Collateralized Transactions - Maturities Schedule (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	$ 1,462,117	$ 1,500,406
Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	472,117	385,406
0 - 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	385,000	510,000
31 - 60 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	365,000	465,000
61 - 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	240,000	140,000
Securities loaned
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	459,533	372,631
Securities loaned | Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	459,533	372,631
Securities loaned | 0 - 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	0	0
Securities loaned | 31 - 60 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	0	0
Securities loaned | 61 - 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	0	0
Financial instruments sold under agreements to repurchase
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	952,584	1,027,775
Financial instruments sold under agreements to repurchase | Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	12,584	12,775
Financial instruments sold under agreements to repurchase | 0 - 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	335,000	410,000
Financial instruments sold under agreements to repurchase | 31 - 60 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	365,000	465,000
Financial instruments sold under agreements to repurchase | 61 - 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	240,000	140,000
Other collateralized financings
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	50,000	100,000
Other collateralized financings | Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	0	0
Other collateralized financings | 0 - 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	50,000	100,000
Other collateralized financings | 31 - 60 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	0	0
Other collateralized financings | 61 - 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	$ 0	$ 0