Debt Notes and Debentures (Details) (USD $)	1 Months Ended
	Apr. 30, 2014	Dec. 31, 2014		Aug. 31, 2014	Mar. 31, 2014	Aug. 31, 2013	Aug. 31, 2012	Mar. 31, 2012
Debt Instrument [Line Items]
Total Debt excluding capital leases		$ 18,471,000,000	[1],[2]
Mortgage Bonds [Member]
Debt Instrument [Line Items]
Total Debt excluding capital leases		81,000,000	[3]
Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Subordinated notes related to financial covenants		500,000,000
Corporate Debt Securities | Three Point Four Zero Zero Percent Debentures Due September 1, 2024 [Member] [Member]
Debt Instrument [Line Items]
Debentures issued amount				700,000,000
Debentures interest rate				3.40%
Corporate Debt Securities | Four Point Five Five Zero Percent Debentures Due September 1, 2044 [Member]
Debt Instrument [Line Items]
Debentures issued amount				800,000,000
Debentures interest rate				4.55%
Corporate Debt Securities | Three Point Seven Five Zero Percent Debentures Due April 1, 2024 [Member] [Member]
Debt Instrument [Line Items]
Debentures issued amount					500,000,000
Debentures interest rate					3.75%
Corporate Debt Securities | Four Point Nine Zero Zero Percent Debentures Due April 1, 2044 [Member] [Member]
Debt Instrument [Line Items]
Debentures issued amount					1,000,000,000
Debentures interest rate					4.90%
Corporate Debt Securities | Three Point Eight Five Zero Percent Debentures Due September 1, 2023 [Member]
Debt Instrument [Line Items]
Debentures issued amount						800,000,000
Debentures interest rate						3.85%
Corporate Debt Securities | Five Point One Five Zero Percent Debentures Due September 1, 2043 [Member]
Debt Instrument [Line Items]
Debentures issued amount						700,000,000
Debentures interest rate						5.15%
Corporate Debt Securities | Three Point Zero Zero Zero Percent Debentures Due March 15, 2023
Debt Instrument [Line Items]
Debentures issued amount					700,000,000
Debentures interest rate					3.00%
Corporate Debt Securities | Four Point Four Five Zero Percent Debentures Due March 15, 2043 [Member]
Debt Instrument [Line Items]
Debentures issued amount					800,000,000
Debentures interest rate					4.45%
Corporate Debt Securities | Three Point Zero Five Zero Percent Debentures Due September 1, 2022 [Member]
Debt Instrument [Line Items]
Debentures issued amount							600,000,000
Debentures interest rate							3.05%
Corporate Debt Securities | Four Point Three Seven Five Percent Debentures Due September 1, 2042 [Member]
Debt Instrument [Line Items]
Debentures issued amount							650,000,000
Debentures interest rate							4.375%
Corporate Debt Securities | Three Point Zero Five Zero Percent Debentures Due March 15, 2022 [Member]
Debt Instrument [Line Items]
Debentures issued amount								625,000,000
Debentures interest rate								3.05%
Corporate Debt Securities | Four Point Four Zero Zero Percent Debentures Due March 15, 2042 [Member]
Debt Instrument [Line Items]
Debentures issued amount								625,000,000
Debentures interest rate								4.40%
SEC Debt Shelf Registration
Debt Instrument [Line Items]
Additional debt securities authorized for issuance by the Board of Managers	3,000,000,000
Remaining amount of debt securities that may be issued		$ 1,750,000,000

[1] Amount also excludes unamortized fair value adjustment under acquisition method accounting related to capital leases.

[2] Amount also excludes unamortized gains on interest rate swaps.

[3] Amounts represent debt outstanding and weighted average effective interest rates for 2014 and 2013, respectively. Maturities are as of December 31, 2014.