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Investment Securities Available for Sale - Amortized Cost and Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands | Sep. 30, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | ---
Amortized cost | $ 117,577 | $ 142,893 | $ 150,476
Gross unrealized gains | 5,538 | 4,345 | 5,095
Gross unrealized losses | (61) | (718) | (1,237)
Investment securities available for sale, at fair value | 123,054 | 146,520 | 154,334
US Government Agencies Debt Securities [Member] |  |  |
Amortized cost | 10,516 | 21,655 | 23,035
Gross unrealized gains | 359 | 245 | 311
Gross unrealized losses | (12) | (271) | (450)
Investment securities available for sale, at fair value | 10,863 | 21,629 | 22,896
US States and Political Subdivisions Debt Securities [Member] | Taxable [Member] |  |  |
Amortized cost | 1,616 | 1,989 | 2,953
Gross unrealized gains | 186 | 134 | 226
Gross unrealized losses |  |  |
Investment securities available for sale, at fair value | 1,802 | 2,123 | 3,179
US States and Political Subdivisions Debt Securities [Member] | Tax Exempt [Member] |  |  |
Amortized cost | 81,829 | 91,940 | 91,916
Gross unrealized gains | 3,899 | 3,402 | 3,803
Gross unrealized losses | (5) | (175) | (553)
Investment securities available for sale, at fair value | 85,723 | 95,167 | 95,166
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |  |
Amortized cost | 20,939 | 24,480 | 29,150
Gross unrealized gains | 485 | 316 | 475
Gross unrealized losses | (44) | (272) | (234)
Investment securities available for sale, at fair value | 21,380 | 24,524 | 29,391
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |  |
Amortized cost | 1,927 | 2,079 | 2,672
Gross unrealized gains | 141 | 184 | 247
Gross unrealized losses |  |  |
Investment securities available for sale, at fair value | 2,068 | 2,263 | 2,919
Debt Securities [Member] |  |  |
Amortized cost | 116,827 | 142,143 | 149,726
Gross unrealized gains | 5,070 | 4,281 | 5,062
Gross unrealized losses | (61) | (718) | (1,237)
Investment securities available for sale, at fair value | 121,836 | 145,706 | 153,551
Equity Securities In Financial Institutions [Member] |  |  |
Amortized cost | 750 | 750 | 750
Gross unrealized gains | 468 | 64 | 33
Gross unrealized losses |  |  |
Investment securities available for sale, at fair value | $ 1,218 | $ 814 | $ 783

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562 + Details
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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 + Details
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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 + Details
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- Definition This item represents the cost of debt and equity 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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