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Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Investment securities |  |  |  |
Unrealized investment gains |  | $ 336,900,000 | $ (83,600,000) | $ 97,500,000
Foreign Currency Transaction Gain (Loss), Unrealized |  | (300,000) | (11,800,000) | 11,700,000
Net unrealized investment gains (losses), pre-tax |  | 336,600,000 | (95,400,000) | 109,200,000
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments |  | (76,400,000) | 9,100,000 | (4,000,000.0)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax |  | 0 | 0 | 0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax |  | (76,400,000) | 9,100,000 | (4,000,000.0)
Unrealized investment gains after tax |  | 260,500,000 | (74,500,000) | 93,500,000
Trading Securities, Unrealized Foreign Currenc Gain (Loss) Net of Tax |  | (300,000) | (11,800,000) | 11,700,000
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax |  | 260,200,000 | (86,300,000) | 105,200,000
Fixed maturity investments |  |  |  |
Investment securities |  |  |  |
Unrealized investment gains |  | 38,600,000 | (8,300,000) | 13,800,000
Foreign Currency Transaction Gain (Loss), Unrealized |  | 0 | (14,800,000) | 12,700,000
Net unrealized investment gains (losses), pre-tax |  | 38,600,000 | (23,100,000) | 26,500,000
Common equity securities |  |  |  |
Investment securities |  |  |  |
Unrealized investment gains |  | 109,800,000 | (105,500,000) | 99,300,000
Foreign Currency Transaction Gain (Loss), Unrealized |  | 0 | 0 | 0
Net unrealized investment gains (losses), pre-tax |  | 109,800,000 | (105,500,000) | 99,300,000
Other long-term investments |  |  |  |
Investment securities |  |  |  |
Unrealized investment gains |  | 188,500,000 | 30,200,000 | (15,600,000)
Foreign Currency Transaction Gain (Loss), Unrealized |  | (300,000) | 3,000,000.0 | (1,000,000.0)
Net unrealized investment gains (losses), pre-tax |  | 188,200,000 | $ 33,200,000 | $ (16,600,000)
MediaAlpha |  |  |  |
Investment securities |  |  |  |
Unrealized investment gain | $ 114,700,000 | $ 180,000,000.0 |  |

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