Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 76.4	$ 26.1	$ 48.4	$ 82.1
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(12.0)	(3.3)	(17.1)	(5.9)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	64.4	22.8	31.3	76.2
Foreign Currency Transaction Gain (Loss), Unrealized	(0.1)	(0.4)	(11.6)	(0.4)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0	0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(0.1)	(0.4)	(11.6)	(0.4)
Net unrealized investment gains, pre-tax	76.3	25.7	36.8	81.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(12.0)	(3.3)	(17.1)	(5.9)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	64.3	22.4	19.7	75.8
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	1.5	2.0	(15.7)	19.4
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	5.1	(14.8)	12.6
Net unrealized investment gains, pre-tax	1.5	7.1	(30.5)	32.0
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	(0.1)	0.0	(0.1)	0.0
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0	0.0	0.0
Net unrealized investment gains, pre-tax	(0.1)	0.0	(0.1)	0.0
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	54.9	26.6	43.7	53.5
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	(3.1)	0.0	0.0
Net unrealized investment gains, pre-tax	54.9	23.5	43.7	53.5
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	20.1	(2.5)	20.5	9.2
Foreign Currency Transaction Gain (Loss), Unrealized	(0.1)	(2.4)	3.2	(13.0)
Net unrealized investment gains, pre-tax	$ 20.0	$ (4.9)	$ 23.7	$ (3.8)

Amount before tax of foreign currency transaction unrealized gain (loss) recognized in the income statement.

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