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Investment Securities (Net Realized and 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
Investments, Debt and Equity Securities [Abstract]
Net realized investment (losses) gains, pre-tax	$ (10.2)	$ 6.8	$ (13.9)	$ 20.8
Net unrealized investment gains, pre-tax	76.3	25.7	36.8	81.7
Net realized and unrealized investment gains, pre-tax	66.1	32.5	22.9	102.5
Income tax expense attributable to net realized and unrealized investment gains	(9.7)	(3.9)	(5.9)	(9.5)
Net realized and unrealized investment gains, after-tax	$ 56.4	$ 28.6	$ 17.0	$ 93.0

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The sum of the realized and unrealized net gain (loss) on trading securities during the period.

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Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax

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The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period.

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Tax on net realized and unrealized gains (loss), trading securities

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Trading Securities, Realized Gain (Loss), Total

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