Held for Sale and Discontinued Operations - (Details) - USD ($) $ / shares in Units, $ in Millions					3 Months Ended			12 Months Ended
	Sep. 28, 2017	Oct. 05, 2016	Jul. 21, 2016	Apr. 18, 2016	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Discontinued Operations [Line Items}
Gain from sale of other discontinued operations, net of tax					$ 0.1	$ 0.1	$ (1.0)
Net (loss) income from discontinued operations					0.0		32.3
OneBeacon
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300.0
Gain from sale of other discontinued operations, net of tax	$ 554.6
Net (loss) income from discontinued operations					32.3
Tranzact
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1.2	$ 221.3
Gain from sale of other discontinued operations, net of tax								$ 51.9
Net (loss) income from discontinued operations							0.0
Discontinued operation, tax effect of discontinued operation							0.0	30.2
Sirius Group
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 2,600.0
Gain from sale of other discontinued operations, net of tax								363.2
Sale of Sirius Group
Discontinued Operations [Line Items}
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary								$ 113.3
Discontinued Operations | OneBeacon
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations							32.3
Discontinued operation, tax effect of discontinued operation							(4.2)
Discontinued Operations | Tranzact
Discontinued Operations [Line Items}
Gain from sale of other discontinued operations, net of tax							$ (1.0)
Discontinued Operations | Sirius Group
Discontinued Operations [Line Items}
Gain from sale of other discontinued operations, net of tax					0.1
Net (loss) income from discontinued operations					0.0
Discontinued operation, tax effect of discontinued operation					0.0
Intact Financial Corporation | OneBeacon
Discontinued Operations [Line Items}
Sale of stock, price per share	$ 18.10
Connecticut | Discontinued Operations, Held-for-sale
Discontinued Operations [Line Items}
Assets held for sale					$ 3.3	$ 3.3