Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share
Net income	$ 18,279	$ 19,474	$ 4,276	$ 22,511	$ 22,409	$ 22,897	$ 24,554	$ 24,383	$ 64,540	$ 94,243	$ 98,927
Income allocated to participating securities									(52)	(55)	(40)
Net income available to common stockholders									$ 64,488	$ 94,188	$ 98,887
Total basic average common shares outstanding	137,705,000	138,018,000	137,968,000	137,898,000	137,801,000	137,720,000	137,635,000	137,551,000	137,896,704	137,676,923	134,698,344
Effect of dilutive stock options									4,484	58,478	60,647
Total diluted average common shares outstanding	137,705,000	138,018,000	138,000,000	137,976,000	137,867,000	137,788,000	137,691,000	137,592,000	137,901,188	137,735,401	134,758,991
Net EPS:
Basic	$ 0.13	$ 0.14	$ 0.03	$ 0.16	$ 0.16	$ 0.17	$ 0.18	$ 0.18	$ 0.47	$ 0.68	$ 0.73
Diluted	$ 0.13	$ 0.14	$ 0.03	$ 0.16	$ 0.16	$ 0.17	$ 0.18	$ 0.18	$ 0.47	$ 0.68	$ 0.73
Antidilutive stock options, excluded from the diluted average common shares outstanding calculation									437,731	470,938	541,418
Average Common Shares Outstanding [Member]
Earnings Per Share
Total basic average common shares outstanding									137,834,304	137,614,465	134,635,886
Average Committed ESOP Shares Outstanding [Member]
Earnings Per Share
Total basic average common shares outstanding									62,400	62,458	62,458