Summary of Significant Accounting Policies - Components of Lease Expense (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Accounting Policies [Abstract]
Operating lease cost	$ 18.4	$ 33.2
Finance lease cost
Amortization of right-of-use assets	0.4	0.8
Interest on lease liabilities	0.0	0.0
Total finance lease cost	0.4	0.8
Short-term lease costs	0.1	0.2
Variable lease cost	2.8	6.7
Sublease income	(0.1)	(0.6)
Total lease cost	$ 21.6	$ 40.3