Financial Instruments And Investments (Company's Decommissioning Trust Funds And Investments In Debt Securities) (Details) - USD ($) $ in Thousands		Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		$ 325,998		$ 276,905
Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		325,998		190,330
Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0		86,575
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		1,587		1,656	$ 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		1,587		1,656
Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		324,267		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		324,267		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
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
Federal Agency Mortgage Backed Securities | Available-for-sale Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value				30,713
Federal Agency Mortgage Backed Securities | 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				0
Federal Agency Mortgage Backed Securities | Available-for-sale Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value				30,713
Federal Agency Mortgage Backed Securities | Available-for-sale Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value				0
U.S. Government Bonds | Available-for-sale Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value				47,607
U.S. Government Bonds | 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
U.S. Government Bonds | Available-for-sale Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value				0
U.S. Government Bonds | Available-for-sale Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value				0
Municipal Bonds | Available-for-sale Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value				14,089
Municipal Bonds | 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				0
Municipal Bonds | Available-for-sale Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value				14,089
Municipal Bonds | Available-for-sale Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value				0
Corporate Debt Obligations | Available-for-sale Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value				41,773
Corporate Debt Obligations | 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				0
Corporate Debt Obligations | Available-for-sale Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value				41,773
Corporate Debt Obligations | Available-for-sale Securities | 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		1,731		6,858
Cash | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		1,731		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
Domestic Equity Securities | Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		295,065	[1]	111,325
Domestic Equity Securities | Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		295,065	[1]	111,325
Domestic Equity Securities | Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0	[1]	0
Domestic Equity Securities | Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0	[1]	0
International Equity Securities | Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		29,202		24,540
International Equity Securities | Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		29,202		24,540
International Equity Securities | Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0		0
International Equity Securities | Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0		$ 0
Exchange Traded Funds | Domestic Equity Securities | Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	[1]	$ 148,100

[1] Includes $148.1 million held in exchange traded funds with underlying investments in debt securities that meet the definition of equity securities with readily determinable fair values.