Securities - Maturities of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized cost of debt securities available for sale, due within one year	$ 2,520
Amortized cost of debt securities available for sale, due in one to five years	28,341
Amortized cost of debt securities available for sale, due in five to ten years	13,137
Amortized cost of debt securities available for sale, due after ten years	6,895
Total amortized cost debt securities available for sale with specific maturities	50,893
Total unrestricted securities available-for-sale at amortized cost	163,703	$ 184,940
Estimated fair value of debt securities available for sale, due within one year	2,522
Estimated fair value of debt securities available for sale, due in one to five years	27,677
Estimated fair value of debt securities available for sale, due in five to ten years	12,734
Estimated fair value of debt securities available for sale, due after ten years	6,822
Total estimated fair value of debt securities available for sale with specific maturities	49,755
Total unrestricted securities available for sale at estimated fair value	159,282	184,791
Amortizing Agency Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost of debt securities available for sale without specific maturities	48,110
Total estimated fair value of debt securities available for sale without specific maturities	46,909
Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost of debt securities available for sale without specific maturities	64,700
Total unrestricted securities available-for-sale at amortized cost	64,700	71,389
Total estimated fair value of debt securities available for sale without specific maturities	62,618
Total unrestricted securities available for sale at estimated fair value	$ 62,618	$ 70,764