Schedule of Fair Value of Assets and Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Carrying Amount
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Company-owned life insurance	$ 137	$ 138
Carrying Amount | Long-Term Debt | New Facilities loans Under Third Amended Credit Agreement due 2025
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, fair value	53	57
Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Company-owned life insurance	137	138
Fair Value | Long-Term Debt | New Facilities loans Under Third Amended Credit Agreement due 2025
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, fair value	53	57
Revenue bonds, due in varying amounts periodically through 2029 | Carrying Amount | Long-Term Debt
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, fair value	75	75
Revenue bonds, due in varying amounts periodically through 2029 | Fair Value | Long-Term Debt
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, fair value	76	76
American & Foreign Power Company, Inc. 5% debentures, due 2030 | Carrying Amount | Long-Term Debt
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, fair value	16	15
American & Foreign Power Company, Inc. 5% debentures, due 2030 | Fair Value | Long-Term Debt
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, fair value	$ 14	$ 14