| Cover Document | Apr. 30, 2020 |
| Cover [Abstract] | |
| Entity Central Index Key | 0001613665 |
| Title of 12(b) Security | Common Stock, $0.01 par value per share |
| Entity File Number | 001-36688 |
| Document Type | 8-K |
| Document Period End Date | Apr. 30, 2020 |
| Entity Registrant Name | GREAT WESTERN BANCORP, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 47-1308512 |
| Entity Address, Address Line One | 225 South Main Avenue |
| Entity Address, Postal Zip Code | 57104 |
| Entity Address, City or Town | Sioux Falls, |
| Entity Address, State or Province | SD |
| City Area Code | 605 |
| Local Phone Number | 334-2548 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Trading Symbol | GWB |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
