Goodwill and Other Intangible Assets (Summary of Goodwill and Intangible Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill, fair value	$ 379.9	$ 25.9
Goodwill	379.9	25.9	$ 25.9
Other intangible assets, accumulated amortization	53.6	34.8
Other intangible assets, net	155.4
Other intangible assets, gross (including goodwill)	591.1	96.9
Goodwill and other intangible assets	537.5	62.1
Goodwill, foreign currency translation gain		0.0
NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, fair value	354.3	0.0
Goodwill	354.3	0.0
Other intangible assets, fair value	140.2	0.0
Other intangible assets, accumulated amortization	8.3	0.0
Other intangible assets, net	131.9	0.0
MediaAlpha
Goodwill [Line Items]
Goodwill, fair value	18.3	18.3
Goodwill	18.3	18.3
Other intangible assets, fair value	69.9	69.9
Other intangible assets, accumulated amortization	44.8	34.5
Other intangible assets, net	25.1	35.4
Buzzmove
Goodwill [Line Items]
Goodwill, fair value	7.3	7.6
Goodwill	7.3	7.6
Other intangible assets, fair value	1.1	1.1
Other intangible assets, accumulated amortization	0.5	0.3
Other intangible assets, net	$ 0.6	0.8
Customer Relationships | NSM Holding Co LLC
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	9 years
Other intangible assets, fair value	$ 85.3	0.0
Other intangible assets, accumulated amortization	6.0	0.0
Other intangible assets, net	$ 79.3	0.0
Customer Relationships | MediaAlpha
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	9 years
Other intangible assets, fair value	$ 26.8	26.8
Other intangible assets, accumulated amortization	4.9	2.4
Other intangible assets, net	$ 21.9	24.4
Trade Names | NSM Holding Co LLC
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	20 years
Other intangible assets, fair value	$ 51.2	0.0
Other intangible assets, accumulated amortization	1.8	0.0
Other intangible assets, net	$ 49.4	0.0
Information Technology | NSM Holding Co LLC
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	5 years
Other intangible assets, fair value	$ 3.7	0.0
Other intangible assets, accumulated amortization	0.5	0.0
Other intangible assets, net	$ 3.2	0.0
Information Technology | MediaAlpha
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	5 years
Other intangible assets, fair value	$ 33.3	33.3
Other intangible assets, accumulated amortization	30.9	24.3
Other intangible assets, net	$ 2.4	9.0
Information Technology | Buzzmove
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	5 years
Other intangible assets, fair value	$ 0.5	0.5
Other intangible assets, accumulated amortization	0.3	0.2
Other intangible assets, net	$ 0.2	0.3
Other | MediaAlpha
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	3 years
Other intangible assets, fair value	$ 9.8	9.8
Other intangible assets, accumulated amortization	9.0	7.8
Other intangible assets, net	0.8	2.0
Other Intangible Assets
Goodwill [Line Items]
Other intangible assets, fair value	211.2	71.0
Other intangible assets, accumulated amortization	53.6	34.8
Other intangible assets, net	157.6	36.2
Other Intangible Assets | NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, foreign currency translation gain	$ (0.7)
Trademarks | Buzzmove
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	7 years
Other intangible assets, fair value	$ 0.6	0.6
Other intangible assets, accumulated amortization	0.2	0.1
Other intangible assets, net	0.4	0.5
Non-controlling interests
Goodwill [Line Items]
Goodwill and other intangible assets	(40.6)	(21.1)
Parent
Goodwill [Line Items]
Goodwill and other intangible assets	496.9	41.0
Goodwill
Goodwill [Line Items]
Goodwill	379.9	$ 25.9
Goodwill, foreign currency translation gain	(2.5)
Goodwill | NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, foreign currency translation gain	(2.2)
Goodwill | Buzzmove
Goodwill [Line Items]
Goodwill, foreign currency translation gain	$ (0.3)