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Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 428.6	$ 25.9	$ 379.9	$ 25.9	$ 25.9
Intangible assets, gross (including goodwill), beginning balance			591.1
Foreign currency translation		0.0		0.0
Goodwill, ending balance	424.8		424.8		379.9
Intangible assets, gross (including goodwill), ending balance	683.4		683.4		591.1
Other Intangible Assets
Goodwill [Roll Forward]
Intangible assets, net (excluding goodwill), beginning balance	239.0	30.7	157.6	36.2	36.2
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	2.2	140.9	29.0	140.9
Acquisitions of businesses			0.0	0.0
Foreign currency translation	(0.5)	(0.4)	0.0	(0.4)
Adjustments to reflect acquisition date fair value	0.0	0.0	5.9	0.0
Amortization	4.6	7.4	(15.4)	(12.9)
Intangible assets, net (excluding goodwill), ending balance	236.1	163.8	236.1	163.8	157.6
Renewal Rights
Goodwill [Roll Forward]
Acquisitions of businesses			82.5
Intangible Asset and Goodwill
Goodwill [Roll Forward]
Intangible assets, gross (including goodwill), beginning balance	667.6	56.6	537.5	62.1	62.1
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	0.0	436.2	0.0	436.2
Acquisitions of businesses			93.8	0.0
Foreign currency translation	(2.1)	(1.6)	(1.6)	(1.6)
Adjustments to reflect acquisition date fair value	0.0	1.4	5.9	1.4
Amortization	4.6	7.4	15.4	12.9
Intangible assets, gross (including goodwill), ending balance	660.9	485.2	660.9	485.2	537.5
Intangible Asset and Goodwill \| Renewal Rights
Goodwill [Roll Forward]
Acquisitions of businesses			82.5
Goodwill
Goodwill [Roll Forward]
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	(2.2)	(295.3)	(29.0)	(295.3)
Acquisitions of businesses			93.8	0.0
Foreign currency translation	(1.6)	(1.2)	(1.6)	(1.2)
Adjustments to reflect acquisition date fair value	0.0	1.4	0.0	1.4
Goodwill, ending balance	424.8	$ 321.4	424.8	321.4
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)	0.0
MediaAlpha \| Goodwill
Goodwill [Roll Forward]
Deconsolidation of MediaAlpha			$ (18.3)	$ 0.0
NSM Holding Co LLC \| Other Intangible Assets
Goodwill [Roll Forward]
Foreign currency translation					(0.7)
NSM Holding Co LLC \| Goodwill
Goodwill [Roll Forward]
Foreign currency translation	$ (1.5)				$ (2.2)