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Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract] |  |  |  |
Net realized investment gains, pre-tax | $ 6.8 | $ 0.7 | $ 20.8 | $ 265.0
Net unrealized investment gains (losses), pre-tax | 25.7 | 10.2 | 81.7 | (237.8)
Net realized and unrealized investment gains, pre-tax | 32.5 | 10.9 | 102.5 | 27.2
Income tax expense attributable to net realized and unrealized investment gains | (3.9) | 0.0 | (9.5) | (4.0)
Net realized and unrealized investment gains, after tax | $ 28.6 | $ 10.9 | $ 93.0 | $ 23.2

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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=73527792&loc=d3e27476-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 + 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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