Derivative Instruments (Detail) - Summary of financial operations derivatives credit exposure (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2012	Dec. 31, 2011
Total liability recorded	$ 15,763	$ 15,763
Underlying obligations credit rating	BB	BB-
Structured Finance [Member]
Principal outstanding	78,763	78,425
Interest outstanding	3,383	3,253
Aggregate outstanding exposure	82,146	81,678
Total liability recorded	$ 10,288	$ 10,288
Weighted average contractual term to maturity	4.5 years	5.0 years
Underlying obligations credit rating	BB	BB

This element represents the amount of insured contractual payments outstanding on the underlying insured obligations for financial guarantee insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

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This element represents the amount of insured contractual payments outstanding attributable to interest accrued on the underlying insured obligations for financial guarantee insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

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- Definition

This element represents the amount of insured contractual payments outstanding attributable to the principal of the underlying insured obligations for financial guarantee insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

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