Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, acquisition date fair value	$ 349.7	$ 379.9
Goodwill	349.7	379.9	$ 25.9
Other intangible assets, accumulated amortization	13.8	53.6
Goodwill and intangible assets, acquisition date fair value	524.0	591.1
Goodwill and intangible assets, net carrying value	510.2	537.5
NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, acquisition date fair value	328.8	354.3
Goodwill	328.8	354.3
Other intangible assets, acquisition date fair value	173.2	140.2
Other intangible assets, accumulated amortization	13.3	8.3
Other intangible assets, net carrying value	159.9	131.9
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	20.9	7.3
Goodwill	20.9	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	110.9	85.3
Other intangible assets, accumulated amortization	9.3	6.0
Other intangible assets, net carrying value	101.6	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	2.7	1.8
Other intangible assets, net carrying value	54.4	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	3.8	3.7
Other intangible assets, accumulated amortization	0.7	0.5
Other intangible assets, net carrying value	3.1	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	1.4
Other intangible assets, accumulated amortization	0.6
Other intangible assets, net carrying value	0.8
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
Other Intangible Assets
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	174.3	211.2
Other intangible assets, accumulated amortization	13.8	53.6
Other intangible assets, net carrying value	160.5	157.6
Non-controlling interest
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	(24.1)	(40.6)
Total
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	$ 486.1	$ 496.9