Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	$ 6,675,155	$ 6,813,011
Securities available for sale, Gross Unrealized Gains	103,299	36,393
Securities available for sale, Gross Unrealized Losses	(7,275)	(42,455)
Total securities available for sale, Fair Value	6,771,179	6,806,949
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	362,766	350,354
Securities available for sale, Gross Unrealized Gains	576	1
Securities available for sale, Gross Unrealized Losses	(5)	(576)
Total securities available for sale, Fair Value	363,337	349,779
U.S. Agencies
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	421,149	667,414
Securities available for sale, Gross Unrealized Gains	507	7
Securities available for sale, Gross Unrealized Losses	(31)	(1,032)
Total securities available for sale, Fair Value	421,625	666,389
Mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	3,553,661	3,598,115
Securities available for sale, Gross Unrealized Gains	53,444	12,420
Securities available for sale, Gross Unrealized Losses	(6,930)	(38,089)
Total securities available for sale, Fair Value	3,600,175	3,572,446
State and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	2,257,535	2,116,543
Securities available for sale, Gross Unrealized Gains	48,687	23,965
Securities available for sale, Gross Unrealized Losses	(243)	(2,095)
Total securities available for sale, Fair Value	2,305,979	2,138,413
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	80,044	80,585
Securities available for sale, Gross Unrealized Gains	85
Securities available for sale, Gross Unrealized Losses	(66)	(663)
Total securities available for sale, Fair Value	$ 80,063	$ 79,922

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
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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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
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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