Selected Quarterly Financial Data (Unaudited) (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	$ 90,385	$ 93,026	$ 95,648	$ 92,411	$ 93,922	$ 91,261	$ 88,315	$ 86,331	$ 371,470	$ 359,829	$ 323,846
Interest expense	17,502	19,498	19,093	17,404	16,589	15,475	14,035	13,054	73,497	59,153	45,644
Net interest income	72,883	73,528	76,555	75,007	77,333	75,786	74,280	73,277	297,973	300,676	278,202
Provision for loan losses	2,900	500	9,500	200	1,800	1,000	15,500	5,400	13,100	23,700	5,600
Net interest income after provision for loan losses	69,983	73,028	67,055	74,807	75,533	74,786	58,780	67,877	284,873	276,976	272,602
Non-interest income	17,725	18,047	15,834	12,188	15,616	15,916	13,837	13,307	63,794	58,676	55,697
Non-interest expense	53,731	49,738	49,694	48,416	49,360	46,659	48,806	46,910	201,579	191,735	187,822
Income before income tax expense	33,977	41,337	33,195	38,579	41,789	44,043	23,811	34,274	147,088	143,917	140,477
Income tax expense	8,026	9,938	8,802	7,689	6,026	8,575	4,568	6,361	34,455	25,530	46,528
Net income	$ 25,951	$ 31,399	$ 24,393	$ 30,890	$ 35,763	$ 35,468	$ 19,243	$ 27,913	$ 112,633	$ 118,387	$ 93,949
Basic earnings per share (in dollars per share)	$ 0.40	$ 0.49	$ 0.38	$ 0.48	$ 0.55	$ 0.55	$ 0.30	$ 0.43	$ 1.74	$ 1.82	$ 1.46
Diluted earnings per share (in dollars per share)	$ 0.40	$ 0.49	$ 0.38	$ 0.48	$ 0.55	$ 0.54	$ 0.30	$ 0.43	$ 1.74	$ 1.82	$ 1.45