Selected Consolidated Quarterly Financial Data - Summary of Consolidated Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 77,075	$ 76,887	$ 78,410	$ 76,422	$ 72,358	$ 71,382	$ 70,078	$ 62,837	$ 308,794	$ 276,654	$ 188,829
Interest expense	13,721	13,495	13,573	12,034	10,517	9,878	8,631	7,126	52,823	36,152	19,611
Net interest income	63,354	63,392	64,837	64,388	61,841	61,504	61,447	55,711	255,971	240,502	169,218
Provision for loan losses	355	305	356	620	506	907	706	1,371	1,636	3,490	4,445
Net interest income after provision for loan losses	62,999	63,087	64,481	63,768	61,335	60,597	60,741	54,340	254,335	237,012	164,773
Other income	11,231	11,543	9,879	9,512	8,748	8,285	8,883	8,910	42,165	34,827	27,072
Operating expenses (excluding merger related and branch consolidation expenses)	43,589	40,884	43,289	41,827	37,794	37,503	42,470	38,508
Merger related and branch consolidation expenses	4,010	2,473	7,626	5,444	1,288	2,030	8,434	18,310
Income before provision for income taxes	26,631	31,273	23,445	26,009	31,001	29,349	18,720	6,432	107,358	85,502	65,325
Provision for income taxes	3,181	6,302	4,465	4,836	4,269	5,278	3,018	1,005	18,784	13,570	22,855
Net income	$ 23,450	$ 24,971	$ 18,980	$ 21,173	$ 26,732	$ 24,071	$ 15,702	$ 5,427	$ 88,574	$ 71,932	$ 42,470
Basic earnings per share (in dollars per share)	$ 0.47	$ 0.50	$ 0.37	$ 0.43	$ 0.56	$ 0.50	$ 0.33	$ 0.12	$ 1.77	$ 1.54	$ 1.32
Diluted earnings per share (in dollars per share)	$ 0.47	$ 0.49	$ 0.37	$ 0.42	$ 0.55	$ 0.50	$ 0.32	$ 0.12	$ 1.75	$ 1.51	$ 1.28