Parent-only Financial Information - Statement of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended													12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	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	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2017
Income:
Loans receivable									$ 43,043					$ 217,774	$ 189,869	$ 164,849
Unrealized gain (loss) on debt securities available for sale									(3,131)					21,067	(5,778)	(11,498)
Interest expense on borrowings									4,609					27,161	22,733	18,865
Net interest income	$ 47,428	$ 41,716	$ 40,843	$ 42,384	$ 43,375	$ 40,583	$ 41,015	$ 39,061	36,929	$ 35,960	$ 35,639	$ 34,777	$ 33,405	172,371	164,034	139,780
Non-interest income:
Gain (loss) on securities transactions									(60)					2,612	116	(1,689)
Change in fair value of equity securities									0					305	0	0
Other non-interest income									1,162					5,922	4,943	4,497
Total non-interest income	8,709	10,115	6,775	6,037	6,405	5,290	5,450	4,543	4,733	1,630	4,645	5,806	5,534	31,636	21,688	17,172
Non-interest expense
Charitable contribution to foundation									0					0	34,767	3,603
Merger-related expenses									0					2,755	0	0
Other non-interest expense									3,780					11,769	5,597	14,207
Total non-interest expense	36,237	31,064	31,841	29,559	31,013	26,590	61,768	26,015	25,601	30,206	24,703	24,903	24,078	128,701	145,386	103,446
Income before income tax expense	17,381	19,610	15,665	18,426	17,990	17,783	(17,703)	15,589	12,661	1,708	15,206	15,305	14,861	71,082	33,659	47,080
Income tax benefit	(3,832)	(5,392)	(3,634)	(3,507)	(3,123)	(6,956)	2,961	(3,805)	(8,983)	(194)	(5,934)	(5,012)	(4,868)	(16,365)	(10,923)	(16,008)
Net income	$ 13,549	$ 14,218	$ 12,031	$ 14,919	14,867	$ 10,827	$ (14,742)	$ 11,784	3,678	$ 1,514	$ 9,272	$ 10,293	$ 9,993	54,717	22,736	31,072
Other comprehensive income									(19,230)					3,710	(6,487)	5,178
Total comprehensive income (loss), net of tax									(15,552)					58,427	16,249	36,250
Parent Company
Income:
Dividends from subsidiary									0					179,000	0	2,000
Loans receivable									0					2,111	1,513	0
Unrealized gain (loss) on debt securities available for sale									41					51	123	162
Interest-earning deposits									0					173	871	1
Total interest income									41					181,335	2,507	2,163
Interest expense on borrowings									1,044					176	3,468	4,177
Net interest income									(1,003)					181,159	(961)	(2,014)
Equity earnings (loss) in subsidiary									4,288					(123,142)	51,401	32,230
Non-interest income:
Gain (loss) on securities transactions									(60)					236	0	0
Change in fair value of equity securities									0					65	0	0
Other non-interest income									0					139	0	0
Total non-interest income									(60)					440	0	0
Non-interest expense
Charitable contribution to foundation									0					0	34,767	0
Merger-related expenses					$ 0				0					1,807		0
Other non-interest expense									2					1,955	425	460
Total non-interest expense									2					3,762	35,192	460
Income before income tax expense									3,223					54,695	15,248	29,756
Income tax benefit									455					22	7,488	1,316
Net income									3,678					54,717	22,736	31,072
Other comprehensive income									(19,230)					3,710	(6,487)	5,178
Total comprehensive income (loss), net of tax									$ (15,552)					$ 58,427	$ 16,249	$ 36,250