Debt Debt (Schedule of Long Term Debt Maturities) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Unsecured Debt		$ 400,000
Long-term Debt, Maturities, Repayments of Principal in Next Rolling Twelve Months	$ 458,371
Long-term Debt, Maturities, Repayments of Principal in Rolling Year Two	250,389
Long-term Debt, Maturities, Repayments of Principal in Year Three	475,408
Long-term Debt, Maturities, Repayments of Principal in Rolling Year Four	75,425
Long-term Debt, Maturities, Repayments of Principal in Rolling Year Five	143,761
Long-term Debt, Maturities, Repayments of Principal in Rolling after Year Five	100,000
Long-term Debt, Gross	1,503,354
Less: unamortized loan costs	4,069	7,076
Debt, Long-term and Short-term, Combined Amount	1,499,285	1,440,465
Private Placement [Member]
Unsecured Debt	400,000	$ 400,000
January 2023 [Member]
Unsecured Debt	$ 125,000
Debt Instrument, Interest Rate, Stated Percentage	3.99%
November 2023 [Member]
Unsecured Debt	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	3.99%
September 2024 [Member]
Unsecured Debt	$ 75,000
Debt Instrument, Interest Rate, Stated Percentage	3.93%
November 2025 [Member]
Unsecured Debt	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	4.33%
January 2027 [Member]
Unsecured Debt	$ 100,000
Debt Instrument, Interest Rate, Stated Percentage	4.51%

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

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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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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