Goodwill and Other Intangible Assets (Goodwill and Intangible Assets Rollforward) (Details) - USD ($) $ in Millions		12 Months Ended
	Oct. 05, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning balance		$ 25.9	$ 25.9
Acquisitions of businesses			0.0
Foreign currency translation			0.0
Ending balance		379.9	25.9	$ 25.9
Finite-lived Intangible Assets [Roll Forward]
Amortization		(18.8)	(10.7)	(10.5)
Purchase of consolidated subsidiaries, net of cash acquired		295.2	27.6	13.4
Healthplans.com
Finite-lived Intangible Assets [Roll Forward]
Purchase of consolidated subsidiaries, net of cash acquired	$ 28.0
Other Intangible Assets
Finite-lived Intangible Assets [Roll Forward]
Beginning balance		36.2	19.3
Acquisitions of businesses		140.9	0.0
Acquisition of asset groups			27.6
Foreign currency translation		(0.7)	0.0
Amortization		(18.8)	(10.7)
Ending balance		157.6	36.2	$ 19.3
Goodwill
Goodwill [Roll Forward]
Beginning balance		25.9
Acquisitions of businesses		356.5
Foreign currency translation		(2.5)
Ending balance		$ 379.9	$ 25.9