Financial Instruments and Trading Securities (Fair Value Of Assets And Liabilities) (Details) - USD ($) $ in Thousands | 3 Months Ended | |
Mar. 31, 2017 | Dec. 31, 2016 |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Redemption Restriction Period | 15 years | |
Nuclear decommissioning trust assets | $ 212,820 | $ 200,122 |
Trading securities | 33,036 | 34,491 |
Total assets measured at fair value | 245,856 | 234,613 |
Level 1 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 172 | 73 |
Trading securities | 0 | 156 |
Total assets measured at fair value | 172 | 229 |
Level 2 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 177,477 | 166,089 |
Trading securities | 33,036 | 34,335 |
Total assets measured at fair value | 210,513 | 200,424 |
Level 3 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Trading securities | 0 | 0 |
Total assets measured at fair value | 0 | 0 |
Domestic Equity Funds [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 64,453 | 61,368 |
Trading securities | 17,165 | 18,364 |
Domestic Equity Funds [Member] | Level 1 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Trading securities | 0 | 0 |
Domestic Equity Funds [Member] | Level 2 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 59,559 | 56,312 |
Trading securities | 17,165 | 18,364 |
Domestic Equity Funds [Member] | Level 3 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Trading securities | 0 | 0 |
International Equity Funds [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 40,287 | 35,944 |
Trading securities | 4,287 | 4,467 |
International Equity Funds [Member] | Level 1 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Trading securities | 0 | 0 |
International Equity Funds [Member] | Level 2 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 40,287 | 35,944 |
Trading securities | 4,287 | 4,467 |
International Equity Funds [Member] | Level 3 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Trading securities | 0 | 0 |
Core Bonds Fund [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 27,912 | 27,423 |
Trading securities | 11,584 | 11,504 |
Core Bonds Fund [Member] | Level 1 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Trading securities | 0 | 0 |
Core Bonds Fund [Member] | Level 2 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 27,912 | 27,423 |
Trading securities | 11,584 | 11,504 |
Core Bonds Fund [Member] | Level 3 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Trading securities | 0 | 0 |
High-Yield Bonds Fund [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 18,836 | 18,188 |
High-Yield Bonds Fund [Member] | Level 1 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
High-Yield Bonds Fund [Member] | Level 2 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 18,836 | 18,188 |
High-Yield Bonds Fund [Member] | Level 3 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Emerging Market Bonds Fund [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 16,275 | 14,738 |
Emerging Market Bonds Fund [Member] | Level 1 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Emerging Market Bonds Fund [Member] | Level 2 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 16,275 | 14,738 |
Emerging Market Bonds Fund [Member] | Level 3 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Combination Debt Equity And Other Fund [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 14,608 | 13,484 |
Combination Debt Equity And Other Fund [Member] | Level 1 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Combination Debt Equity And Other Fund [Member] | Level 2 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 14,608 | 13,484 |
Combination Debt Equity And Other Fund [Member] | Level 3 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Alternative Funds [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 20,107 | 18,958 |
Alternative Funds [Member] | Level 1 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Alternative Funds [Member] | Level 2 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Alternative Funds [Member] | Level 3 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Real Estate Securities Fund [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 10,170 | 9,946 |
Real Estate Securities Fund [Member] | Level 1 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Real Estate Securities Fund [Member] | Level 2 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Real Estate Securities Fund [Member] | Level 3 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Cash Equivalents [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 172 | 73 |
Trading securities | | 156 |
Cash Equivalents [Member] | Level 1 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 172 | 73 |
Trading securities | | 156 |
Cash Equivalents [Member] | Level 2 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Trading securities | | 0 |
Cash Equivalents [Member] | Level 3 [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Trading securities | | 0 |
Fair Value, Inputs, Net Asset Value [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 35,171 | 33,960 |
Trading securities | 0 | 0 |
Total assets measured at fair value | 35,171 | 33,960 |
Fair Value, Inputs, Net Asset Value [Member] | Domestic Equity Funds [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 4,894 | 5,056 |
Trading securities | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | International Equity Funds [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Trading securities | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | Core Bonds Fund [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Trading securities | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | High-Yield Bonds Fund [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | Emerging Market Bonds Fund [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | Combination Debt Equity And Other Fund [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | Alternative Funds [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 20,107 | 18,958 |
Fair Value, Inputs, Net Asset Value [Member] | Real Estate Securities Fund [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | 10,170 | 9,946 |
Fair Value, Inputs, Net Asset Value [Member] | Cash Equivalents [Member] | | |
Financial and Derivative Instruments and Trading Securities [Line Items] | | |
Nuclear decommissioning trust assets | $ 0 | 0 |
Trading securities | | $ 0 |