Financial Instruments And Investments (Company's Decommissioning Trust Funds And Investments In Debt Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	$ 297,030
Cash and cash equivalents	15,965	$ 6,990	$ 7,060	$ 8,420
Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	215,361
Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	81,669
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	10,245
Cash | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	10,245
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
U.S. Government Bonds | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		58,181
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		58,181
U.S. Government Bonds | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
U.S. Government Bonds | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Municipal Obligations | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		16,584
Municipal 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
Municipal Obligations | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		16,584
Municipal Obligations | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Corporate Obligations | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		34,674
Corporate 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 Obligations | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		34,674
Corporate 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	1,681	1,735
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	1,681	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	156,195
Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	156,195
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	127,966
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	127,966
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	28,229
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	28,229
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	130,590
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	48,921
Available-for-sale Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	81,669
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	27,207
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	27,207
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 | U.S. Government Bonds
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	48,921
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	48,921
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
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
Available-for-sale Securities | Municipal Obligations
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	14,463
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
Available-for-sale Securities | Municipal Obligations | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	14,463
Available-for-sale Securities | Municipal Obligations | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Corporate Obligations
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	39,999
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
Available-for-sale Securities | Corporate Obligations | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	39,999
Available-for-sale Securities | Corporate 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