Employee Bargaining Agreements and Benefit Plans - Plan's Assets Carried at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	$ 18	$ 15
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	10	8
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	3	3
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	5	4
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	13	11
Level 1 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	10	8
Level 1 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	3	3
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	5	4
Level 2 [Member] | Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	$ 5	$ 4