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Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (1) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Investment Securities [Line Items] |  |
Amortized Cost | $ 829,188 | $ 740,846
Securities available for sale, at fair value | 812,877 | 731,358
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax | 4,474 | 4,854
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax | 20,785 | 14,342
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 1,100 | 1,098
Securities available for sale, at fair value | 1,098 | 1,097
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax | 2 | $ 1
US Government Agencies Residential Mortgage-backed Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 83,500 |
Securities available for sale, at fair value | 82,800 |
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 681,300 |
Securities available for sale, at fair value | 664,800 |
Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 827,518 |
Securities available for sale, at fair value | $ 811,207 |

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- Definition Investment Securities [Line Items] + References No definition available. + Details
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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=82897076&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=82897076&loc=d3e27161-111563 + 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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