Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012
Fair Value Disclosures [Abstract]
Maximum remaining term for Investment purchased for estimating at cost	90 days
Decrease in valuation due to net credit risk adjustment	$ 111
Decrease in valuation due to liquidity adjustments	$ 107

Maximum remaining term for investment purchased for estimating at cost.

No definition available.

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