Fair Value Measurements - Additional Information (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of debt instruments	$ 8,200,000,000	$ 7,800,000,000
Outstanding principal amount of debt instrument	7,200,000,000	6,966,000,000
Unamortized discount on long-term debt	$ 892,000,000	$ 506,000,000