Discontinued Operations (Details) - USD ($) $ in Millions		3 Months Ended					12 Months Ended
	Apr. 18, 2016	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Discontinued Operations [Line Items}
Payments to Acquire Other Investments		$ 22.4				$ 10.9
Investments		5,339.0	$ 5,334.8				$ 5,334.8
Net realized and unrealized investment gains		51.3				29.5
Net (loss) gain on sale of discontinued operations, net of tax		(1.0)				0.0
Net (loss) income from discontinued operations		0.0				1.1
Net change in foreign currency translation		0.9				0.1
Other liabilities		178.5	199.9				199.9
Income Tax Expense (Benefit)		3.9				(9.7)
Net Realized and Unrealized Gain (Loss) on Trading Securities		51.3				29.5
Sirius Group
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated 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						4.1
Net (loss) gain on sale of discontinued operations, net of tax				$ 4.0			363.2
Net change in foreign currency translation		0.0				37.2
Sale of Sirius Group [Member]
Discontinued Operations [Line Items}
Change in foreign currency translation and other, net of tax							113.3
Tranzact [Member]
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold			$ 1.2	221.3
Net (loss) gain on sale of discontinued operations, net of tax		(1.0)		51.9			51.9
Discontinued Operation, Tax Effect of Discontinued Operation				$ 30.2			30.2
Discontinued Operations [Member]
Discontinued Operations [Line Items}
Net realized and unrealized investment gains		0.0				(8.8)
Net (loss) income from discontinued operations		0.0				1.1
Net change in foreign currency translation		0.0				37.2
Discontinued Operation, Tax Effect of Discontinued Operation		0.0				(0.1)
Discontinued Operations [Member] | Sirius Group
Discontinued Operations [Line Items}
Net realized and unrealized investment gains		0.0				(8.8)
Net (loss) income from discontinued operations		0.0				(0.9)
Net change in foreign currency translation		0.0				37.2
Discontinued Operation, Tax Effect of Discontinued Operation		$ 0.0				0.9
Discontinued Operations [Member] | Tranzact [Member]
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations						$ (2.0)
Income Tax Expense (Benefit)							(21.4)
Tax Effect of Extraordinary Item, Gain (Loss)							30.2
Discontinued Operation, Tax Effect of Discontinued Operation							(8.8)
Discontinued Operations, Held-for-sale | Star & Shield LLC [Member]
Discontinued Operations [Line Items}
Other Expenses							$ 21.0