Schedule of Fair Value of Assets and Liabilities (Parenthetical) (Detail) - Recourse Debt	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 30, 2017
Term Loan, due 2022 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, due date	2022	2022
Term Loan, due 2022 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, due date	2022	2022
Revenue bonds, due in varying amounts periodically through 2029 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, due date	2029	2029
Revenue bonds, due in varying amounts periodically through 2029 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, due date	2029	2029
American & Foreign Power Company, Inc. 5% debentures, due 2030 | Carrying Value
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%
American & Foreign Power Company, Inc. 5% debentures, due 2030 | Fair Value
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%