Cover Page - USD ($)	12 Months Ended
	Sep. 30, 2020	Nov. 17, 2020	Mar. 31, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	001-36688
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, City or Town	Sioux Falls,
Entity Address, State or Province	SD
Entity Address, Postal Zip Code	57104
City Area Code	605
Local Phone Number	334-2548
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol	GWB
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
ICFR Auditor Attestation Flag	true
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 1,126,685,245
Entity Common Stock, Shares Outstanding		55,014,727
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement for the annual meeting of shareholders to be held on February 9, 2021, and to be filed pursuant to Regulation 14A within 120 days after the registrant’s fiscal year ended September 30, 2020, are incorporated by reference under Part III.
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Central Index Key	0001613665
Current Fiscal Year End Date	--09-30