Fair Value of Financial Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 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	16	(5)
Payments & Maturities	0	0
Ending Balance	1,443	1,423
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	16	(5)
Payments & Maturities	0	0
Ending Balance	$ 1,443	$ 1,423