Segment Information (Details) - USD ($)	3 Months Ended								10 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment reporting information
Earned insurance premiums										$ 16,300,000	$ 13,900,000	$ 10,400,000
Net investment income										79,700,000	59,000,000.0	56,000,000.0
Net realized and unrealized investment gains										433,200,000	(108,300,000)	133,300,000
Net realized and unrealized investment gains										318,500,000
Commission revenues										249,100,000	389,200,000	167,000,000.0
Other revenue										47,600,000	15,300,000	7,100,000
Total revenues	$ 175,300,000	$ 155,500,000	$ 129,000,000.0	$ 433,600,000	$ 6,000,000.0	$ 198,700,000	$ 122,300,000	$ 42,100,000		893,400,000	369,100,000	373,800,000
Loss and loss adjustment expenses												1,100,000
Insurance acquisition expense										5,700,000	5,300,000	4,100,000
Other underwriting expenses										400,000	400,000	400,000
Change in fair value of contingent consideration earnout liabilities										2,100,000	2,700,000
Cost of sales										48,100,000	248,700,000	139,400,000
General and administrative expenses										327,800,000	233,500,000	208,000,000.0
Broker commission expense										64,800,000	28,400,000
Amortization of other intangible assets										21,800,000	18,800,000	10,700,000
Interest expense										17,600,000	9,500,000	2,300,000
Total expenses	118,800,000	104,400,000	113,200,000	151,900,000	150,100,000	154,400,000	134,700,000	108,100,000		488,300,000	547,300,000	366,000,000.0
Pre-tax income (loss)	$ 56,500,000	$ 51,100,000	$ 15,800,000	$ 281,700,000	$ (144,100,000)	$ 44,300,000	$ (12,400,000)	$ (66,000,000.0)		405,100,000	(178,200,000)	7,800,000
HG Global-BAM
Segment reporting information
Net investment income										21,600,000	16,700,000	12,300,000
Net realized and unrealized investment gains											(7,500,000)	600,000
Net realized and unrealized investment gains										27,100,000
Commission revenues										0
Other revenue										1,600,000	1,200,000	1,000,000.0
Total revenues										66,600,000	24,300,000	23,300,000
Insurance acquisition expense										5,700,000	5,300,000	4,000,000.0
Other underwriting expenses										400,000	400,000	400,000
Cost of sales										0
General and administrative expenses										50,500,000	48,000,000.0	42,900,000
Broker commission expense										0
Amortization of other intangible assets										0
Interest expense										0
Total expenses										56,600,000	53,700,000	47,300,000
Pre-tax income (loss)										10,000,000.0	(29,400,000)	(24,000,000.0)
NSM Holding Co LLC
Segment reporting information
Earned insurance premiums										0
Net investment income										0
Net realized and unrealized investment gains										0
Commission revenues										193,400,000	89,600,000	0
Other revenue										39,700,000	12,000,000.0	0
Total revenues										233,100,000	101,600,000	0
Insurance acquisition expense										0
Other underwriting expenses										0
Change in fair value of contingent consideration earnout liabilities										2,100,000	2,700,000	0
Cost of sales										0
General and administrative expenses										132,200,000	59,400,000	0
Broker commission expense										64,800,000	28,400,000	0
Amortization of other intangible assets										19,400,000	8,300,000	0
Interest expense										16,700,000	8,000,000.0	0
Total expenses										235,200,000	106,800,000	0
Pre-tax income (loss)										$ (2,100,000)	$ (5,200,000)
Percentage of commission revenue										17.00%	33.00%
Kudu
Segment reporting information
Earned insurance premiums										$ 0
Net investment income										14,700,000	$ 0	0
Net realized and unrealized investment gains											0	0
Net realized and unrealized investment gains										6,300,000
Commission revenues										0
Other revenue										200,000	0	0
Total revenues										21,200,000	0	0
Insurance acquisition expense										0
Other underwriting expenses										0
Cost of sales										0
General and administrative expenses										10,100,000	0	0
Broker commission expense										0
Amortization of other intangible assets										200,000	0	0
Interest expense										100,000	0	0
Total expenses										10,400,000	0	0
Pre-tax income (loss)										10,800,000
MediaAlpha
Segment reporting information
Commission revenues										48,800,000	295,500,000	163,200,000
Other revenue										0	1,600,000	0
Total revenues										48,800,000	297,100,000	163,200,000
Cost of sales										40,600,000	245,000,000.0	135,900,000
General and administrative expenses										12,500,000	31,700,000	16,200,000
Amortization of other intangible assets										1,600,000	10,300,000	10,500,000
Interest expense										200,000	1,200,000	1,000,000.0
Total expenses										54,900,000	288,200,000	163,600,000
Pre-tax income (loss)										(6,100,000)	$ 8,900,000	$ (400,000)
Percentage of advertising revenue											29.00%	27.00%
Other
Segment reporting information
Earned insurance premiums										0	$ 0	$ 1,000,000.0
Net investment income										43,400,000	42,300,000	43,700,000
Net realized and unrealized investment gains											(100,800,000)	132,700,000
Net realized and unrealized investment gains										285,100,000
Realized gain and unrealized investment gain from the MediaAlpha Transaction											0	0
Commission revenues										6,900,000	4,100,000	3,800,000
Other revenue										6,100,000	500,000	6,100,000
Total revenues										523,700,000	(53,900,000)	187,300,000
Loss and loss adjustment expenses										0	0	1,100,000
Insurance acquisition expense										0	0	100,000
Other underwriting expenses										0
Cost of sales										7,500,000	3,700,000	3,500,000
General and administrative expenses										122,500,000	94,400,000	148,900,000
Broker commission expense										0
Amortization of other intangible assets										600,000	200,000	200,000
Interest expense										600,000	300,000	1,300,000
Total expenses										131,200,000	98,600,000	155,100,000
Pre-tax income (loss)										392,500,000	$ (152,500,000)	$ 32,200,000
MediaAlpha
Segment reporting information
Gain from deconsolidation of MediaAlpha										182,200,000
MediaAlpha | MediaAlpha
Segment reporting information
Total revenues									$ 359,200,000
MediaAlpha | Other
Segment reporting information
Realized gain and unrealized investment gain from the MediaAlpha Transaction										$ 182,200,000