Goodwill and Other Intangible Assets - Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	$ 492.8	$ 62.1
NSM Holding Co LLC
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	436.2	0.0
MediaAlpha
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	48.3	53.7
Other
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	8.3	8.4
Non-controlling interest
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	(41.7)	(21.1)
Total
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	$ 451.1	$ 41.0