Securities - Maturities of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of debt securities available for sale, due within one year	$ 3,103
Amortized cost of debt securities available for sale, due in one to five years	29,802
Amortized cost of debt securities available for sale, due in five to ten years	16,013
Amortized cost of debt securities available for sale, due after ten years	7,226
Total amortized cost debt securities available for sale with specific maturities	56,144
Total unrestricted securities available-for-sale at amortized cost	182,379	$ 184,940
Estimated fair value of debt securities available for sale, due within one year	3,123
Estimated fair value of debt securities available for sale, due in one to five years	29,394
Estimated fair value of debt securities available for sale, due in five to ten years	15,885
Estimated fair value of debt securities available for sale, due after ten years	7,643
Total estimated fair value of debt securities available for sale with specific maturities	56,045
Total unrestricted securities available for sale at estimated fair value	180,212	184,791
Amortizing Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost of debt securities available for sale without specific maturities	54,463
Total estimated fair value of debt securities available for sale without specific maturities	54,022
Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost of debt securities available for sale without specific maturities	71,772
Total unrestricted securities available-for-sale at amortized cost	71,772	71,389
Total estimated fair value of debt securities available for sale without specific maturities	70,145
Total unrestricted securities available for sale at estimated fair value	$ 70,145	$ 70,764

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing after the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of investment in available-for-sale debt securities with a single maturity date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the next fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of available-for-sale debt securities at cost, net of adjustments, that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the cost over several maturity groupings. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.

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