Acquisition Activity - Schedule of Total Contractually Required Principal and Interest (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016	May 16, 2016	Dec. 31, 2015	Sep. 30, 2015
Business Acquisition [Line Items]
Accretable yield	$ (39,758)	$ (38,124)		$ (41,882)	$ (44,489)
HF Financial Corporation | Acquired receivables subject to ASC 310-30
Business Acquisition [Line Items]
Contractually required principal and interest			$ 83,710
Non-accretable difference			(28,516)
Cash flows expected to be collected			55,194
Accretable yield			(3,662)
Total purchased credit impaired loans acquired			$ 51,532