Debt - Debt Redemption (Details) - Senior Notes	12 Months Ended
	Sep. 30, 2016
2017 Notes
Debt Instrument [Line Items]
Redemption price, additional basis spread (percent)	0.05%
2020 Notes
Debt Instrument [Line Items]
Redemption price, additional basis spread (percent)	0.10%
2022 Notes
Debt Instrument [Line Items]
Redemption price, additional basis spread (percent)	0.125%
2025 Notes
Debt Instrument [Line Items]
Redemption price, additional basis spread (percent)	0.15%
2035 Notes
Debt Instrument [Line Items]
Redemption price, additional basis spread (percent)	0.20%
2045 Notes
Debt Instrument [Line Items]
Redemption price, additional basis spread (percent)	0.20%