Borrowings Line of Credit Facilities (Details) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)
Line of Credit Facility
Line of Credit Facility, Amount Outstanding	$ 0	$ 0
Line of Credit Facility, Remaining Borrowing Capacity	12,250	12,250
Line of Credit Facility, Maximum Borrowing Capacity	12,250	12,250
Existing Line of Credit 3
Line of Credit Facility
Line of Credit Facility, Initial Borrowing Capacity	2,500	2,500
Line of Credit Facility, Amount Outstanding	0	0
Line of Credit Facility, Remaining Borrowing Capacity	6,000	$ 6,000
Line of Credit Facility, Expiration Date		Mar. 31, 2019
Line of Credit Facility, Maximum Borrowing Capacity	6,000	$ 6,000
Line of Credit Facility, Increase (Decrease), Net	6,000
Existing Line of Credit 2
Line of Credit Facility
Line of Credit Facility, Initial Borrowing Capacity	2,300	2,300
Line of Credit Facility, Amount Outstanding	0	0
Line of Credit Facility, Remaining Borrowing Capacity	4,000	$ 4,000
Line of Credit Facility, Expiration Date		Mar. 31, 2023
Line of Credit Facility, Maximum Borrowing Capacity	4,000	$ 4,000
Line of Credit Facility, Increase (Decrease), Net	4,000
Existing Line Of Credit 1
Line of Credit Facility
Line of Credit Facility, Amount Outstanding	0	0
Line of Credit Facility, Remaining Borrowing Capacity	2,250	$ 2,250
Line of Credit Facility, Expiration Date		Mar. 31, 2021
Line of Credit Facility, Maximum Borrowing Capacity	$ 2,250	$ 2,250