Financial Instruments And Investments (Company's Decommissioning Trust Funds And Investments In Debt Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	$ 276,905
Cash and Cash Equivalents	12,900	$ 6,990	$ 8,420	$ 8,149
Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	190,330
Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	86,575
Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Cash
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	6,858
Cash | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	6,858
Cash | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Cash | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		286,866
Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		214,876
Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		71,990
Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Federal Agency Mortgage Backed Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		20,732
Federal Agency 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
Federal Agency Mortgage Backed Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		20,732
Federal Agency Mortgage Backed Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
US Treasury and Government | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		58,181
US Treasury and Government | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		58,181
US Treasury and Government | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
US Treasury and Government | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Municipal Bonds | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		16,584
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
Municipal Bonds | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		16,584
Municipal Bonds | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Corporate Debt Obligations | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		34,674
Corporate Debt Obligations | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Corporate Debt Obligations | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		34,674
Corporate Debt Obligations | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Debt Securities
Financial Instruments and Investments [Line Items]
Trading Securities	1,656	1,735	$ 1,421
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	1,656	1,735
Debt Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		130,171
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		58,181
Debt Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		71,990
Debt Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	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	135,865
Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Equity Securities | Domestic Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	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	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
Equity Securities | Domestic Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Equity Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		149,831
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		149,831
Equity Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Equity Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Equity Securities | International Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	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	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
Equity Securities | International Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Cash | Held in trust Decommissioning Fund Investment
Financial Instruments and Investments [Line Items]
Cash and Cash Equivalents	0	6,864
Cash | Held in trust Decommissioning Fund Investment | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Cash and Cash Equivalents		6,864
Cash | Held in trust Decommissioning Fund Investment | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Cash and Cash Equivalents		0
Cash | Held in trust Decommissioning Fund Investment | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Cash and Cash Equivalents		0
Available-for-sale Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	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	47,607
Available-for-sale Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	86,575
Available-for-sale Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Federal Agency Mortgage Backed Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	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
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	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
Available-for-sale Securities | US Treasury and Government
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	47,607
Available-for-sale Securities | US Treasury and Government | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	47,607
Available-for-sale Securities | US Treasury and Government | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | US Treasury and Government | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Municipal Bonds
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	14,089
Available-for-sale Securities | Municipal Bonds | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	14,089
Available-for-sale Securities | Municipal Bonds | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Corporate Debt Obligations
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	41,773
Available-for-sale Securities | Corporate Debt Obligations | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Corporate Debt Obligations | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	41,773
Available-for-sale Securities | Corporate Debt Obligations | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	$ 0
Domestic Equity Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		121,027
Domestic 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		121,027
Domestic Equity Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Domestic Equity Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
International Equity Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		28,804
International 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		28,804
International Equity Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
International Equity Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		$ 0