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Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2017 | Mar. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Net realized investment gains, pre-tax | $ 1.1 | $ 256.8
Net unrealized investment gains (losses), pre-tax | 50.2 | (227.3)
Net realized and unrealized investment gains, pre-tax | 51.3 | 29.5
Income tax expense attributable to net realized and unrealized investment gains | (7.2) | (8.5)
Net realized and unrealized investment gains, after tax | 44.1 | 21.0
Fixed maturity investments. |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Net realized investment gains, pre-tax | (2.1) | (1.1)
Net unrealized investment gains (losses), pre-tax | 19.2 | 21.7
Common equity securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Net realized investment gains, pre-tax | 1.3 | 257.6
Net unrealized investment gains (losses), pre-tax | 29.6 | (247.4)
Other long-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Net realized investment gains, pre-tax | 1.9 | 0.3
Net unrealized investment gains (losses), pre-tax | $ 4.4 | $ (1.6)

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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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