SECURITIES - Amortized Cost and Fair Value of Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment Securities Held To Maturity [Line Items]
Amortized Cost	$ 3,206,345	$ 3,242,268
Gross Unrealized Gains	598	17,357
Gross Unrealized Losses	(173,996)	(41,246)
Fair Value	3,032,947	3,218,379
U.S. Treasury [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	500	500
Gross Unrealized Gains	95	134
Gross Unrealized Losses	0	0
Fair Value	595	634
US Government Agencies [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	1,987
Gross Unrealized Gains	58
Gross Unrealized Losses	0
Fair Value	2,045
U.S. Government-Sponsored Entities [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	230,011	247,310
Gross Unrealized Gains	0	93
Gross Unrealized Losses	(6,170)	(4,388)
Fair Value	223,841	243,015
Agency Mortgage-Backed Securities [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	1,078,270	1,219,802
Gross Unrealized Gains	225	3,475
Gross Unrealized Losses	(40,969)	(9,058)
Fair Value	1,037,526	1,214,219
Agency Collateralized Mortgage Obligations [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	797,940	777,146
Gross Unrealized Gains	219	32
Gross Unrealized Losses	(37,797)	(20,095)
Fair Value	760,362	757,083
Commercial Mortgage-Backed Securities [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	76,818	80,786
Gross Unrealized Gains	1	414
Gross Unrealized Losses	(1,864)	(575)
Fair Value	74,955	80,625
States of the U.S. and Political Subdivisions [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	1,020,819	916,724
Gross Unrealized Gains	0	13,209
Gross Unrealized Losses	(87,196)	(7,130)
Fair Value	$ 933,623	$ 922,803

Investment Securities Held To Maturity [Line Items]

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- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-Publisher FASB
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-Topic 320
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-Section 50
-Paragraph 5
-Subparagraph (a)
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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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