Fair Value Measurement of Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets:
Cash and cash equivalents	$ 37	$ 58	$ 46
Estimate of Fair Value Measurement [Member]
Assets:
Total assets:	14	2
Liabilities:
Total liabilities:	2	13
Estimate of Fair Value Measurement [Member] | Mutual And Bond Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	2	2
Energy Hedges [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	12
Liabilities:
Derivative Liability	0	13
Energy Hedges [Member] | Estimate of Fair Value Measurement [Member] | Energy Hedges [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0
Interest rate swap | Fair Value, Inputs, Level 2 [Member]
Liabilities:
Derivative Liability	$ 2	$ 0