Shareholders' Equity - Common Stock and Debt Repurchase Authorizations (Details) - USD ($) shares in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019	Nov. 07, 2019	Jul. 26, 2019	Nov. 02, 2018
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
Remaining authorized repurchase amount under debt repurchase	$ 76,000,000		76,000,000		$ 76,000,000
Common Stock
Class of Stock [Line Items]
Authorized repurchase of securities, with debt reduction target (up to)						$ 325,000,000	$ 250,000,000	$ 200,000,000
Number of shares authorized to be repurchased (in shares)	5.6	4.9			1.3
Shares repurchased during period	$ 141,000,000	$ 100,000,000			$ 25,000,000
Remaining authorized repurchase amount under the stock repurchase program	$ 59,000,000		$ 59,000,000