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Investment Securities (Net Unrealized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended
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Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012
Investment securities |  |  |  |
Net realized and unrealized gain (loss) on trading securities | $ (72.9) | $ (36.2) | $ 12.1 | $ 47.5
Trading securities, tax on realized and unrealized holding gain (loss) on investments | 20.3 | 5.6 | 4.4 | (15.0)
Trading securities realized and unrealized holding gain (loss) on investments after tax | (52.6) | (30.6) | 16.5 | 32.5
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax | 35.6 | 28.1 | 25.8 | 3.0
Trading securities, unrealized foreign currency transaction gain (loss) tax | (7.8) | (7.6) | (5.6) | (1.0)
Trading securities, unrealized foreign currency gain (loss) net of tax | 27.8 | 20.5 | 20.2 | 2.0
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities | (37.3) | (8.1) | 37.9 | 50.5
Net realized and unrealized gain (loss) and foreign currency gain (loss) on trading securities tax | 12.5 | (2.0) | (1.2) | (16.0)
Net realized and unrealized gain (loss) and foreign currency gain (loss) on trading securities net of tax | (24.8) | (10.1) | 36.7 | 34.5
Fixed maturity investments. |  |  |  |
Investment securities |  |  |  |
Net realized and unrealized gain (loss) on trading securities | (74.0) | 19.0 | (82.0) | 49.7
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax | 32.5 | 24.2 | 22.5 | 2.8
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities | (41.5) | 43.2 | (59.5) | 52.5
Short-term investments |  |  |  |
Investment securities |  |  |  |
Net realized and unrealized gain (loss) on trading securities | 0.2 | (0.1) | 0.2 | (0.1)
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax | 0.5 | (0.1) | 0.6 | (0.4)
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities | 0.7 | (0.2) | 0.8 | (0.5)
Common equity securities |  |  |  |
Investment securities |  |  |  |
Net realized and unrealized gain (loss) on trading securities | 2.3 | (36.3) | 86.8 | 0.3
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax | 1.2 | 0 | 1.0 | 0
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities | 3.5 | (36.3) | 87.8 | 0.3
Convertible Debt Securities |  |  |  |
Investment securities |  |  |  |
Net realized and unrealized gain (loss) on trading securities | (5.1) | (6.4) | (3.5) | (0.2)
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax | 0.3 | 0 | 0.1 | 0
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities | (4.8) | (6.4) | (3.4) | (0.2)
Other long-term investments |  |  |  |
Investment securities |  |  |  |
Net realized and unrealized gain (loss) on trading securities | 3.9 | (12.4) | 10.4 | (2.2)
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax | 1.1 | 4.0 | 1.6 | 0.6
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities | 5.0 | (8.4) | 12.0 | (1.6)
Forward Contracts |  |  |  |
Investment securities |  |  |  |
Net realized and unrealized gain (loss) on trading securities | (0.2) | 0 | 0.2 | 0
Net unrealized and realized 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 | $ (0.2) | $ 0 | $ 0.2 | $ 0

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- Definition The sum of the realized and unrealized net gain (loss) on trading securities during the period. + References Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 + Details
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- Definition The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. + 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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- Definition Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax + References No definition available. + Details
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- Definition Represents the net realized and unrealized investment gains (losses) after income taxes. + References No definition available. + Details
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- Definition Represents the income taxes attributable to realized and unrealized gains (losses) on trading securities. + References No definition available. + Details
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- Definition The after-tax gain (loss) attributable to the foreign currency transaction gain (loss) on trading securities held at each balance sheet date, that was included in earnings in the period. + References No definition available. + Details
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- Definition The unrealized foreign currency transaction gain (loss) as of the balance sheet date which is included in net income before tax during the reporting period. + References No definition available. + Details
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- Definition The tax expense (benefit) attributable to the foreign currency transaction gain (loss) on trading securities held at each balance sheet date, that was included in earnings in the period. + References No definition available. + Details
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