Financial Instruments and Investments (Carrying Amount and Fair Value Amounts) (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		$ 1,499,751	$ 1,434,426
Carrying (Reported) Amount, Fair Value Disclosure | Pollution Control Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1]	157,965	157,769
Carrying (Reported) Amount, Fair Value Disclosure | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		1,118,371	1,117,943
Carrying (Reported) Amount, Fair Value Disclosure | RGRT Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[2]	109,614	109,507
Carrying (Reported) Amount, Fair Value Disclosure | Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[2]	113,801	49,207
Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		1,793,972	1,566,874
Estimate of Fair Value Measurement | Pollution Control Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1]	167,256	161,917
Estimate of Fair Value Measurement | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		1,398,645	1,244,310
Estimate of Fair Value Measurement | RGRT Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[2]	114,270	111,440
Estimate of Fair Value Measurement | Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[2]	$ 113,801	$ 49,207

[1] On February 1, 2019 and April 1, 2019, the Company purchased in lieu of redemption the 2009 Series A 7.25% PCBs with an aggregate principal amount of $63.5 million and the 2009 Series B 7.25% PCBs with an aggregate principal amount of $37.1 million, respectively, utilizing funds borrowed under the RCF. On May 22, 2019, the Company reoffered and sold $63.5 million aggregate principal amount of 2009 Series A 7.25% PCBs and $37.1 million aggregate principal amount of 2009 Series B 7.25% PCBs with a fixed interest rate of 3.60% per annum until the PCBs mature on February 1, 2040 and April 1, 2040, respectively. See Part II, Item 8, Financial Statements and Supplementary Data, Note J of Notes to Financial Statements for further discussion.

[2] Nuclear fuel financing, as of December 31, 2019 and December 31, 2018, is funded through $110 million RGRT Senior Notes and $29.8 million and $26.2 million, respectively, under the RCF. As of December 31, 2019, $84.0 million was outstanding under the RCF for working capital or general corporate purposes. As of December 31, 2018, $23.0 million was outstanding under the RCF for working capital or general corporate purposes. The interest rate on the Company’s borrowings under the RCF is reset throughout the period reflecting current market rates. Consequently, the carrying value approximates fair value. See Part II, Item 8, Financial Statements and Supplementary Data, Note J of Notes to Financial Statements for further discussion.