Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	3 Months Ended					6 Months Ended	9 Months Ended	12 Months Ended			0 Months Ended	12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013 Segment	Mar. 31, 2012 Segment	Jun. 30, 2012 Segment	Dec. 31, 2012 Segment	Dec. 31, 2013 Segment	Dec. 31, 2012	Dec. 31, 2011	Jun. 28, 2013 NASDAQ OMX [Member]	Dec. 31, 2013 NASDAQ OMX [Member]	Dec. 31, 2013 Software, Including Software Development Costs [Member]	Dec. 31, 2013 Minimum [Member]	Dec. 31, 2013 Minimum [Member] Computer and Communications Equipment [Member]	Dec. 31, 2013 Maximum [Member]	Dec. 31, 2013 Maximum [Member] Computer and Communications Equipment [Member]
Significant Accounting Policies [Line Items]
Other revenues								$ 44,643,000	$ 4,495,000	$ 4,174,000		$ 39,500,000
Purchase price in cash								750,000,000			750,000,000
Contingent consideration earn-out period												15 years
Consolidated gross revenues target												25,000,000
Aggregate number of shares to be issued											14,883,705
Gain on divestiture and sale of investments								723,147,000				723,100,000
Proceed from sale of investment in LME									52,500,000
Consideration paid for shares acquired in London Metals Exchange (LME)									0
Carrying value of shares in London Metals Exchange (LME)									0
Number of reportable segments				2	1	2	2	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.
Marketable securities	45,002,000						0	45,002,000	0			39,500,000
Allowance for doubtful accounts on accrued commissions receivable	15,400,000						13,500,000	15,400,000	13,500,000
Software amortization period													3 years
Useful life of computer software															3 years		5 years
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 good standing after five to ten years
Restricted stock saleable period														5 years		10 years
Payout period for post-termination awards								4 equal yearly installment
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%