Investment Securities (Disclosure Regarding Length of Time on Investment Securities with Temporary Impairment) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Less than 12 months, fair value	$ 216,158	$ 50,800
Less than 12 months, gross unrealized losses	(3,764)	(465)
12 months or longer, fair value	57,408	49,488
12 months or longer, gross unrealized losses	(3,268)	(1,443)
Total, fair value	273,566	100,288
Total, gross unrealized losses	(7,032)	(1,908)
Agency Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than 12 months, fair value	4,828	3,821
Less than 12 months, gross unrealized losses	(160)	(87)
12 months or longer, fair value	0	0
12 months or longer, gross unrealized losses	0	0
Total, fair value	4,828	3,821
Total, gross unrealized losses	(160)	(87)
State And Municipal Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than 12 months, fair value	201,848	37,317
Less than 12 months, gross unrealized losses	(3,398)	(295)
12 months or longer, fair value	57,408	49,488
12 months or longer, gross unrealized losses	(3,268)	(1,443)
Total, fair value	259,256	86,805
Total, gross unrealized losses	(6,666)	(1,738)
Corporate Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than 12 months, fair value	9,482	9,662
Less than 12 months, gross unrealized losses	(206)	(83)
12 months or longer, fair value	0	0
12 months or longer, gross unrealized losses	0	0
Total, fair value	9,482	9,662
Total, gross unrealized losses	$ (206)	$ (83)