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Financing: Long-Term Debt: Debt Transactions (Details) - USD ($) $ in Millions				12 Months Ended
	Nov. 30, 2023	Sep. 15, 2023	Mar. 07, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Less unamortized deferred financing costs				$ 24.8	$ 22.3
Repayment of long term debt				$ 525.0		$ 8.4
Senior Unsecured Notes Due 2034
Debt Instrument [Line Items]
Stated interest rate (percent)		6.15%
Less unamortized deferred financing costs		$ 7.6
Senior Unsecured Notes Due 2034 | Corporate, Non-Segment
Debt Instrument [Line Items]
Proceeds from Issuance of Debt		$ 450.0
Senior Unsecured Notes Due 2023
Debt Instrument [Line Items]
Repayment of long term debt	$ 525.0
Senior Unsecured Notes Due 2028
Debt Instrument [Line Items]
Stated interest rate (percent)			5.95%
Debt Instrument, Term			5 years
Less unamortized deferred financing costs			$ 4.2
Senior Unsecured Notes Due 2028 | Corporate, Non-Segment
Debt Instrument [Line Items]
Proceeds from Issuance of Debt			$ 350.0

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Contractual interest rate for funds borrowed, under the debt agreement.

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