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Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Investment Securities [Line Items] |  |  |
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis |  | $ 997 |
Amortized Cost |  | 784,875 | $ 790,082
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax |  | 7,051 | 7,076
Gross Unrealized Losses |  | (13,282) | (18,546)
Securities available for sale, at fair value |  | 778,644 | 778,612
Due within one year, Estimated Fair Value |  | 997 |
Due after one but within five years, Amortized Cost |  | 14,088 |
Due after one but within five years, Estimated Fair Value |  | 14,214 |
Due after five but within ten years, Amortized Cost |  | 27,081 |
Due after five but within ten years, Estimated Fair Value |  | 27,515 |
Due after ten years, Amortized Cost |  | 42,509 |
Due after ten years, Estimated Fair Value |  | 37,092 |
Due within one year, Amortized Cost |  | 84,675 |
Available-for-sale Securities, Debt Maturities, Single Maturity Date |  | 79,818 |
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis | [1] | 698,530 |
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value | [1] | 697,156 |
US Government Agencies Residential Mortgage-backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 35,700 |
Securities available for sale, at fair value |  | 36,385 |
Residential Mortgage Backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 660,771 | 676,853
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 0 | 1
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 1,097 | 16,631
Obligations of States and Political Subdivisions [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 27,514 | 27,229
Corporate Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 16,578 | 6,319
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 34,629 | 33,292
Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 783,205 |
Securities available for sale, at fair value |  | 776,974 |
US Government Agency Debt Securities Commercial [Domain] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 24,684 | 0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax |  | 0 | 0
Gross Unrealized Losses |  | (217) | 0
Securities available for sale, at fair value |  | 24,467 | 0
Equity Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 1,670 | 1,670
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax |  | 0 | 0
Gross Unrealized Losses |  | 0 | 0
Securities available for sale, at fair value |  | 1,670 | 1,670
Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 783,205 | 788,412
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax |  | 7,051 | 7,076
Gross Unrealized Losses |  | (13,282) | (18,546)
Securities available for sale, at fair value |  | 776,974 | 776,942
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 40,594 | 39,989
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax |  | 516 | 427
Gross Unrealized Losses |  | (6,481) | (7,124)
Securities available for sale, at fair value |  | 34,629 | 33,292
Corporate Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 15,903 | 5,903
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax |  | 675 | 416
Gross Unrealized Losses |  | 0 | 0
Securities available for sale, at fair value |  | 16,578 | 6,319
Obligations of States and Political Subdivisions [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 27,081 | 27,075
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax |  | 433 | 195
Gross Unrealized Losses |  | 0 | (41)
Securities available for sale, at fair value |  | 27,514 | 27,229
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 1,097 | 16,700
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax |  | 0 | 0
Gross Unrealized Losses |  | 0 | (69)
Securities available for sale, at fair value |  | 1,097 | 16,631
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 0 | 1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax |  | 0 | 0
Gross Unrealized Losses |  | 0 | 0
Securities available for sale, at fair value |  | 0 | 1
Residential Mortgage Backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 662,858 | 683,601
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax |  | 4,497 | 4,557
Gross Unrealized Losses |  | (6,584) | (11,305)
Securities available for sale, at fair value |  | 660,771 | 676,853
US Government Agencies Residential Mortgage-backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 10,988 | 15,143
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax |  | 930 | 1,481
Gross Unrealized Losses |  | 0 | (7)
Securities available for sale, at fair value |  | $ 11,918 | $ 16,617
[1] | Mortgage-Backed Securities include an amortized cost of $35.7 million and a fair value of $36.4 million for Obligations of U.S.

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt 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 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 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. + 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27198-111563 + Details
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