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Segment Information - Schedule of Gross Premiums Written, Net Premiums Earned and Net Change in Fair Value of Credit Derivatives (Detail) (USD $) In Thousands, 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
Segment Information [Line Items] |  |  |  |
Gross Premiums Written | $ (55,310) | $ 242,573 | $ (175,777) | $ (125,712)
Net Premiums Earned | 113,074 | 102,055 | 311,066 | 293,125
Net Change in Fair Value of Credit Derivatives | 27,440 | 4,505 | 12,803 | 19,889
United States [Member] |  |  |  |
Segment Information [Line Items] |  |  |  |
Gross Premiums Written | (40,748) | 26,998 | (165,408) | (328,181)
Net Premiums Earned | 92,132 | 82,291 | 235,672 | 232,420
Net Change in Fair Value of Credit Derivatives | 15,845 | 3,223 | 7,577 | 13,741
United Kingdom [Member] |  |  |  |
Segment Information [Line Items] |  |  |  |
Gross Premiums Written | (1,060) | 187,895 | 11,325 | 198,778
Net Premiums Earned | 14,763 | 12,220 | 55,355 | 35,006
Net Change in Fair Value of Credit Derivatives | 716 | (602) | (3,721) | 251
Other International [Member] |  |  |  |
Segment Information [Line Items] |  |  |  |
Gross Premiums Written | (13,502) | 27,680 | (21,694) | 3,691
Net Premiums Earned | 6,179 | 7,544 | 20,039 | 25,699
Net Change in Fair Value of Credit Derivatives | $ 10,879 | $ 1,884 | $ 8,947 | $ 5,897

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- Definition Premiums written for direct and assumed insurance contracts. + References No definition available. + Details
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- Definition Premiums earned on the income statement for all insurance and reinsurance contracts after subtracting any amounts ceded to another insurer and adding premiums assumed from other insurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(c)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27 -Subparagraph a, b -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. + Details
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