Segment Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		5 Months Ended
	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2018
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		$ 68.5	$ 99.4
Net investment income		22.9	16.0
Net realized and unrealized investment gains		(156.7)	245.6
Total revenues		(59.2)	433.6
Total expenses		106.8	151.9
Pre-tax income (loss)		(166.0)	281.7
Other Revenue		(127.7)	334.2
Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Total revenues		(17.4)
Other Revenue		(17.4)
Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		3.5	1.3
Total revenues		(124.3)	313.6
Other Revenue		(127.8)	312.3
Kudu investments
Segment reporting information
Earned insurance premiums		0.0
Net investment income		7.3	0.0
Net realized and unrealized investment gains		(24.8)	0.0
Commission revenues		0.0
Other revenue		0.1
Total revenues		(17.4)	0.0
Insurance acquisition expenses		0.0
Cost of sales		0.0
General and administrative expenses		2.5	0.0
Amortization		0.1	0.0
Interest expense		1.4	0.0
Total expenses		4.0	0.0
Pre-tax income (loss)		(21.4)
HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0	0.0
Earned insurance premiums		5.4	4.2
Net investment income		5.5	5.3
Net realized and unrealized investment gains		6.1	11.8
Commission revenues		0.0	0.0
Other revenue		0.5	0.6
Total revenues		17.5	21.9
Insurance acquisition expenses		1.7	1.3
Cost of sales			0.0
Cost of sales		0.0
General and administrative expenses		14.7	14.4
Amortization		0.0	0.0
Interest expense		0.0
Total expenses		16.4	15.7
Pre-tax income (loss)		1.1	6.2
Other Revenue		17.5	21.9
NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		65.0	49.3
Earned insurance premiums		0.0
Net investment income		0.0
Net realized and unrealized investment gains		0.0
Commission revenues		53.0	43.3
Other revenue		12.0	6.0
Total revenues		65.0	49.3
Insurance acquisition expenses		0.0
Cost of sales		0.0
General and administrative expenses		39.6	25.3
Broker commission expenses		18.3	15.5
Change in fair value of contingent consideration earnout liabilities		(0.6)	1.3
Amortization		4.8	5.0
Interest expense		4.3	3.7
Total expenses		66.4	50.8
Pre-tax income (loss)		(1.4)	(1.5)
Other Revenue		0.0	0.0
MediaAlpha
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			48.8
Earned insurance premiums			0.0
Net investment income			0.0
Net realized and unrealized investment gains			0.0
Commission revenues		0.0	48.8
Other revenue		0.0	0.0
Total revenues		0.0	48.8
Insurance acquisition expenses			0.0
Cost of sales		0.0	40.6
General and administrative expenses		0.0	12.5
Amortization		0.0	1.6
Interest expense		0.0	0.2
Total expenses		0.0	54.9
Pre-tax income (loss)			(6.1)
Other Revenue			0.0
Other Segment
Segment reporting information
Earned insurance premiums		0.0	0.0
Net investment income		10.1	10.7
Net realized and unrealized investment gains		(138.0)	119.1
Realized gain and unrealized investment gain from the MediaAlpha Transaction			182.2
Commission revenues		2.1	1.3
Other revenue		1.5	0.3
Total revenues		(124.3)	313.6
Insurance acquisition expenses		0.0	0.0
Cost of sales		2.0	1.1
General and administrative expenses		17.5	29.4
Amortization		0.2	0.0
Interest expense		0.3	0.0
Total expenses		20.0	30.5
Pre-tax income (loss)		(144.3)	283.1
Total
Segment reporting information
Earned insurance premiums		5.4	4.2
Net investment income		22.9	16.0
Net realized and unrealized investment gains		(156.7)	130.9
Realized gain and unrealized investment gain from the MediaAlpha Transaction			182.2
Commission revenues		55.1	93.4
Other revenue		14.1	6.9
Total revenues		(59.2)	433.6
Insurance acquisition expenses		1.7	1.3
Cost of sales			41.7
Cost of sales		2.0	81.6
General and administrative expenses		74.3	15.5
Broker commission expenses		18.3	1.3
Change in fair value of contingent consideration earnout liabilities		(0.6)	6.6
Amortization		5.1	3.9
Interest expense		6.0
Total expenses		106.8	151.9
Pre-tax income (loss)		(166.0)	281.7
Kudu investments | Kudu investments
Segment reporting information
Other revenue			0.0
NSM Holding Co LLC
Segment reporting information
Change in fair value of contingent consideration earnout liabilities		(0.6)	1.3	$ 2.6
MediaAlpha | MediaAlpha
Segment reporting information
Total revenues	$ 33.5
Commission and Other Revenue [Member]
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		66.4	50.6
Commission and Other Revenue [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Commission and Other Revenue [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		1.4	1.3
Commission and Other Revenue [Member] | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0	0.0
Commission and Other Revenue [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		65.0	49.3
Commission and Other Revenue [Member] | MediaAlpha
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			0.0
Real Estate [Member]
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		11.3	9.0
Real Estate [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Real Estate [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0	0.0
Real Estate [Member] | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0	0.0
Real Estate [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		11.3	9.0
Real Estate [Member] | MediaAlpha
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			0.0
Social Services [Member]
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		6.3	5.8
Social Services [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Social Services [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0	0.0
Social Services [Member] | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0	0.0
Social Services [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		6.3	5.8
Social Services [Member] | MediaAlpha
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			0.0
Other
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		6.7	5.8
Other | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Other | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		1.4	1.3
Other | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0	0.0
Other | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		5.3	4.5
Other | MediaAlpha
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			0.0
Product [Member]
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Product [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Product [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		2.1
Product [Member] | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Product [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Pet [Member]
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		11.8
Pet [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Pet [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Pet [Member] | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Pet [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		11.8
Advertising [Member]
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			48.8
Advertising [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			0.0
Advertising [Member] | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			0.0
Advertising [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			0.0
Advertising [Member] | MediaAlpha
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			48.8
Specialty Transportation [Member]
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		20.3	17.9
Specialty Transportation [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
Specialty Transportation [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0	0.0
Specialty Transportation [Member] | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0	0.0
Specialty Transportation [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		20.3	17.9
Specialty Transportation [Member] | MediaAlpha
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			0.0
UNITED KINGDOM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		10.0	12.1
UNITED KINGDOM | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0
UNITED KINGDOM | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0	0.0
UNITED KINGDOM | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		0.0	0.0
UNITED KINGDOM | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax		$ 10.0	12.1
UNITED KINGDOM | MediaAlpha
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax			$ 0.0