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Investment Securities (Net Realized Investment Gains (Losses)) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Investment securities |  |  |  |
Trading Securities, Realized Gain (Loss) | $ 23.7 | $ 38.3 | $ 40.5 | $ 67.7
Trading Securities, Realized Gain (Loss) Tax | 7.4 | 8.3 | 12.8 | 20.7
Trading Securities, Realized Gain (Loss) Net of Tax | 16.3 | 30.0 | 27.7 | 47.0
Fixed maturity investments. |  |  |  |
Investment securities |  |  |  |
Trading Securities, Realized Gain (Loss) | 29.0 | 14.1 | 69.5 | 3.3
Short-term investments |  |  |  |
Investment securities |  |  |  |
Trading Securities, Realized Gain (Loss) | (3.4) | 0.4 | (3.9) | (10.9)
Common equity securities |  |  |  |
Investment securities |  |  |  |
Trading Securities, Realized Gain (Loss) | 3.9 | 23.1 | (2.2) | 41.0
Convertible Debt Securities [Member] |  |  |  |
Investment securities |  |  |  |
Trading Securities, Realized Gain (Loss) | 1.1 | (0.7) | 3.2 | 6.3
Other long-term investments |  |  |  |
Investment securities |  |  |  |
Trading Securities, Realized Gain (Loss) | (7.2) | 1.4 | (26.4) | 28.0
Forward Contracts [Member] |  |  |  |
Investment securities |  |  |  |
Trading Securities, Realized Gain (Loss) | $ 0.3 | $ 0 | $ 0.3 | $ 0

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