Defined Benefit Pension And Other Postretirement Plans (Reconciliation Of Beginning And Ending Balances Of Plan Assets Classified As Level 3) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at January 1	$ 1,840
Actual return on plan assets:
Fair value of plan assets at December 31	2,089	$ 1,840
Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at January 1	51
Actual return on plan assets:
Fair value of plan assets at December 31	41	51
Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at January 1	34
Actual return on plan assets:
Fair value of plan assets at December 31	29	34
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at January 1	105	118
Actual return on plan assets:
Realized	11	14
Unrealized	1	0
Purchases	2	4
Sales	(31)	(31)
Fair value of plan assets at December 31	88	105
Level 3 | Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at January 1	51	60
Actual return on plan assets:
Realized	9	11
Unrealized	2	(1)
Purchases	1	2
Sales	(22)	(21)
Fair value of plan assets at December 31	41	51
Level 3 | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at January 1	34	39
Actual return on plan assets:
Realized	2	3
Unrealized	(1)	0
Purchases	1	1
Sales	(7)	(9)
Fair value of plan assets at December 31	29	34
Level 3 | Fixed income corporate and other plan assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at January 1	20	19
Actual return on plan assets:
Realized	0	0
Unrealized	0	1
Purchases	0	1
Sales	(2)	(1)
Fair value of plan assets at December 31	$ 18	$ 20