Parent Company Only Financial Information - Condensed Statements of Cash Flows (Detail) - USD ($) $ 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
Condensed Financial Statements, Captions [Line Items]
Net income	$ 10,484	$ 10,357	$ 9,767	$ 9,473	$ 8,945	$ 9,236	$ 8,441	$ 7,097	$ 40,081	$ 33,719	$ 23,860
Adjustments to reconcile net income to net cash provided by operating activities:
Net unrealized gains on trading securities									(1,888)	451	(881)
Decrease (increase) in other assets									1,072	(3,079)	(1,175)
Increase in other liabilities									3,610	4,613	698
Net Cash Provided By Operating Activities									51,907	43,768	30,763
Cash flows from investing activities
Net Cash Used In Investing Activities									(368,601)	(451,588)	(203,651)
Cash flows from financing activities:
Dividends paid									(10,358)	(10,237)	(10,094)
Purchase of treasury stock									(1,291)	(2,454)	(1,877)
Net proceeds from issuance of common stock									1,423	0	19,294
Net Cash Provided By Financing Activities									464,105	418,038	179,050
Net Increase in Cash and Cash Equivalents									147,411	10,218	6,162
Cash and Cash Equivalents, Beginning				45,563				35,345	45,563	35,345	29,183
Cash and Cash Equivalents, Ending	192,974				45,563				192,974	45,563	35,345
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income									40,081	33,719	23,860
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed net income of bank subsidiary									(27,580)	(24,031)	(17,430)
(Equity in undistributed) distributions in excess of net income of non–bank subsidiaries									(3,914)	1,485	1,537
Net unrealized gains on trading securities									(24)	0	(12)
Decrease (increase) in other assets									177	872	(549)
Increase in other liabilities									1,267	1,089	830
Net Cash Provided By Operating Activities									10,007	13,134	8,236
Cash flows from investing activities
Purchase of trading securities									(36)	(44)	(39)
Investment in non-bank subsidiaries									0	(400)	0
Investment in bank subsidiaries									0	0	(15,400)
Net Cash Used In Investing Activities									(36)	(444)	(15,439)
Cash flows from financing activities:
Dividends paid									(10,358)	(10,237)	(10,094)
Purchase of treasury stock									(1,291)	(2,454)	(1,877)
Net proceeds from issuance of common stock									1,423	0	19,294
Net advance (to) from subsidiary									650	(1,200)	0
Net Cash Provided By Financing Activities									(9,576)	(13,891)	7,323
Net Increase in Cash and Cash Equivalents									395	(1,201)	120
Cash and Cash Equivalents, Beginning				$ 355				$ 1,556	355	1,556	1,436
Cash and Cash Equivalents, Ending	$ 750				$ 355				$ 750	$ 355	$ 1,556