Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, acquisition date fair value	$ 424.8		$ 379.9
Goodwill	424.8	$ 428.6	379.9	$ 25.9	$ 25.9
Other intangible assets, accumulated amortization	22.5		53.6
Goodwill and intangible assets, acquisition date fair value	683.4		591.1
Goodwill and intangible assets, net carrying value	660.9		537.5
NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, acquisition date fair value	397.0		354.3
Goodwill	397.0		354.3
Other intangible assets, acquisition date fair value	255.4		140.2
Other intangible assets, accumulated amortization	21.8		8.3
Other intangible assets, net carrying value	233.6		131.9
Kudu investments
Goodwill [Line Items]
Goodwill, acquisition date fair value	7.6		0.0
Goodwill	7.6		0.0
MediaAlpha
Goodwill [Line Items]
Goodwill, acquisition date fair value	0.0		18.3
Goodwill	0.0		18.3
Other intangible assets, acquisition date fair value	0.0		69.9
Other intangible assets, accumulated amortization	0.0		44.8
Other intangible assets, net carrying value	0.0		25.1
Other Operations
Goodwill [Line Items]
Goodwill, acquisition date fair value	20.2		7.3
Goodwill	20.2		7.3
Other intangible assets, acquisition date fair value	1.0		1.1
Other intangible assets, accumulated amortization	0.5		0.5
Other intangible assets, net carrying value	0.5		0.6
Customer relationships | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	110.5		85.3
Other intangible assets, accumulated amortization	15.5		6.0
Other intangible assets, net carrying value	$ 95.0		79.3
Acquired finite-lived intangible assets, weighted average useful life	9 years
Customer relationships | MediaAlpha
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 0.0		26.8
Other intangible assets, accumulated amortization	0.0		4.9
Other intangible assets, net carrying value	$ 0.0		21.9
Acquired finite-lived intangible assets, weighted average useful life	9 years
Trade names | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 57.1		51.2
Other intangible assets, accumulated amortization	4.2		1.8
Other intangible assets, net carrying value	$ 52.9		49.4
Acquired finite-lived intangible assets, weighted average useful life	19 years
Trade names | Kudu investments
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 2.2		0.0
Other intangible assets, accumulated amortization	0.2		0.0
Other intangible assets, net carrying value	$ 2.0		0.0
Acquired finite-lived intangible assets, weighted average useful life	7 years
Trade names | Other Operations
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 0.6		0.6
Other intangible assets, accumulated amortization	0.2		0.2
Other intangible assets, net carrying value	$ 0.4		0.4
Acquired finite-lived intangible assets, weighted average useful life	7 years
Information technology | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 3.6		3.7
Other intangible assets, accumulated amortization	1.1		0.5
Other intangible assets, net carrying value	$ 2.5		3.2
Acquired finite-lived intangible assets, weighted average useful life	5 years
Information technology | MediaAlpha
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 0.0		33.3
Other intangible assets, accumulated amortization	0.0		30.9
Other intangible assets, net carrying value	$ 0.0		2.4
Acquired finite-lived intangible assets, weighted average useful life	5 years
Information technology | Other Operations
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 0.4		0.5
Other intangible assets, accumulated amortization	0.3		0.3
Other intangible assets, net carrying value	$ 0.1		0.2
Acquired finite-lived intangible assets, weighted average useful life	5 years
Other Operations | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 1.7
Other intangible assets, accumulated amortization	0.7
Other intangible assets, net carrying value	$ 1.0
Acquired finite-lived intangible assets, weighted average useful life	3 years
Other Operations | MediaAlpha
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 0.0		9.8
Other intangible assets, accumulated amortization	0.0		9.0
Other intangible assets, net carrying value	$ 0.0		0.8
Acquired finite-lived intangible assets, weighted average useful life	3 years
Renewal Rights | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 82.5
Other intangible assets, accumulated amortization	0.3
Other intangible assets, net carrying value	$ 82.2
Acquired finite-lived intangible assets, weighted average useful life	12 years
Other Intangible Assets
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 258.6		211.2
Other intangible assets, accumulated amortization	22.5		53.6
Other intangible assets, net carrying value	236.1		157.6
Non-controlling interest
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	(25.0)		(40.6)
Total
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	$ 635.9		$ 496.9