Notes Payable (Details)	3 Months Ended
	Dec. 31, 2017 USD ($) borrowing	Sep. 30, 2017 USD ($)	Sep. 30, 2015 USD ($)
Revolving Credit Facility | Unsecured Debt
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 2,000,000,000.0
Credit facility, term			5 years
Draw downs from lines of credit		$ 800,000,000
Interest rate during period		2.36%
Borrowings outstanding	$ 0
Available borrowing capacity	2,000,000,000
Accordion feature, increase in maximum borrowing capacity			$ 500,000,000
Revolving Credit Facility | Uncommitted Credit Facilities
Line of Credit Facility [Line Items]
Maximum borrowing capacity	250,000,000
Borrowings outstanding	0
Available borrowing capacity	250,000,000
Revolving Credit Facility | London Interbank Offered Rate (LIBOR) | Unsecured Debt
Line of Credit Facility [Line Items]
Basis spread on variable rate		1.125%
Letter of Credit Sub-Facility | Unsecured Debt
Line of Credit Facility [Line Items]
Maximum borrowing capacity			150,000,000
Swingline Sub-Facility | Unsecured Debt
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 150,000,000
Unsecured Debt | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 3,000,000,000.0
Credit facility, term	364 days
Maximum number of borrowings | borrowing	3
Draw downs from lines of credit	$ 1,000,000,000
Interest rate during period	2.78%
Borrowings outstanding	$ 1,000,000,000
Available borrowing capacity	$ 2,000,000,000
Unsecured Debt | Revolving Credit Facility | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.125%