Discontinued Operations (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended	54 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Mar. 31, 2015	Jun. 30, 2014	Sep. 30, 2015	Mar. 31, 2016	Dec. 31, 2015
Discontinued Operations [Line Items}
Payments to Acquire Other Investments		$ 10.9	$ 6.5
Investments		3,639.7				$ 3,639.7	$ 4,280.7
Net realized and unrealized investment gains		29.5	25.0
Net gain on sale of discontinued operations		0.0	8.0
Net (loss) income from discontinued operations, net of tax		(0.9)	65.8
Net Realized and Unrealized Gain (Loss) on Trading Securities		29.5	25.0
Sirius Group
Discontinued Operations [Line Items}
Proceeds from Divestiture of Interest in Subsidiaries and Affiliates	$ 2,600.0
Payments to Acquire Other Investments	$ (161.8)
Investments		161.8				161.8	686.2
Net realized and unrealized investment gains		4.1	0.3
Symetra | Sirius Group
Discontinued Operations [Line Items}
Investments							$ 528.6
Discontinued Operations [Member]
Discontinued Operations [Line Items}
Net realized and unrealized investment gains		(8.8)	78.4
Net loss from discontinued operations, net of tax		(0.9)	73.8
Net gain on sale of discontinued operations		(0.9)	65.9	$ 2.7
Discontinued Operations [Member] | Sirius Group
Discontinued Operations [Line Items}
Net realized and unrealized investment gains		(8.8)	78.4
Net loss from discontinued operations, net of tax		(0.9)	65.9
Net gain on sale of discontinued operations		(0.9)	65.9
Discontinued Operations [Member] | OneBeacon Runoff [Member]
Discontinued Operations [Line Items}
Net loss from discontinued operations, net of tax			(0.1)
Net gain on sale of discontinued operations		0.0	(0.1)
Discontinued Operations [Member] | Esurance
Discontinued Operations [Line Items}
Net gain on sale of discontinued operations		$ 0.0	$ 8.0		$ 10.3	$ 28.3