Financial Instruments And Investments (Company's Decommissioning Trust Funds And Investments In Debt Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	$ 255,708	$ 239,035
Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	197,703	176,130
Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	58,005	62,905
Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Debt Securities
Financial Instruments and Investments [Line Items]
Trading Securities	1,421	1,543	$ 1,653
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	1,421	1,543
Debt Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	119,873	113,304
Debt Securities | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	61,868	50,399
Debt Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	58,005	62,905
Debt Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
U.S. Government Bonds | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	61,868	50,399
U.S. Government Bonds | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	61,868	50,399
U.S. Government Bonds | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
U.S. Government Bonds | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Collateralized Mortgage Backed Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	19,448	20,085
Collateralized Mortgage Backed Securities | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Collateralized Mortgage Backed Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	19,448	20,085
Collateralized Mortgage Backed Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Municipal Bonds | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	16,144	24,345
Municipal Bonds | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Municipal Bonds | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	16,144	24,345
Municipal Bonds | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Corporate Debt Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	22,413	18,475
Corporate Debt Securities | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Corporate Debt Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	22,413	18,475
Corporate Debt Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Equity Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	64,644	76,220
Equity Securities | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	64,644	76,220
Equity Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Equity Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Equity Funds | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	42,244	18,853
Equity Funds | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	42,244	18,853
Equity Funds | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Equity Funds | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Institutional Funds-International Equity | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	22,945	22,454
Institutional Funds-International Equity | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	22,945	22,454
Institutional Funds-International Equity | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Institutional Funds-International Equity | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities	0	0
Cash | Held in trust Decommissioning Fund Investment
Financial Instruments and Investments [Line Items]
Cash	6,002	8,204
Cash | Held in trust Decommissioning Fund Investment | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Cash	6,002	8,204
Cash | Held in trust Decommissioning Fund Investment | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Cash	0	0
Cash | Held in trust Decommissioning Fund Investment | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Cash	$ 0	$ 0