INVESTMENT SECURITIES - Roll-Forward of Credit Losses on Debt Securities Held by Corporation (Detail) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Beginning balance of credit losses on debt securities held for which a portion of an OTTI was recognized in OCI	$ 6,792,000	$ 28,981,000	$ 28,981,000	$ 22,294,000
Additions:
Credit losses on debt securities for which an OTTI was no previously recognized	0	0		0
Credit losses on debt securities for which an OTTI was previously recognized	0	0	12,231,000	6,687,000
Reductions:
Securities sold during the period	0		(34,420,000)
Ending balance of credit losses on debt securities held for which a portion of an OTTI was recognized in OCI	6,792,000	28,981,000	6,792,000	28,981,000
Mortgage Backed Securities Issued By Private Enterprises [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Beginning balance of credit losses on debt securities held for which a portion of an OTTI was recognized in OCI	6,792,000	6,792,000	6,792,000	6,405,000
Additions:
Credit losses on debt securities for which an OTTI was no previously recognized	0	0		0
Credit losses on debt securities for which an OTTI was previously recognized	0	0	0	387,000
Reductions:
Securities sold during the period	0		0
Ending balance of credit losses on debt securities held for which a portion of an OTTI was recognized in OCI	6,792,000	6,792,000	6,792,000	6,792,000
US States And Political Subdivisions Member [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Beginning balance of credit losses on debt securities held for which a portion of an OTTI was recognized in OCI		22,189,000	22,189,000	15,889,000
Additions:
Credit losses on debt securities for which an OTTI was no previously recognized		0		0
Credit losses on debt securities for which an OTTI was previously recognized		0	12,231,000	6,300,000
Reductions:
Securities sold during the period			(34,420,000)
Ending balance of credit losses on debt securities held for which a portion of an OTTI was recognized in OCI	$ 0	$ 22,189,000	$ 0	$ 22,189,000

Additions.

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Reductions.

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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

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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

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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

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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 because of the sale of impaired securities during the period.

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