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Investment Securities (Net Realized and Unrealized Gains (Losses) After Tax) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investments in and Advances to Affiliates [Line Items] |  |  |
Net change in pre-tax unrealized investment losses on investments in unconsolidated affiliates | $ 0.0 | $ 0.0 | $ (39.2)
Income tax benefit | 0.0 | 0.0 | 2.9
Net change in unrealized investment losses on investments in unconsolidated affiliates, after-tax | 0.0 | 0.0 | (36.3)
Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 120.4 | (24.7) | 212.8
Total investment gains (losses) recorded during the period, after-tax | 120.4 | (24.7) | 177.9
Symetra |  |  |
Investments in and Advances to Affiliates [Line Items] |  |  |
Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra | 0.0 | 0.0 | 1.4
Net change in unrealized investment losses on investments in unconsolidated affiliates, after-tax | $ 0.0 | $ 0.0 | $ (34.9)

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- Definition The net unrealized gains (losses) from Symetra's fixed maturity portfolio. + References No definition available. + Details
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- Definition Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax + References No definition available. + Details
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