Parent-only Financial Information - Statement of Comprehensive Income (Loss) (Details) - USD ($) $ 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
Income:
Unrealized (losses) gains on securities available for sale					$ (3,131)									$ (5,778)	$ (11,498)	$ 4,674
Borrowings					4,609									22,733	18,865	19,900
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
Expense:
Charitable contribution to foundation					0									34,767	3,603	347
Other non-interest expense					3,780									16,103	14,207	13,986
Income before income tax expense					12,661									33,659	47,080	49,756
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
Other comprehensive (loss) income					(19,230)									(6,487)	5,178	(11,287)
Total comprehensive income (loss), net of tax					(15,552)									16,249	36,250	21,666
Parent Company
Income:
Dividend from subsidiary					0									0	2,000	3,000
Unrealized (losses) gains on securities available for sale					41									123	162	157
Interest earnings					0									2,384	1	0
Total income					41									2,507	2,163	3,157
Borrowings					1,044									3,468	4,177	4,177
Net interest income					(1,003)									(961)	(2,014)	(1,020)
Equity earnings in subsidiary					4,288									51,401	32,230	32,743
Expense:
Loss on securities available for sale					60									0	0	0
Charitable contribution to foundation					0									34,767	0	0
Other non-interest expense					2									425	460	355
Total expense					62									35,192	460	355
Income before income tax expense					3,223									15,248	29,756	31,368
Income tax expense					455									7,488	1,316	1,585
Net income					3,678									22,736	31,072	32,953
Other comprehensive (loss) income					(19,230)									(6,487)	5,178	(11,287)
Total comprehensive income (loss), net of tax					$ (15,552)									$ 16,249	$ 36,250	$ 21,666