Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,141,773	$ 6,334,018
Gross Unrealized Gains	1,756	13,879
Gross Unrealized Losses	(210,788)	(89,320)
Available for sale	5,932,741	6,258,577
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40,101	40,092
Gross Unrealized Losses	(2,163)	(1,449)
Available for sale	37,938	38,643
U.S. Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	200	14,762
Gross Unrealized Losses	(2)	(10)
Available for sale	198	14,752
Mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,778,032	3,719,369
Gross Unrealized Gains	337	1,914
Gross Unrealized Losses	(157,991)	(72,040)
Available for sale	3,620,378	3,649,243
State and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,323,440	2,546,517
Gross Unrealized Gains	1,419	11,965
Gross Unrealized Losses	(50,632)	(15,809)
Available for sale	$ 2,274,227	2,542,673
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		13,278
Gross Unrealized Losses		(12)
Available for sale		$ 13,266

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as 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

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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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

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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.

+ References

No definition available.

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