Summary of Significant Accounting Policies - Components of Lease Expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Accounting Policies [Abstract]
Operating lease cost	$ 19.0	$ 52.2
Finance lease cost
Amortization of right-of-use assets	0.4	1.2
Interest on lease liabilities	0.2	0.2
Total finance lease cost	0.6	1.4
Short-term lease costs	0.2	0.4
Variable lease cost	5.1	11.8
Sublease income	(0.1)	(0.7)
Total lease cost	$ 24.8	$ 65.1