Investments - Investments in Variable Interest Entities (Detail) (Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	$ 18,582	[1]	$ 24,309	[1]
Investment	$ 1,602	[1]	$ 7,329	[1]

[1] (1) In addition to its equity method investments, the Company has entered into a credit agreement to lend one of its VIEs (ELX) up to $16.0 million. The commitment period for such credit facility extends through March 28, 2015. Additionally, the Company has entered into a subordinated loan agreement with another of its VIEs (Aqua), whereby the Company agreed to lend the principal sum of $980 thousand. As of September 30, 2014, the Company's maximum exposure to loss with respect to its VIEs is the sum of its equity investment in such VIEs plus the $16 million credit facility and the $980 thousand subordinated loan. Additionally, in connection with the sale of eSpeed (see Note 2-"Divestiture"), the Company has guaranteed all payment obligations of ELX through December 31, 2014 under the Amended and Restated Technology Services Agreement, dated as of March 28, 2012, by and between eSpeed Technology Services L.P. and ELX Futures L.P.