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Financing: Short-Term Debt: (Details)		3 Months Ended	12 Months Ended
	Feb. 24, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($) credit_extension	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 19, 2021 USD ($)
Short-term Debt [Line Items]
Notes payable			$ 420,180,000	$ 234,040,000
Letters of Credit Outstanding, Amount			27,209,000	24,730,000
Commercial Paper, Maximum Borrowing Capacity			750,000,000
Net borrowings (payments) of Revolving Credit Facility and CP Program			186,140,000	(115,460,000)	$ 163,880,000
Term Loan - borrowings			800,000,000	0	0
Repayments of Other Short-term Debt			800,000,000	0	$ 0
Commercial Paper
Short-term Debt [Line Items]
Notes payable			420,180,000	234,040,000
Letters of Credit Outstanding, Amount			$ 0	0
Debt Instrument, Term			397 days
Debt, Weighted Average Interest Rate			0.30%
Line of Credit | Senior Unsecured Notes Due 2021
Short-term Debt [Line Items]
Debt Instrument, Term	9 months
Term Loan - borrowings	$ 800,000,000
Prepayment penalty			$ 0
Repayments of Other Short-term Debt		$ 200,000,000
Revolving Credit Facility
Short-term Debt [Line Items]
Notes payable			0	0
Letters of Credit Outstanding, Amount			$ 27,209,000	$ 24,730,000
Line of Credit Facility, Current Borrowing Capacity						$ 750,000,000
Number Of One-Year Extension Options | credit_extension			2
Debt Instrument, Term			1 year
Line of Credit Facility, Maximum Borrowing Capacity						$ 1,000,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage			0.175%
Debt Issuance Cost, Gross, Noncurrent			$ 8,900,000
Revolving Credit Facility | Base Rate
Short-term Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End			0.125%
Revolving Credit Facility | Eurodollar
Short-term Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End			1.125%
Revolving Credit Facility | Letter of Credit
Short-term Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End			1.125%

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