Discontinued Operations (Details) - USD ($) $ in Millions		3 Months Ended									6 Months Ended	12 Months Ended
	Apr. 18, 2016	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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												$ 2,646.2	$ 24.0	$ 0.0
Payments to Acquire Other Investments												39.5	78.0	114.7
Income Tax Expense (Benefit)		$ 23.0	$ 6.7	$ 6.0	$ 9.7	$ 1.0	$ 1.6	$ 2.2	$ (4.6)			45.4	0.2	14.8
Net realized and unrealized investment gains												10.3	225.4	78.5
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax													0.0	0.0
Total investments		5,334.8				4,271.1						5,334.8	4,271.1
OCI, Change in foreign currency transaction and translation adjustment, net of tax												0.3	(0.5)	(10.7)
Gain (loss) from sale of other discontinued operations, net of tax												415.1	18.2	(1.6)
Loss and loss adjustment expense reserves		1,365.6				1,389.8				$ 1,342.2		1,365.6	1,389.8	1,342.2	$ 1,054.3
Net (loss) income from discontinued operations, net of tax		(6.8)	61.9	358.6	$ 1.1	19.6	$ 6.5	$ (0.5)	$ 71.3			(0.3)	78.7	260.6
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			2,600.0
Payments to Acquire Other Investments	$ 161.8			161.8
Net realized and unrealized investment gains												3.7	205.0	2.7
Total investments						686.2							686.2
Gain (loss) from sale of other discontinued operations, net of tax				$ 363.2								363.2
Sirius Group | Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net realized and unrealized investment gains													200.8
Total investments						528.6							528.6
Sale of Sirius Group [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax											$ 113.3	113.3
OCI, Change in foreign currency transaction and translation adjustment, net of tax												32.0	(65.0)	(169.5)
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
Discontinued Operation, Tax Effect of Discontinued Operation			30.2
Gain (loss) from sale of other discontinued operations, net of tax			$ 51.9									51.9
OneBeacon Runoff
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										$ 36.1
Esurance [Member]
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	17.9	3.2
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net realized and unrealized investment gains												(1.5)	15.1	205.4
OCI, Change in foreign currency transaction and translation adjustment, net of tax													(65.0)	(169.5)
Discontinued Operation, Tax Effect of Discontinued Operation												(9.5)	26.4	69.4
Total income from discontinued operations												414.8	96.9	259.0
Net (loss) income from discontinued operations, net of tax												(0.3)	78.7	260.6
Discontinued Operations | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net realized and unrealized investment gains												(1.5)	15.1	205.4
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax												113.3
OCI, Change in foreign currency transaction and translation adjustment, net of tax												32.0	(65.0)	(169.5)
Discontinued Operation, Tax Effect of Discontinued Operation												(3.1)	27.1	70.4
Gain (loss) from sale of other discontinued operations, net of tax												363.2		0.0
Total income from discontinued operations												358.9	81.1	262.0
Net (loss) income from discontinued operations, net of tax												(4.3)	81.1	262.0
Discontinued Operations | Tranzact
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income Tax Expense (Benefit)												21.4
Tax Effect of Discontinued Operations												30.2
Discontinued Operation, Tax Effect of Discontinued Operation												8.8
Gain (loss) from sale of other discontinued operations, net of tax												51.9
Net (loss) income from discontinued operations, net of tax												6.1	(1.9)	0.4
Discontinued Operations | OneBeacon Runoff
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													0.3	$ (18.8)
Loss and loss adjustment expense reserves		$ 18.3				$ 20.4						18.3	20.4
Percentage of quota share reinsurance agreement														100.00%
Total income from discontinued operations													(0.5)
Discontinued Operations | Esurance [Member]
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	$ 3.2
Discontinued Operations | Fireman's Fund Insurance Company [Member]
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														$ 14.0
Discontinued Operations, Held-for-sale [Member] | Star & Shield LLC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Other Expenses												$ 21.0