Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended				12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015 USD ($) Segment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | Segment					2
Maturity days for highly liquid investments					The Company considers all highly liquid investments with original maturities of 90 days or less at the date of acquisition that are not segregated under regulatory requirements, other than those used for trading purposes, to be cash equivalents.
Allowance for doubtful accounts on accrued commissions receivable	$ 21,500,000				$ 21,500,000	$ 26,300,000
Ownership interest					The Company's policy is to consolidate all entities of which it owns more than 50% unless it does not have control over the entity.
Impairment of goodwill or indefinite-lived intangible assets					$ 0	$ 0	$ 0
Restricted stock saleable period Description					Shares of restricted stock are generally saleable by partners in five to ten years.
Payout period for post-termination awards					Four equal yearly installments
Limited partnership units exchangeability into Class A common stock					Certain limited partnership units are granted exchangeability into Class A common stock on a one-for-one basis (subject to adjustment).
Percentage to preferred units	0.6875%	0.6875%	0.6875%	0.6875%	2.75%
Software, Including Software Development Costs [Member]
Significant Accounting Policies [Line Items]
Software amortization period					3 years
Minimum [Member]
Significant Accounting Policies [Line Items]
Restricted stock saleable period					5 years
Minimum [Member] | Computer and Communications Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of computer software					3 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Restricted stock saleable period					10 years
Maximum [Member] | Computer and Communications Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of computer software					5 years