Commitments and Contingencies - Additional Information (Narrative) (Details) - USD ($)						12 Months Ended
		Jan. 27, 2017	Feb. 17, 2015	Feb. 09, 2015	Jan. 27, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Contingencies [Line Items]
Operating leases expiring at various dates through						2028
Rent expense						$ 45,600,000	$ 44,400,000	$ 45,900,000
Unfunded Commitments						1,400,000
Eligible investor subject to future buyback principal value						25,300,000
Auction rate securities
Loss Contingencies [Line Items]
Total amount of ARS the firm purchased and hold						113,900,000
Amount of ARS committed to purchase from clients						11,000,000
SEC
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 | Unfavorable Regulatory Action, Disgorgement
Loss Contingencies [Line Items]
Litigation Settlement, Expense					4,200,000
SEC | Unfavorable Regulatory Action, Prejudgment Interest
Loss Contingencies [Line Items]
Litigation Settlement Interest					753,500
SEC | Unfavorable Regulatory Action, Civil Penalties
Loss Contingencies [Line Items]
Litigation Settlement, Expense					5,100,000
United States Department of the Treasury
Loss Contingencies [Line Items]
Litigation Settlement, Amount Awarded to Other Party					$ 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						37,000,000
Private Equity Funds [Member]
Loss Contingencies [Line Items]
Unfunded Commitments	[1]					$ 1,400,000

[1] Includes private equity funds and private equity fund of funds with a focus on diversified portfolios, real estate and global natural resources. Due to the illiquid nature of these funds, investors are not permitted to make withdrawals without the consent of the general partner. The lock-up period of the private equity funds can extend to 10 years.