Regulatory Matters - Summary of Capital Amounts and Ratios (Details) $ in Thousands	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Tier 1 risk based capital (to risk-weighted assets):
For Capital Adequacy Purposes, Percentage	0.085
Total risk based capital (to risk-weighted assets):
For Capital Adequacy Purposes, Percentage	0.105
Common Equity Tier 1 risk based capital (to risk-weighted assets):
For Capital Adequacy Purposes, Percentage	7.00%
Great Western Bancorp, Inc.
Tier 1 risk based capital (to risk-weighted assets):
Actual, Amount	$ 1,195,453	$ 1,225,355
Actual, Percentage	0.118	0.117
For Capital Adequacy Purposes, Amount	$ 609,080	$ 627,493
For Capital Adequacy Purposes, Percentage	0.060	0.060
Total risk based capital (to risk-weighted assets):
Actual, Amount	$ 1,350,658	$ 1,331,611
Actual, Percentage	0.133	0.127
For Capital Adequacy Purposes, Amount	$ 812,107	$ 836,658
For Capital Adequacy Purposes, Percentage	0.080	0.080
Tier 1 leverage capital (to average assets):
Actual, Amount	$ 1,195,453	$ 1,225,355
Actual, Percentage	0.094	0.101
For Capital Adequacy Purposes, Actual	$ 511,248	$ 483,487
For Capital Adequacy Purposes, Percentage	0.040	0.040
Common Equity Tier 1 risk based capital (to risk-weighted assets):
Actual, Amount	$ 1,121,621	$ 1,151,658
Actual, Percentage	11.00%	11.00%
For Capital Adequacy Purposes, Actual	$ 456,810	$ 470,620
For Capital Adequacy Purposes, Percentage	4.50%	4.50%
Great Western Bank
Tier 1 risk based capital (to risk-weighted assets):
Actual, Amount	$ 1,187,905	$ 1,201,476
Actual, Percentage	0.117	0.115
For Capital Adequacy Purposes, Amount	$ 608,916	$ 627,311
For Capital Adequacy Purposes, Percentage	0.060	0.060
To Be Well Capitalized Under Prompt Corrective Action Provisions, Actual	$ 811,888	$ 836,415
To Be Well Capitalized Under Prompt Corrective Action Provisions, Percentage	0.080	0.080
Total risk based capital (to risk-weighted assets):
Actual, Amount	$ 1,315,077	$ 1,272,733
Actual, Percentage	0.130	0.122
For Capital Adequacy Purposes, Amount	$ 811,888	$ 836,415
For Capital Adequacy Purposes, Percentage	0.080	0.080
To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 1,014,860	$ 1,045,519
To Be Well Capitalized Under Prompt Corrective Action Provisions, Percentage	0.100	0.100
Tier 1 leverage capital (to average assets):
Actual, Amount	$ 1,187,905	$ 1,201,476
Actual, Percentage	0.093	0.099
For Capital Adequacy Purposes, Actual	$ 509,649	$ 483,957
For Capital Adequacy Purposes, Percentage	0.040	0.040
To Be Well Capitalized Under Prompt Corrective Action Provisions, Actual	$ 637,062	$ 604,946
To Be Well Capitalized Under Prompt Corrective Action Provisions, Percentage	0.050	0.050
Common Equity Tier 1 risk based capital (to risk-weighted assets):
Actual, Amount	$ 1,187,905	$ 1,201,476
Actual, Percentage	11.70%	11.50%
For Capital Adequacy Purposes, Actual	$ 456,687	$ 470,483
For Capital Adequacy Purposes, Percentage	4.50%	4.50%
To Be Well Capitalized Under Prompt Corrective Action Provisions, Actual	$ 659,659	$ 679,587
To Be Well Capitalized Under Prompt Corrective Action Provisions, Percentage	6.50%	6.50%