Carrying Values and Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 10,767	$ 10,897
Estimate of Fair Value, Fair Value Disclosure
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	11,180	11,371
Estimate of Fair Value, Fair Value Disclosure | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	8,309	8,646
Estimate of Fair Value, Fair Value Disclosure | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	$ 2,871	$ 2,725