Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	$ 6,622,170	$ 6,558,715
Securities available for sale, Gross Unrealized Gains	18,475	14,569
Securities available for sale, Gross Unrealized Losses	(89,052)	(106,950)
Total securities available for sale, Fair Value	6,551,593	6,466,334
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	65,069	95,315
Securities available for sale, Gross Unrealized Gains	29	37
Securities available for sale, Gross Unrealized Losses	(1,404)	(1,526)
Total securities available for sale, Fair Value	63,694	93,826
U.S. Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	82,104	198,158
Securities available for sale, Gross Unrealized Gains	25	67
Securities available for sale, Gross Unrealized Losses	(6)	(48)
Total securities available for sale, Fair Value	82,123	198,177
Mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	3,948,616	3,773,090
Securities available for sale, Gross Unrealized Gains	6,424	7,069
Securities available for sale, Gross Unrealized Losses	(64,890)	(68,460)
Total securities available for sale, Fair Value	3,890,150	3,711,699
State and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	2,459,630	2,425,155
Securities available for sale, Gross Unrealized Gains	11,997	7,391
Securities available for sale, Gross Unrealized Losses	(22,662)	(36,789)
Total securities available for sale, Fair Value	2,448,965	2,395,757
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	66,751	66,997
Securities available for sale, Gross Unrealized Gains		5
Securities available for sale, Gross Unrealized Losses	(90)	(127)
Total securities available for sale, Fair Value	$ 66,661	$ 66,875

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
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-Publisher FASB
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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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