Assets and Liabilities Measured at Fair Value - Additional Information (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Estimated fair value of long-term debt	$ 31,858	$ 31,411
Carrying amounts of long-term debt	30,733	30,655
Investments of insurance subsidiaries	477	482	$ 482
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments of insurance subsidiaries	$ 5	$ 6