Significant Transactions (Details)										3 Months Ended					5 Months Ended	6 Months Ended		12 Months Ended
	May 07, 2020 USD ($)	Apr. 07, 2020 USD ($)	Jun. 28, 2019 USD ($)	Apr. 04, 2019 USD ($)	Apr. 01, 2019 USD ($)	Feb. 26, 2019 USD ($)	Dec. 03, 2018 USD ($)	May 18, 2018 USD ($)	May 11, 2018 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($) earnout_period	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	May 31, 2019 USD ($)	Apr. 03, 2019 USD ($)	Dec. 31, 2018 USD ($)	Feb. 05, 2018 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of Intangible Assets, Finite-lived																		$ (10,400,000)
Ownership interest (as a percent)										53.80%	50.00%	50.00%				53.80%		50.00%
Commitment to Fund																						$ 125,000,000.0
Payments to Acquire Businesses and Interest in Affiliates																$ 120,300,000	$ 243,400,000
Stock Issued During Period, Value, Acquisitions	$ 15,000,000.0
Payments to Acquire Intangible Assets													$ 82,500,000				82,500,000
Payments to acquire business, net				$ 81,400,000
Goodwill										$ 518,200,000		$ 394,700,000				518,200,000		$ 394,700,000
Total commitment under revolving credit facility		$ 291.4									$ 234,000,000.0
NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Change in fair value of contingent consideration earnout liabilities										(1,700,000)			6,300,000		$ 2,600,000	2,300,000	7,600,000
Payment for contingent consideration liability													2,600,000			6,400,000	2,600,000
Contingent consideration earnout liabilities										11,700,000		20,600,000				11,700,000		20,600,000
White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Other Investments										898,100,000		$ 856,300,000				898,100,000		856,300,000
MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)						48.30%															61.00%
Fully diluted ownership interest (as a percent)						42.00%															58.90%
Proceeds from dividends received						$ 89,300,000
Net gain from sale of discontinued operations, net of tax														$ 67,500,000
Assets, fair value						$ 114,700,000								114,700,000
Unrealized investment gain										$ 15,000,000.0				114,700,000		$ 45,000,000.0	114,700,000	$ 180,000,000.0
Total realized and unrealized gain on sale of minority stake											182,200,000
Elementum Holdings, L.P.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)										30.00%		30.00%				30.00%		30.00%	30.00%
Other Investments																			$ 55,100,000
Insurance-Linked Securities Funds
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Commitment to Fund																			$ 50,000,000.0
Elementum Funds
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost										$ 52,100,000						$ 52,100,000
NSM Insurance HoldCo, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)										96.60%		96.40%				96.60%		96.40%
Fully diluted ownership interest (as a percent)										89.60%		88.40%				89.60%		88.40%
AIG Collector Car Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Intangible assets acquired			$ 82,500,000
Payments to Acquire Intangible Assets			82,500,000
AIG Collector Car Business | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from borrowings			22,500,000
AIG Collector Car Business | White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates			59,100,000
Kudu investments
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)				99.10%								49.50%						49.50%		49.50%		49.50%
Assets acquired				$ 155,500,000
Liabilities acquired				800,000
Noncontrolling interest acquired				$ 1,500,000
Fully diluted ownership interest (as a percent)				85.40%								42.70%						42.70%		42.70%		42.70%
Unfunded capital commitment				$ 250,000,000.0						$ 94,500,000		$ 129,000,000.0				$ 94,500,000		$ 129,000,000.0		$ 250,000,000.0
Goodwill				7,600,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles				$ 2,200,000
Increase (Decrease) in Capital Commitment												100,000,000.0
Funded Capital Commitment												350,000,000.0						350,000,000.0
NSM Bank Facility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt Instrument, Face Amount									$ 100,000,000.0
NSM Bank Facility | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt Instrument, Face Amount										273,400,000	220,700,000	221,300,000	222,300,000	186,400,000		273,400,000	222,300,000	221,300,000			$ 180,400,000
NSM Bank Facility | Term Loan | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt Instrument, Face Amount		276,400,000								273,400,000	$ 224,000,000.0	221,300,000				273,400,000		221,300,000
Proceeds from borrowings										52,400,000			42,900,000			52,400,000	42,900,000
Kudu Debt [Member] | Delayed Draw Term Loans [Member] | Kudu
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total commitment under revolving credit facility												125,000,000.0						125,000,000.0
Line of Credit Facility, Remaining Borrowing Capacity										51,000,000.0		68,000,000.0				51,000,000.0		68,000,000.0
NSM Insurance HoldCo, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)									95.00%
Payments to Acquire Businesses and Interest in Affiliates							$ 29,000,000.0		$ 274,200,000
Fully diluted ownership interest (as a percent)									83.60%
KBK Insurance Group, Inc. & KBK Premium Services, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates							60,000,000.0
Goodwill and intangible assets acquired							59,400,000							32,600,000
Proceeds from borrowings							$ 30,100,000
Intangible assets acquired														32,700,000
Contingent consideration earnout liabilities														5,900,000
Fresh Insurance Services Group Limited
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)								100.00%
Equity method investment, aggregate cost								$ 49,600,000
Purchase price adjustment																	700,000
Number of earnout periods | earnout_period											2
Proceeds from borrowings								$ 51,000,000.0
Embrace Pet Insurance
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill and intangible assets acquired					$ 67,600,000
Intangible assets acquired					15,400,000
Goodwill					52,200,000
Embrace Pet Insurance | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates					58,200,000
Proceeds from borrowings					$ 20,400,000
Percentage of voting interests acquired					100.00%
Payments to acquire businesses, gross					$ 71,500,000
Embrace Pet Insurance | NSM Bank Facility | Term Loan | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from borrowings													20,400,000
Kingsbridge [Member] | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates		80,300,000
Goodwill and intangible assets acquired		$ 125,100,000
Percentage of voting interests acquired		100.00%
Kingsbridge [Member] | NSM Bank Facility | Term Loan | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from borrowings										52,400,000
Other Intangible Assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisitions of businesses										0			0			0	26,800,000
Goodwill, Acquired During Period																0	0
Finite Lived Intangible Assets, Foreign Currency Translation Gain (Loss)										100,000			200,000			(400,000)	500,000
Measurement Period Adjustments										0			0			0	5,900,000
Impairment of Intangible Assets, Finite-lived										(6,200,000)						(6,200,000)		(2,800,000)
Intangible Assets, Net (Excluding Goodwill)										241,700,000	$ 254,400,000	260,000,000.0	239,000,000.0	160,500,000		241,700,000	239,000,000.0	260,000,000.0			157,600,000
Deconsolidation of Business																	(23,500,000)
Amortization										(6,600,000)			(4,200,000)			(11,700,000)	(10,800,000)
Renewal Rights
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisitions of businesses																0
Goodwill and intangible assets acquired			$ 82,500,000
Payments to Acquire Intangible Assets													82,500,000				82,500,000
Intangible Asset and Goodwill
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisitions of businesses										125,100,000			80,400,000			0	0
Goodwill, Acquired During Period																125,100,000	93,800,000
Finite Lived Intangible Assets, Foreign Currency Translation Gain (Loss)										800,000			(1,000,000.0)			(2,200,000)	500,000
Measurement Period Adjustments										0			(300,000)			200,000	5,900,000
Impairment of Intangible Assets, Finite-lived										(6,200,000)						(6,200,000)
Intangible Assets, Gross (Including Goodwill)										759,900,000	646,800,000	654,700,000	667,600,000	510,200,000		759,900,000	667,600,000	654,700,000			537,500,000
Deconsolidation of Business																	(41,800,000)
Amortization										(6,600,000)			(4,200,000)			(11,700,000)	(10,800,000)
Goodwill
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisitions of businesses																0	(26,800,000)
Goodwill, Acquired During Period										125,100,000			80,400,000			125,100,000	93,800,000
Foreign currency translation										700,000			(1,200,000)			(1,800,000)	0
Measurement Period Adjustments										0			(300,000)			200,000
Goodwill										$ 518,200,000	$ 392,400,000	$ 394,700,000	$ 428,600,000	$ 349,700,000		518,200,000	428,600,000	394,700,000			$ 379,900,000
Deconsolidation of Business																	$ (18,300,000)
Goodwill | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Foreign currency translation																$ (1,800,000)		$ 1,400,000
United Kingdom, Pounds | NSM Bank Facility | Term Loan | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt Instrument, Face Amount		$ 42,500,000
United Kingdom, Pounds | Kingsbridge [Member] | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates		107,200,000
Proceeds from borrowings		42,500,000
United States of America, Dollars | NSM Bank Facility | Term Loan | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt Instrument, Face Amount		52,400,000
United States of America, Dollars | Kingsbridge [Member] | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates		132,000,000
Proceeds from borrowings		$ 52,400,000
Measurement Input, Revenue Multiple [Member] | Other [Member] | Maximum | Standard Poors NR Rating
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity Securities, FV-NI, Measurement Input										4		4				4		4