Segment Information (Revenue) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment reporting information
Total revenues from contracts with customers									$ 294.3	$ 401.2	$ 163.6
Other revenues									599.1	(32.1)	210.2
Revenues	$ 175.3	$ 155.5	$ 129.0	$ 433.6	$ 6.0	$ 198.7	$ 122.3	$ 42.1	893.4	369.1	373.8
HG Global-BAM
Segment reporting information
Total revenues from contracts with customers									0.0	0.0	0.0
Other revenues									66.6	24.3	23.3
Revenues									66.6	24.3	23.3
NSM Holding Co LLC
Segment reporting information
Total revenues from contracts with customers									233.1	101.6
Other revenues									0.0	0.0
Revenues									233.1	101.6	0.0
Kudu
Segment reporting information
Total revenues from contracts with customers									0.0
Other revenues									21.2
Revenues									21.2
MediaAlpha
Segment reporting information
Total revenues from contracts with customers									48.8	295.5	163.2
Other revenues									0.0	1.6	0.0
Revenues									48.8	297.1	163.2
Other Operations
Segment reporting information
Total revenues from contracts with customers									12.4	4.1	0.4
Other revenues									511.3	(58.0)	186.9
Revenues									523.7	(53.9)	187.3
Specialty Transportation
Segment reporting information
Total revenues from contracts with customers									77.6	28.3
Specialty Transportation | HG Global-BAM
Segment reporting information
Total revenues from contracts with customers										0.0
Specialty Transportation | NSM Holding Co LLC
Segment reporting information
Total revenues from contracts with customers									77.6	28.3
Specialty Transportation | Kudu
Segment reporting information
Total revenues from contracts with customers									0.0
Specialty Transportation | MediaAlpha
Segment reporting information
Total revenues from contracts with customers									0.0	0.0
Specialty Transportation | Other Operations
Segment reporting information
Total revenues from contracts with customers									0.0	0.0
Real Estate
Segment reporting information
Total revenues from contracts with customers									34.7	17.8
Real Estate | HG Global-BAM
Segment reporting information
Total revenues from contracts with customers										0.0
Real Estate | NSM Holding Co LLC
Segment reporting information
Total revenues from contracts with customers									34.7	17.8
Real Estate | Kudu
Segment reporting information
Total revenues from contracts with customers									0.0
Real Estate | MediaAlpha
Segment reporting information
Total revenues from contracts with customers									0.0	0.0
Real Estate | Other Operations
Segment reporting information
Total revenues from contracts with customers									0.0	0.0
Social Services
Segment reporting information
Total revenues from contracts with customers									25.9	14.7
Social Services | HG Global-BAM
Segment reporting information
Total revenues from contracts with customers										0.0
Social Services | NSM Holding Co LLC
Segment reporting information
Total revenues from contracts with customers									25.9	14.7
Social Services | Kudu
Segment reporting information
Total revenues from contracts with customers									0.0
Social Services | MediaAlpha
Segment reporting information
Total revenues from contracts with customers									0.0	0.0
Social Services | Other Operations
Segment reporting information
Total revenues from contracts with customers									0.0	0.0
Pet
Segment reporting information
Total revenues from contracts with customers									30.0
Pet | NSM Holding Co LLC
Segment reporting information
Total revenues from contracts with customers									30.0
Pet | Kudu
Segment reporting information
Total revenues from contracts with customers									0.0
Pet | MediaAlpha
Segment reporting information
Total revenues from contracts with customers									0.0
Pet | Other Operations
Segment reporting information
Total revenues from contracts with customers									0.0
Other
Segment reporting information
Total revenues from contracts with customers									25.9	16.9	0.4
Other | HG Global-BAM
Segment reporting information
Total revenues from contracts with customers										0.0	0.0
Other | NSM Holding Co LLC
Segment reporting information
Total revenues from contracts with customers									19.0	12.8
Other | Kudu
Segment reporting information
Total revenues from contracts with customers									0.0
Other | MediaAlpha
Segment reporting information
Total revenues from contracts with customers									0.0	0.0	0.0
Other | Other Operations
Segment reporting information
Total revenues from contracts with customers									6.9	4.1	0.4
Total Commission and Other Revenue
Segment reporting information
Total revenues from contracts with customers									240.0	105.7	0.4
Total Commission and Other Revenue | HG Global-BAM
Segment reporting information
Total revenues from contracts with customers										0.0	0.0
Total Commission and Other Revenue | NSM Holding Co LLC
Segment reporting information
Total revenues from contracts with customers									233.1	101.6
Total Commission and Other Revenue | Kudu
Segment reporting information
Total revenues from contracts with customers									0.0
Total Commission and Other Revenue | MediaAlpha
Segment reporting information
Total revenues from contracts with customers									0.0	0.0	0.0
Total Commission and Other Revenue | Other Operations
Segment reporting information
Total revenues from contracts with customers									6.9	4.1	0.4
Advertising revenues
Segment reporting information
Total revenues from contracts with customers									48.8	295.5	163.2
Advertising revenues | HG Global-BAM
Segment reporting information
Total revenues from contracts with customers									0.0	0.0	0.0
Advertising revenues | NSM Holding Co LLC
Segment reporting information
Total revenues from contracts with customers									0.0	0.0
Advertising revenues | Kudu
Segment reporting information
Total revenues from contracts with customers									0.0
Advertising revenues | MediaAlpha
Segment reporting information
Total revenues from contracts with customers									48.8	295.5	163.2
Advertising revenues | Other Operations
Segment reporting information
Total revenues from contracts with customers									0.0	0.0	$ 0.0
Products
Segment reporting information
Total revenues from contracts with customers									5.5
Products | HG Global-BAM
Segment reporting information
Total revenues from contracts with customers									0.0
Products | NSM Holding Co LLC
Segment reporting information
Total revenues from contracts with customers									0.0
Products | Kudu
Segment reporting information
Total revenues from contracts with customers									0.0
Products | MediaAlpha
Segment reporting information
Total revenues from contracts with customers									0.0
Products | Other Operations
Segment reporting information
Total revenues from contracts with customers									5.5
United Kingdom
Segment reporting information
Total revenues from contracts with customers									45.9	28.0
United Kingdom | HG Global-BAM
Segment reporting information
Total revenues from contracts with customers									0.0	0.0
United Kingdom | NSM Holding Co LLC
Segment reporting information
Total revenues from contracts with customers									45.9	28.0
United Kingdom | Kudu
Segment reporting information
Total revenues from contracts with customers									0.0
United Kingdom | MediaAlpha
Segment reporting information
Total revenues from contracts with customers									0.0	0.0
United Kingdom | Other Operations
Segment reporting information
Total revenues from contracts with customers									$ 0.0	$ 0.0