Debt Notes and Debentures (Details) - USD ($) $ in Millions		Dec. 31, 2015	Aug. 31, 2015	Apr. 30, 2015	Mar. 31, 2015	Aug. 31, 2014	Mar. 31, 2014	Aug. 31, 2013	Mar. 31, 2013
Debt Instrument [Line Items]
Total Debt excluding capital leases	[1],[2]	$ 21,033
Mortgage Bonds [Member]
Debt Instrument [Line Items]
Total Debt excluding capital leases	[3]	81
Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Subordinated notes related to financial covenants		500
Corporate Debt Securities | Three Point Six Five Zero Percent Debentures Due September 1, 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument issued amount			$ 350
Debt instrument interest rate			3.65%
Corporate Debt Securities | Four Point Seven Zero Zero Percent Debentures Due September 1, 2045 [Member]
Debt Instrument [Line Items]
Debt Instrument issued amount			$ 650
Debt instrument interest rate			4.70%
Corporate Debt Securities | Three Point Zero Zero Zero Percent Debentures Due April 1, 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument issued amount					$ 500
Debt instrument interest rate					3.00%
Corporate Debt Securities | Four Point One Five Zero Percent Debentures Due April 1, 2045 [Member]
Debt Instrument [Line Items]
Debt Instrument issued amount					$ 1,000
Debt instrument interest rate					4.15%
Corporate Debt Securities | Three Point Four Zero Zero Percent Debentures Due September 1, 2024 [Member] [Member]
Debt Instrument [Line Items]
Debt Instrument issued amount						$ 700
Debt instrument interest rate						3.40%
Corporate Debt Securities | Four Point Five Five Zero Percent Debentures Due September 1, 2044 [Member]
Debt Instrument [Line Items]
Debt Instrument issued amount						$ 800
Debt instrument interest rate						4.55%
Corporate Debt Securities | Three Point Seven Five Zero Percent Debentures Due April 1, 2024 [Member] [Member]
Debt Instrument [Line Items]
Debt Instrument issued amount							$ 500
Debt instrument interest rate							3.75%
Corporate Debt Securities | Four Point Nine Zero Zero Percent Debentures Due April 1, 2044 [Member] [Member]
Debt Instrument [Line Items]
Debt Instrument issued amount							$ 1,000
Debt instrument interest rate							4.90%
Corporate Debt Securities | Three Point Eight Five Percent Debentures Due September 1, 2023 [Member] [Domain]
Debt Instrument [Line Items]
Debt Instrument issued amount								$ 800
Debt instrument interest rate								3.85%
Corporate Debt Securities | Five Point One Five Percent Debentures Due September 1, 2043 [Member] [Domain]
Debt Instrument [Line Items]
Debt Instrument issued amount								$ 700
Debt instrument interest rate								5.15%
Corporate Debt Securities | Three Point Zero Zero Percent Debentures Due March 15, 2023 [Member] [Domain]
Debt Instrument [Line Items]
Debt Instrument issued amount									$ 700
Debt instrument interest rate									3.00%
Corporate Debt Securities | Four Point Four Five Percent Debentures Due March 15, 2043 [Member] [Domain]
Debt Instrument [Line Items]
Debt Instrument issued amount									$ 800
Debt instrument interest rate									4.45%
SEC Debt Shelf Registration
Debt Instrument [Line Items]
The board of managers authorized additional debt securities for issuance				$ 3,000
Remaining amount of debt securities that may be issued		$ 2,250

[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 2015 and 2014, respectively. Maturities are as of December 31, 2015.