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Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Investment Securities [Line Items] |  |
Amortized Cost | $ 740,846 | $ 790,082
Gross Unrealized Gains | 4,854 | 7,076
Gross Unrealized Losses | (14,342) | (18,546)
Estimated Fair Value of Available for Sale | 731,358 | 778,612
US States and Political Subdivisions Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 27,083 | 27,075
Gross Unrealized Gains | 327 | 195
Gross Unrealized Losses | 0 | (41)
Estimated Fair Value of Available for Sale | 27,410 | 27,229
Corporate Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 15,907 | 5,903
Gross Unrealized Gains | 590 | 416
Gross Unrealized Losses | (4) | 0
Estimated Fair Value of Available for Sale | 16,493 | 6,319
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 27,499 | 39,989
Gross Unrealized Gains | 526 | 427
Gross Unrealized Losses | (4,379) | (7,124)
Estimated Fair Value of Available for Sale | 23,646 | 33,292
Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 739,176 | 788,412
Gross Unrealized Gains | 4,854 | 7,076
Gross Unrealized Losses | (14,342) | (18,546)
Estimated Fair Value of Available for Sale | 729,688 | 776,942
Mortgage - Backed Securities - Residential [Member] | US Government Agencies Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 10,556 | 15,143
Gross Unrealized Gains | 789 | 1,481
Gross Unrealized Losses | (7) | (7)
Estimated Fair Value of Available for Sale | 11,338 | 16,617
Mortgage - Backed Securities - Residential [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 632,422 | 683,601
Gross Unrealized Gains | 2,622 | 4,557
Gross Unrealized Losses | (9,489) | (11,305)
Estimated Fair Value of Available for Sale | 625,555 | 676,853
Mortgage - Backed Securities - Commercial [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 0 | 1
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value of Available for Sale | 0 | 1
Other Government - Sponsored Enterprises [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 1,098 | 16,700
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (1) | (69)
Estimated Fair Value of Available for Sale | 1,097 | 16,631
Equity Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 1,670 | 1,670
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value of Available for Sale | 1,670 | 1,670
US Government Agencies Debt Securities [Member] | Mortgage - Backed Securities - Commercial [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 24,611 | 0
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (462) | 0
Estimated Fair Value of Available for Sale | $ 24,149 | $ 0

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- Definition Investment Securities [Line Items] + References No definition available. + Details
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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=75030427&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=82897076&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=82896770&loc=d3e26610-111562 + 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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- Definition Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities. + 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=82897076&loc=d3e27161-111563 + Details
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