Fair Value - Reconciliation of All Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3			$ 4,000
Transfers into Level 3				$ 4,100
Significant Unobservable Inputs (Level 3) [Member] | Pooled Trust Preferred Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 7,836	$ 8,205	8,495	8,448
Accretion (amortization) of discount	3	3	9	13
Sales/payments received	(14)	(17)	(46)	(322)
Increase (decrease) in fair value of securities	(130)	881	(763)	933
Balance at end of period	7,695	9,072	7,695	9,072
Significant Unobservable Inputs (Level 3) [Member] | States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	40	4,028	4,108
Accretion (amortization) of discount		(19)		(37)
Sales/payments received			(35)
Increase (decrease) in fair value of securities		(30)		(45)
Transfers out of Level 3			(4,033)
Transfers into Level 3				4,061
Balance at end of period	$ 40	$ 3,979	$ 40	$ 3,979