Significant Transactions (Dispositions) (Details) - USD ($) $ / shares in Units, $ 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, 2014	Mar. 31, 2013	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Feb. 01, 2016	Dec. 23, 2014
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax									$ 415.1	$ 18.2	$ (1.6)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax										0.0	0.0
Total investments		$ 5,334.8							5,334.8	4,271.1
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
Repayments of debt, principal									404.6	76.1	65.2
Sales of common equity securities									$ 1,200.8	461.4	$ 609.8
OneBeacon
Significant Transactions
Percentage of quota share reinsurance agreement									100.00%
Symetra
Significant Transactions
Sales of common equity securities					$ 658.0
Dividends Payable, Amount Per Share												$ 32.00
Sirius Group
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax				$ 363.2					$ 363.2
Total investments										686.2
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
Sale of Sirius Group [Member]
Significant Transactions
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax								$ 113.3	113.3
Tranzact
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax			$ 51.9						$ 51.9
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1.2	221.3
Repayments of debt, principal			56.3
Discontinued Operation, Tax Effect of Discontinued Operation			30.2
Increase of Book Value			$ 82.1
OneBeacon Runoff
Significant Transactions
Surplus notes													$ 101.0
Gain (loss) from sale of other discontinued operations, net of tax						$ 36.1
Essentia | OneBeacon
Significant Transactions
Gain (Loss) on Disposition of Subsidiary							$ 23.0
Gain (Loss) on Disposition of Subsidiary, After Tax							$ 15.0
Gain (Loss) from Adjustments to Prior Period Extraordinary Items										(3.7)
Symetra
Significant Transactions
Sales of common equity securities					$ 658.0
Symetra | Sirius Group
Significant Transactions
Total investments										528.6
Fair Value [Member] | Other long-term investments | OneBeacon Runoff
Significant Transactions
Surplus notes													$ 64.9
Crop Business
Significant Transactions
Proceeds related from termination of agreement										$ 3.0
Percentage of quota share reinsurance agreement										100.00%