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Held for Sale and Discontinued Operations (Details) - USD ($) $ / shares in Units, $ in Millions				3 Months Ended								9 Months Ended		12 Months Ended
	Sep. 28, 2017	Jul. 21, 2016	Apr. 18, 2016	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 28, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Discontinued Operations
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold														$ 0.0	$ 0.0	$ 1,131.0
Gain (loss) on sale of other discontinued operations, net of tax														0.8	(17.2)	557.0
Net income from discontinued operations, net of tax				$ (0.8)	$ 0.9	$ 0.0	$ 0.7	$ 0.0	$ (17.3)	$ 0.0	$ 0.1			0.0	0.0	20.5
Payments to acquire other investments														177.7	95.9	84.1
OneBeacon
Discontinued Operations
Sale of Stock, Price Per Share	$ 18.10												$ 18.10
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300.0
Gain (loss) on sale of other discontinued operations, net of tax	$ 554.6
Net income from discontinued operations, net of tax													$ 20.5
OneBeacon | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of other discontinued operations, net of tax																554.5
Net income from discontinued operations, net of tax																20.5
Tranzact
Discontinued Operations
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 221.3
Gain (loss) on sale of other discontinued operations, net of tax															3.2	3.2
Tranzact | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of other discontinued operations, net of tax																3.2
Net income from discontinued operations, net of tax																0.0
Sirius Group
Discontinued Operations
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold			$ 2,600.0
Gain (loss) on sale of other discontinued operations, net of tax															(17.3)
Payments to acquire other investments			$ 161.8
Discontinued operations, liability				16.5				17.3						16.5	17.3
Sirius Group | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of other discontinued operations, net of tax														0.8	(17.2)	(0.7)
Net income from discontinued operations, net of tax																0.0
Connecticut | Sirius Group, Tranzact, and Star and Shield | Discontinued Operations, Held-for-sale
Discontinued Operations
Other expenses																$ 3.7
Assets held for sale				$ 3.0				$ 3.3						3.0	3.3
OneBeacon | Intact Financial Corporation
Discontinued Operations
Sale of Stock, Price Per Share	$ 18.10												$ 18.10
Swedish Tax Authority | Sirius Group | Sirius Group
Discontinued Operations
Net income from discontinued operations, net of tax												$ (17.3)		0.8
Loss contingency accrual, period increase (decrease)														$ (0.8)	$ 17.3

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