Business Combinations, Goodwill and Intangible Assets, Identifiable Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Identifiable intangible assets
Original cost	$ 43.9	$ 43.9
Accumulated amortization	(14.3)	(11.5)
Net book value	29.6	32.4
Amortization of definite-lived intangible assets	$ 2.8	$ 1.8	$ 1.4
Noncompete Agreements
Finite-Lived Intangible Assets
Weighted-Average Amortization Period (in years)	5 years	5 years
Identifiable intangible assets
Original cost	$ 5.4	$ 5.4
Accumulated amortization	(1.3)	(0.3)
Net book value	$ 4.1	$ 5.1
Trade Names
Finite-Lived Intangible Assets
Weighted-Average Amortization Period (in years)	10 years	10 years
Identifiable intangible assets
Original cost	$ 2.4	$ 2.4
Accumulated amortization	(0.3)	(0.1)
Net book value	$ 2.1	$ 2.3
Customer relationships
Finite-Lived Intangible Assets
Weighted-Average Amortization Period (in years)	24 years	24 years
Identifiable intangible assets
Original cost	$ 36.1	$ 36.1
Accumulated amortization	(12.7)	(11.1)
Net book value	$ 23.4	$ 25.0