Significant Transactions (Details)					3 Months Ended
	Feb. 26, 2019 USD ($)	Dec. 03, 2018 USD ($)	May 11, 2018 USD ($)	Jan. 24, 2018 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($) earnout_period	Mar. 31, 2018 USD ($)	Apr. 04, 2019 USD ($)	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						$ 9,600,000	$ 0
Net gain from sale of discontinued operations, net of tax						66,200,000	$ 0
NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contingent consideration						1,300,000
Contingent consideration earnout liabilities						27,500,000			$ 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%
Proceeds from Dividends Received	$ 88,000,000.0
Fully diluted ownership interest (as a percent)	42.00%
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
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%			95.30%			95.50%
Purchases of consolidated subsidiaries, net of cash acquired			$ 274,200,000
Purchase price adjustment						$ 2,100,000
Number of earnout periods | earnout_period						2
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
Fully diluted ownership interest (as a percent)						84.80%			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%
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
Equity method investment, aggregate cost										49,600,000
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
Intangible assets acquired						$ 32,700,000
Stock issued during period, value, acquisitions		29,000,000.0
Proceeds from borrowings		$ 30,100,000
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%
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%
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%
Equity method investment, aggregate cost				$ 41,800,000
Payments to acquire business, net				$ 28,300,000
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%
Kudu Investment Management, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)						49.50%			49.50%
Equity method investment, aggregate cost											$ 125,000,000.0
Commitment for working capital						$ 3,800,000
Additional funding						82,200,000			$ 63,000,000.0
Fully diluted ownership interest (as a percent)									42.70%
Kudu Investment Management, LLC | White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost						72,600,000
Additional funding						41,100,000			$ 31,500,000
Scenario, Forecast | MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax					$ 1,300,000
Subsequent Event | 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%
Equity method investment, aggregate cost								$ 81,400,000
Unfunded capital commitment								$ 104,700,000
Fully diluted ownership interest (as a percent)								85.40%
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			$ 181,100,000			$ 186,400,000			$ 180,400,000	$ 51,000,000.0