Collateralized Transactions - Agreements to Repurchase (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	$ 459,533	$ 372,631
Financial instruments sold under agreements to repurchase	952,584	1,027,775
Equities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	459,533	369,168
Financial instruments sold under agreements to repurchase	940,000	989,812
U.S. government obligations
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	0	0
Financial instruments sold under agreements to repurchase	12,584	12,775
Corporate debt
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned		3,463
Financial instruments sold under agreements to repurchase		25,188
Other collateralized financings
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	50,000	100,000
Other collateralized financings | Equities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	50,000	76,176
Other collateralized financings | U.S. government obligations
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	$ 0	0
Other collateralized financings | Corporate debt
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned		$ 23,824