Securities - Amortized Cost and Fair Value of Securities Available for Sale and Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized cost within one year	$ 13,347	$ 7,455
Amortized cost one to five years	40,468	37,483
Amortized cost for five to ten years	50,473	21,112
Amortized cost for after ten years	63,306	60,566
Total amortized cost	167,594	126,616
Total available for sale investment securities, Amortized Cost	513,602	445,839
Within one year, amortized cost	1,948
One to five years, amortized cost	40,603	24,594
Five to ten years, amortized cost	89,801	87,645
After ten years, amortized cost	47,484	53,368
Total amortized cost	179,836	165,607
Total held to maturity investment securities, amortized cost	200,448	193,194
Fair value within one year	13,326	7,480
Fair value for one to five years	40,193	37,479
Fair value for five to ten years	51,156	20,984
Fair value for after ten years	64,326	59,967
Total fair value	169,001	125,910
Investment securities, available for sale	509,665	439,831
Within one year, fair value	1,934
One to five years, fair value	41,531	25,271
five to ten years, fair value	91,249	88,805
After ten years, fair value	45,683	51,746
Total fair value	180,397	165,822
Held-to-maturity, Fair Value	201,085	194,086
Federal Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	132,871	139,040
Total held to maturity investment securities, amortized cost	5,734	6,530
Investment securities, available for sale	130,365	137,195
Held-to-maturity, Fair Value	5,682	6,490
Federal Agency Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	211,487	180,183
Total held to maturity investment securities, amortized cost	14,878	21,057
Investment securities, available for sale	208,657	176,726
Held-to-maturity, Fair Value	15,006	$ 21,774
Private Labeled Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	1,650
Investment securities, available for sale	$ 1,642

Available for sale securities debt maturities rolling fair value.

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Held to maturity securities debt maturities amortized cost.

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Held To Maturity Securities Debt Maturities Fair Value Aggregate

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the next rolling twelve months following the latest balance sheet. 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. For 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 rolling twelve months following the latest balance sheet. 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. For 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 rolling twelve months following the latest balance sheet. For 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 sixth through tenth rolling twelve months following the latest balance sheet. 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. For 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 in the sixth through tenth rolling twelve months following the latest balance sheet. For 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 through fifth rolling twelve months following the latest balance sheet. 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. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Fair value of held-to-maturity debt securities maturing in the next rolling twelve months following the latest balance sheet. For 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 held-to-maturity debt securities maturing after the tenth rolling twelve months following the latest balance sheet. For 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 investments in debt securities classified as held-to-maturity.

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Amortized cost of held-to-maturity debt securities maturing in the next rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amortized cost of held-to-maturity debt securities maturing after the tenth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amortized cost of held-to-maturity debt securities maturing in the sixth through tenth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amortized cost of held-to-maturity debt securities maturing in the second through fifth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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