Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		5 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2020	Jun. 30, 2019
Segment reporting information
Net investment income	$ 19.6	$ 21.0		$ 42.5	$ 37.0
Net realized and unrealized investment gains				(3.8)	280.8
Total revenues	259.6	129.0		200.4	562.6
Total expenses	126.9	113.2		233.7	265.1
Pre-tax income (loss)	132.7	15.8		(33.3)	297.5
Other Revenue	179.4	61.0		50.9	395.2
Revenues from Contract with Customer and Products, Including Assessed Tax					167.4
Revenue from Contract with Customer, Including Assessed Tax	80.2	68.0		149.5	166.8
Kudu
Segment reporting information
Earned insurance premiums		0.0
Commission revenues		0.0
Total revenues	22.1	4.4		4.7	4.4
Insurance acquisition expenses		0.0
Cost of sales		0.0
Amortization		0.0
Interest expense		0.0
Total expenses		2.2
Pre-tax income (loss)		2.2
Other Revenue	22.1	4.4		4.7	4.4
Revenues from Contract with Customer and Products, Including Assessed Tax					0.0
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Other Operations
Segment reporting information
Total revenues	135.3	36.9		11.0	350.5
Other Revenue	131.2	35.0		2.6	347.3
Revenues from Contract with Customer and Products, Including Assessed Tax					3.2
Revenue from Contract with Customer, Including Assessed Tax	4.1	1.9		8.4	2.6
Kudu investments
Segment reporting information
Earned insurance premiums	0.0			0.0
Net investment income	5.6	4.0		12.9	4.0
Net realized and unrealized investment gains	16.5	0.4		(8.3)	0.4
Commission revenues	0.0			0.0
Other revenue	0.0	0.0		0.1	0.0
Total revenues	22.1	4.4		4.7	4.4
Insurance acquisition expenses	0.0			0.0
Cost of sales	0.0			0.0
Cost of sales	0.0			0.0
General and administrative expenses	2.8	2.2		5.3	2.2
Amortization	0.1	0.0		0.2	0.0
Interest expense	1.5	0.0		2.9	0.0
Total expenses	4.4	2.2		8.4	2.2
Pre-tax income (loss)	17.7			(3.7)	2.2
HG Global-BAM
Segment reporting information
Earned insurance premiums	5.6	4.3		11.0	8.5
Net investment income	4.9	5.4		10.4	10.7
Net realized and unrealized investment gains	14.4	11.5		20.5	23.3
Commission revenues	0.0	0.0		0.0
Other revenue	1.2	0.4		1.7	1.0
Total revenues	26.1	21.6		43.6	43.5
Insurance acquisition expenses	2.1	1.4		3.8	2.7
Cost of sales	0.2	0.1		0.2	0.2
Cost of sales	0.0	0.0		0.0
General and administrative expenses	12.5	12.3		27.2	26.6
Amortization	0.0	0.0		0.0
Interest expense	0.0			0.0
Total expenses	14.8	13.8		31.2	29.5
Pre-tax income (loss)	11.3	7.8		12.4	14.0
Other Revenue	26.1	21.6		43.6	43.5
Revenues from Contract with Customer and Products, Including Assessed Tax					0.0
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
NSM Holding Co LLC
Segment reporting information
Earned insurance premiums	0.0			0.0
Net investment income	0.0			0.0
Net realized and unrealized investment gains	0.0			0.0
Commission revenues	63.0	54.9		116.0	98.2
Other revenue	13.1	11.2		25.1	17.2
Total revenues	76.1	66.1		141.1	115.4
Insurance acquisition expenses	0.0
Cost of sales	0.0			0.0
Cost of sales	0.0			0.0
General and administrative expenses	48.5	32.8		88.1	58.1
Broker commission expenses	21.0	18.1		39.3	33.6
Change in fair value of contingent consideration earnout liabilities	(1.7)	6.3		(2.3)	7.6
Amortization	6.3	4.1		11.1	9.1
Interest expense	5.7	4.1		10.0	7.8
Total expenses	79.8	65.4		146.2	116.2
Pre-tax income (loss)	(3.7)	0.7		(5.1)	(0.8)
Other Revenue	0.0	0.0		0.0	0.0
Revenues from Contract with Customer and Products, Including Assessed Tax					115.4
Revenue from Contract with Customer, Including Assessed Tax	76.1	66.1		141.1	115.4
MediaAlpha
Segment reporting information
Commission revenues	0.0	0.0		0.0	48.8
Total revenues	0.0	0.0		0.0	48.8
Cost of sales	0.0	0.0		0.0	40.6
General and administrative expenses	0.0	0.0		0.0	12.5
Amortization	0.0	0.0		0.0	1.6
Interest expense	0.0	0.0		0.0	0.2
Total expenses	0.0	0.0		0.0	54.9
Pre-tax income (loss)					(6.1)
Other Revenue					0.0
Revenues from Contract with Customer and Products, Including Assessed Tax					48.8
Revenue from Contract with Customer, Including Assessed Tax					48.8
Other Segment
Segment reporting information
Earned insurance premiums	0.0	0.0		0.0
Net investment income	9.1	11.6		19.2	22.3
Net realized and unrealized investment gains	122.0	23.3		(16.0)	142.4
Realized gain and unrealized investment gain from the MediaAlpha Transaction		0.0		0.0	182.2
Commission revenues	1.9	1.3		4.0	2.6
Other revenue	2.3	0.7		3.8	1.0
Total revenues	135.3	36.9		11.0	350.5
Insurance acquisition expenses	0.0	0.0		0.0
Cost of sales	0.0			0.0
Cost of sales	2.2	1.6		4.2	2.7
General and administrative expenses	25.3	30.1		42.8	59.5
Amortization	0.2	0.1		0.4	0.1
Interest expense	0.2	0.0		0.5	0.0
Total expenses	27.9	31.8		47.9	62.3
Pre-tax income (loss)	107.4	5.1		(36.9)	288.2
Total
Segment reporting information
Earned insurance premiums	5.6	4.3		11.0	8.5
Net investment income	19.6	21.0		42.5	37.0
Net realized and unrealized investment gains	152.9	35.2		(3.8)	166.1
Realized gain and unrealized investment gain from the MediaAlpha Transaction					182.2
Commission revenues	64.9	56.2		120.0	149.6
Other revenue	16.6	12.3		30.7	19.2
Total revenues	259.6	129.0		200.4	562.6
Insurance acquisition expenses	2.1	1.4		3.8	2.7
Cost of sales	0.2	0.1		0.2	0.2
Cost of sales	2.2	1.6		4.2	43.3
General and administrative expenses	89.1	77.4		163.4	158.9
Broker commission expenses	21.0	18.1		39.3	33.6
Change in fair value of contingent consideration earnout liabilities	(1.7)	6.3		(2.3)	7.6
Amortization	6.6	4.2		11.7	10.8
Interest expense	7.4	4.1		13.4	8.0
Total expenses	126.9	113.2		233.7	265.1
Pre-tax income (loss)	132.7	15.8		(33.3)	297.5
NSM Holding Co LLC
Segment reporting information
Change in fair value of contingent consideration earnout liabilities	(1.7)	6.3	$ 2.6	2.3	7.6
Commission and Other Revenue [Member]
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	77.9	67.4		145.1	118.0
Commission and Other Revenue [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Commission and Other Revenue [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	1.8	1.3		4.0	2.6
Commission and Other Revenue [Member] | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Commission and Other Revenue [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	76.1	66.1		141.1	115.4
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	15.4	11.5		26.7	20.5
Real Estate [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Real Estate [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		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		0.0	0.0
Real Estate [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	15.4	11.5		26.7	20.5
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	7.3	6.5		13.6	12.3
Social Services [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Social Services [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		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		0.0	0.0
Social Services [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	7.3	6.5		13.6	12.3
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	5.8	6.0		13.1	11.8
Other | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Other | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	1.8	1.3		4.0	2.6
Other | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Other | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	4.0	4.7		9.1	9.2
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	0.6		4.4	0.6
Product [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Product [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	2.3	0.6		4.4	0.6
Product [Member] | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Product [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Product [Member] | MediaAlpha
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	13.2	9.5		25.1	9.5
Pet [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Pet [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Pet [Member] | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Pet [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	13.2	9.5		25.1	9.5
Pet [Member] | MediaAlpha
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax					0.0
Advertising [Member]
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax					48.8
Advertising [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax					0.0
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	23.8	21.7		44.2	39.6
Specialty Transportation [Member] | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
Specialty Transportation [Member] | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		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		0.0	0.0
Specialty Transportation [Member] | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	23.8	21.7		44.2	39.6
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	12.4	12.2		22.4	24.3
UNITED KINGDOM | Kudu
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
UNITED KINGDOM | Other Operations
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
UNITED KINGDOM | HG Global-BAM
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0		0.0	0.0
UNITED KINGDOM | NSM Holding Co LLC
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax	$ 12.4	$ 12.2		$ 22.4	24.3
UNITED KINGDOM | MediaAlpha
Segment reporting information
Revenue from Contract with Customer, Including Assessed Tax					$ 0.0