Financial Instruments and Investments (Carrying Amount and Fair Value Amounts) (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		$ 1,434,426	$ 1,369,521
Carrying (Reported) Amount, Fair Value Disclosure | Pollution Control Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		157,769	157,676
Carrying (Reported) Amount, Fair Value Disclosure | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1]	1,117,943	993,426
Carrying (Reported) Amount, Fair Value Disclosure | RGRT Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1],[2]	109,507	44,886
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]	49,207	173,533
Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		1,566,874	1,601,711
Estimate of Fair Value Measurement | Pollution Control Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		161,917	169,186
Estimate of Fair Value Measurement | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1]	1,244,310	1,211,922
Estimate of Fair Value Measurement | RGRT Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1],[2]	111,440	47,070
Estimate of Fair Value Measurement | Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[2]	$ 49,207	$ 173,533

[1] On June 28, 2018, the Company issued $125 million in aggregate principal amount of 4.22% Senior Notes due August 15, 2028 and guaranteed the issuance by the RGRT of $65 million in aggregate principal amount of 4.07% Senior Guaranteed Notes due August 15, 2025. See Part II, Item 8, Financial Statements and Supplementary Data, Note J of Notes to Financial Statements.

[2] Nuclear fuel financing, as of December 31, 2018 and December 31, 2017, is funded through $110 million and $45 million RGRT Senior Notes and $26.2 million and $88.5 million, respectively under the RCF. As of December 31, 2018, $23 million was outstanding under the RCF for working capital or general corporate purposes. As of December 31, 2017, $85.0 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