Selected Quarterly Financial Data (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 74,349	$ 73,013	$ 72,188	$ 72,231	$ 73,800	$ 73,652	$ 67,386	$ 64,523	$ 291,781	$ 279,361	$ 252,777
Interest expense	10,618	10,466	10,514	10,303	10,482	10,683	9,985	9,322	41,901	40,472	36,767
Net interest income	63,731	62,547	61,674	61,928	63,318	62,969	57,401	55,201	249,880	238,889	216,010
Provision for loan losses	1,250	1,400	1,100	600	1,250	1,500	1,500	400	4,350	4,650	5,500
Net interest income after provision for loan losses	62,481	61,147	60,574	61,328	62,068	61,469	55,901	54,801	245,530	234,239	210,510
Non-interest income	15,867	12,110	16,942	10,303	11,416	11,309	10,327	8,116	55,222	41,168	44,153
Non-interest expense	47,419	43,614	46,119	43,437	42,297	45,833	43,671	38,190	180,589	169,991	148,763
Income before income tax expense	30,929	29,643	31,397	28,194	31,187	26,945	22,557	24,727	120,163	105,416	105,900
Income tax expense	9,414	9,034	9,601	8,392	9,968	7,913	6,206	7,698	36,441	31,785	35,366
Net income	$ 21,515	$ 20,609	$ 21,796	$ 19,802	$ 21,219	$ 19,032	$ 16,351	$ 17,029	$ 83,722	$ 73,631	$ 70,534
Basic earnings per share (in dollars per share)	$ 0.34	$ 0.33	$ 0.35	$ 0.32	$ 0.34	$ 0.30	$ 0.28	$ 0.30	$ 1.33	$ 1.22	$ 1.23
Diluted earnings per share (in dollars per share)	$ 0.34	$ 0.33	$ 0.35	$ 0.32	$ 0.34	$ 0.30	$ 0.28	$ 0.30	$ 1.33	$ 1.22	$ 1.23