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Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
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Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Investment securities |  |  |  |
Net unrealized gains (losses), pre-tax | $ 93.3 | $ (110.0) | $ 122.3 | $ (110.3)
Trading Securities, Unrealized Gain (Loss) Tax | 16.7 | (26.4) | 25.9 | (28.5)
Trading Securities, Unrealized Gain (Loss), Net of Tax | 76.6 | (83.6) | 96.4 | (81.8)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax | (44.3) | 74.6 | (39.6) | 79.0
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax | (11.8) | 19.5 | (10.4) | 20.5
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax | (32.5) | 55.1 | (29.2) | 58.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 49.0 | (35.4) | 82.7 | (31.3)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax | 4.9 | (6.9) | 15.5 | (8.0)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 44.1 | (28.5) | 67.2 | (23.3)
Fixed maturity investments. |  |  |  |
Investment securities |  |  |  |
Net unrealized gains (losses), pre-tax | 27.2 | 3.5 | 35.5 | 12.6
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax | (40.6) | 67.5 | (36.9) | 77.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (13.4) | 71.0 | (1.4) | 90.3
Short-term investments |  |  |  |
Investment securities |  |  |  |
Net unrealized gains (losses), pre-tax | 0 | (0.1) | 0 | (0.1)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax | 0.1 | (0.4) | 0.1 | (1.4)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 0.1 | (0.5) | 0.1 | (1.5)
Common equity securities |  |  |  |
Investment securities |  |  |  |
Net unrealized gains (losses), pre-tax | 65.1 | (101.0) | 71.5 | (95.1)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax | (0.1) | (0.9) | (0.1) | (1.8)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 65.0 | (101.9) | 71.4 | (96.9)
Convertible Debt Securities [Member] |  |  |  |
Investment securities |  |  |  |
Net unrealized gains (losses), pre-tax | (0.6) | (11.2) | (2.9) | (19.3)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax | 0 | 0 | 0 | 0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (0.6) | (11.2) | (2.9) | (19.3)
Other long-term investments |  |  |  |
Investment securities |  |  |  |
Net unrealized gains (losses), pre-tax | 1.6 | (1.2) | 18.2 | (8.4)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax | (3.7) | 8.4 | (2.7) | 4.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | $ (2.1) | $ 7.2 | $ 15.5 | $ (3.9)

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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of trading securities held at each balance sheet date, that was included in earnings for the period. Trading securities are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) and the change in net unrealized holding gain (loss) is included in earnings for the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 21 -Subparagraph e -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 13 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27357-111563 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 12 -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c)) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=16383099&loc=d3e24584-111560 + Details
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