Debt: - Pollution Control Revenue Bonds (Details) - Long-term debt	Dec. 28, 2017 USD ($) item
First mortgage notes issued in connection with the sale of pollution control revenue bonds: Series 2017 CDEF Burke
Debt
Principal amount	$ 399,785,000
Number of banks \| item	2
Repayments of debt, defeasance	$ 399,785,000
Pollution Control Revenue Bonds Series 2017C and Series 2017D
Debt
Principal amount	$ 200,000,000
Interest rate (as a percent)	4.125%
Pollution Control Revenue Bonds Series 2017E
Debt
Principal amount	$ 100,000,000
Interest rate (as a percent)	3.25%
Pollution Control Revenue Bonds Series 2017F
Debt
Principal amount	$ 99,785,000
Interest rate (as a percent)	3.00%

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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