Core Deposits and Other Intangibles - Intangible Assets Amortization Expense (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 16,341	$ 16,341
Accumulated amortization	(7,393)	(6,967)
Total	8,948	9,374
Core Deposit Intangible
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	7,339	7,339
Accumulated amortization	(1,847)	(1,579)
Total	5,492	5,760
Brand Intangible
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	8,464	8,464
Accumulated amortization	(5,405)	(5,264)
Total	3,059	3,200
Other Intangible
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	538	538
Accumulated amortization	(141)	(124)
Total	$ 397	$ 414