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Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract] |  |  |
Net realized investment gains, pre-tax | $ 24.1 | $ 270.0 | $ 30.0
Net unrealized investment gains (losses), pre-tax | 109.2 | (297.4) | 230.5
Net realized and unrealized investment gains (losses), pre-tax | 133.3 | (27.4) | 260.5
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses) | (12.9) | 2.7 | (47.7)
Net realized and unrealized investment gains (losses), after-tax | $ 120.4 | $ (24.7) | $ 212.8

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- References No definition available. + Details
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- Definition The sum of the realized and unrealized net gain (loss) on trading securities during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27476-111563 + Details
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- Definition Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax + References No definition available. + Details
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- Definition The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. + References No definition available. + Details
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- Definition Tax on net realized and unrealized gains (loss), trading securities + References No definition available. + Details
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- Definition Trading Securities, Realized Gain (Loss), Total + References No definition available. + Details
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