Benefit Plans - Plan Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - Pension Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, beginning balance	$ 2,500	$ 2,713
Fair value of plan assets, ending balance	2,423	2,500
Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, beginning balance	465
Fair value of plan assets, ending balance	510	465
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, beginning balance	10	15
Actual Return on Assets Still Held at December 31	0	0
Actual Return on Assets Sold During the Year Ended December 31	0	0
Net Purchases, Sales, and Settlements	0	0
Net Transfers In (Out) of Level 3		(5)
Fair value of plan assets, ending balance	10	10
Level 3 [Member] | Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, beginning balance	10	15
Actual Return on Assets Still Held at December 31	0	0
Actual Return on Assets Sold During the Year Ended December 31	0	0
Net Purchases, Sales, and Settlements	0	0
Net Transfers In (Out) of Level 3		(5)
Fair value of plan assets, ending balance	$ 10	$ 10