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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 72,358	$ 71,382	$ 70,078	$ 62,837	$ 47,906	$ 48,030	$ 46,879	$ 46,014	$ 276,654	$ 188,829	$ 133,425
Interest expense	10,517	9,878	8,631	7,126	5,401	4,974	4,705	4,531	36,152	19,611	13,163
Net interest income	61,841	61,504	61,447	55,711	42,505	43,056	42,174	41,483	240,502	169,218	120,262
Provision for loan losses	506	907	706	1,371	1,415	1,165	1,165	700	3,490	4,445	2,623
Net interest income after provision for loan losses	61,335	60,597	60,741	54,340	41,090	41,891	41,009	40,783	237,012	164,773	117,639
Other income	8,748	8,285	8,883	8,910	6,745	7,359	6,973	5,995
Operating expenses (excluding merger related expenses)	37,794	37,503	42,470	38,508	26,434	27,580	28,527	29,481	186,337	126,520	102,852
Merger related expenses	1,288	2,030	8,434	18,310	1,259	3,153	8,606	1,480	26,911	8,293	16,534
Income before provision for income taxes	31,001	29,439	18,720	6,432	20,142	18,517	10,849	15,817
Provision for income taxes	4,269	5,278	3,018	1,005	10,186	5,700	3,170	3,799	13,570	22,855	12,153
Net income	$ 26,732	$ 24,071	$ 15,702	$ 5,427	$ 9,956	$ 12,817	$ 7,679	$ 12,018	$ 71,932	$ 42,470	$ 23,046
Basic earnings per share (in dollars per share)	$ 0.56	$ 0.50	$ 0.33	$ 0.12	$ 0.31	$ 0.40	$ 0.24	$ 0.38	$ 1.54	$ 1.32	$ 1.00
Diluted earnings per share (in dollars per share)	$ 0.55	$ 0.50	$ 0.32	$ 0.12	$ 0.30	$ 0.39	$ 0.23	$ 0.36	$ 1.51	$ 1.28	$ 0.98