Debt and Interest Expense - Outstanding Debt Table (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Debt Disclosure [Abstract]
Total debt at face value	$ 15,337		$ 14,952
Net unamortized discounts and premiums	(1,674)	[1]	(1,683)	[1]
Total long-term debt	$ 13,663		$ 13,269

[1] Unamortized discounts and premiums are amortized over the term of the related debt.