Derivatives (Change in the Fair Value for Interest Rate Contracts and the Related Hedged Items) (Detail) - Fair Value Hedging - Interest Rate Contract - Interest Expense, Long-Term Debt - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value
Change in fair value of interest rate swaps hedging long-term debt	$ (33)	$ (59)	$ (29)
Change in fair value of hedged long-term debt	$ 31	$ 54	$ 25