Commitments and Contingencies - Additional Information (Narrative) (Details) ₪ in Millions					1 Months Ended		3 Months Ended
	Jan. 27, 2017 USD ($)	Feb. 17, 2015 USD ($)	Feb. 09, 2015 USD ($)	Jan. 27, 2015 USD ($)	Aug. 31, 2013 USD ($)	Aug. 31, 2013 ILS (₪)	Mar. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Eligible investor subject to future buyback principal value							$ 29,400,000
Auction rate securities
Loss Contingencies [Line Items]
Total amount of ARS the firm purchased and hold							92,300,000
Amount of ARS committed to purchase from clients							26,400,000
Israel Value-Added Tax Authority Case [Member]
Loss Contingencies [Line Items]
Loss Contingency Accrual, Provision							6,400,000
SEC [Member]
Loss Contingencies [Line Items]
Litigation Settlement, Amount				$ (10,000,000)
Payments for Legal Settlements	$ 5,000,000	$ 5,000,000
SEC [Member] | Unfavorable Regulatory Action, Disgorgement
Loss Contingencies [Line Items]
Litigation Settlement, Amount				(4,200,000)
SEC [Member] | Unfavorable Regulatory Action, Prejudgment Interest
Loss Contingencies [Line Items]
Litigation Settlement, Amount				(753,500)
SEC [Member] | Unfavorable Regulatory Action, Civil Penalties
Loss Contingencies [Line Items]
Litigation Settlement, Amount				(5,100,000)
United States Department of the Treasury [Member]
Loss Contingencies [Line Items]
Litigation Settlement, Amount				$ (10,000,000)
Payments for Legal Settlements	$ 5,000,000		$ 5,000,000
Minimum [Member]
Loss Contingencies [Line Items]
Maximum estimated range of aggregate loss for legal proceedings							0
Eligible investor subject to future buyback potential additional losses related to valuation adjustments							0
Maximum [Member]
Loss Contingencies [Line Items]
Maximum estimated range of aggregate loss for legal proceedings							$ 50,000,000
Pending Litigation [Member] | Israel Value-Added Tax Authority Case [Member]
Loss Contingencies [Line Items]
Litigation Settlement, Amount					$ 5,800,000	₪ 21