Segment Information - Reconciliation of Revenue from Segments to Consolidated (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Revenues from contracts with customers	$ 117.8	$ 83.6
Other Revenue	231.1	94.6
Total revenues	348.9	178.2
Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Other Revenue	(31.3)	(6.7)
Total revenues	(31.3)	(6.7)
Ark
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Other Revenue	175.7	109.1
Total revenues	175.7	109.1
NSM Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenues from contracts with customers	88.5	74.8
Other Revenue	0.0	0.0
Total revenues	88.5	74.8
Asset Management (Kudu)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Other Revenue	34.9	24.1
Total revenues	34.9	24.1
Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	29.3	8.8
Other Revenue	51.8	(31.9)
Total revenues	81.1	(23.1)
United Kingdom
Segment Reporting Information [Line Items]
Revenues from contracts with customers	12.6	13.1
United Kingdom | Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
United Kingdom | Ark
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
United Kingdom | NSM Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenues from contracts with customers	12.6	13.1
United Kingdom | Asset Management (Kudu)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
United Kingdom | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Specialty Transportation
Segment Reporting Information [Line Items]
Revenues from contracts with customers	26.5	20.7
Specialty Transportation | Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Specialty Transportation | Ark
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Specialty Transportation | NSM Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenues from contracts with customers	26.5	20.7
Specialty Transportation | Asset Management (Kudu)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Specialty Transportation | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Pet
Segment Reporting Information [Line Items]
Revenues from contracts with customers	23.4	17.1
Pet | Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Pet | Ark
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Pet | NSM Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenues from contracts with customers	23.4	17.1
Pet | Asset Management (Kudu)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Pet | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Real Estate
Segment Reporting Information [Line Items]
Revenues from contracts with customers	10.5	9.8
Real Estate | Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Real Estate | Ark
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Real Estate | NSM Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenues from contracts with customers	10.5	9.8
Real Estate | Asset Management (Kudu)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Real Estate | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Social Services
Segment Reporting Information [Line Items]
Revenues from contracts with customers	7.0	6.7
Social Services | Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Social Services | Ark
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Social Services | NSM Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenues from contracts with customers	7.0	6.7
Social Services | Asset Management (Kudu)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Social Services | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Diversified B2B
Segment Reporting Information [Line Items]
Revenues from contracts with customers	8.5	7.4
Diversified B2B | Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Diversified B2B | Ark
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Diversified B2B | NSM Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenues from contracts with customers	8.5	7.4
Diversified B2B | Asset Management (Kudu)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Diversified B2B | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Other
Segment Reporting Information [Line Items]
Revenues from contracts with customers	2.9	2.3
Other | Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Other | Ark
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Other | NSM Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Other | Asset Management (Kudu)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Other | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	2.9	2.3
Total commission and other revenue
Segment Reporting Information [Line Items]
Revenues from contracts with customers	91.4	77.1
Total commission and other revenue | Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Total commission and other revenue | Ark
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Total commission and other revenue | NSM Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenues from contracts with customers	88.5	74.8
Total commission and other revenue | Asset Management (Kudu)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Total commission and other revenue | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	2.9	2.3
Product and service revenues
Segment Reporting Information [Line Items]
Revenues from contracts with customers	26.4	6.5
Product and service revenues | Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Product and service revenues | Ark
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Product and service revenues | NSM Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Product and service revenues | Asset Management (Kudu)
Segment Reporting Information [Line Items]
Revenues from contracts with customers	0.0	0.0
Product and service revenues | Other Operations
Segment Reporting Information [Line Items]
Revenues from contracts with customers	$ 26.4	$ 6.5