Retirement Plans - Change in Benefit Obligation, Fair Value of Plan Assets, and Funded Status of Pension Plans and Postretirement Medical Plan (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Lump-sum settlement payment included in Benefit payments	$ 41	$ 55
Change in Discount Rate
Defined Benefit Plan Disclosure [Line Items]
Actuarial (gain) loss resulted from a change		330
Change in Mortality Assumptions
Defined Benefit Plan Disclosure [Line Items]
Actuarial (gain) loss resulted from a change		$ 125