Employee Benefits - Summarizes Changes for Qualified Pension Plan Investment Assets Measured at Fair Value Using Significant Unobservable Inputs (Level 3) (Detail) - Other [Member] - Level 3 [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Balance at beginning of period	$ 1	$ 1	$ 2
Unrealized gains (losses) relating to assets still held at end of year			(1)
Purchases, sales, and settlements, net	1
Balance at end of period	$ 2	$ 1	$ 1