Parent Company Only Financial Statements - Condensed Statements of Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income
Dividends on securities	$ 42,653	$ 41,510	$ 29,171
Expenses
Interest on subordinated debentures and subordinated notes payable	4,516	5,540	5,040
Salaries and employee benefits	149,441	136,305	135,352
Professional fees	21,961	14,579	17,891
Other	31,632	20,293	19,171
Income tax benefit	(25,510)	48,230	74,119
Net income	(680,808)	167,365	157,916
Great Western Bancorp, Inc.
Income
Dividends from subsidiary bank	75,079	148,128	111,159
Dividends on securities	2	19	51
Other	87	120	103
Total income	75,168	148,267	111,313
Expenses
Interest on subordinated debentures and subordinated notes payable	4,515	5,540	5,040
Salaries and employee benefits	5,295	6,288	5,849
Professional fees	1,291	1,035	1,038
Other	2,913	2,653	2,456
Total expense	14,014	15,516	14,383
Income before income tax and equity in undistributed net income of subsidiaries	61,154	132,751	96,930
Income tax benefit	(1,114)	(2,965)	(3,075)
Income before equity in undistributed net income of subsidiaries	62,268	135,716	100,005
Equity in undistributed net income of subsidiaries	(743,076)	31,649	57,911
Net income	$ (680,808)	$ 167,365	$ 157,916