Borrowings - Additional Information (Details) $ in Millions	3 Months Ended
	Dec. 31, 2016 USD ($)
Line of Credit Facility
Line of Credit Facility, Interest Rate Description	All of the above bank facilities allow for borrowings at LIBOR-based rates plus a spread depending on the credit default swap spread applicable to the Company’s debt, subject to a cap and floor that vary with the Company’s debt rating assigned by Moody’s Investors Service and Standard and Poor’s. The spread above LIBOR can range from 0.23% to 1.63%.
Line of Credit Facility, Maximum Borrowing Capacity	$ 6,000
Letters of Credit Outstanding, Amount	172
Letter of Credit
Line of Credit Facility
Line of Credit Facility, Maximum Borrowing Capacity	$ 800
Minimum
Line of Credit Facility
Variable Spread Above LIBOR	0.23%
Maximum
Line of Credit Facility
Variable Spread Above LIBOR	1.63%