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Investment Securities (Details 2) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2012 | Mar. 31, 2011
Net Realized and Unrealized Investment Gains and Losses |  |
Net realized investment gains (losses), pre-tax | $ 39.2 | $ (1.2)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings | 19.4 | (8.0)
Net realized and unrealized investment gains (losses), pre-tax | 58.6 | (9.2)
Income taxes attributable to realized and unrealized investment gains (losses) | (14.0) | 0.9
Net realized and unrealized investment gains (losses), after-tax | 44.6 | (8.3)
Net realized investment gains (losses) |  |
Net realized investment gains (losses), pre-tax | 39.2 | (1.2)
Income taxes attributable to realized investment gains (losses) | (11.7) | (5.0)
Net realized investment gains (losses), after-tax | 27.5 | (6.2)
Net unrealized investment gains (losses) |  |
Net unrealized gains (losses), pre-tax | 42.0 | 13.8
Income taxes attributable to unrealized investment gains (losses) | (8.2) | 0.2
Net unrealized investment gains (losses), after-tax | 33.8 | 14.0
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax | (22.6) | (21.8)
Income taxes attributable to unrealized investment gains, net foreign exchange gains (losses) | 5.9 | 5.7
Net unrealized investment gains (losses), net foreign currency gains (losses), after-tax | (16.7) | (16.1)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings | 19.4 | (8.0)
Income taxes attributable to unrealized investment gains (losses), total changes in fair value reflected in earnings | (2.3) | 5.9
Net unrealized investment gains (losses) after-tax, total changes in fair value reflected in earnings | 17.1 | (2.1)
Fixed maturity investments. |  |
Net Realized and Unrealized Investment Gains and Losses |  |
Net realized investment gains (losses), pre-tax | 31.5 | (13.6)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings | (22.2) | (23.3)
Net realized investment gains (losses) |  |
Net realized investment gains (losses), pre-tax | 31.5 | (13.6)
Net unrealized investment gains (losses) |  |
Net unrealized gains (losses), pre-tax | (2.9) | (8.0)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax | (19.3) | (15.3)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings | (22.2) | (23.3)
Short-term investments |  |
Net Realized and Unrealized Investment Gains and Losses |  |
Net realized investment gains (losses), pre-tax | (0.3) | (12.2)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings |  | (0.6)
Net realized investment gains (losses) |  |
Net realized investment gains (losses), pre-tax | (0.3) | (12.2)
Net unrealized investment gains (losses) |  |
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax |  | (0.6)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings |  | (0.6)
Common equity securities |  |
Net Realized and Unrealized Investment Gains and Losses |  |
Net realized investment gains (losses), pre-tax | 4.2 | 2.3
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings | 32.4 | 23.9
Net realized investment gains (losses) |  |
Net realized investment gains (losses), pre-tax | 4.2 | 2.3
Net unrealized investment gains (losses) |  |
Net unrealized gains (losses), pre-tax | 32.4 | 24.7
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax |  | (0.8)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings | 32.4 | 23.9
Convertible fixed maturity investments |  |
Net Realized and Unrealized Investment Gains and Losses |  |
Net realized investment gains (losses), pre-tax | 1.5 | 4.1
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings | 4.7 | (2.1)
Net realized investment gains (losses) |  |
Net realized investment gains (losses), pre-tax | 1.5 | 4.1
Net unrealized investment gains (losses) |  |
Net unrealized gains (losses), pre-tax | 4.7 | (2.1)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings | 4.7 | (2.1)
Other long-term investments |  |
Net Realized and Unrealized Investment Gains and Losses |  |
Net realized investment gains (losses), pre-tax | 2.3 | 18.2
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings | 4.5 | (5.9)
Net realized investment gains (losses) |  |
Net realized investment gains (losses), pre-tax | 2.3 | 18.2
Net unrealized investment gains (losses) |  |
Net unrealized gains (losses), pre-tax | 7.8 | (0.8)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax | (3.3) | (5.1)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings | $ 4.5 | $ (5.9)

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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 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 -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 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 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 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 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=6956420&loc=d3e24584-111560 + Details
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