Changes in Fair Value Measurements that Used Significant Unobservable Inputs (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Beginning of period	$ 3,698
End of period	4,501
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Beginning of period	8
Purchases, sales and settlements	8
End of period	16
Common/Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Beginning of period	565
End of period	900
Common/Collective Trust Funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Beginning of period	6
End of period	6
Other Holdings
Defined Benefit Plan Disclosure [Line Items]
Beginning of period	101
End of period	117
Other Holdings | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Beginning of period	2
Purchases, sales and settlements	8
End of period	$ 10