Parent Company Only Financial Information - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net income	$ 8,945	$ 9,236	$ 8,441	$ 7,097	$ 3,451	$ 7,246	$ 6,683	$ 6,480	$ 33,719	$ 23,860	$ 20,540
Adjustments to reconcile net income to net cash provided by operating activities:
Decrease (increase) in other assets									(3,079)	(1,175)	(3,906)
Increase in other liabilities									4,613	698	2,095
Net Cash Provided By Operating Activities									43,768	30,763	26,764
Cash flows from investing activities
Investment in non-bank subsidiaries									0	0	(2,866)
Net Cash Used In Investing Activities									(451,588)	(203,651)	(150,883)
Cash flows from financing activities:
Dividends paid									(10,237)	(10,094)	(9,546)
Proceeds from issuance of long term debt									50,000	160,000	80,000
Purchase of treasury stock									(2,454)	(1,877)	(44)
Net proceeds from issuance of common stock									0	19,294	0
Net Cash Provided By Financing Activities									418,038	179,050	126,041
Net Increase in Cash and Cash Equivalents									10,218	6,162	1,922
Cash and Cash Equivalents, Beginning				35,345				29,183	35,345	29,183	27,261
Cash and Cash Equivalents, Ending	45,563				35,345				45,563	35,345	29,183
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income									33,719	23,860	20,540
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed net income of bank subsidiary									(24,031)	(17,430)	(13,184)
Distributions in excess of net income of non–bank subsidiaries									1,485	1,537	976
Net unrealized gains on trading securities									0	(12)	(58)
Decrease (increase) in other assets									872	(549)	(1,277)
Increase in other liabilities									1,045	791	1,645
Net Cash Provided By Operating Activities									13,090	8,197	8,642
Cash flows from investing activities
Investment in non-bank subsidiaries									(400)	0	0
Investment in bank subsidiaries									0	(15,400)	(49,223)
Net Cash Used In Investing Activities									(400)	(15,400)	(49,223)
Cash flows from financing activities:
Dividends paid									(10,237)	(10,094)	(9,546)
Proceeds from issuance of long term debt									0	0	50,000
Purchase of treasury stock									(2,454)	(1,877)	(44)
Net proceeds from issuance of common stock									0	19,294	0
Net advance (to) from subsidiary									(1,200)	0	50
Net Cash Provided By Financing Activities									(13,891)	7,323	40,460
Net Increase in Cash and Cash Equivalents									(1,201)	120	(121)
Cash and Cash Equivalents, Beginning				$ 1,556				$ 1,436	1,556	1,436	1,557
Cash and Cash Equivalents, Ending	$ 355				$ 1,556				$ 355	$ 1,556	$ 1,436