Property, Equipment, and Software - Allocation of Depreciation and Amortization Expense between Cost of Services and Selling, General and Administrative Expenses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Property, Plant and Equipment [Line Items]
Total depreciation and amortization	$ 4.9	$ 3.2
Cost of services
Property, Plant and Equipment [Line Items]
Total depreciation and amortization	4.6	2.9
Selling, general and administrative
Property, Plant and Equipment [Line Items]
Total depreciation and amortization	$ 0.3	$ 0.3