Retirement Plans - Changes in Financial Assets that are Measured at Fair Value Based on Level 3 Inputs that are held by Institutional Funds (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	$ 2,732
Ending balance	2,568	$ 2,732
Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	157
Ending balance	160	157
Level 3
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	4	10
Actual return on plan assets	1	0
Purchases, sales or other settlements	56	(5)
Net transfers in (out) of Level 3	0	(1)
Ending balance	61	4
Level 3 | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	2	2
Actual return on plan assets	0	0
Purchases, sales or other settlements	(2)	1
Net transfers in (out) of Level 3	0	(1)
Ending balance	0	2
Level 3 | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	2	8
Actual return on plan assets	1	0
Purchases, sales or other settlements	58	(6)
Net transfers in (out) of Level 3	0	0
Ending balance	$ 61	$ 2