Employee Benefit Plans (Fair Values of the Company's Pension Plan Assets by Asset Category) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total assets in the fair value hierarchy	$ 2,056	$ 1,177
Common-collective trusts	827	1,384
Fair values of pension plan assets	2,883	2,561
Equity Securities | U.S. | US Large Cap
Fair values of pension plan assets	83	80
Equity Securities | U.S. | US Small Cap
Fair values of pension plan assets	29	0
Equity Securities | U.S. | Treasury Stock
Fair values of pension plan assets	0	53
Equity Securities | Developed Markets | Developed Markets
Fair values of pension plan assets	194	155
Debt Securities | Cash and cash equivalents
Fair values of pension plan assets	11	6
Debt Securities | Corporate | Developed Markets
Fair values of pension plan assets	43	42
Debt Securities | Corporate | UNITED STATES
Fair values of pension plan assets	287	91
Debt Securities | Diversified
Fair values of pension plan assets	124	71
Debt Securities | Government | Developed Markets
Fair values of pension plan assets	101	78
Debt Securities | Government | UNITED STATES
Fair values of pension plan assets	69	0
Debt Securities | Other Debt Securities
Fair values of pension plan assets	432	15
Real Estate And Other
Fair values of pension plan assets	683	586
Level 1
Total assets in the fair value hierarchy	212	133
Fair values of pension plan assets	212	133
Level 1 | Equity Securities | U.S. | US Large Cap
Fair values of pension plan assets	0	0
Level 1 | Equity Securities | U.S. | US Small Cap
Fair values of pension plan assets	29	0
Level 1 | Equity Securities | U.S. | Treasury Stock
Fair values of pension plan assets	0	53
Level 1 | Equity Securities | Developed Markets | Developed Markets
Fair values of pension plan assets	115	80
Level 1 | Debt Securities | Cash and cash equivalents
Fair values of pension plan assets	0	0
Level 1 | Debt Securities | Corporate | Developed Markets
Fair values of pension plan assets	0	0
Level 1 | Debt Securities | Corporate | UNITED STATES
Fair values of pension plan assets	0	0
Level 1 | Debt Securities | Diversified
Fair values of pension plan assets	0	0
Level 1 | Debt Securities | Government | Developed Markets
Fair values of pension plan assets	0	0
Level 1 | Debt Securities | Government | UNITED STATES
Fair values of pension plan assets	68	0
Level 1 | Debt Securities | Other Debt Securities
Fair values of pension plan assets	0	0
Level 1 | Real Estate And Other
Fair values of pension plan assets	0	0
Level 2
Total assets in the fair value hierarchy	842	529
Fair values of pension plan assets	842	529
Level 2 | Equity Securities | U.S. | US Large Cap
Fair values of pension plan assets	83	80
Level 2 | Equity Securities | U.S. | US Small Cap
Fair values of pension plan assets	0	0
Level 2 | Equity Securities | U.S. | Treasury Stock
Fair values of pension plan assets	0	0
Level 2 | Equity Securities | Developed Markets | Developed Markets
Fair values of pension plan assets	79	75
Level 2 | Debt Securities | Cash and cash equivalents
Fair values of pension plan assets	11	6
Level 2 | Debt Securities | Corporate | Developed Markets
Fair values of pension plan assets	43	42
Level 2 | Debt Securities | Corporate | UNITED STATES
Fair values of pension plan assets	201	5
Level 2 | Debt Securities | Diversified
Fair values of pension plan assets	124	71
Level 2 | Debt Securities | Government | Developed Markets
Fair values of pension plan assets	101	78
Level 2 | Debt Securities | Government | UNITED STATES
Fair values of pension plan assets	1	0
Level 2 | Debt Securities | Other Debt Securities
Fair values of pension plan assets	14	0
Level 2 | Real Estate And Other
Fair values of pension plan assets	185	172
Level 3
Total assets in the fair value hierarchy	1,002	515
Fair values of pension plan assets	1,002	515	$ 635
Level 3 | Equity Securities | U.S. | US Large Cap
Fair values of pension plan assets	0	0
Level 3 | Equity Securities | U.S. | US Small Cap
Fair values of pension plan assets	0	0
Level 3 | Equity Securities | U.S. | Treasury Stock
Fair values of pension plan assets	0	0
Level 3 | Equity Securities | Developed Markets | Developed Markets
Fair values of pension plan assets	0	0
Level 3 | Debt Securities | Cash and cash equivalents
Fair values of pension plan assets	0	0
Level 3 | Debt Securities | Corporate | Developed Markets
Fair values of pension plan assets	0	0
Level 3 | Debt Securities | Corporate | UNITED STATES
Fair values of pension plan assets	86	86
Level 3 | Debt Securities | Diversified
Fair values of pension plan assets	0	0
Level 3 | Debt Securities | Government | Developed Markets
Fair values of pension plan assets	0	0
Level 3 | Debt Securities | Government | UNITED STATES
Fair values of pension plan assets	0	0
Level 3 | Debt Securities | Other Debt Securities
Fair values of pension plan assets	418	15
Level 3 | Real Estate And Other
Fair values of pension plan assets	$ 498	$ 414