Fair Value Option - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Options [Abstract]
Fixed rate debt accounted for under fair value option	$ 10,100,000,000	$ 10,200,000,000
Long-term debt carried at fair value	9,600,000,000	9,700,000,000
Due To Affiliate Carried At Fair Value	511,000,000	514,000,000
Fair value option not elected related to fixed rate long-term debt	7,600,000,000	8,100,000,000
Aggregate unpaid principal balance of fixed rate debt accounted for under fair value option	9,300,000,000	9,400,000,000
Increased the debt balance	137,000,000	247,000,000
Cumulative increased value of debt through fair value option adjustments	$ 779,000,000	$ 824,000,000