Business Segments - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net interest income	$ 71,739	$ 63,161	$ 207,655	$ 191,633
(Recapture of) provision for loan losses	(1,500)	(2,000)	(7,000)	(2,000)
Net interest income after (recapture of) provision for loan losses	73,239	65,161	214,655	193,633
Noninterest income	10,038	9,183	29,536	27,140
Noninterest expense	34,706	33,006	105,696	101,808
Earnings before income taxes	48,571	41,338	138,495	118,965
Goodwill	116,564	88,174	116,564	88,174	$ 89,533
Segment assets	8,304,012	8,044,993	8,304,012	8,044,993	$ 8,073,707
Operating Segments [Member] | Centers [Member]
Segment Reporting Information [Line Items]
Net interest income	50,539	45,499	144,879	132,316
(Recapture of) provision for loan losses	772	(1,164)	2,158	2,251
Net interest income after (recapture of) provision for loan losses	49,767	46,663	142,721	130,065
Noninterest income	5,764	5,182	16,274	15,335
Noninterest expense	12,963	12,423	38,607	37,924
Earnings before income taxes	42,568	39,422	120,388	107,476
Goodwill	116,564	88,174	116,564	88,174
Segment assets	7,166,356	6,945,214	7,166,356	6,945,214
Operating Segments [Member] | Dairy & Livestock and Agribusiness [Member]
Segment Reporting Information [Line Items]
Net interest income	2,728	2,008	7,241	5,927
(Recapture of) provision for loan losses	(66)	1,089	(3,186)	(264)
Net interest income after (recapture of) provision for loan losses	2,794	919	10,427	6,191
Noninterest income	84	73	188	180
Noninterest expense	479	474	1,484	1,452
Earnings before income taxes	2,399	518	9,131	4,919
Goodwill	0	0	0	0
Segment assets	377,670	376,862	377,670	376,862
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net interest income	18,472	15,654	55,535	53,390
(Recapture of) provision for loan losses	(2,206)	(1,925)	(5,972)	(3,987)
Net interest income after (recapture of) provision for loan losses	20,678	17,579	61,507	57,377
Noninterest income	4,190	3,928	13,074	11,625
Noninterest expense	21,264	20,109	65,605	62,432
Earnings before income taxes	3,604	1,398	8,976	6,570
Goodwill	0	0	0	0
Segment assets	$ 759,986	$ 722,917	$ 759,986	$ 722,917