Condensed Quarterly Statements of Income (Unaudited) - Consolidated Selected Quarterly Statements of Income (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
Income Statement [Abstract]
Interest and dividend income	$ 128,309	$ 117,348	$ 119,543	$ 119,053	$ 121,387	$ 108,393	$ 98,888	$ 86,417	$ 484,253	$ 414,957
Interest expense	21,345	21,228	21,083	20,692	19,620	18,460	16,541	13,125	84,349	67,721
Net interest income	106,964	96,120	98,460	98,361	101,767	89,933	82,347	73,292	399,904	347,236
Provision for credit losses	1,824	4,121	2,747	2,507	2,854	1,035	1,708	2,168	11,198	7,764	$ 9,986
Net interest income after provision for credit losses	105,140	91,999	95,713	95,854	98,913	88,898	80,639	71,124	388,706	339,472
Non-interest income	30,318	26,715	28,247	27,116	27,864	26,140	23,050	24,019	112,396	101,176
Net securities (losses) gains	520	235	2,909	657	(1,303)	84	358	(39)	4,320	(900)
Non-interest expense	92,556	73,268	71,952	74,432	70,990	76,120	63,543	54,571	312,208	265,224	220,860
Income before provision for income taxes	43,422	45,681	54,917	49,195	54,484	39,002	40,504	40,533	193,214	174,524	148,289
Provision for income taxes	7,046	8,334	10,103	8,858	10,556	6,516	7,335	7,004	34,341	31,412	53,807
Net income	$ 36,376	$ 37,347	$ 44,814	$ 40,337	$ 43,928	$ 32,486	$ 33,169	$ 33,529	$ 158,873	$ 143,112	$ 94,482
Earnings per common share—basic	$ 0.60	$ 0.68	$ 0.82	$ 0.74	$ 0.80	$ 0.65	$ 0.71	$ 0.76	$ 2.83	$ 2.93	$ 2.15
Earnings per common share—diluted	$ 0.60	$ 0.68	$ 0.82	$ 0.74	$ 0.80	$ 0.64	$ 0.71	$ 0.76	$ 2.83	$ 2.92	$ 2.14