Commitments and Contingencies - Additional Information (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 07, 2019
Loss Contingencies [Line Items]
Lease Expiration Date	Feb. 28, 2034
Rent expense	$ 44,700,000	$ 45,600,000	$ 44,400,000
Unfunded Commitments	1,399,000
Eligible investor subject to future buyback principal value	7,500,000
Auction rate securities
Loss Contingencies [Line Items]
Total amount of ARS the firm purchased and hold	40,700,000
Amount of ARS committed to purchase from clients	7,300,000
Subsequent Event [Member]
Loss Contingencies [Line Items]
Offer of Settlement with the SEC - SCSD Initiative				$ 3,500,000
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	30,000,000
Private Equity Funds [Member]
Loss Contingencies [Line Items]
Unfunded Commitments	$ 1,399,000