Parent Company Only Financial Statements - Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Assets [Abstract]
Cash and cash equivalents	$ 432,887	$ 243,474
Net deferred tax assets	47,709	7,286
Other assets	197,558	132,417
Total assets	12,604,439	12,788,301
Liabilities and stockholders’ equity
Subordinated debentures and subordinated notes payable	108,832	108,636
Accrued expenses and other liabilities	63,389	70,085
Total liabilities	11,441,506	10,888,052
Stockholders’ equity
Common stock	550	563
Additional paid-in capital	1,183,647	1,228,714
Retained earnings	(57,169)	657,475
Accumulated other comprehensive income	35,905	13,497
Total stockholders' equity	1,162,933	1,900,249	$ 1,840,551	$ 1,755,000
Total liabilities and stockholders' equity	12,604,439	12,788,301
Great Western Bancorp, Inc.
Assets [Abstract]
Cash and cash equivalents	36,420	57,543
Investment in subsidiaries	1,229,227	1,950,077
Net deferred tax assets	1,395	1,716
Other assets	6,086	689
Total assets	1,273,128	2,010,025
Liabilities and stockholders’ equity
Subordinated debentures and subordinated notes payable	108,832	108,636
Accrued expenses and other liabilities	1,363	1,140
Total liabilities	110,195	109,776
Stockholders’ equity
Common stock	550	563
Additional paid-in capital	1,183,647	1,228,714
Retained earnings	(57,169)	657,475
Accumulated other comprehensive income	35,905	13,497
Total stockholders' equity	1,162,933	1,900,249
Total liabilities and stockholders' equity	$ 1,273,128	$ 2,010,025