Collateralized Transactions - Agreements to Repurchase (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	$ 741,732	$ 707,744
Financial instruments sold under agreements to repurchase	995,667	933,576
Equities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	726,413	696,162
Financial instruments sold under agreements to repurchase	879,472	832,614
U.S. Government Obligations [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	0	0
Financial instruments sold under agreements to repurchase	85,667	73,576
Corporate Debt [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned	15,319	11,582
Financial instruments sold under agreements to repurchase	$ 30,528	$ 27,386