Fair Value Measurements - Quantitative Information Related to Significant Unobservable Inputs (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2013	Dec. 31, 2012
Privately Issued Mortgage-Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments	$ 5,272	$ 1,023,886
Valuation technique	Two independent price quotes	Discounted cash flow
Unobservable inputs/assumptions		Probability of default Loss severity
Probability of default weighted-average		19.00%
Loss severity weighted-average		51.00%
Privately Issued Mortgage-Backed Securities [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default		1.00%
Loss severity		32.00%
Privately Issued Mortgage-Backed Securities [Member] \| Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default		40.00%
Loss severity		82.00%
Collateralized Debt Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments	59,916	61,869
Valuation technique	Discounted cash flow	Discounted cash flow
Unobservable inputs/assumptions	Probability of default Loss severity	Probability of default Loss severity
Probability of default weighted-average	48.00%	47.00%
Collateralized Debt Obligations [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default	21.00%	22.00%
Collateralized Debt Obligations [Member] \| Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default	69.00%	69.00%
Loss severity	100.00%	100.00%
Net Other Assets (Liabilities) [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments	$ 7,408	$ 47,859
Valuation technique	Discounted cash flow	Discounted cash flow
Unobservable inputs/assumptions	Commitment expirations	Commitment expirations
Commitment expirations weighted-average	18.00%	20.00%
Net Other Assets (Liabilities) [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Commitment expirations	0.00%	0.00%
Net Other Assets (Liabilities) [Member] \| Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Commitment expirations	64.00%	69.00%

Percentage of fair value inputs commitment expirations.

No definition available.

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- Definition

Percentage of fair value inputs loss severity.

+ References

No definition available.

+ Details

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- Definition

Percentage of fair value inputs probability of default.

+ References

No definition available.

+ Details

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- Definition

Weighted average percentage of fair value inputs commitment expirations.

+ References

No definition available.

+ Details

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- Definition

Weighted average percentage of fair value inputs loss severity.

+ References

No definition available.

+ Details

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- Definition

Weighted average percentage of fair value inputs probability of default

+ References

No definition available.

+ Details

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- Details

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- Definition

Description of significant assumptions used by management to measure fair value.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611

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- Definition

Description of the inputs and valuation technique(s) used to measure fair value.

+ References

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- Definition

This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c

-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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