Shareholders' Equity - Narrative (Details) - USD ($)		12 Months Ended
	Nov. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Oct. 04, 2018	Mar. 28, 2016
Class of Stock [Line Items]
Common stock shares authorized (in shares)	4,558,404	200,000,000	200,000,000
Proceeds from issuance of common stock gross	$ 400,000,000
Net proceeds from equity offering	$ 395,100,000	$ 395,092,000	$ 98,755,000	$ 98,574,000
Subsequent Event
Class of Stock [Line Items]
Debt face amount					$ 600,000,000
Shelf Registration Statement
Class of Stock [Line Items]
Debt and equity securities authorized for issuance						$ 2,500,000,000.0
Debt and equity securities authorized for issuance value remaining		650,000,000
At-The-Market
Class of Stock [Line Items]
Value of shares authorized for issuance		$ 500,000,000
Common stock sold (in shares)		0