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Fair Value Measurements - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Credit valuation adjustment included in fair value of credit derivative liabilities	$ 191,683		$ 261,203
Credit valuation adjustment included in fair value of derivative liabilities other than credit derivatives	91,781		121,928
Percentage of CDS gross par outstanding used in the determination of CDS fair values	80.00%
Percentage of CDS derivative liability used in the determination of CDS fair values	76.00%
Percentage of CDS gross par outstanding using internally estimated reference obligation prices in determination of fair value	20.00%
Percentage of CDS liability using internally estimated reference obligation prices in determination of fair value	24.00%
Additional basis point fee need to receive for issuing a CDS on obligation	0.05%
Additional basis points fee increase on change of reference obligation spread	0.15%
Revised relative change ratio of CDS fee to cash market spread	73.20%
Estimated remaining life	8 years
CVA percentage	50.00%
Notional outstanding	10,204,738		11,281,777
Derivative liabilities	203,307		213,585
Percentage of profit margin		20.00%
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	4.80%		7.60%
Fixed income securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Percentage of investment portfolio valued using dealer quotes	9.00%		10.00%
Corporate obligations [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of investments in corporate obligation securities classified as level 3 in the fair value hierarchy	3,652		3,656
Collateralized debt obligations [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of investments in corporate obligation securities classified as level 3 in the fair value hierarchy	3,905		6,482
Other commercial asset-backed [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of investments in corporate obligation securities classified as level 3 in the fair value hierarchy	50,234		50,264
European ABS transactions [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of investments in corporate obligation securities classified as level 3 in the fair value hierarchy	$ 1,910,589		$ 2,956,501
Credit derivatives [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
CVA percentage	48.50%		55.00%
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Basis point fee received for issuing a CDS on reference obligation base fee	0.20%
Credit spread of reference obligation of a base transaction	0.80%
Relative change ratio of CDS fee to cash market spread	25.00%
Minimum [Member] | Fixed income securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Percentage of investment portfolio valued using internal valuation models	1.00%		1.00%
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Basis point fee received for issuing a CDS on reference obligation hypothetical fee	0.25%
Relative change ratio of CDS fee to cash market spread	35.00%
Credit spread of reference obligation of revised transaction	1.00%
Maximum [Member] | Fixed income securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Percentage of investment portfolio valued using external pricing services	90.00%		89.00%
Internal credit rating [Member] | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Revised relative change ratio of CDS fee to cash market spread	33.00%
Internal credit rating [Member] | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Full Relative change ratio of CDS fee to cash market spread	100.00%

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- Definition

Amount of credit risk valuation adjustment to derivative liabilities other than credit derivatives, to properly reflect the credit quality of the entity.

+ References

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Weighted average discounted rate of estimated future premium payments to be paid by the VIEs.

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- Definition

Hypothetical base fee received on a credit derivative contract used in the description of the valuation processes in place for fair value measurement of credit derivatives.

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Hypothetical base reference obligation spread used in the description of the valuation processes in place for fair value measurement of credit derivatives.

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Hypothetical base relative change ratio for a credit derivative contract used in the description of the valuation processes in place for fair value measurement of credit derivatives.

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Full relative change ratio comparable to certain default used in the description of the valuation processes in place for fair value measurement of credit derivative.

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Hypothetical incremental credit derivative fee used in the description of the valuation processes in place for fair value measurement of credit derivatives.

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Hypothetical incremental credit derivative fee given a certain credit downgrade used in the description of the valuation processes in place for fair value measurement of credit derivative.

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Hypothetical remaining contract life used in the description of the valuation processes in place for fair value measurement of credit derivatives.

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Hypothetical revised fee required on credit derivative contract used in the description of the valuation processes in place for fair value measurement of credit derivatives.

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Hypothetical revised fee required on credit derivative given a certain credit downgrade used in the description of the valuation processes in place for fair value measurement of credit derivatives.

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Hypothetical revised reference obligation spread (second example) used in the description of the valuation processes in place for fair value measurement of credit derivatives.

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Hypothetical revised relative change ratio used in the description of the valuation processes in place for fair value measurement of credit derivatives.

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Fair value of investments in securities valued using unobservable inputs by asset class.

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- Definition

Percentage of credit derivative gross par outstanding using internally estimated reference obligation prices in determination of fair value.

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- Definition

Percentage of credit derivative liability using internally estimated reference obligation prices in determination of fair value.

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- Definition

Percentage of credit derivatives gross par outstanding using third party reference obligation quotes in determination of fair value.

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Percentage of credit derivatives liability using third party reference obligation quotes ion determination of fair value.

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- Definition

Credit valuation adjustment (CVA) as a percentage of the credit derivative liability calculated excluding the CVA.

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Investment portfolio valued using dealer quotes.

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Percentage of investment portfolio valued using internal valuation models.

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Amount of credit risk valuation adjustment to derivative liabilities to properly reflect the credit quality of the entity.

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Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 205G

-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.



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-Publisher FASB

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-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.



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- Definition

Ratio of an entity's operating income net of tax to its revenue, used as an input to measure fair value.

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-Name Accounting Standards Codification

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Aggregate notional amount of credit risk derivatives.

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