Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share
Net income	$ 4,276	$ 22,511	$ 24,554	$ 24,383	$ 26,787	$ 48,937
Income allocated to participating securities	(3)		(10)		(22)	(19)
Net income available to common stockholders	$ 4,273		$ 24,544		$ 26,765	$ 48,918
Total basic average common shares outstanding	137,968,327		137,634,820		137,932,976	137,592,409
Effect of dilutive stock options	32,007		55,897		55,673	48,717
Total diluted average common shares outstanding	138,000,334		137,690,717		137,988,649	137,641,126
Net EPS:
Basic	$ 0.03		$ 0.18		$ 0.19	$ 0.36
Diluted	$ 0.03		$ 0.18		$ 0.19	$ 0.36
Antidilutive stock options, excluded from the diluted average common shares outstanding calculation	382,894		494,395		387,979	529,261
Average Common Shares Outstanding [Member]
Earnings Per Share
Total basic average common shares outstanding	137,926,574		137,593,062		137,911,988	137,571,533
Average Committed ESOP Shares Outstanding [Member]
Earnings Per Share
Total basic average common shares outstanding	41,753		41,758		20,988	20,876

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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