Parent Company Only Financial Statements - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Operating activities
Net income	$ (680,808)	$ 167,365	$ 157,916
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation and amortization	26,354	13,655	13,093
Stock-based compensation	3,705	4,582	4,419
Deferred income taxes	(47,754)	8,124	21,302
Changes in:
Accrued interest payable and other liabilities	(5,398)	2,949	(2,742)
Net cash provided by operating activities	82,464	191,659	182,429
Financing activities
Common stock repurchased	(39,983)	(94,351)	0
Dividends paid	(42,456)	(62,904)	(53,002)
Net cash provided by financing activities	476,340	451,587	289,182
Net increase (decrease) in cash and cash equivalents	189,413	(55,222)	(61,700)
Cash and cash equivalents, beginning of period	243,474	298,696	360,396
Cash and cash equivalents, end of period	432,887	243,474	298,696
Great Western Bancorp, Inc.
Operating activities
Net income	(680,808)	167,365	157,916
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation and amortization	196	168	166
Stock-based compensation	3,705	4,582	4,419
Deferred income taxes	321	(227)	13
Changes in:
Other assets	(5,397)	1,367	6,459
Accrued interest payable and other liabilities	223	268	(140)
Equity in undistributed net income of subsidiaries	743,076	(31,649)	(57,911)
Net cash provided by operating activities	61,316	141,874	110,922
Financing activities
Common stock repurchased	(39,983)	(94,351)	0
Dividends paid	(42,456)	(62,904)	(53,002)
Net cash provided by financing activities	(82,439)	(157,255)	(53,002)
Net increase (decrease) in cash and cash equivalents	(21,123)	(15,381)	57,920
Cash and cash equivalents, beginning of period	57,543	72,924	15,004
Cash and cash equivalents, end of period	$ 36,420	$ 57,543	$ 72,924