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Debt (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Gross debt	$ 425,766	$ 286,113
Less unamortized debt issuance costs	1,008	1,400
Debt less unamortized debt issuance costs	424,758	284,713
Mortgage loans
Debt Instrument [Line Items]
Gross debt	$ 0	$ 36,113
Mortgage loans | LIBOR
Debt Instrument [Line Items]
Effective interest rate on debt		1.58%
Basis spread on variable rate for debt	1.325%	1.325%
Senior notes due 2023 | Senior notes due 2023
Debt Instrument [Line Items]
Gross debt	$ 250,000	$ 250,000
Stated interest rate on debt	5.30%	5.30%
Contribution Certificates
Debt Instrument [Line Items]
Gross debt	$ 175,766	$ 0
Stated interest rate on debt	3.00%
Effective interest rate on debt	4.35%

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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