Document and Entity Information - shares	3 Months Ended
	Dec. 31, 2016	Feb. 03, 2017
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-Q
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding		58,756,598