Held for Sale and Discontinued Operations (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
Discontinued Operations
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
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
Tax benefit (expense)					3.6	$ 3.6	$ (2.5)	$ (0.7)	2.5	$ 4.0	$ 1.0	$ 0.3			4.0	7.8	32.9
Payments to acquire other investments															95.9	84.1	38.5
OneBeacon
Discontinued Operations
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
Net income from discontinued operations, net of tax														$ 20.5			108.6
OneBeacon | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of other discontinued operations, net of tax															0.0	554.5	0.0
Net income from discontinued operations, net of tax																20.5	108.6
Tax effect of discontinued operation																(5.7)	(12.5)
Recognition of foreign currency translation from sale of Sirius Group, net of tax																2.9	0.0
Tranzact
Discontinued Operations
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
Tax effect of discontinued operation																	30.2
Tranzact | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of other discontinued operations, net of tax															0.0	3.2	51.9
Net income from discontinued operations, net of tax																0.0	6.1
Tax benefit (expense)																	21.4
Tax effect of extraordinary item, loss																	30.2
Tax effect of discontinued operation																0.0	(6.4)
Recognition of foreign currency translation from sale of Sirius Group, net of tax																0.0	0.0
Sirius Group
Discontinued Operations
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
Recognition of foreign currency translation from sale of Sirius Group, net of tax																	113.3
Sirius Group | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of other discontinued operations, net of tax															(17.2)	(0.7)	363.2
Net income from discontinued operations, net of tax																0.0	(6.4)
Tax effect of discontinued operation																0.0	(3.1)
Recognition of foreign currency translation from sale of Sirius Group, net of tax																0.0	113.3
Sale of Sirius Group
Discontinued Operations
Recognition of foreign currency translation from sale of Sirius Group, net of tax																	113.3
Sale of Sirius Group | Discontinued Operations
Discontinued Operations
Net income from discontinued operations, net of tax																	(4.3)
Net gain (loss) from sale of discontinued operations | Tranzact | Discontinued Operations
Discontinued Operations
Tax effect of discontinued operation																	$ 8.8
Connecticut | Sirius Group, Tranzact, and Star and Shield | Discontinued Operations, Held-for-sale
Discontinued Operations
Assets held for sale					$ 3.3				$ 3.3						3.3	3.3
Other expenses																$ 3.7
Sirius Group | Swedish Tax Authority | Sirius Group
Discontinued Operations
Net income from discontinued operations, net of tax															$ (17.3)