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Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Investment Securities [Line Items] |  |  |
Due within 1 year, Amortized Cost |  | $ 1,098 |
Due after 1 but within 5 years, Amortized Cost |  | 13,989 |
Due after 5 but within 10 years, Amortized Cost |  | 27,083 |
Due after 10 years, Amortized Cost |  | 29,417 |
Debt securities gross, Amortized Cost |  | 71,587 |
Amortized Cost |  | 740,846 | $ 790,082
Total debt securities, Amortized Cost |  | 739,176 |
Due within 1 year, Estimated Fair Value |  | 1,097 |
Due after 1 but within 5 years, Estimated Fair Value |  | 14,020 |
Due after 5 but within 10 years, Estimated Fair Value |  | 27,410 |
Due after 10 years, Estimated Fair Value |  | 26,119 |
Debt securities gross, Estimated Fair Value |  | 68,646 |
Estimated Fair Value of Available for Sale |  | 731,358 | $ 778,612
Total debt securities, Estimated Fair Value |  | 729,688 |
Mortgage-Backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost | [1] | 667,589 |
Estimated Fair Value of Available for Sale | [1] | $ 661,042 |
[1] | Mortgage Backed Securities include an amortized cost of $35.2 million and a fair value of $35.5 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $632.4 million and a fair value of $625.6 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac.

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- Definition This item represents the cost of debt 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 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 2: 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 after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth 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 (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 + 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=82897076&loc=d3e27198-111563 + Details
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