Liquidity Facilities and Letters of Credit - Additional Information (Details) - USD ($)	1 Months Ended
	Nov. 30, 2017	Mar. 31, 2018	Mar. 31, 2017
364 Day Credit Agreement Expiring 2019 [Member]
Line Of Credit Facility [Line Items]
Initiation date	Nov. 30, 2017
Maximum borrowing capacity	$ 5,000,000,000
Credit facilities amount outstanding		$ 0
Three Year Agreement Expiring 2021 [Member]
Line Of Credit Facility [Line Items]
Initiation date	Nov. 30, 2017
Maximum borrowing capacity	$ 5,000,000,000
Credit facilities amount outstanding		0
Five Year Agreement Expiring 2023 [Member]
Line Of Credit Facility [Line Items]
Initiation date	Nov. 30, 2017
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		5,500,000,000
Committed Bank Credit Facility Expiring 2019 [Member]
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		2,400,000,000
Credit facilities amount outstanding		0	$ 0
Committed Bank Credit Facility Expiring 2020 [Member]
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		450,000,000
Credit facilities amount outstanding		0	0
Committed Bank Credit Facility Expiring 2021 [Member]
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		2,600,000,000
Credit facilities amount outstanding		$ 0	$ 0