Liquidity Facilities and Letters of Credit - Additional Information (Details) - USD ($)	1 Months Ended
	Nov. 30, 2015	Sep. 30, 2016	Mar. 31, 2016
364 Day Credit Agreement Expiring 2017 [Member]
Line Of Credit Facility [Line Items]
Initiation date	Nov. 30, 2015
Maximum borrowing capacity	$ 5,000,000,000
Credit facilities amount outstanding		$ 0	$ 0
Three Year Agreement Expiring 2019 [Member]
Line Of Credit Facility [Line Items]
Initiation date	Nov. 30, 2015
Maximum borrowing capacity	$ 5,000,000,000
Credit facilities amount outstanding		0	0
Five Year Agreement Expiring 2021 [Member]
Line Of Credit Facility [Line Items]
Initiation date	Nov. 30, 2015
Maximum borrowing capacity	$ 5,000,000,000
Credit facilities amount outstanding		0	0
Total Committed Bank Credit Facilities [Member]
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		5,400,000,000
Committed Bank Credit Facility Expiring 2017 [Member]
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		1,900,000,000
Credit facilities amount outstanding		0	0
Committed Bank Credit Facility Expiring 2018 [Member]
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		775,000,000
Credit facilities amount outstanding		0	0
Committed Bank Credit Facility Expiring 2019 [Member]
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		1,700,000,000
Credit facilities amount outstanding		0	0
Committed Bank Credit Facility Expiring 2020 [Member]
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		1,000,000,000
Credit facilities amount outstanding		$ 0	$ 0