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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, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract] |  |  |
Net realized investment gains, pre-tax | $ 280.3 | $ 77.3 | $ 166.8
Net unrealized investment (losses) gains, pre-tax | (270.0) | 148.1 | (88.3)
Net realized and unrealized investment gains, pre-tax | 10.3 | 225.4 | 78.5
Income tax expense attributable to net realized and unrealized investment gains | (8.8) | (35.2) | (18.0)
Net realized and unrealized investment gains, after tax | $ 1.5 | $ 190.2 | $ 60.5

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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=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 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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