Goodwill and Other Intangible Assets (Goodwill and Intangible Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Aug. 04, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill	$ 25.9	$ 25.9		$ 18.6
Intangible assets, gross (excluding goodwill)	71.0	43.5
Amortization of Intangible Assets	34.8	24.2
Intangible assets, net (excluding goodwill)	36.2	19.3
Intangible assets, gross (including goodwill)	96.9	69.4
Goodwill and intangible assets	62.1	45.2
MediaAlpha
Goodwill [Line Items]
Goodwill	18.3	18.3
Intangible assets, gross (excluding goodwill)	69.9	42.4
Amortization of Intangible Assets	34.5	24.1
Intangible assets, net (excluding goodwill)	35.4	18.3
Buzzmove
Goodwill [Line Items]
Goodwill	7.6	7.6	$ 7.6
Intangible assets, gross (excluding goodwill)	1.1	1.1
Amortization of Intangible Assets	0.3	0.1
Intangible assets, net (excluding goodwill)	$ 0.8	1.0	$ 1.1
Customer relationships | MediaAlpha
Goodwill [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	8 years
Intangible assets, gross (excluding goodwill)	$ 36.6	10.0
Amortization of Intangible Assets	10.2	6.2
Intangible assets, net (excluding goodwill)	$ 26.4	3.8
Information technology | MediaAlpha
Goodwill [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years
Intangible assets, gross (excluding goodwill)	$ 33.3	32.4
Amortization of Intangible Assets	24.3	17.9
Intangible assets, net (excluding goodwill)	$ 9.0	14.5
Information technology | Buzzmove
Goodwill [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years
Intangible assets, gross (excluding goodwill)	$ 0.5	0.5
Amortization of Intangible Assets	0.2	0.0
Intangible assets, net (excluding goodwill)	$ 0.3	0.5
Trademark | Buzzmove
Goodwill [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years
Intangible assets, gross (excluding goodwill)	$ 0.6	0.6
Amortization of Intangible Assets	0.1	0.1
Intangible assets, net (excluding goodwill)	0.5	0.5
Other intangible assets
Goodwill [Line Items]
Intangible assets, net (excluding goodwill)	$ 36.2	$ 19.3		$ 26.9