Regulatory Matters - Summary of Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Great Western Bancorp, Inc.
Tier 1 risk based capital (to risk-weighted assets):
Actual, Amount	$ 1,001,873	$ 825,211
Actual, Percentage	11.10%	10.90%
For Capital Adequacy Purposes, Amount	$ 541,553	$ 456,338
For Capital Adequacy Purposes, Percentage	6.00%	6.00%
Total risk based capital (to risk-weighted assets):
Actual, Amount	$ 1,101,997	$ 917,446
Actual, Percentage	12.20%	12.10%
For Capital Adequacy Purposes, Amount	$ 722,621	$ 608,084
For Capital Adequacy Purposes, Percentage	8.00%	8.00%
Tier 1 leverage capital (to average assets):
Actual, Amount	$ 1,001,873	$ 825,211
Actual, Percentage	9.50%	9.10%
For Capital Adequacy Purposes, Actual	$ 421,841	$ 361,538
For Capital Adequacy Purposes, Percentage	4.00%	4.00%
Common Equity Tier 1 risk based capital (to risk-weighted assets):
Actual, Amount	$ 924,718	$ 769,128
Actual, Percentage	10.20%	10.10%
For Capital Adequacy Purposes, Actual	$ 407,964	$ 342,004
For Capital Adequacy Purposes, Percentage	4.50%	4.50%
Great Western Bank
Tier 1 risk based capital (to risk-weighted assets):
Actual, Amount	$ 1,023,386	$ 850,464
Actual, Percentage	11.30%	11.20%
For Capital Adequacy Purposes, Amount	$ 543,391	$ 455,606
For Capital Adequacy Purposes, Percentage	6.00%	6.00%
To Be Well Capitalized Under Prompt Corrective Action Provisions, Actual	$ 724,521	$ 607,474
To Be Well Capitalized Under Prompt Corrective Action Provisions, Percentage	8.00%	8.00%
Total risk based capital (to risk-weighted assets):
Actual, Amount	$ 1,088,511	$ 907,700
Actual, Percentage	12.00%	12.00%
For Capital Adequacy Purposes, Amount	$ 725,674	$ 607,665
For Capital Adequacy Purposes, Percentage	8.00%	8.00%
To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 907,093	$ 759,582
To Be Well Capitalized Under Prompt Corrective Action Provisions, Percentage	10.00%	10.00%
Tier 1 leverage capital (to average assets):
Actual, Amount	$ 1,023,386	$ 850,464
Actual, Percentage	9.70%	9.40%
For Capital Adequacy Purposes, Actual	$ 422,015	$ 361,131
For Capital Adequacy Purposes, Percentage	4.00%	4.00%
To Be Well Capitalized Under Prompt Corrective Action Provisions, Actual	$ 527,519	$ 451,414
To Be Well Capitalized Under Prompt Corrective Action Provisions, Percentage	5.00%	5.00%
Common Equity Tier 1 risk based capital (to risk-weighted assets):
Actual, Amount	$ 1,023,386	$ 850,464
Actual, Percentage	11.30%	11.20%
For Capital Adequacy Purposes, Actual	$ 407,543	$ 341,704
For Capital Adequacy Purposes, Percentage	4.50%	4.50%
To Be Well Capitalized Under Prompt Corrective Action Provisions, Actual	$ 588,673	$ 493,573
To Be Well Capitalized Under Prompt Corrective Action Provisions, Percentage	6.50%	6.50%