Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended													12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Quarterly Financial Information Disclosure [Abstract]
Total interest income	$ 61,785	$ 57,695	$ 55,019	$ 51,791	$ 49,169	$ 47,820	$ 46,850	$ 45,428	$ 44,129	$ 43,011	$ 42,421	$ 41,929	$ 41,616	$ 226,290	$ 184,226	$ 168,977
Total interest expense	18,410	17,112	14,004	12,730	12,240	11,860	11,211	10,651	10,724	11,123	11,003	10,840	10,996	62,256	44,446	43,962
Net interest income	43,375	40,583	41,015	39,061	36,929	35,960	35,639	34,777	33,405	31,888	31,418	31,089	30,620	164,034	139,780	125,015
Provision for loan losses	777	1,500	2,400	2,000	3,400	5,676	375	375	0	0	0	417	0
Net interest income after provision for loan losses	42,598	39,083	38,615	37,061	33,529	30,284	35,264	34,402	33,405	31,888	31,418	30,672	30,620	157,357	133,354	124,598
Total non-interest income	6,405	5,290	5,450	4,543	4,733	1,630	4,645	5,806	5,534	4,702	4,525	4,738	4,537	21,688	17,172	18,927
Total non-interest expense	31,013	26,590	61,768	26,015	25,601	30,206	24,703	24,903	24,078	23,482	23,658	22,816	23,388	145,386	103,446	93,769
Income (loss) before income tax expense	17,990	17,783	(17,703)	15,589	12,661	1,708	15,206	15,305	14,861	13,108	12,285	12,594	11,769
Income tax expense	3,123	6,956	(2,961)	3,805	8,983	194	5,934	5,012	4,868	4,611	4,052	4,281	3,859	10,923	16,008	16,803
Net income	$ 14,867	$ 10,827	$ (14,742)	$ 11,784	$ 3,678	$ 1,514	$ 9,272	$ 10,293	$ 9,993	$ 8,497	$ 8,233	$ 8,313	$ 7,910	$ 22,736	$ 31,072	$ 32,953
Basic and diluted earnings per share (in dollars per share)	$ 0.13	$ 0.10	$ (0.13)											$ 0.20