Parent Company Only Financial Information - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net income	$ 3,451	$ 7,246	$ 6,683	$ 6,480	$ 5,040	$ 6,416	$ 4,065	$ 5,019	$ 23,860	$ 20,540	$ 22,197
Adjustments to reconcile net income to net cash provided by operating activities:
Net unrealized gains on trading securities									(881)	(503)	213
Decrease (increase) in other assets									(1,175)	(3,906)	93
(Decrease) increase in other liabilities									698	2,095	5,435
Net Cash Provided By Operating Activities									30,763	26,764	33,636
Cash flows from investing activities
Net cash used in investing activities									(203,651)	(150,883)	(100,714)
Cash Flows From Financing Activities:
Dividends paid									(10,094)	(9,546)	(9,515)
Proceeds from issuance of long term debt									160,000	80,000	50,000
Purchase of treasury stock									(1,877)	(44)	(868)
Net proceeds from issuance of common stock									19,294	0	0
Net Cash Provided By Financing Activities									179,050	126,041	66,411
Net increase (decrease) in cash									6,162	1,922	(667)
Cash and Cash Equivalents, Beginning				29,183				27,261	29,183	27,261	27,928
Cash and Cash Equivalents, Ending	35,345				29,183				35,345	29,183	27,261
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									23,860	20,540	22,197
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed net income of bank subsidiary									(17,430)	(13,184)	(11,889)
Equity in undistributed net income (distributions in excess of net income) of non-bank subsidiaries									1,537	976	(585)
Net unrealized gains on trading securities									(12)	(58)	(11)
Decrease (increase) in other assets									(549)	(1,277)	9
(Decrease) increase in other liabilities									791	1,645	663
Net Cash Provided By Operating Activities									8,197	8,642	10,384
Cash flows from investing activities
Investment in bank subsidiaries									(15,400)	(49,223)	0
Net cash used in investing activities									(15,400)	(49,223)	0
Cash Flows From Financing Activities:
Dividends paid									(10,094)	(9,546)	(9,515)
Proceeds from issuance of long term debt									0	50,000	0
Purchase of treasury stock									(1,877)	(44)	(868)
Net proceeds from issuance of common stock									19,294	0	0
Net advance from subsidiary									0	50	0
Net Cash Provided By Financing Activities									7,323	40,460	(10,383)
Net increase (decrease) in cash									120	(121)	1
Cash and Cash Equivalents, Beginning				$ 1,436				$ 1,557	1,436	1,557	1,556
Cash and Cash Equivalents, Ending	$ 1,556				$ 1,436				$ 1,556	$ 1,436	$ 1,557