Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		4 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2018
Segment reporting information
Net investment income	$ 21.0	$ 11.8		$ 37.0	$ 31.5
Net realized and unrealized investment gains (losses)				280.8	(43.2)
Total revenues	129.0	122.3		562.6	164.4
Broker commission expense		6.6			6.6
Amortization	4.2	0.0		10.8
Total expenses	113.2	134.7		265.1	242.8
Pre-tax income (loss)	15.8	(12.4)		297.5	(78.4)
Kudu
Segment reporting information
Earned insurance premiums	0.0
Net investment income	4.0	0.0		4.0	0.0
Net realized and unrealized investment gains (losses)	0.4	0.0		0.4	0.0
Commission revenues	0.0
Other revenue	0.0			0.0
Total revenues	4.4	0.0		4.4	0.0
Insurance acquisition expenses	0.0
Other underwriting expenses	0.0
Cost of sales	0.0
General and administrative expenses	2.2	0.0		2.2	0.0
Broker commission expense	0.0
Change in fair value of contingent consideration earnout liabilities	0.0
Amortization	0.0
Total expenses	2.2	0.0		2.2	0.0
Pre-tax income (loss)	2.2			2.2
HG Global-BAM
Segment reporting information
Earned insurance premiums	4.3	3.4		8.5	6.4
Net investment income	5.4	4.0		10.7	7.7
Net realized and unrealized investment gains (losses)	11.5	(2.4)		23.3	(10.3)
Commission revenues	0.0	0.0		0.0	0.0
Other revenue	0.4	0.4		1.0	0.6
Total revenues	21.6	5.4		43.5	4.4
Insurance acquisition expenses	1.4	1.3		2.7	2.7
Other underwriting expenses	0.1	0.1		0.2	0.2
Cost of sales	0.0	0.0		0.0	0.0
General and administrative expenses	12.3	13.4		26.6	25.2
Broker commission expense	0.0			0.0
Change in fair value of contingent consideration earnout liabilities	0.0			0.0
Amortization	0.0	0.0		0.0	0.0
Interest expense	0.0	0.0		0.0	0.0
Total expenses	13.8	14.8		29.5	28.1
Pre-tax income (loss)	7.8	(9.4)		14.0	(23.7)
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 (losses)	0.0			0.0
Commission revenues	54.9	22.6		98.2	22.6
Other revenue	11.2	0.9		17.2	0.9
Total revenues	66.1	23.5		115.4	23.5
Insurance acquisition expenses	0.0			0.0
Other underwriting expenses	0.0			0.0
Cost of sales	0.0			0.0
General and administrative expenses	32.7	12.1		57.8	12.1
Broker commission expense	18.2	6.6		33.9	6.6
Change in fair value of contingent consideration earnout liabilities	6.3	0.0		7.6	0.0
Amortization	4.1	0.0		9.1	0.0
Interest expense	4.1	1.6		7.8	1.6
Total expenses	65.4	20.3		116.2	20.3
Pre-tax income (loss)	0.7			(0.8)	3.2
MediaAlpha
Segment reporting information
Earned insurance premiums		0.0		0.0	0.0
Net investment income		0.0		0.0	0.0
Net realized and unrealized investment gains (losses)		0.0		0.0	0.0
Commission revenues	0.0	71.8		48.8	141.9
Other revenue	0.0	0.0		0.0	1.6
Total revenues	0.0	71.8		48.8	143.5
Insurance acquisition expenses		0.0		0.0	0.0
Other underwriting expenses		0.0		0.0	0.0
Cost of sales	0.0	59.9		40.6	117.3
General and administrative expenses	0.0	4.6		12.5	15.8
Broker commission expense				0.0
Change in fair value of contingent consideration earnout liabilities				0.0
Amortization	0.0	2.5		1.6	5.4
Interest expense	0.0	0.3		0.2	0.7
Total expenses	0.0	67.3		54.9	139.2
Pre-tax income (loss)		$ 4.5		(6.1)	$ 4.3
Percentage of advertising revenue		32.00%			32.00%
Other Segment
Segment reporting information
Earned insurance premiums	0.0	$ 0.0		0.0	$ 0.0
Net investment income	11.6	7.8		22.3	23.8
Net realized and unrealized investment gains (losses)	23.3	12.9		142.4	(32.9)
Realized and unrealized gains from the MediaAlpha Transaction					0.0
Commission revenues	1.3	1.0		2.6	1.9
Other revenue	0.7	(0.1)		1.0	0.2
Total revenues	36.9	21.6		350.5	(7.0)
Insurance acquisition expenses	0.0			0.0
Other underwriting expenses	0.0			0.0
Cost of sales	1.6	1.1		2.7	1.8
General and administrative expenses	30.1	31.0		59.5	53.0
Broker commission expense	0.0			0.0
Change in fair value of contingent consideration earnout liabilities	0.0			0.0
Amortization	0.1	0.1		0.1	0.1
Interest expense	0.0	0.1		0.0	0.3
Total expenses	31.8	32.3		62.3	55.2
Pre-tax income (loss)	5.1	(10.7)		288.2	(62.2)
Total
Segment reporting information
Earned insurance premiums	4.3	3.4		8.5	6.4
Net investment income	21.0	11.8		37.0	31.5
Net realized and unrealized investment gains (losses)	35.2	10.5		166.1	(43.2)
Commission revenues	56.2	95.4		149.6	166.4
Other revenue	12.3	1.2		19.2	3.3
Total revenues	129.0	122.3		562.6	164.4
Insurance acquisition expenses	1.4	1.3		2.7	2.7
Other underwriting expenses	0.1	0.1		0.2	0.2
Cost of sales	1.6	61.0		43.3	119.1
General and administrative expenses	77.3	61.1		158.6	106.1
Broker commission expense	18.2			33.9
Change in fair value of contingent consideration earnout liabilities	6.3			7.6
Amortization		2.6			5.5
Interest expense	4.1	2.0		8.0	2.6
Total expenses	113.2	134.7		265.1	242.8
Pre-tax income (loss)	15.8	$ (12.4)		297.5	$ (78.4)
NSM Holding Co LLC
Segment reporting information
Change in fair value of contingent consideration earnout liabilities	6.3			7.6
MediaAlpha | MediaAlpha
Segment reporting information
Total revenues	$ 89.1		$ 122.7
MediaAlpha | HG Global-BAM
Segment reporting information
Realized and unrealized gains from the MediaAlpha Transaction				0.0
MediaAlpha | NSM Holding Co LLC
Segment reporting information
Realized and unrealized gains from the MediaAlpha Transaction				0.0
MediaAlpha | Other Segment
Segment reporting information
Realized and unrealized gains from the MediaAlpha Transaction				182.2
MediaAlpha | Total
Segment reporting information
Realized and unrealized gains from the MediaAlpha Transaction				$ 182.2