Discontinued Operations (Narrative) (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	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 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													$ 1,131.0	$ 2,646.2	$ 24.0
Gain (loss) from sale of other discontinued operations, net of tax													557.0	415.1	18.2
Net (loss) income from discontinued operations, net of tax					$ 4.3	$ 539.1	$ 2.8	$ 31.3	$ 8.0	$ 84.4	$ 383.6	$ 47.4	20.5	108.3	116.9
Income Tax (expense) benefit					2.5	$ 4.0	$ 1.0	$ 0.3	10.2	17.1	$ 4.0	$ 1.6	7.8	32.9	(12.7)
Payments to acquire other investments													84.1	38.5	73.9
Total investments					$ 3,380.7				$ 2,714.4				3,380.7	2,714.4
Net realized and unrealized investment gains													133.3	(27.4)	260.5
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	$ 1,300.0
Gain (loss) from sale of other discontinued operations, net of tax	$ 554.6												554.5	0.0	0.0
Net (loss) income from discontinued operations, net of tax													20.5	108.6	38.2
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		$ 1.2	$ 221.3
Gain (loss) from sale of other discontinued operations, net of tax		51.9											3.2	51.9	0.0
Discontinued operation, tax effect of discontinued operation		$ 30.2
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				$ 2,600.0
Gain (loss) from sale of other discontinued operations, net of tax										$ (4.0)				363.2
Payments to acquire other investments				$ 161.8
Total investments															686.2
Net realized and unrealized investment gains														3.7	205.0
Sirius Group | Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total investments															528.6
Net realized and unrealized investment gains															200.8
Sale of Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) from sale of other discontinued operations, net of tax													(0.7)	363.2	0.0
Recognition of other comprehensive income from sale of subsidiary														113.3
Esurance
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) from sale of other discontinued operations, net of tax													0.0	0.0	17.9
Discontinued Operations | OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) from sale of other discontinued operations, net of tax													554.5	0.0
Net (loss) income from discontinued operations, net of tax													20.5	108.6	38.2
Discontinued operation, tax effect of discontinued operation													(5.7)	(12.5)	(12.9)
Recognition of other comprehensive income from sale of subsidiary													2.9	0.0
Net realized and unrealized investment gains													38.8	37.7	(35.1)
Net change in pension liability and other accumulated comprehensive items													0.3	1.0	0.0
Discontinued Operations | Tranzact
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) from sale of other discontinued operations, net of tax													3.2	51.9
Net (loss) income from discontinued operations, net of tax													0.0	6.1	(1.9)
Income Tax (expense) benefit														21.4
Tax effect of discontinued operations														30.2
Discontinued operation, tax effect of discontinued operation													0.0	(6.4)
Recognition of other comprehensive income from sale of subsidiary													0.0	0.0
Net realized and unrealized investment gains													0.0	0.0
Net change in pension liability and other accumulated comprehensive items													0.0	0.0
Discontinued Operations | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) from sale of other discontinued operations, net of tax													(0.7)	363.2
Net (loss) income from discontinued operations, net of tax													0.0	(6.4)
Discontinued operation, tax effect of discontinued operation													0.0	(3.1)	27.1
Recognition of other comprehensive income from sale of subsidiary													0.0	113.3
Net realized and unrealized investment gains													0.0	(1.5)	15.1
Net change in pension liability and other accumulated comprehensive items													$ 0.0	32.0	(65.0)
Discontinued Operations | Sale of Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net (loss) income from discontinued operations, net of tax														(4.3)	81.1
Net change in pension liability and other accumulated comprehensive items														32.0
Discontinued Operations | Esurance
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) from sale of other discontinued operations, net of tax														28.3	$ 17.9
OneBeacon | Intact Financial Corporation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of stock, price per share	$ 18.10
Net gain (loss) from sale of discontinued operations | 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														$ 8.8