Significant Accounting Policies - Depreciation and Amortization (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Depreciation and Amortization Expense [Line Items]
Depreciation and amortization	$ 217.0	$ 206.5	$ 208.3
Deferred contract costs	32.9	25.7	25.5
Accelerated amortization of internally developed software	1.7
Accelerated amortization of deferred charges	3.4	3.3	4.1
Property and equipment
Schedule of Depreciation and Amortization Expense [Line Items]
Depreciation and amortization	32.4	29.0	28.4
Computer software
Schedule of Depreciation and Amortization Expense [Line Items]
Depreciation and amortization	94.5	84.0	78.0
Other
Schedule of Depreciation and Amortization Expense [Line Items]
Depreciation and amortization	$ 57.2	$ 67.8	$ 76.4