Significant Transactions (Dispositions) (Details) - USD ($) $ in Millions					3 Months Ended	12 Months Ended
	Sep. 28, 2017	Oct. 05, 2016	Jul. 21, 2016	Apr. 18, 2016	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold						$ 1,131.0	$ 2,646.2	$ 24.0
Gain (loss) from sale of other discontinued operations, net of tax						557.0	415.1	18.2
Repayments of debt, principal						365.0	404.6	76.1
Payments to acquire other investments						84.1	38.5	73.9
OneBeacon
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300.0
Gain (loss) from sale of other discontinued operations, net of tax	$ 554.6					554.5	0.0	0.0
Tranzact
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1.2	$ 221.3
Gain (loss) from sale of other discontinued operations, net of tax		51.9				3.2	51.9	0.0
Repayments of debt, principal			56.3
Discontinued operation, tax effect of discontinued operation		30.2
Increase of book value		$ 82.1	$ 82.1
Sirius Group
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 2,600.0
Gain (loss) from sale of other discontinued operations, net of tax					$ (4.0)		363.2
Payments to acquire other investments				$ 161.8
Sale of Sirius Group
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax						(0.7)	363.2	0.0
Recognition of benefit plan assets and obligations from sale of OneBeacon							113.3
Discontinued Operations | OneBeacon
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax						554.5	0.0
Discontinued operation, tax effect of discontinued operation						(5.7)	(12.5)	(12.9)
Recognition of benefit plan assets and obligations from sale of OneBeacon						2.9	0.0
Discontinued Operations | Tranzact
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax						3.2	51.9
Discontinued operation, tax effect of discontinued operation						0.0	(6.4)
Recognition of benefit plan assets and obligations from sale of OneBeacon						0.0	0.0
Discontinued Operations | Sirius Group
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax						(0.7)	363.2
Discontinued operation, tax effect of discontinued operation						0.0	(3.1)	$ 27.1
Recognition of benefit plan assets and obligations from sale of OneBeacon						$ 0.0	$ 113.3