Collateralized Transactions - Maturities Schedule (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	$ 1,500,406	$ 1,418,279
Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	385,406	518,279
0 - 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	510,000	635,000
31 - 60 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	465,000	150,000
61 - 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	140,000	115,000
Securities loaned
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	372,631	463,377
Securities loaned | Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	372,631	463,377
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	1,027,775	954,902
Financial instruments sold under agreements to repurchase | Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	12,775	54,902
Financial instruments sold under agreements to repurchase | 0 - 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	410,000	635,000
Financial instruments sold under agreements to repurchase | 31 - 60 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	465,000	150,000
Financial instruments sold under agreements to repurchase | 61 - 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	140,000	$ 115,000
Other collateralized financings
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	100,000
Other collateralized financings | Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	0
Other collateralized financings | 0 - 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	100,000
Other collateralized financings | 31 - 60 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	0
Other collateralized financings | 61 - 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	$ 0