Financial Instruments And Investments (Carrying Amount and Fair Value Amounts) (Details) - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
Carrying (Reported) Amount, Fair Value Disclosure    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt Instrument, Fair Value Disclosure [1] $ 1,333,670 $ 1,264,398
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] 190,706 190,499
Carrying (Reported) Amount, Fair Value Disclosure | Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt Instrument, Fair Value Disclosure [1] 993,004 837,475
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] 94,768 94,686
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],[2] 55,192 141,738
Estimate of Fair Value Measurement    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt Instrument, Fair Value Disclosure 1,573,869 1,284,571
Estimate of Fair Value Measurement | Pollution Control Bonds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt Instrument, Fair Value Disclosure 211,050 212,624
Estimate of Fair Value Measurement | Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt Instrument, Fair Value Disclosure 1,207,162 829,864
Estimate of Fair Value Measurement | RGRT Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt Instrument, Fair Value Disclosure [2] 100,465 100,345
Estimate of Fair Value Measurement | Revolving Credit Facility    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt Instrument, Fair Value Disclosure [2] $ 55,192 $ 141,738
[1] The Company implemented ASU 2015-03, Interest - Imputation of Interest, which requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability. ASU 2015-03 is effective for financial statements issued for fiscal years beginning after December 15, 2015, and interim periods within those fiscal years. The impact of ASU 2015-03 on the Company's Balance Sheet was to reclassify $11.6 million of other deferred charges to long-term debt, net of current portion at December 31, 2015.
[2] Nuclear fuel financing, as of September 30, 2016 and December 31, 2015, is funded through the $95 million RGRT Senior Notes and $36.2 million and $33.7 million, respectively under the RCF. As of September 30, 2016, $19.0 million was outstanding under the RCF for working capital or general corporate purposes. As of December 31, 2015, $108.0 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 quarter reflecting current market rates. Consequently, the carrying value approximates fair value.