Goodwill and Intangible Assets - Change in Acquired Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019
Accumulated Amortization & Impairment
Amortization	$ (6.6)	$ (6.6)
Net Book Value
Amortization	(6.6)	(6.6)
Ending balance	64.1
Amortization of acquired intangibles	6.6	6.6	$ 6.6
Estimated annual amortization expense for each of next five fiscal years
2020				$ 6.6
2021				6.6
2022				6.5
2023				6.4
2024				6.4
Thereafter				31.6
Total	64.1			64.1
Client 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	108.3
Accumulated Amortization & Impairment
Beginning Balance	(37.6)	(31.0)
Amortization	(6.6)	(6.6)
Disposals	0.0	0.0
Ending Balance	(44.2)	(37.6)	(31.0)
Net Book Value
Beginning balance	70.7	77.3
Amortization	(6.6)	(6.6)
Disposals	0.0	0.0
Ending balance	64.1	70.7	77.3
Estimated annual amortization expense for each of next five fiscal years
Total	$ 64.1	70.7	$ 77.3	$ 64.1
Client relationships | Minimum
Net Book Value
Useful life	3 years
Client relationships | Maximum
Net Book Value
Useful life	10 years
Trade name
Gross Book Value
Additions	$ 1.0	$ 1.0