Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment reporting information
Earned insurance premiums	$ 3.4	$ 2.2	$ 6.4	$ 5.2
Net investment income	11.8	14.7	31.5	27.5
Net realized and unrealized investment gains	10.5	33.7	(43.2)	70.0
Advertising and commission revenues	95.4	31.3	166.4	65.1
Other revenue	1.2	1.6	3.3	4.5
Total revenues	122.3	83.5	164.4	172.3
Loss and loss adjustment expenses				1.1
Insurance acquisition expenses	1.3	0.9	2.7	2.1
Other underwriting expenses	0.1	0.1	0.2	0.2
Cost of sales	61.0	26.8	119.1	55.6
General and administrative expenses	61.1	54.9	106.1	113.3
Broker commission expense	6.6		6.6
Amortization of other intangible assets	2.6	2.5	5.5	4.9
Interest expense	2.0	0.5	2.6	0.9
Total expenses	134.7	85.7	242.8	178.1
Pre-tax income (loss)	$ (12.4)	$ (2.2)	$ (78.4)	$ (5.8)
MediaAlpha
Segment reporting information
Percentage of advertising revenue	32.00%	26.00%	32.00%	27.00%
HG Global-BAM | HG Global-BAM
Segment reporting information
Earned insurance premiums	$ 3.4	$ 2.2	$ 6.4	$ 4.2
Net investment income	4.0	3.0	7.7	5.6
Net realized and unrealized investment gains	(2.4)	1.1	(10.3)	2.4
Advertising and commission revenues	0.0	0.0	0.0	0.0
Other revenue	0.4	0.2	0.6	0.6
Total revenues	5.4	6.5	4.4	12.8
Loss and loss adjustment expenses				0.0
Insurance acquisition expenses	1.3	0.9	2.7	2.1
Other underwriting expenses	0.1	0.1	0.2	0.2
Cost of sales		0.0	0.0	0.0
General and administrative expenses	13.4	10.3	25.2	20.9
Broker commission expense	0.0		0.0
Amortization of other intangible assets	0.0	0.0	0.0
Interest expense		0.0	0.0	0.0
Total expenses	14.8	11.3	28.1	23.2
Pre-tax income (loss)	(9.4)	(4.8)	(23.7)	(10.4)
NSM Holding Co LLC | 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	22.6	0.0	22.6	0.0
Other revenue	0.9	0.0	0.9	0.0
Total revenues	23.5	0.0	23.5	0.0
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	12.1	0.0	12.1	0.0
Broker commission expense	6.6	0.0	6.6	0.0
Amortization of other intangible assets	0.0		0.0
Interest expense	1.6	0.0	1.6	0.0
Total expenses	20.3	0.0	20.3	0.0
Pre-tax income (loss)	3.2		3.2
MediaAlpha | 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		0.0	0.0	0.0
Advertising and commission revenues	71.8	30.8	141.9	63.3
Other revenue	0.0	0.0	1.6	0.0
Total revenues	71.8	30.8	143.5	63.3
Loss and loss adjustment expenses				0.0
Insurance acquisition expenses			0.0	0.0
Other underwriting expenses		0.0	0.0	0.0
Cost of sales	59.9	26.1	117.3	53.8
General and administrative expenses	4.6	3.7	15.8	6.9
Broker commission expense	0.0		0.0
Amortization of other intangible assets	2.5	2.5	5.4	4.9
Interest expense	0.3	0.3	0.7	0.5
Total expenses	67.3	32.6	139.2	66.1
Pre-tax income (loss)	4.5	(1.8)	4.3	(2.8)
Other | Other Segment
Segment reporting information
Earned insurance premiums	0.0	0.0	0.0	1.0
Net investment income	7.8	11.7	23.8	21.9
Net realized and unrealized investment gains	12.9	32.6	(32.9)	67.6
Advertising and commission revenues	1.0	0.5	1.9	1.8
Other revenue	(0.1)	1.4	0.2	3.9
Total revenues	21.6	46.2	(7.0)	96.2
Loss and loss adjustment expenses	0.0	0.0	0.0	1.1
Insurance acquisition expenses	0.0	0.0	0.0	0.0
Other underwriting expenses	0.0	0.0	0.0	0.0
Cost of sales	1.1	0.7	1.8	1.8
General and administrative expenses	31.0	40.9	53.0	85.5
Broker commission expense	0.0		0.0
Amortization of other intangible assets	0.1	0.0	0.1	0.0
Interest expense	0.1	0.2	0.3	0.4
Total expenses	32.3	41.8	55.2	88.8
Pre-tax income (loss)	$ (10.7)	$ 4.4	$ (62.2)	$ 7.4