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Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (1) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Investment Securities [Line Items] |  |
Amortized Cost | $ 915,350 | $ 740,846
Securities available for sale, at fair value | 889,056 | 731,358
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 2,000 | 4,854
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 28,294 | 14,342
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 100 | 1,098
Securities available for sale, at fair value | 100 | 1,097
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | $ 1
US Government Agencies Residential Mortgage-backed Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 179,400 |
Securities available for sale, at fair value | 177,000 |
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 685,700 |
Securities available for sale, at fair value | 661,600 |
Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 913,680 |
Securities available for sale, at fair value | $ 887,386 |

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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 investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 No definition available. + 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 No definition available. + Details
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