Fair Value - Reconciliation of All Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - Significant Unobservable Inputs (Level 3) [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pooled Trust Preferred Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 8,205	$ 8,097	$ 8,448	$ 8,119
Accretion (amortization) of discount	3	5	13	13
Sales/payments received	(17)	(161)	(322)	(373)
Increase (decrease) in fair value of securities	881	329	933	511
Balance at end of period	9,072	$ 8,270	9,072	$ 8,270
States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	4,028
Accretion (amortization) of discount	(19)		(37)
Increase (decrease) in fair value of securities	(30)		(45)
Transfers into Level 3			4,061
Balance at end of period	$ 3,979		$ 3,979