Acquisition Activity - Schedule of Pro Forma Information (Details) - HF Financial Corp - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Business Acquisition [Line Items]
Net interest income	$ 415,149	$ 370,778	$ 352,991
Net income	$ 133,685	$ 114,731	$ 112,387
Basic earnings per share (in dollars per share)	$ 2.36	$ 2.00	$ 1.94
Fully diluted earnings per share (in dollars per share)	$ 2.36	$ 2.00	$ 1.94