Liquidity Facilities and Letters of Credit - Additional Information (Details) - USD ($)	1 Months Ended
	Nov. 30, 2016	Jun. 30, 2017	Mar. 31, 2017
364 Day Credit Agreement Expiring 2018 [Member]
Line Of Credit Facility [Line Items]
Initiation date	Nov. 30, 2016
Maximum borrowing capacity	$ 5,000,000,000
Credit facilities amount outstanding		$ 0	$ 0
Three Year Agreement Expiring 2020 [Member]
Line Of Credit Facility [Line Items]
Initiation date	Nov. 30, 2016
Maximum borrowing capacity	$ 5,000,000,000
Credit facilities amount outstanding		0	0
Five Year Agreement Expiring 2022 [Member]
Line Of Credit Facility [Line Items]
Initiation date	Nov. 30, 2016
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		5,600,000,000
Committed Bank Credit Facility Expiring 2018 [Member]
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		2,200,000,000
Credit facilities amount outstanding		0	0
Committed Bank Credit Facility Expiring 2019 [Member]
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		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		2,300,000,000
Credit facilities amount outstanding		0	0
Committed Bank Credit Facility Expiring 2021 [Member]
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		725,000,000
Credit facilities amount outstanding		$ 0	$ 0