Significant Transactions (Details)							3 Months Ended	6 Months Ended
	Jun. 28, 2019 USD ($)	Apr. 01, 2019 USD ($)	Feb. 26, 2019 USD ($)	Dec. 03, 2018 USD ($)	May 11, 2018 USD ($)	Jan. 24, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($) earnout_period	Jun. 30, 2018 USD ($)	Jun. 03, 2019 USD ($)	Apr. 04, 2019	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]
Purchases of consolidated subsidiaries, net of cash acquired								$ 243,400,000	$ 252,100,000
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							$ 6,300,000	7,600,000
Payment for contingent consideration liability								2,600,000
Contingent consideration earnout liabilities							30,900,000	30,900,000					$ 20,200,000
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	114,700,000
Unrealized investment gain								114,700,000
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%
Equity method investment, aggregate cost										$ 55,100,000
Elementum Advisors
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost										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
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	$ 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	$ 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
Acquired finite-lived intangible assets, weighted average useful life (in years)	12 years
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%	50.00%	50.00%					50.00%
DavidShield | White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)							25.00%	25.00%					25.00%
PassportCard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)						50.00%	25.00%	25.00%					25.00%
DavidShield & PassportCard
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						$ 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%
Equity method investment, aggregate cost															$ 125,000,000.0
Fully diluted ownership interest (as a percent)											85.40%	42.70%	42.70%
Additional funding							$ 145,200,000	$ 145,200,000					$ 63,000,000.0
Assets under management							17,000,000,000	17,000,000,000
Unfunded capital commitment							105,000,000.0	105,000,000.0				$ 250,000,000.0
Funded capital commitment							82,200,000	82,200,000
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
Funded capital commitment							41,100,000	41,100,000
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		222,300,000	222,300,000					$ 180,400,000	$ 51,000,000.0
Proceeds from borrowings							42,900,000	42,900,000
Embrace Pet Insurance | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill and intangible assets acquired		$ 70,600,000
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	$ 20,400,000