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Employment Benefit Plans - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified | 12 Months Ended |  | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2012 Additions [Member] | Dec. 31, 2012 RSU and other stock [Member] | Dec. 31, 2011 RSU and other stock [Member] | Dec. 31, 2012 Unvested RSUs granted [Member] | Dec. 31, 2020 Subsequent Event [Member]
Postretirement And Other Employee Benefits [Line Items] |  |  |  |  |  |  |
Postretirement and postemployment benefits expense | $ 2,099 | $ 4,037 |  |  |  |  |
Unfunded accumulated postretirement benefit obligation | 17,485 |  |  |  |  |  |
Assumed health care cost trend rates range | 8.50% |  |  |  |  |  | 5.00%
One percentage point increase on accumulated postretirement benefit obligation | 3,476 |  |  |  |  |  |
One percentage point decrease on accumulated postretirement benefit obligation | 2,713 |  |  |  |  |  |
One percentage point increase on accumulated postretirement benefit expense | 454 |  |  |  |  |  |
One percentage point decrease on accumulated postretirement benefit expense | 345 |  |  |  |  |  |
Projected plan benefit obligations | 4.00% | 4.50% |  |  |  |  |
Additional contribution by employer | 100.00% |  | 50.00% |  |  |  |
Additional to base compensation | 3.00% |  | 2.00% |  |  |  |
Employee Savings Incentive Plan cost | 1,058 | 610 |  |  |  |  |
Fair market value of Ambac's Common Stock on grant date | 7 years |  |  |  |  |  |
Total unrecognized compensation costs | 0 |  |  |  |  | 0 |
Gross stock option expense | 0 | (12,576) |  |  |  |  |
Net income effect from stock options | 0 | (11,297) |  |  |  |  |
Vesting period |  |  |  | 4 years |  |  |
Shares outstanding, Restricted stock units | 104,389 |  |  |  |  |  |
Shares outstanding, Equity instruments other than options nonvested | 70,530 |  |  |  |  |  |
Shares outstanding, Equity instruments restricted stock units vested | 33,859 |  |  |  |  |  |
Gross restricted stock units expense |  |  |  | 0 | (2,988) |  |
Restricted stock units expense, net of tax |  |  |  | 0 | (1,163) |  |
Fair value for RSUs vested | $ 0 | $ 93 |  |  |  |  |

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- Definition The interest rate used to adjust for the time value of money for the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph j -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 -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 + Details
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- Definition The effect of a one-percentage-point increase in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph m -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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- Definition The effect of a one-percentage-point increase in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph m -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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- Definition The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph l -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 Accounting Standards Codification -Glossary Health Care Cost Trend Rate -URI http://asc.fasb.org/extlink&oid=6514820 + Details
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