Collateralized Transactions - Agreements to Repurchase (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	$ 372,631	$ 463,377
Financial instruments sold under agreements to repurchase	1,027,775	954,902
Equities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	369,168	451,085
Financial instruments sold under agreements to repurchase	989,812	855,632
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	12,775	54,902
Corporate debt
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	3,463	12,292
Financial instruments sold under agreements to repurchase	25,188	44,368
Other collateralized financings
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	100,000	0
Other collateralized financings | Equities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	76,176	0
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	$ 0