Significant Transactions (Details) $ / shares in Units, $ in Millions							3 Months Ended							6 Months Ended		12 Months Ended
	May 02, 2017 $ / shares	Oct. 05, 2016 USD ($)	Aug. 04, 2016 USD ($)	Jul. 21, 2016 USD ($)	Apr. 18, 2016 USD ($)	Feb. 01, 2016 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2015 $ / shares	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets									$ 6,786.2					$ 6,786.2		$ 6,544.7
Goodwill									31.7	$ 31.7		$ 24.1		31.7	$ 24.1	31.7	$ 24.1	$ 24.1
Liabilities									3,022.1					3,022.1		2,807.5
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold														0.0	2,248.5
Repayments of debt														13.3	150.5
Net (loss) gain on sale of discontinued operations, net of tax									(0.6)			$ 366.6		(1.6)	366.6
Payments to acquire other investments														55.1	12.9
Sales of common equity securities														$ 183.9	$ 676.9
Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)			70.90%
Assets			$ 11.5
Goodwill			7.6
Intangible assets, net (excluding goodwill)			1.1
Liabilities			0.1
United Kingdom, Pounds | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to acquire interest in subsidiaries and affiliates			6.1
United States of America, Dollars | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to acquire interest in subsidiaries and affiliates			$ 8.1
Intact Financial Corporation | OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of stock, price per share | $ / shares	$ 18.10
Transaction book value multiple	1.65
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
Repayments of debt				56.3
Net (loss) gain on sale of discontinued operations, net of tax		51.9								$ (1.0)						51.9
Discontinued operation, tax effect of discontinued operation		30.2														30.2
Increase of book value		$ 82.1		$ 82.1
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
Net (loss) gain on sale of discontinued operations, net of tax									$ (0.6)		$ 4.0					363.2
Payments to acquire other investments					$ 161.8
Sale of Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of Sirius Group																$ 113.3
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities						$ 658.0
Dividends payable, amount per share | $ / shares						$ 32.00
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities						$ 658.0
Common stock, dividends, per share, cash paid | $ / shares						$ 0.50							$ 0.50
OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)									75.70%					75.70%
Business acquisition, percentage of voting interests acquired									96.90%					96.90%
Scenario, Forecast
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from acquisition							$ 1,300.0	$ 1,300.0