Fair Value Measurements - Estimated Fair Values of Assets Measured on Nonrecurring Basis (Detail) - USD ($) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2021	Jun. 30, 2021	Jun. 30, 2020
Fair Value, Assets and Liabilities Measured On Recurring and Nonrecurring Basis [Line Items]
Impairment Loss, Property and equipment		$ 5	$ 5	$ 88
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured On Recurring and Nonrecurring Basis [Line Items]
Fair value, Property and equipment	[1]	77	77
Fair Value, Total		$ 77	77
Impairment Loss, Property and equipment	[1]		5
Impairment Loss, Total			$ 5

[1] Fair value as of June 30, 2021 is based upon the contracted sales price for a property, less costs to sell, as applicable (Level 2).