Significant Transactions (Details)								3 Months Ended				5 Months Ended	9 Months Ended
	Jun. 28, 2019 USD ($)	Apr. 04, 2019 USD ($)	Apr. 01, 2019 USD ($)	Feb. 26, 2019 USD ($)	Dec. 03, 2018 USD ($)	May 11, 2018 USD ($)	Jan. 24, 2018 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) earnout_period	Sep. 30, 2018 USD ($)	May 31, 2019 USD ($)	Apr. 03, 2019 USD ($)	Dec. 31, 2018 USD ($)	May 18, 2018 USD ($)	Feb. 05, 2018 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)								50.00%					50.00%				50.00%
Equity method investment, aggregate cost																			$ 125,000,000.0
Purchases of consolidated subsidiaries, net of cash acquired													$ 243,400,000	$ 252,100,000
Payments to acquire business, net		$ 81,400,000
Net gain from sale of discontinued operations, net of tax													67,500,000	$ 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								$ (2,000,000.0)		$ 2,600,000		$ 2,600,000	5,600,000
Payment for contingent consideration liability													2,600,000
Contingent consideration earnout liabilities								28,700,000					28,700,000				$ 20,200,000
White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Other Investments								773,700,000					773,700,000				$ 325,600,000
MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)				48.30%
Fully diluted ownership interest (as a percent)				42.00%
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								$ 35,300,000	114,700,000				150,000,000.0
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%
Other Investments															$ 55,100,000
Insurance-Linked Securities Funds
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Other Commitment															$ 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								$ 40,000,000.0					$ 40,000,000.0
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%		96.40%					96.40%				95.50%
Purchases of consolidated subsidiaries, net of cash acquired						$ 274,200,000
Purchase price adjustment								$ 2,100,000
Assets acquired						495,200,000
Goodwill and intangible assets acquired						383,000,000.0
Liabilities acquired						204,600,000
Contingent consideration						10,200,000
Noncontrolling interest acquired						14,400,000
Transaction costs											$ 6,300,000
Number of earnout periods | earnout_period													2
Fully diluted ownership interest (as a percent)								88.40%					88.40%				85.00%
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
Assets acquired																		72,600,000
Goodwill and intangible assets acquired																		54,600,000
Liabilities acquired																		22,300,000
Contingent consideration																		7,500,000
Fresh Insurance Services Group Limited | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase price adjustment									5,900,000
KBK Insurance Group, Inc. & KBK Premium Services, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired					$ 60,000,000.0
Goodwill and intangible assets acquired					59,400,000				32,600,000
Stock issued during period, value, acquisitions					29,000,000.0
Proceeds from borrowings					$ 30,100,000
Intangible assets acquired									$ 32,700,000
AIG Collector Car Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired	$ 82,500,000
Intangible assets acquired	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]
Stock issued during period, value, acquisitions	$ 59,100,000
DavidShield
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)							50.00%
PassportCard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)							50.00%
DavidShield & PassportCard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost							$ 41,800,000
Payments to acquire business, net							$ 28,300,000
Kudu Investment Management, LLC
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%
Fully diluted ownership interest (as a percent)		85.40%							42.70%							42.70%	42.70%
Additional funding																	$ 63,000,000.0
Assets under management								$ 30,000,000,000					$ 30,000,000,000
Unfunded capital commitment		$ 250,000,000.0						28,100,000					28,100,000			$ 250,000,000
Amount deployed													221,900,000
Kudu Investment Management, LLC | White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Additional funding																	31,500,000
NSM Bank Facility | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term line of credit						100,000,000.0
NSM Bank Facility | Term Loan | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term line of credit						$ 224,000,000.0		$ 221,800,000					221,800,000				$ 180,400,000	$ 51,000,000.0
Proceeds from borrowings													42,900,000
Embrace Pet Insurance
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill and intangible assets acquired			$ 70,600,000
Embrace Pet Insurance | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Stock issued during period, value, acquisitions			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