Significant Transactions (Details) - USD ($)								3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 28, 2017	Aug. 01, 2017	Oct. 05, 2016	Aug. 04, 2016	Jul. 21, 2016	Apr. 18, 2016	Feb. 01, 2016	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2015
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												$ 1,131,000,000	$ 2,657,200,000
Gain from sale of other discontinued operations, net of tax								$ 554,300,000		$ 47,900,000		552,700,000	414,500,000
Assets								3,637,100,000				3,637,100,000		$ 6,520,200,000
Goodwill								25,900,000		25,900,000		25,900,000	25,900,000	25,900,000	$ 25,900,000	$ 18,300,000	$ 18,600,000
Liabilities								$ 299,900,000				299,900,000		2,804,200,000
Repayments of debt												364,600,000	401,800,000
Payments to acquire other investments												67,700,000	36,600,000
Sales of common equity securities												$ 407,800,000	683,000,000
Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)				70.90%				77.10%				77.10%
Assets				$ 11,500,000
Goodwill				7,600,000
Intangible Assets, Net (Excluding Goodwill)				1,100,000
Liabilities				100,000
United Kingdom, Pounds | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary				6,100,000
United States of America, Dollars | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary				$ 8,100,000
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
Gain from sale of other discontinued operations, net of tax	$ 554,600,000
Tranzact
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			$ 1,200,000		$ 221,300,000
Gain from sale of other discontinued operations, net of tax			51,900,000						$ (1,000,000)					51,900,000
Repayments of debt					56,300,000
Discontinued operation, tax effect of discontinued operation			30,200,000							15,900,000			13,800,000	30,200,000
Increase of book value			$ 82,100,000		$ 82,100,000
Sirius Group
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						$ 2,600,000,000
Gain from sale of other discontinued operations, net of tax										(4,000,000)				363,200,000
Payments to acquire other investments						$ 161,800,000
Sale of Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary														113,300,000
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities							$ 658,000,000
Dividends payable, amount per share							$ 32.00
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities							$ 658,000,000
Common stock, dividends, per share, cash paid							$ 0.50				$ 0.50
Preferred Stock | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary		$ 5,000,000
Stock issued during period, shares, acquisitions		37,409
Preferred Stock | United Kingdom, Pounds | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary		$ 4,000,000
Common Stock | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary		$ 700,000
Stock issued during period, shares, acquisitions		5,808
Common Stock | United Kingdom, Pounds | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary		$ 500,000
Discontinued Operations | OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued operation, tax effect of discontinued operation								$ 4,100,000		(10,400,000)		$ 5,700,000	(300,000)
Discontinued Operations | Tranzact
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued operation, tax effect of discontinued operation														$ 30.2
Discontinued Operations | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain from sale of other discontinued operations, net of tax								(800,000)				$ 1,000,000
Discontinued operation, tax effect of discontinued operation								$ 0		$ 0			$ 3,100,000