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Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investment securities |  |  |
Net realized investment gains (losses), pre-tax | $ 35.3 | $ 269.7 | $ 30.0
Net realized investment gains (losses), foreign, pre-tax | (11.2) | 0.3 | 0.0
Net realized investment gains, pre-tax, total | 24.1 | 270.0 | 30.0
Income tax expense attributable to net realized investment gains (losses) | (8.9) | (45.6) | (6.4)
Income tax expense attributable to net realized investment gains (losses), foreign | 0.0 | 0.0 | 0.0
Income tax expense attributable to net realized investment gains, total | (8.9) | (45.6) | (6.4)
Net realized investment gains (losses), after-tax | 26.4 | 224.1 | 23.6
Net realized investment gains (losses), foreign, after-tax | (11.2) | 0.3 | 0.0
Net realized investment gains (losses), total, after-tax | 15.2 | 224.4 | 23.6
Fixed maturity investments |  |  |
Investment securities |  |  |
Net realized investment gains (losses), pre-tax | (1.6) | (1.9) | (0.2)
Net realized investment gains (losses), foreign, pre-tax | 4.1 | 0.3 | 0.0
Net realized investment gains, pre-tax, total | 2.5 | (1.6) | (0.2)
Short-term investments |  |  |
Investment securities |  |  |
Net realized investment gains (losses), pre-tax | (0.3) | 0.4 |
Net realized investment gains (losses), foreign, pre-tax | 0.0 | 0.0 |
Net realized investment gains, pre-tax, total | (0.3) | 0.4 |
Common equity securities |  |  |
Investment securities |  |  |
Net realized investment gains (losses), pre-tax | 18.1 | 268.5 | 31.0
Net realized investment gains (losses), foreign, pre-tax | 6.0 | 0.0 | 0.0
Net realized investment gains, pre-tax, total | 24.1 | 268.5 | 31.0
Other long-term investments |  |  |
Investment securities |  |  |
Net realized investment gains (losses), pre-tax | 19.1 | 2.7 | (0.8)
Net realized investment gains (losses), foreign, pre-tax | (21.3) | 0.0 | 0.0
Net realized investment gains, pre-tax, total | $ (2.2) | $ 2.7 | $ (0.8)

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- Definition Amount before tax of foreign currency transaction realized gain (loss) recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=64836518&loc=d3e30690-110894 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64836518&loc=d3e30700-110894 + Details
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