Schedule of Fair Value of Assets and Liabilities (Parenthetical) (Detail) - Long-Term Debt	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Carrying Amount | New Facilities loans Under Third Amended Credit Agreement due 2025
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, due date	2025	2025
Carrying Amount | Revenue bonds, due in varying amounts periodically through 2029
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, due date	2029	2029
Carrying Amount | American & Foreign Power Company, Inc. 5% debentures, due 2030
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, due date	2030	2030
Long-term debt, interest rate	5.00%	5.00%
Fair Value | New Facilities loans Under Third Amended Credit Agreement due 2025
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, due date	2025	2025
Fair Value | Revenue bonds, due in varying amounts periodically through 2029
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, due date	2029	2029
Fair Value | American & Foreign Power Company, Inc. 5% debentures, due 2030
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, due date	2030	2030
Long-term debt, interest rate	5.00%	5.00%