Parent Company Only Financial Information - Condensed Statements of Income (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
Dividends from:
Income tax benefit									$ (12,392)	$ (7,171)	$ (8,292)
Net Income	$ 3,451	$ 7,246	$ 6,683	$ 6,480	$ 5,040	$ 6,416	$ 4,065	$ 5,019	23,860	20,540	22,197
Parent Company [Member]
Dividends from:
Bank subsidiary									8,840	8,700	10,404
Non-bank subsidiaries									2,250	1,200	323
Other									220	230	157
Total income									11,310	10,130	10,884
Expenses									(4,957)	(2,611)	(1,708)
Income before income taxes and equity in undistributed net income of subsidiaries:									6,353	7,519	9,176
Income tax benefit									1,614	813	547
Equity in undistributed net income of bank subsidiary									17,430	13,184	11,889
Distributions in excess of net income of non-bank subsidiaries									(1,537)	(976)	585
Net Income									$ 23,860	$ 20,540	$ 22,197