Liquidity Facilities and Letters of Credit - Additional Information (Details) (USD $)	1 Months Ended
	Nov. 30, 2014	Mar. 31, 2015	Mar. 31, 2014
364 Day Credit Agreement Expiring 2016 [Member]
Line Of Credit Facility [Line Items]
Initiation Date	Nov. 30, 2014
Maximum Borrowing Capacity	$ 5,000,000,000
Credit facilities amount outstanding		0
Three Year Agreement Expiring 2018 [Member]
Line Of Credit Facility [Line Items]
Initiation Date	Nov. 30, 2014
Maximum Borrowing Capacity	5,000,000,000
Credit facilities amount outstanding		0
Five Year Agreement Expiring 2020 [Member]
Line Of Credit Facility [Line Items]
Initiation Date	Nov. 30, 2014
Maximum Borrowing Capacity	5,000,000,000
Credit facilities amount outstanding		0
Total Committed Bank Credit Facilities [Member]
Line Of Credit Facility [Line Items]
Maximum Borrowing Capacity		5,700,000,000
Committed Bank Credit Facility Expiring 2016 [Member]
Line Of Credit Facility [Line Items]
Maximum Borrowing Capacity		3,200,000,000
Credit facilities amount outstanding		0	0
Committed Bank Credit Facility Expiring 2018 [Member]
Line Of Credit Facility [Line Items]
Maximum Borrowing Capacity		2,100,000,000
Credit facilities amount outstanding		0	0
Committed Bank Credit Facility Expiring 2020 [Member]
Line Of Credit Facility [Line Items]
Maximum Borrowing Capacity		375,000,000
Credit facilities amount outstanding		$ 0	$ 0