Amounts Recognized in Consolidated Balance Sheet and Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Amounts recognized in the consolidated balance sheets consist of:
Noncurrent asset	$ 2	$ 49
Current liability	(17)	(2)
Noncurrent liability	(2,226)	(1,650)
Total liability	(2,241)	(1,603)
Amounts recognized in accumulated other comprehensive income (loss) consist of:
Net actuarial gain (loss)	(982)	(162)
Prior service credit (loss)	(1)
Total accumulated other comprehensive income (loss)	(983)	(162)
Other Postretirement Benefits
Amounts recognized in the consolidated balance sheets consist of:
Current liability	(62)	(59)
Noncurrent liability	(1,933)	(1,703)
Total liability	(1,995)	(1,762)
Amounts recognized in accumulated other comprehensive income (loss) consist of:
Net actuarial gain (loss)	233	555
Prior service credit (loss)	278	306
Total accumulated other comprehensive income (loss)	$ 511	$ 861