Significant Transactions (Details)											3 Months Ended					9 Months Ended
	May 07, 2020 USD ($)	Apr. 07, 2020 GBP (£)	Apr. 07, 2020 USD ($)	Jun. 28, 2019 USD ($)	Apr. 04, 2019 USD ($)	Apr. 01, 2019 USD ($)	Feb. 26, 2019 USD ($)	Dec. 03, 2018 USD ($)	May 18, 2018 USD ($) earnout_period	May 11, 2018 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	May 31, 2019 USD ($)	Apr. 03, 2019 USD ($)	Dec. 31, 2018	Feb. 05, 2018 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Commitment to fund																					$ 125,000,000.0
Cash consideration to acquire businesses and interest in affiliates																$ 129,400,000	$ 243,400,000
Goodwill											$ 540,100,000		$ 394,700,000			540,100,000
Change in fair value of contingent consideration earnout liabilities											700,000			$ (2,000,000.0)		(1,600,000)	5,600,000
Payments to acquire business, net					$ 81,400,000
Total commitment under revolving credit facility			$ 291,400,000									$ 234,000,000.0
Specialty Insurance Distribution (NSM)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contingent consideration earnout liabilities											11,800,000		20,600,000			11,800,000
Payment for contingent consideration liability											600,000	6,400,000		2,600,000		7,000,000.0	2,600,000
Change in fair value of contingent consideration earnout liabilities											$ 700,000			(2,000,000.0)		$ (1,600,000)	5,600,000
Marketing Technology (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%
Unrealized gain (loss) on investments																	114,700,000
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
Unrealized investment gain															114,700,000
Total realized and unrealized gain on sale of minority stake												$ 182,200,000
Passport/DavidShield
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)											53.80%	50.00%				53.80%
Stock issued during period, value, acquisitions	$ 15,000,000.0
NSM Bank Facility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount borrowed										$ 100,000,000.0
NSM Bank Facility | Specialty Insurance Distribution (NSM)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount borrowed											$ 275,000,000.0					$ 275,000,000.0
NSM Bank Facility | Specialty Insurance Distribution (NSM) | Revolving credit loan
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from borrowings											0			$ 0		0	6,500,000
Total commitment under revolving credit facility			15,000,000.0									$ 10,000,000.0
Kudu Debt | Kudu | Revolving credit loan
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
Remaining borrowing capacity under revolving credit facility											51,000,000.0		$ 68,000,000.0			51,000,000.0
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%
Fully diluted ownership interest (as a percent)										83.60%
Cash consideration to acquire businesses and interest in affiliates								$ 29,000,000.0		$ 274,200,000
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
Proceeds from borrowings									$ 51,000,000.0
Purchase price adjustment																	$ 700,000
Number of earnout periods | earnout_period									2
KBK Insurance Group, Inc. & KBK Premium Services, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration to acquire businesses and interest in affiliates								60,000,000.0
Proceeds from borrowings								30,100,000
Goodwill and intangible assets acquired								$ 59,400,000
Goodwill															32,600,000
Intangible assets acquired															32,700,000
Contingent consideration earnout liabilities															$ 5,900,000
Embrace Pet Insurance
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill						$ 52,200,000
Intangible assets acquired						15,400,000
Embrace Pet Insurance | Specialty Insurance Distribution (NSM)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration 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
Kingsbridge Group Limited | Specialty Insurance Distribution (NSM)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration to acquire businesses and interest in affiliates		£ 107,200,000	132,000,000
Proceeds from borrowings		£ 42,500,000	$ 52,400,000
Goodwill and intangible assets acquired											$ 125,100,000					$ 125,100,000
Percentage of voting interests acquired			100.00%
Kingsbridge Group Limited | White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration to acquire businesses and interest in affiliates			$ 80,300,000
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%
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											$ 52,700,000					$ 52,700,000
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%
Fully diluted ownership interest (as a percent)											89.60%		88.40%			89.60%
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 | Specialty Insurance Distribution (NSM)
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]
Cash consideration to acquire businesses and interest in affiliates				$ 59,100,000
Asset Management (Kudu)
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%
Fully diluted ownership interest (as a percent)					85.40%														42.70%		42.70%
Goodwill					$ 7,600,000
Intangible assets acquired					2,200,000
Unfunded capital commitment					250,000,000.0						$ 94,500,000		$ 129,000,000.0			$ 94,500,000			$ 250,000,000.0
Assets acquired					155,500,000
Liabilities acquired					800,000
Noncontrolling interest acquired					$ 1,500,000
Increase (decrease) in capital commitment													100,000,000.0
Funded capital commitment													$ 350,000,000.0