Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		5 Months Ended	7 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018
Segment reporting information
Net investment income	$ 18.4	$ 13.4			$ 55.4	$ 44.9
Net realized and unrealized investment gains (losses)					347.8	22.9
Total revenues	155.5	198.7			718.1	363.1
Loss and loss adjustment expenses						0.0
Total expenses	104.4	154.4			369.5	397.2
Pre-tax income (loss)	51.1	44.3			348.6	(34.1)
Kudu investments
Segment reporting information
Earned insurance premiums	0.0
Net investment income	4.6	0.0			8.6	0.0
Net realized and unrealized investment gains (losses)	2.7	0.0			3.1	0.0
Commission revenues	0.0
Other revenue	0.2				0.2	0.0
Total revenues	7.5	0.0			11.9	0.0
Insurance acquisition expenses	0.0
Other underwriting expenses	0.0
Cost of sales	0.0
General and administrative expenses	4.5	0.0			6.7	0.0
Amortization	0.2	0.0			0.2	0.0
Total expenses	4.7	0.0			6.9	0.0
Pre-tax income (loss)	2.8				5.0
HG Global-BAM
Segment reporting information
Earned insurance premiums	5.2	3.3			13.7	9.7
Net investment income	5.4	4.8			16.1	12.5
Net realized and unrealized investment gains (losses)	7.1	(4.1)			30.4	(14.4)
Commission revenues	0.0	0.0			0.0	0.0
Other revenue	0.3	0.2			1.3	0.8
Total revenues	18.0	4.2			61.5	8.6
Insurance acquisition expenses	1.4	1.2			4.1	3.9
Other underwriting expenses	0.1	0.1			0.3	0.3
Cost of sales	0.0	0.0			0.0	0.0
General and administrative expenses	12.5	11.3			39.1	36.5
Amortization	0.0	0.0			0.0	0.0
Interest expense	0.0	0.0			0.0	0.0
Total expenses	14.0	12.6			43.5	40.7
Pre-tax income (loss)	4.0	(8.4)			18.0	(32.1)
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	49.4	35.0			147.6	56.4
Other revenue	10.5	4.7			27.7	6.8
Total revenues	59.9	39.7			175.3	63.2
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	33.3	23.5			91.4	35.7
Broker commission expense	15.3	10.6			48.9	17.1
Change in fair value of contingent consideration earnout liabilities	(2.0)	2.6			5.6	2.6
Amortization	4.4	5.0			13.5	5.0
Interest expense	4.4	3.2			12.2	4.8
Total expenses	55.4	44.9			171.6	65.2
Pre-tax income (loss)	4.5	(5.2)			3.7	(2.0)
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	74.5			48.8	216.4
Other revenue	0.0	0.0			0.0	1.6
Total revenues	0.0	74.5			48.8	218.0
Insurance acquisition expenses		0.0			0.0	0.0
Other underwriting expenses		0.0			0.0	0.0
Cost of sales	0.0	61.8			40.6	179.1
General and administrative expenses	0.0	5.4			12.5	21.2
Amortization	0.0	2.4			1.6	7.8
Interest expense	0.0	0.2			0.2	0.9
Total expenses	0.0	69.8			54.9	209.0
Pre-tax income (loss)		$ 4.7			(6.1)	$ 9.0
Percentage of advertising revenue		30.00%				31.00%
Other Segment
Segment reporting information
Earned insurance premiums	0.0	$ 0.0			0.0	$ 0.0
Net investment income	8.4	8.6			30.7	32.4
Net realized and unrealized investment gains (losses)	57.2	70.2			199.6	37.3
Realized gain and unrealized investment gain from the MediaAlpha Transaction					182.2	0.0
Commission revenues	1.9	1.1			4.5	3.0
Other revenue	2.6	0.4			3.6	0.6
Total revenues	70.1	80.3			420.6	73.3
Loss and loss adjustment expenses						0.0
Insurance acquisition expenses	0.0				0.0	0.0
Other underwriting expenses	0.0				0.0	0.0
Cost of sales	2.3	1.1			5.0	2.9
General and administrative expenses	27.7	26.0			87.2	79.0
Amortization	0.0	0.0			0.1	0.1
Interest expense	0.3	0.0			0.3	0.3
Total expenses	30.3	27.1			92.6	82.3
Pre-tax income (loss)	39.8	53.2			328.0	(9.0)
Total
Segment reporting information
Earned insurance premiums	5.2	3.3			13.7	9.7
Net investment income	18.4	13.4			55.4	44.9
Net realized and unrealized investment gains (losses)	67.0	66.1			233.1	22.9
Realized gain and unrealized investment gain from the MediaAlpha Transaction					182.2
Commission revenues	51.3	110.6			200.9	275.8
Other revenue	13.6	5.3			32.8	9.8
Total revenues	155.5	198.7			718.1	363.1
Insurance acquisition expenses	1.4	1.2			4.1	3.9
Other underwriting expenses	0.1	0.1			0.3	0.3
Cost of sales	2.3	62.9			45.6	182.0
General and administrative expenses	78.0	66.2			236.9	172.4
Broker commission expense	15.3	10.6			48.9	17.1
Change in fair value of contingent consideration earnout liabilities	(2.0)	2.6			5.6	2.6
Amortization	4.6	7.4			15.4	12.9
Interest expense	4.7	3.4			12.7	6.0
Total expenses	104.4	154.4			369.5	397.2
Pre-tax income (loss)	51.1	44.3			348.6	(34.1)
Kudu investments | Kudu investments
Segment reporting information
Other revenue		0.0
HG Global-BAM | HG Global-BAM
Segment reporting information
Loss and loss adjustment expenses						0.0
NSM Holding Co LLC
Segment reporting information
Change in fair value of contingent consideration earnout liabilities	(2.0)	$ 2.6	$ 2.6		5.6
MediaAlpha | MediaAlpha
Segment reporting information
Total revenues	$ 110.5			$ 233.2
Loss and loss adjustment expenses						$ 0.0
MediaAlpha | HG Global-BAM
Segment reporting information
Realized gain and unrealized investment gain from the MediaAlpha Transaction					0.0
MediaAlpha | NSM Holding Co LLC
Segment reporting information
Realized gain and unrealized investment gain from the MediaAlpha Transaction					$ 0.0