Intangible Assets Subject to Amortization - Finite-Lived Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Intangible Assets Subject to Amortization
Balance at the beginning of the period	$ 2.9	$ 3.6
Acquisitions	0.2	1.1
Amortization	(1.0)	(1.8)	$ (2.3)
Balance at the end of the period	2.1	2.9	3.6
Customer relationships
Intangible Assets Subject to Amortization
Balance at the beginning of the period	0.5	1.5
Acquisitions	0.1
Amortization	(0.5)	(1.0)
Balance at the end of the period	0.1	0.5	1.5
Other
Intangible Assets Subject to Amortization
Balance at the beginning of the period	2.4	2.1
Acquisitions	0.1	1.1
Amortization	(0.5)	(0.8)
Balance at the end of the period	$ 2.0	$ 2.4	$ 2.1