Segment Information - Reconciliation of Revenue from Segments to Consolidated (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues from contracts with customers	$ 74.8	$ 64.3	$ 224.3	$ 231.7
Other Revenue	384.7	91.2	435.6	486.4
Total revenues	459.5	155.5	659.9	718.1
HG Global/ BAM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Other Revenue	14.5	18.0	58.1	61.5
Total revenues	14.5	18.0	58.1	61.5
NSM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	70.7	59.9	211.8	175.3
Other Revenue	0.0	0.0	0.0	0.0
Total revenues	70.7	59.9	211.8	175.3
Kudu
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Other Revenue	16.3	7.5	21.0	11.9
Total revenues	16.3	7.5	21.0	11.9
MediaAlpha
Segment Reporting Information [Line Items]
Revenues from contracts with customers				48.8
Other Revenue				0.0
Total revenues	0.0	0.0	0.0	48.8
Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	4.1	4.4	12.5	7.6
Other Revenue	353.9	65.7	356.5	413.0
Total revenues	358.0	70.1	369.0	420.6
United Kingdom
Segment Reporting Information [Line Items]
Revenues from contracts with customers	13.9	11.2	36.3	35.5
United Kingdom | HG Global/ BAM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
United Kingdom | NSM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	13.9	11.2	36.3	35.5
United Kingdom | Kudu
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
United Kingdom | MediaAlpha
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
United Kingdom | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Specialty Transportation
Segment Reporting Information [Line Items]
Revenues from contracts with customers	21.9	20.6	66.1	60.3
Specialty Transportation | HG Global/ BAM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Specialty Transportation | NSM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	21.9	20.6	66.1	60.3
Specialty Transportation | Kudu
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Specialty Transportation | MediaAlpha
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Specialty Transportation | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Real Estate
Segment Reporting Information [Line Items]
Revenues from contracts with customers	6.0	4.9	32.7	25.4
Real Estate | HG Global/ BAM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Real Estate | NSM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	6.0	4.9	32.7	25.4
Real Estate | Kudu
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Real Estate | MediaAlpha
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Real Estate | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Social Services
Segment Reporting Information [Line Items]
Revenues from contracts with customers	8.6	7.4	22.2	19.7
Social Services | HG Global/ BAM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Social Services | NSM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	8.6	7.4	22.2	19.7
Social Services | Kudu
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Social Services | MediaAlpha
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Social Services | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Pet
Segment Reporting Information [Line Items]
Revenues from contracts with customers	14.7	10.4	39.8	19.9
Pet | HG Global/ BAM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Pet | NSM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	14.7	10.4	39.8	19.9
Pet | Kudu
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Pet | MediaAlpha
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Pet | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Other
Segment Reporting Information [Line Items]
Revenues from contracts with customers	7.7	7.3	20.8	19.0
Other | HG Global/ BAM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Other | NSM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	5.6	5.4	14.7	14.5
Other | Kudu
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Other | MediaAlpha
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Other | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	2.1	1.9	6.1	4.5
Total commission and other revenue
Segment Reporting Information [Line Items]
Revenues from contracts with customers	72.8	61.8	217.9	179.8
Total commission and other revenue | HG Global/ BAM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Total commission and other revenue | NSM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	70.7	59.9	211.8	175.3
Total commission and other revenue | Kudu
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Total commission and other revenue | MediaAlpha
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Total commission and other revenue | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	2.1	1.9	6.1	4.5
Advertising revenues
Segment Reporting Information [Line Items]
Revenues from contracts with customers				48.8
Advertising revenues | HG Global/ BAM
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Advertising revenues | NSM
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Advertising revenues | Kudu
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Advertising revenues | MediaAlpha
Segment Reporting Information [Line Items]
Revenues from contracts with customers				48.8
Advertising revenues | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Revenues from contracts with customers
Segment Reporting Information [Line Items]
Revenues from contracts with customers				228.6
Revenues from contracts with customers | HG Global/ BAM
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Revenues from contracts with customers | NSM
Segment Reporting Information [Line Items]
Revenues from contracts with customers				175.3
Revenues from contracts with customers | Kudu
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Revenues from contracts with customers | MediaAlpha
Segment Reporting Information [Line Items]
Revenues from contracts with customers				48.8
Revenues from contracts with customers | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers				4.5
Product revenues
Segment Reporting Information [Line Items]
Revenues from contracts with customers	2.0	2.5	6.4	3.1
Product revenues | HG Global/ BAM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Product revenues | NSM
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Product revenues | Kudu
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Product revenues | MediaAlpha
Segment Reporting Information [Line Items]
Revenues from contracts with customers				0.0
Product revenues | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	$ 2.0	$ 2.5	$ 6.4	$ 3.1