Stock-based Payments - Summary of RSU Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-IPO stock-based compensation	$ 10.8	$ 6.2	$ 25.7	$ 20.5
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-IPO stock-based compensation	15.2	7.3	30.8	23.4
Unrecognized compensation expense related to RSUs	44.2		44.2
Time-Based RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-IPO stock-based compensation	14.5	5.4	27.3	17.8
Unrecognized compensation expense related to RSUs	41.6		41.6
Co-Investment RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-IPO stock-based compensation	0.1	0.3	(0.1)	1.0
Unrecognized compensation expense related to RSUs	0.2		0.2
Performance-Based RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-IPO stock-based compensation	0.0	0.0	0.0	0.0
Unrecognized compensation expense related to RSUs	2.4		2.4
Restricted Stock Units (RSUs), Equity Classification
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-IPO stock-based compensation	14.6	5.7	27.2	18.8
Unrecognized compensation expense related to RSUs	44.2		44.2
Restricted Stock Units (RSUs), Liability Classification
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-IPO stock-based compensation	0.6	$ 1.6	3.6	$ 4.6
Unrecognized compensation expense related to RSUs	$ 0.0		$ 0.0