Goodwill and Other Intangible Assets (Summary of Goodwill and Intangible Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill	$ 25.9	$ 25.9	$ 18.6
Intangible assets, net (excluding goodwill)	36.2	19.3
Goodwill and intangible assets	62.1	45.2
MediaAlpha
Goodwill [Line Items]
Goodwill	18.3	18.3
Buzzmove
Goodwill [Line Items]
Goodwill	7.6	7.6
Other intangible assets
Goodwill [Line Items]
Intangible assets, net (excluding goodwill)	36.2	19.3	$ 26.9
Other intangible assets | MediaAlpha
Goodwill [Line Items]
Intangible assets, net (excluding goodwill)	35.4	18.3
Other intangible assets | Buzzmove
Goodwill [Line Items]
Intangible assets, net (excluding goodwill)	0.8	1.0
Discontinued Operations, Held-for-sale
Goodwill [Line Items]
Goodwill and intangible assets	0.0	1.2
Non-controlling interests
Goodwill [Line Items]
Goodwill and intangible assets	(21.1)	(17.1)
Parent
Goodwill [Line Items]
Goodwill and intangible assets	$ 41.0	$ 29.3