Goodwill and Intangible Assets - Change in Definite-Lived Acquired Intangible Assets (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Net Book Value
Ending balance	$ 59.0
Customer Relationships
Gross Book Value
Beginning balance	108.3	$ 108.3
Additions	0.0	0.0
Amortization	0.0	0.0
Disposals	0.0	0.0
Ending balance	108.3	108.3
Accumulated Amortization & Impairment
Beginning balance	(44.2)	(37.6)
Amortization	(5.1)	(4.9)
Disposals	0.0	0.0
Ending balance	(49.3)	(42.5)
Net Book Value
Beginning balance	64.1	70.7
Amortization	(5.1)	(4.9)
Disposals	0.0	0.0
Ending balance	$ 59.0	$ 65.8