Tax-Exempt Bonds (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Long-term Debt	$ 2,015	$ 1,486
Tax Exempt Bonds due 2024 to 2042 [Member]
Debt Instrument [Line Items]
Long-term Debt	335	0
Offering Expense	7
Massachusetts Series 2012A [Member] | Tax Exempt Bonds due 2024 to 2042 [Member]
Debt Instrument [Line Items]
Long-term Debt	20
Debt Instrument Maturity Year	2027
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Massachusetts Series 2012B [Member] | Tax Exempt Bonds due 2024 to 2042 [Member]
Debt Instrument [Line Items]
Long-term Debt	67
Debt Instrument Maturity Year	2042
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Massachusetts Series 2012C [Member] | Tax Exempt Bonds due 2024 to 2042 [Member]
Debt Instrument [Line Items]
Long-term Debt	83
Debt Instrument Maturity Year	2042
Debt Instrument, Interest Rate, Stated Percentage	5.25%
Niagara Series 2012A [Member] | Tax Exempt Bonds due 2024 to 2042 [Member]
Debt Instrument [Line Items]
Long-term Debt	130
Debt Instrument Maturity Year	2042
Debt Instrument, Interest Rate, Stated Percentage	5.25%
Niagara Series 2012B [Member] | Tax Exempt Bonds due 2024 to 2042 [Member]
Debt Instrument [Line Items]
Long-term Debt	35
Debt Instrument Maturity Year	2024
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Debt Offering Costs Expensed [Member] | Tax Exempt Bonds due 2024 to 2042 [Member]
Debt Instrument [Line Items]
Offering Expense	3
Debt Offering Costs Deferred [Member] [Member] | Tax Exempt Bonds due 2024 to 2042 [Member]
Debt Instrument [Line Items]
Offering Expense	$ 4