Retirement Plans - Changes in Financial Assets that are Measured at Fair Value Based on Level 3 Inputs that are held by Institutional Funds (Detail) - Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	$ 389	$ 393
Actual return on plan assets	32	5
Purchases, sales or other settlements	2	(9)
Net transfers in (out) of Level 3	(1)	0
Ending balance	422	389
Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	2	3
Actual return on plan assets	0	0
Purchases, sales or other settlements	0	(1)
Net transfers in (out) of Level 3	(1)	0
Ending balance	1	2
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	209	216
Actual return on plan assets	3	(7)
Purchases, sales or other settlements	(25)	0
Net transfers in (out) of Level 3	0	0
Ending balance	187	209
Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	126	122
Actual return on plan assets	18	5
Purchases, sales or other settlements	(4)	(1)
Net transfers in (out) of Level 3	0	0
Ending balance	140	126
Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	52	52
Actual return on plan assets	11	7
Purchases, sales or other settlements	31	(7)
Net transfers in (out) of Level 3	0	0
Ending balance	$ 94	$ 52