Financial Instruments And Investments (Company's Decommissioning Trust Funds And Investments In Debt Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	$ 308,064	$ 276,905
Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	221,204	190,330
Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	86,860	86,575
Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Cash
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	9,176	6,858
Cash | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	9,176	6,858
Cash | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Cash | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Debt Securities
Financial Instruments and Investments [Line Items]
Trading Securities	1,622	1,656
Debt Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Trading Securities	0	0
Debt Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Trading Securities	0	0
Debt Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Trading Securities	1,622	1,656
Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	159,645	135,865
Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	159,645	135,865
Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Equity Securities | Domestic Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	132,183	111,325
Equity Securities | Domestic Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	132,183	111,325
Equity Securities | Domestic Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Equity Securities | Domestic Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Equity Securities | International Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	27,462	24,540
Equity Securities | International Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	27,462	24,540
Equity Securities | International Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Equity Securities | International Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Available-for-sale Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	139,243	134,182
Available-for-sale Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	52,383	47,607
Available-for-sale Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	86,860	86,575
Available-for-sale Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Available-for-sale Securities | Federal Agency Mortgage Backed Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	29,484	30,713
Available-for-sale Securities | Federal Agency Mortgage Backed Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Available-for-sale Securities | Federal Agency Mortgage Backed Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	29,484	30,713
Available-for-sale Securities | Federal Agency Mortgage Backed Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Available-for-sale Securities | U.S. Government Bonds
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	52,383	47,607
Available-for-sale Securities | U.S. Government Bonds | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	52,383	47,607
Available-for-sale Securities | U.S. Government Bonds | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Available-for-sale Securities | U.S. Government Bonds | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Available-for-sale Securities | Municipal Obligations
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	10,027	14,089
Available-for-sale Securities | Municipal Obligations | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Available-for-sale Securities | Municipal Obligations | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	10,027	14,089
Available-for-sale Securities | Municipal Obligations | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Available-for-sale Securities | Corporate Obligations
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	47,349	41,773
Available-for-sale Securities | Corporate Obligations | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0	0
Available-for-sale Securities | Corporate Obligations | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	47,349	41,773
Available-for-sale Securities | Corporate Obligations | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	$ 0	$ 0