Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, acquisition date fair value	$ 428.6		$ 379.9
Goodwill	428.6	$ 349.7	379.9	$ 25.9	$ 25.9
Other intangible assets, accumulated amortization	17.9		53.6
Goodwill and intangible assets, acquisition date fair value	685.5		591.1
Goodwill and intangible assets, net carrying value	667.6		537.5
NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, acquisition date fair value	398.4		354.3
Goodwill	398.4		354.3
Other intangible assets, acquisition date fair value	255.8		140.2
Other intangible assets, accumulated amortization	17.4		8.3
Other intangible assets, net carrying value	238.4		131.9
Kudu
Goodwill [Line Items]
Goodwill, acquisition date fair value	9.8		0.0
Goodwill	9.8		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	20.4		7.3
Goodwill	20.4		7.3
Other intangible assets, acquisition date fair value	1.1		1.1
Other intangible assets, accumulated amortization	0.5		0.5
Other intangible assets, net carrying value	0.6		0.6
Customer relationships | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	110.7		85.3
Other intangible assets, accumulated amortization	12.4		6.0
Other intangible assets, net carrying value	98.3		79.3
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
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	3.5		1.8
Other intangible assets, net carrying value	53.6		49.4
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
Information technology | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	3.7		3.7
Other intangible assets, accumulated amortization	0.9		0.5
Other intangible assets, net carrying value	2.8		3.2
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
Information technology | Other Operations
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	0.5		0.5
Other intangible assets, accumulated amortization	0.3		0.3
Other intangible assets, net carrying value	0.2		0.2
Other Operations | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	1.8
Other intangible assets, accumulated amortization	0.6
Other intangible assets, net carrying value	1.2
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
Renewal Rights | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	82.5
Other intangible assets, accumulated amortization	0.0
Other intangible assets, net carrying value	82.5
Other Intangible Assets
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	256.9		211.2
Other intangible assets, accumulated amortization	17.9		53.6
Other intangible assets, net carrying value	239.0		157.6
Non-controlling interest
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	(25.4)		(40.6)
Total
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	$ 642.2		$ 496.9