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Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | $ (7.4) | $ 58.0 | $ 49.3 | $ 97.1
Trading Securities, Realized Gain (Loss), Total | (7.4) | 57.9 | 49.7 | 97.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (36.5) | (85.4) | (83.6) | (32.6)
Net Realized and Unrealized Gain (Loss) on Trading Securities | (43.9) | (27.5) | (33.9) | 64.4
Income tax benefit (expense) attributable to net realized and unrealized investment gains (losses) | 9.4 | 5.6 | 6.9 | (12.2)
Realized and unrealized gain | (34.5) | (21.9) | (27.0) | 52.2
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | (0.1) | 0.4 | (0.1)
Trading securities, Tax on Realized Holding Gain (Loss) on Investments | 1.5 | (7.0) | (15.7) | (13.3)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax | 0.0 | 0.0 | 0.0 | 0.0
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 1.5 | (7.0) | (15.7) | (13.3)
Trading Securities Realized Holding Gain (Loss) on Investments after Tax | (5.9) | 51.0 | 33.6 | 83.8
Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax | 0.0 | (0.1) | 0.4 | (0.1)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | (5.9) | 50.9 | 34.0 | 83.7
Fixed maturity investments. |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | (0.1) | 2.3 | 2.0 | 4.7
Trading Securities, Realized Gain (Loss), Total | (0.1) | 2.2 | 2.0 | 4.6
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 3.3 | (10.0) | 1.3 | 12.3
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | (0.1) | 0.0 | (0.1)
Short-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 0.0 | 0.0 | 0.0 | 0.0
Trading Securities, Realized Gain (Loss), Total | 0.0 | 0.0 | 0.0 | 0.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 0.0 | 0.0 | 0.0 | 0.0
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.0
Common equity securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | (5.9) | 51.4 | 41.6 | 83.2
Trading Securities, Realized Gain (Loss), Total | (5.9) | 51.4 | 42.0 | 83.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (30.8) | (69.2) | (58.5) | (48.5)
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.4 | 0.0
Convertible fixed maturity investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 0.0 | 1.6 | (4.3) | 6.5
Trading Securities, Realized Gain (Loss), Total | 0.0 | 1.6 | (4.3) | 6.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 1.1 | (3.4) | 2.0 | (5.1)
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.0
Other long-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | (1.4) | 2.7 | 10.0 | 2.7
Trading Securities, Realized Gain (Loss), Total | (1.4) | 2.7 | 10.0 | 2.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (10.1) | (2.8) | (28.4) | 8.7
Foreign Currency Transaction Gain (Loss), Realized | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0

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