Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Increase in investments	$ 100,726,000	$ 125,413,000
Decrease in deferred tax assets, net	(222,631,000)	(234,015,000)
Increase in retained earnings	272,780,000	211,678,000
Maximum maturity of short-term investments	90 days
Minimum [Member]
Property, Plant and Equipment [Line Items]
Amortization period, in years	1 year
Fixed assets, useful life	3 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Amortization period, in years	9 years
Fixed assets, useful life	7 years
Software [Member]
Property, Plant and Equipment [Line Items]
Amortization period, in years	3 years
Investment Income and Other, Net [Member]
Property, Plant and Equipment [Line Items]
Non-cash gain on investment resulting from the merger		128,000,000
Foreign Currency Transactions [Member]
Property, Plant and Equipment [Line Items]
Investment gains (losses)	(2,000,000)	(4,100,000)	200,000
BATS and DE [Member] | Change in Ownership Interest [Member]
Property, Plant and Equipment [Line Items]
Increase in investments		3,400,000
Decrease in deferred tax assets, net		1,100,000
Increase in retained earnings		2,300,000
Increase in investment income and other, net		7,200,000
Increase in net income		$ 6,100,000