Parent Company Only Financial Statements - Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Assets
Cash and cash equivalents	$ 360,396	$ 524,611	$ 237,770	$ 256,639
Accrued interest receivable	53,176	49,531
Net deferred tax assets	42,400	38,346
Other assets	52,041	59,563
Total assets	11,690,011	11,531,180
Liabilities and stockholders’ equity
Subordinated debentures and subordinated notes payable	108,302	111,873
Accrued interest payable	4,405	4,074
Accrued expenses and other liabilities	51,734	52,812
Total liabilities	9,935,011	9,867,789
Stockholders’ equity
Common stock	588	587
Additional paid-in capital	1,314,039	1,312,347
Retained earnings	445,747	344,923
Accumulated other comprehensive income	(5,374)	5,534
Total stockholders’ equity	1,755,000	1,663,391	1,459,346	1,421,090
Total liabilities and stockholders’ equity	11,690,011	11,531,180
Great Western Bancorp, Inc.
Assets
Cash and cash equivalents	15,004	6,521	$ 2,274	$ 5,753
Investment in subsidiaries	1,839,293	1,762,070
Accrued interest receivable	9	8
Net deferred tax assets	1,502	855
Other assets	8,506	7,685
Total assets	1,864,314	1,777,139
Liabilities and stockholders’ equity
Subordinated debentures and subordinated notes payable	108,302	111,873
Accrued interest payable	465	431
Accrued expenses and other liabilities	547	1,444
Total liabilities	109,314	113,748
Stockholders’ equity
Common stock	588	587
Additional paid-in capital	1,314,039	1,312,347
Retained earnings	445,747	344,923
Accumulated other comprehensive income	(5,374)	5,534
Total stockholders’ equity	1,755,000	1,663,391
Total liabilities and stockholders’ equity	$ 1,864,314	$ 1,777,139