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Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
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Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Investments, Debt and Equity Securities [Abstract] |  |  |  |
Net realized investment gains, pre-tax | $ 13.4 | $ 1.6 | $ 14.0 | $ 264.3
Net unrealized investment gains (losses), pre-tax | 20.3 | 1.8 | 56.0 | (248.0)
Net realized and unrealized investment gains, pre-tax | 33.7 | 3.4 | 70.0 | 16.3
Income tax expense attributable to net realized and unrealized investment gains | (1.7) | (1.4) | (5.5) | (3.9)
Net realized and unrealized investment gains, after tax | $ 32.0 | $ 2.0 | $ 64.5 | $ 12.4

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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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