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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Interest and dividend income	$ 80,819	$ 70,092	$ 67,585	$ 67,601	$ 67,660	$ 66,935	$ 66,729	$ 60,379	$ 286,097	$ 261,712	$ 215,991
Interest expense	9,131	8,066	7,811	7,759	7,040	6,326	5,936	5,424	32,767	24,725	22,763
Net interest income	71,688	62,026	59,774	59,842	60,620	60,609	60,793	54,955	253,330	236,987	193,228
Provision for credit losses	2,128	2,214	1,811	2,324	2,585	1,798	2,681	1,289	8,478	8,353	6,405
Net interest income after provision for credit losses	69,560	59,812	57,963	57,518	58,035	58,811	58,112	53,666	244,852	228,634	186,823
Non-interest income	21,357	20,419	19,006	18,282	19,146	18,139	18,072	18,168	79,142	73,518
Net securities gains	63	598	585	1,111	880	47		22	2,357	948	903
Non-interest expense	58,298	57,601	47,360	45,343	46,894	46,981	46,589	53,441	208,680	193,923	161,633
Income before provision for income taxes	32,682	23,228	30,194	31,568	31,167	30,016	29,595	18,415	117,671	109,177	93,694
Provision for income taxes	8,464	5,793	8,085	8,694	8,165	7,768	7,962	4,528	31,036	28,415	23,720
Net income	$ 24,218	$ 17,435	$ 22,109	$ 22,874	$ 23,002	$ 22,248	$ 21,633	$ 13,887	$ 86,635	$ 80,762	$ 69,974
Earnings per common share - basic	$ 0.55	$ 0.44	$ 0.58	$ 0.60	$ 0.60	$ 0.58	$ 0.56	$ 0.40	$ 2.16	$ 2.15	$ 2.39
Earnings per common share - diluted	$ 0.55	$ 0.44	$ 0.58	$ 0.60	$ 0.60	$ 0.58	$ 0.56	$ 0.40	$ 2.16	$ 2.15	$ 2.39