Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment Banking [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer		$ 2,015,179
Investment Banking [Member] | Advisory Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	[1]	1,743,473	$ 1,324,412	$ 1,096,829
Investment Banking [Member] | Underwriting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	[1]	71,691	45,827	36,264
Investment Banking [Member] | Commissions and Related Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	[1]	200,015	$ 205,630	$ 230,913
Investment Management [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer		48,246
Investment Management [Member] | Wealth Management [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer		44,875
Investment Management [Member] | Institutional Asset Management [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer		$ 3,371

[1] Certain balances in prior periods were reclassified to conform to their current presentation. See Note 2 for further information.