Document and Entity Information - USD ($)	12 Months Ended
	Sep. 30, 2016	Nov. 18, 2016	Mar. 31, 2016
Document And Entity Information [Abstract]
Entity Registrant Name	Great Western Bancorp, Inc.
Entity Central Index Key	0001613665
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 1,506,535,977
Entity Common Stock, Shares Outstanding (in shares)		58,714,523