Shareholders' Equity - Common Stock and Debt Repurchase Authorizations (Details) - USD ($) shares in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Nov. 02, 2018	Sep. 30, 2018	Jul. 21, 2016
Class of Stock [Line Items]
Dividends declared or paid	$ 0	$ 0
Debt Securities
Class of Stock [Line Items]
Debt repurchase program, amount authorized			$ 100,000,000		$ 150,000,000
Remaining authorized repurchase amount under debt repurchase	$ 100,000,000			$ 50,000,000
Common Stock
Class of Stock [Line Items]
Authorized repurchase of securities, with debt reduction target			$ 200,000,000		$ 100,000,000
Stock repurchased during the period (in shares)	3.0
Stock repurchased during period	$ 50,000,000
Remaining authorized repurchase amount under the stock repurchase program	$ 150,000,000