Summary of Significant Accounting Policies - Capitalized Software Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Capitalized Software Costs
Capitalized software costs, estimated useful life	3 years
Capitalized software costs, amount capitalized during the period	$ 24.3	$ 20.9	$ 16.5
Amortization of capitalized software costs	17.6	$ 13.2	$ 9.1
Capitalized software costs, estimated amortization expense within the next year	19.1
Capitalized software costs, estimated amortization expense within two years	12.5
Capitalized software costs, estimated amortization expense within three years	5.7
Capitalized software costs, estimated amortization expense within four years	$ 0.1