Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 379.9	$ 25.9	$ 25.9
Intangible assets, gross (including goodwill), beginning balance	591.1
Amortization	(6.6)
Goodwill, ending balance	349.7		379.9
Intangible assets, gross (including goodwill), ending balance	524.0		591.1
Other Intangible Assets
Goodwill [Roll Forward]
Intangible assets, net (excluding goodwill), beginning balance	157.6	36.2	36.2
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	26.8	0.0
Acquisitions of businesses	0.0
Foreign currency translation	0.3
Adjustments to reflect acquisition date fair value	5.9
Amortization	(6.6)	(2.9)
Intangible assets, net (excluding goodwill), ending balance	160.5	33.3	157.6
Intangible Asset and Goodwill
Goodwill [Roll Forward]
Intangible assets, gross (including goodwill), beginning balance	537.5	62.1	62.1
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	0.0	0.0
Acquisitions of businesses	13.4
Foreign currency translation	1.5
Adjustments to reflect acquisition date fair value	6.2
Amortization	6.6	2.9
Intangible assets, gross (including goodwill), ending balance	510.2	59.2	537.5
Goodwill
Goodwill [Roll Forward]
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	(26.8)	0.0
Acquisitions of businesses	13.4
Foreign currency translation	1.2
Adjustments to reflect acquisition date fair value	0.3
Goodwill, ending balance	349.7	25.9
MediaAlpha \| Other Intangible Assets
Goodwill [Roll Forward]
Deconsolidation of MediaAlpha	(23.5)	$ 0.0
MediaAlpha \| Intangible Asset and Goodwill
Goodwill [Roll Forward]
Deconsolidation of MediaAlpha	(41.8)
MediaAlpha \| Goodwill
Goodwill [Roll Forward]
Deconsolidation of MediaAlpha	(18.3)
NSM Holding Co LLC \| Other Intangible Assets
Goodwill [Roll Forward]
Foreign currency translation	0.3		(0.7)
NSM Holding Co LLC \| Goodwill
Goodwill [Roll Forward]
Foreign currency translation	$ 1.0		$ (2.2)

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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