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, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	$ 393	$ 457
Actual return on plan assets	5	22
Purchases, sales or other settlements	(9)	(86)
Net transfers in (out) of Level 3	0	0
Ending balance	389	393
Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	3	2
Actual return on plan assets	0	0
Purchases, sales or other settlements	(1)	1
Net transfers in (out) of Level 3	0	0
Ending balance	2	3
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	216	302
Actual return on plan assets	(7)	(5)
Purchases, sales or other settlements	0	(81)
Net transfers in (out) of Level 3	0	0
Ending balance	209	216
Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	122	105
Actual return on plan assets	5	18
Purchases, sales or other settlements	(1)	(1)
Net transfers in (out) of Level 3	0	0
Ending balance	126	122
Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	52	48
Actual return on plan assets	7	9
Purchases, sales or other settlements	(7)	(5)
Net transfers in (out) of Level 3	0	0
Ending balance	$ 52	$ 52