Significant Transactions (Dispositions) (Details) - USD ($) $ in Millions					3 Months Ended									9 Months Ended	12 Months Ended
	Sep. 28, 2017	Oct. 05, 2016	Jul. 21, 2016	Apr. 18, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 28, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold															$ 0.0	$ 1,131.0	$ 2,646.2
Gain (loss) on sale of other discontinued operations, net of tax															(17.2)	557.0	415.1
Other comprehensive (loss) income, net of tax															(4.8)	0.3	(0.7)
Repayments of debt, principal															15.4	365.0	404.6
Payments to acquire other investments															95.9	84.1	38.5
Net income from discontinued operations, net of tax					$ 0.0	$ (17.3)	$ 0.0	$ 0.1	$ 4.3	$ 539.1	$ 2.8	$ 31.3			0.0	20.5	108.3
OneBeacon
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300.0
Gain (loss) on sale of other discontinued operations, net of tax	554.6
Other comprehensive (loss) income, net of tax	$ 2.9
Net income from discontinued operations, net of tax														$ 20.5			108.6
Tranzact
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1.2	$ 221.3
Gain (loss) on sale of other discontinued operations, net of tax																3.2	51.9
Repayments of debt, principal			56.3
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) on sale of other discontinued operations, net of tax													$ (4.0)				363.2
Payments to acquire other investments				$ 161.8
Net change in pension liability and other accumulated comprehensive items																	113.3
Sale of Sirius Group
Significant Transactions
Net change in pension liability and other accumulated comprehensive items																	113.3
Discontinued Operations | OneBeacon
Significant Transactions
Gain (loss) on sale of other discontinued operations, net of tax															0.0	554.5	0.0
Tax effect of discontinued operation																(5.7)	(12.5)
Net change in pension liability and other accumulated comprehensive items																2.9	0.0
Net income from discontinued operations, net of tax																20.5	108.6
Discontinued Operations | Tranzact
Significant Transactions
Gain (loss) on sale of other discontinued operations, net of tax															0.0	3.2	51.9
Tax effect of discontinued operation																0.0	(6.4)
Net change in pension liability and other accumulated comprehensive items																0.0	0.0
Net income from discontinued operations, net of tax																0.0	6.1
Discontinued Operations | Sirius Group
Significant Transactions
Gain (loss) on sale of other discontinued operations, net of tax															(17.2)	(0.7)	363.2
Tax effect of discontinued operation																0.0	(3.1)
Net change in pension liability and other accumulated comprehensive items																0.0	113.3
Net income from discontinued operations, net of tax																$ 0.0	(6.4)
Discontinued Operations | Sale of Sirius Group
Significant Transactions
Net income from discontinued operations, net of tax																	$ (4.3)
Swedish Tax Authority | Sirius Group | Sirius Group
Significant Transactions
Net income from discontinued operations, net of tax															$ (17.3)