Employee Benefit Plans - Plan's Assets Carried at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	$ 588	$ 543
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	161	181
Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	284	262
Other Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	143	100
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	527	490
Fair Value, Inputs, Level 2 [Member] \| Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	161	181
Fair Value, Inputs, Level 2 [Member] \| Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	284	262
Fair Value, Inputs, Level 2 [Member] \| Other Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	82	47
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	61	53	$ 99
Fair Value, Inputs, Level 3 [Member] \| Other Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	$ 61	$ 53