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Significant Transactions (Details) £ in Millions					3 Months Ended		6 Months Ended
	May 11, 2018 USD ($)	Jan. 24, 2018 USD ($)	Oct. 05, 2017 USD ($) shares	Sep. 28, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	May 18, 2018 GBP (£)	May 18, 2018 USD ($)	Feb. 05, 2018 USD ($)	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired							$ 252,100,000	$ 0
Noncontrolling interest acquired	$ 14,300,000
(Loss) gain from sale of discontinued operations, net of tax					$ 0	$ (600,000)	$ 100,000	$ (1,600,000)
Healthplans.com
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired			$ 28,000,000
MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)					62.30%		62.30%					64.40%
OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 1,300,000,000
(Loss) gain from sale of discontinued operations, net of tax				$ 554,600,000
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%
Purchases of consolidated subsidiaries, net of cash acquired	$ 274,200,000.0
Assets acquired	494,100,000
Goodwill and intangible assets acquired	381,900,000
Liabilities acquired	$ 205,700,000
Transaction costs					$ 6,300,000
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%	100.00%
Assets acquired										$ 69,900,000
Goodwill and intangible assets acquired										54,300,000
Liabilities acquired										20,300,000
DavidShield & PassportCard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)		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)					50.00%		50.00%					0.00%
Equity method investment, aggregate cost					$ 1,800,000		$ 1,800,000				$ 127,500,000
Capital Unit, Class A | MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Capital units, purchased | shares			131,579
Capital units, value			$ 12,500,000
United Kingdom, Pounds | Fresh Insurance Services Group Limited
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost | £									£ 36.6
Contingent consideration | £									£ 5.6
United States of America, Dollars | Fresh Insurance Services Group Limited
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost										49,600,000
Contingent consideration										$ 7,500,000

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