Discontinued Operations (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
	Apr. 18, 2016	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Discontinued Operations [Line Items}
Sales of consolidated and unconsolidated subsidiaries, net of cash sold						$ 2,248.5	$ 24.0
Payments to Acquire Other Investments						13.3	28.6
Investments			$ 5,521.3			5,521.3		$ 4,271.1
Net realized and unrealized investment gains (losses)			28.1		$ (15.0)	57.6	10.0
Net gain on sale of discontinued operations			366.6		(0.1)	366.6	7.9
Net (loss) income from discontinued operations, net of tax			(8.0)		(0.4)	(6.9)	62.9
Net Realized and Unrealized Gain (Loss) on Trading Securities			28.1		(15.0)	57.6	10.0
Sirius Group
Discontinued Operations [Line Items}
Sales of consolidated and unconsolidated subsidiaries, net of cash sold	$ 2,600.0		2,600.0
Payments to Acquire Other Investments	$ 161.8		161.8
Investments								686.2
Net realized and unrealized investment gains (losses)			0.4		2.4	3.7	2.7
Net gain on sale of discontinued operations						366.6
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease			(5.2)		44.0	32.0	(43.6)
Increase of Book Value						479.9
Sale of Sirius Group [Member]
Discontinued Operations [Line Items}
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease			113.3		0.0	113.3	0.0
Symetra | Sirius Group
Discontinued Operations [Line Items}
Investments								$ 528.6
Discontinued Operations [Member]
Discontinued Operations [Line Items}
Net realized and unrealized investment gains (losses)			7.3		(46.1)	(1.5)	32.3
Net gain on sale of discontinued operations			(8.0)		(0.4)	(6.9)	62.9
Discontinued Operations [Member] | Sirius Group
Discontinued Operations [Line Items}
Net realized and unrealized investment gains (losses)			7.3		(46.1)	(1.5)	32.3
Net gain on sale of discontinued operations			(3.4)		3.8	(4.3)	69.7
Net (loss) income from discontinued operations, net of tax			(3.4)			(4.3)
Discontinued Operations [Member] | Tranzact [Member]
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations, net of tax			$ (0.5)		(6.5)	(2.5)	(4.0)
Discontinued Operations [Member] | OneBeacon Runoff [Member]
Discontinued Operations [Line Items}
Net gain on sale of discontinued operations					0.3		0.3
Discontinued Operations [Member] | Esurance
Discontinued Operations [Line Items}
Net gain on sale of discontinued operations				$ 10.3	$ (0.3)	$ 28.3	$ 7.6
Sale of Tranzact [Member]
Discontinued Operations [Line Items}
Sales of consolidated and unconsolidated subsidiaries, net of cash sold		$ 221.3
Sale of Tranzact [Member] | Tranzact [Member]
Discontinued Operations [Line Items}
Sales of consolidated and unconsolidated subsidiaries, net of cash sold		$ 221.3