Intangible Assets and Goodwill - Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Amortizable intangible assets
Carrying Amount	$ 401.6	$ 232.1
Accumulated Amortization	118.5	89.5
Client relationships
Amortizable intangible assets
Carrying Amount	373.3	203.6
Accumulated Amortization	97.3	71.0
Other
Amortizable intangible assets
Carrying Amount	28.3	28.5
Accumulated Amortization	$ 21.2	$ 18.5