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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, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract] |  |  |
Net realized investment gains, pre-tax | $ 77.3 | $ 166.8 | $ 95.5
Net unrealized investment gains (losses), pre-tax | 148.1 | (88.3) | 38.4
Net realized and unrealized investment gains, pre-tax | 225.4 | 78.5 | 133.9
Income tax expense attributable to net realized and unrealized investment gains | (35.2) | (18.0) | (12.8)
Net realized and unrealized investment gains, after tax | $ 190.2 | $ 60.5 | $ 121.1

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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=27724398&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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