Contingencies - Narrative (Details) - USD ($)					9 Months Ended
	Jan. 27, 2017	Feb. 17, 2015	Feb. 09, 2015	Jan. 27, 2015	Sep. 30, 2017
Loss Contingencies [Line Items]
Auction Rate Securities Committed To Purchase related to settlements with regulators					$ 5,000,000
Amount Of Auction Rate Securities Firm Purchased And Hold Net of Redemptions					109,000,000
Auction Rate Securities Committed To Purchase From Clients under Legal Settlements and Awards					10,500,000
Eligible investor subject to future buyback potential additional losses related to valuation adjustments					30,600,000
Eligible Investor Subject To Future Buyback Potential Additional Losses Related To Valuation Adjustments					0
SEC [Member]
Loss Contingencies [Line Items]
Litigation Settlement, Amount Awarded to Other Party				$ 10,000,000
Payments for legal settlements	$ 5,000,000	$ 5,000,000
SEC [Member] | Disgorgement [Member]
Loss Contingencies [Line Items]
Litigation Settlement, Amount Awarded to Other Party				4,200,000
SEC [Member] | Prejudgment Interest [Member]
Loss Contingencies [Line Items]
Litigation Settlement, Amount Awarded to Other Party				753,500
SEC [Member] | Civil Penalties [Member]
Loss Contingencies [Line Items]
Litigation Settlement, Amount Awarded to Other Party				5,100,000
United States Department of the Treasury [Member]
Loss Contingencies [Line Items]
Litigation Settlement, Amount Awarded to Other Party	$ 5,000,000			$ 10,000,000
Payments for legal settlements			$ 5,000,000
Minimum [Member]
Loss Contingencies [Line Items]
Maximum estimated range of aggregate loss for legal proceedings					0
Maximum [Member]
Loss Contingencies [Line Items]
Maximum estimated range of aggregate loss for legal proceedings					$ 29,000,000
Maximum [Member] | Auction Rate Securities [Member]
Loss Contingencies [Line Items]
Commitment to purchase ARS period maximum					2020