Shareholders' Equity - Narrative (Details) - USD ($)				3 Months Ended	6 Months Ended
	Feb. 19, 2019	Nov. 30, 2018	Nov. 30, 2017	Mar. 31, 2019	Mar. 31, 2019	Mar. 31, 2018	Nov. 19, 2018	Nov. 13, 2018
Class of Stock [Line Items]
Net proceeds from equity offering					$ 494,085,000	$ 395,092,000
Shelf Registration Statement
Class of Stock [Line Items]
Debt and equity securities authorized for issuance								$ 3,000,000,000
Debt and equity securities authorized for issuance value remaining				$ 1,300,000,000	1,300,000,000
At-The-Market
Class of Stock [Line Items]
Value of shares authorized for issuance							$ 500,000,000
Value of shares available for issuance				$ 340,000,000	340,000,000
At-The-Market | Forward Sales Equity Agreement, February 2019
Class of Stock [Line Items]
Forward sales equity agreement, shares	1,670,509
Forward sales equity agreement, settlement date				Mar. 31, 2020
Forward sales equity agreement, settlements				$ 0	0
Forward sales equity agreement, settlement alternatives, cash, at fair value				159,000,000	$ 159,000,000
Forward sales equity agreement, forward rate per share	$ 95.46				$ 95.19
Common Stock Block Trade
Class of Stock [Line Items]
Stock Issued During Period, New Issues (in shares)		5,390,836	4,558,404
Gross proceeds from equity offering		$ 500,000,000	$ 400,000,000
Net proceeds from equity offering		$ 494,100,000	$ 395,100,000
Common Stock Block Trade | Forward Sales Equity Agreement, November 2018
Class of Stock [Line Items]
Forward sales equity agreement, shares		2,668,464
Forward sales equity agreement, settlement date					Mar. 31, 2020
Forward sales equity agreement, settlements				0	$ 0
Forward sales equity agreement, settlement alternatives, cash, at fair value				$ 244,800,000	$ 244,800,000
Forward sales equity agreement, forward rate per share					$ 91.75