Held for Sale and Discontinued Operations - (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
	Sep. 28, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Discontinued Operations [Line Items}
(Loss) gain from sale of discontinued operations, net of tax		$ 0.0	$ (0.6)	$ 0.1	$ (1.6)
Net (loss) income from discontinued operations		0.0	3.4	0.0	35.7
OneBeacon
Discontinued Operations [Line Items}
Sale of stock, price per share	$ 18.10
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300.0
(Loss) gain from sale of discontinued operations, net of tax	$ 554.6
Net (loss) income from discontinued operations			3.4		35.7
Discontinued Operations | OneBeacon
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations			3.4		35.7
Discontinued Operations | Tranzact
Discontinued Operations [Line Items}
(Loss) gain from sale of discontinued operations, net of tax					(1.0)
Discontinued Operations | Sirius Group
Discontinued Operations [Line Items}
(Loss) gain from sale of discontinued operations, net of tax			(0.6)	0.1	(0.6)
Net (loss) income from discontinued operations			$ 0.0		$ 0.0
Connecticut | Discontinued Operations, Held-for-sale
Discontinued Operations [Line Items}
Assets held for sale		$ 3.3		$ 3.3		$ 3.3