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Impairment of Investment Securities - Cumulative Roll Forward of Credit Losses Recognized in Earnings for Debt Securities Held and Not Intended to be Sold (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Impairment of Investment Securities Disclosure [Abstract] |  |  |  |  |  |
Balance, beginning (a) | $ 16,610 | $ 24,310 | $ 17,056 | [1] | $ 24,851 | [1]
Credit losses on debt securities for which other-than-temporary impairment was not previously recognized | 0 | 0 | 0 |  | 0 |
Additional credit losses on debt securities for which other-than-temporary impairment was previously recognized | 0 | 0 | 0 |  | 0 |
Balance, ending | 16,391 | 24,040 | 16,391 |  | 24,040 |
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Cash Flows | 0 | 0 | 0 |  | 0 |
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Period Increase (Decrease) | $ 219 | $ 270 | $ 665 |  | $ 811 |
[1] | The beginning balance represents credit related losses included in other-than-temporary impairment charges recognized on debt securities in prior periods.

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- Definition Impairment of Investment Securities Disclosure [Abstract] + References No definition available. + Details
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- Definition This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=73527792&loc=SL6284393-111563 + Details
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- Definition This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=73527792&loc=SL6284393-111563 + Details
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- Definition The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=73527792&loc=SL6284393-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=73527792&loc=SL6284393-111563 + Details
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- Definition Amount of increase (decrease) in the cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -URI http://asc.fasb.org/extlink&oid=73527792&loc=SL6284393-111563 + Details
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- Definition This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held due to increases in cash flows expected to be collected and recognized over the remaining life of the security. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=73527792&loc=SL6284393-111563 + Details
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