Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash and cash equivalents
Maximum maturity of short-term investments	90 days
JV
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Ownership percentage	50.00%
VIE
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Ownership percentage	50.00%
Minimum
Property, Plant and Equipment [Line Items]
Fixed assets, useful life	3 years
Goodwill and intangible assets
Amortization period	1 year
Maximum
Property, Plant and Equipment [Line Items]
Fixed assets, useful life	7 years
Goodwill and intangible assets
Amortization period	8 years
Investment income and other, net
Investments
Non-cash gain recognized on investment in Knight Common Stock			$ 128.0
Foreign currency translation and foreign currency forward contracts
Investment losses	$ 0.7	$ 2.0	$ 4.1
Software
Goodwill and intangible assets
Amortization period	3 years