Financial Instruments And Investments (Carrying Amount and Fair Value Amounts) (Details) - USD ($) $ in Thousands		Mar. 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		$ 1,429,177	$ 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		157,699	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		993,514	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]	44,897	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	[1]	233,067	173,533
Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		1,593,970	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		166,779	169,186
Estimate of Fair Value Measurement | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		1,147,504	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]	46,620	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	[1]	$ 233,067	$ 173,533

[1] Nuclear fuel financing, as of March 31, 2018 and December 31, 2017, is funded through $45 million Rio Grande Resources Trust ("RGRT") Senior Notes and $89.1 million and $88.5 million, respectively, under the RCF. As of March 31, 2018 and December 31, 2017, $144.0 million and $85.0 million, respectively, 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 quarter reflecting current market rates. Consequently, the carrying value approximates fair value.