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Investment Securities Available for Sale - Gross Unrealized Losses and Fair Value (Detail) - USD ($) $ in Thousands | Sep. 30, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | ---
Less than Twelve Months, Fair Value | $ 2,683 | $ 13,811 | $ 2,406
Less than Twelve Months, Gross Unrealized Losses | 0 | (152) | (10)
Twelve Months or Greater, Fair Value | 6,174 | 26,951 | 50,740
Twelve Months or Greater, Gross Unrealized Losses | (61) | (566) | (1,227)
Total, Fair Value | 8,857 | 40,762 | 53,146
Total, Gross Unrealized Losses | (61) | (718) | (1,237)
US Government Agencies Debt Securities [Member] |  |  |
Less than Twelve Months, Fair Value | 2,000 | 3,818 |
Less than Twelve Months, Gross Unrealized Losses | 0 | (57) |
Twelve Months or Greater, Fair Value | 1,465 | 10,872 | 15,734
Twelve Months or Greater, Gross Unrealized Losses | (12) | (214) | (450)
Total, Fair Value | 3,465 | 14,690 | 15,734
Total, Gross Unrealized Losses | (12) | (271) | (450)
US States and Political Subdivisions Debt Securities [Member] | Tax Exempt [Member] |  |  |
Less than Twelve Months, Fair Value | 608 | 1,268 | 2,406
Less than Twelve Months, Gross Unrealized Losses | 0 | (9) | (10)
Twelve Months or Greater, Fair Value | 0 | 9,394 | 18,232
Twelve Months or Greater, Gross Unrealized Losses | (5) | (166) | (543)
Total, Fair Value | 608 | 10,662 | 20,638
Total, Gross Unrealized Losses | (5) | (175) | (553)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |  |
Less than Twelve Months, Fair Value | 0 | 8,725 |
Less than Twelve Months, Gross Unrealized Losses | 0 | (86) |
Twelve Months or Greater, Fair Value | 4,709 | 6,685 | 16,774
Twelve Months or Greater, Gross Unrealized Losses | (44) | (186) | (234)
Total, Fair Value | 4,709 | 15,410 | 16,774
Total, Gross Unrealized Losses | (44) | $ (272) | $ (234)
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |  |
Less than Twelve Months, Fair Value | 75 |  |
Less than Twelve Months, Gross Unrealized Losses | 0 |  |
Twelve Months or Greater, Fair Value | 0 |  |
Twelve Months or Greater, Gross Unrealized Losses | 0 |  |
Total, Fair Value | 75 |  |
Total, Gross Unrealized Losses | $ 0 |  |

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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563 + Details
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