Fair Value of Financial Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
State and Local Governments Taxable[Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 1,427	$ 1,428
Change in Market Value	78	(18)
Payments & Maturities	0	0
Ending Balance	1,505	1,410
State and Local Governments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,427	1,428
Change in Market Value	78	(18)
Payments & Maturities	0	0
Ending Balance	$ 1,505	$ 1,410