Long-Term Debt (Debt Issuances) (Details) (USD $)	Dec. 31, 2014
Debt Instrument [Line Items]
Debt face amount	$ 950,000,000
June 2014 3.77% Mortgage Bonds Maturing in 2026 [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Debt instrument stated interest rate	3.77%
Debt face amount	100,000,000
June 2014 4.60% Mortgage Bonds Maturing in 2044 [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Debt instrument stated interest rate	4.60%
Debt face amount	150,000,000
July 2014 3.375% Mortgage Bonds Maturing in 2025 [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Debt instrument stated interest rate	3.375%
Debt face amount	350,000,000
July 2014 4.30% Mortgage Bonds Maturing in 2044 [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Debt instrument stated interest rate	4.30%
Debt face amount	$ 350,000,000