Assets at Fair Value
Total financial instruments owned, at fair value		964
Level 3 | Investments
Assets at Fair Value
Investments	17,900	20,478
Total
Assets at Fair Value
Total financial instruments owned, at fair value		1,096,181
Total		1,116,659
Liabilities at Fair Value
Total financial liabilities, at fair value		132,730
Total | Agency mortgage-backed securities
Assets at Fair Value
Total financial instruments owned, at fair value		905,038
Total | Loans
Assets at Fair Value
Total financial instruments owned, at fair value		77,573
Total | Federal agency obligations
Assets at Fair Value
Total financial instruments owned, at fair value		46,021
Total | Corporate debt securities
Assets at Fair Value
Total financial instruments owned, at fair value		30,246
Liabilities at Fair Value
Securities sold, but not yet purchased		2,520
Total | Residential mortgage-backed securities
Assets at Fair Value
Total financial instruments owned, at fair value		23,226
Total | Commercial mortgage-backed securities
Assets at Fair Value
Total financial instruments owned, at fair value		4,898
Total | Preferred stock
Assets at Fair Value
Total financial instruments owned, at fair value		2,439
Total | U.S. government obligations
Assets at Fair Value
Total financial instruments owned, at fair value		2,096
Liabilities at Fair Value
Securities sold, but not yet purchased		128,504
Total | Other debt obligations
Assets at Fair Value
Total financial instruments owned, at fair value		2,074
Total | Equity securities
Assets at Fair Value
Total financial instruments owned, at fair value		703
Liabilities at Fair Value
Securities sold, but not yet purchased		2
Total | Collateralized debt obligations
Assets at Fair Value
Total financial instruments owned, at fair value		671
Total | Derivatives
Assets at Fair Value
Total financial instruments owned, at fair value		1,196
Liabilities at Fair Value
Securities sold, but not yet purchased		1,704
Total | Investments
Assets at Fair Value
Investments		$ 20,478