Securities (Year End Securities Held to Maturity and Available for Sale) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 2,663,009	$ 2,926,486
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	53,842	48,939
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	6,166	9,035
Held to Maturity, Estimated Fair Value	2,710,685	2,966,390
Available for Sale, Amortized Cost	9,019,570	8,261,947
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	197,488	203,121
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	10,700	3,814
Available for Sale, Estimated Fair Value	9,206,358	8,461,254
US Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	249,441	249,009
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	7,776	14,604
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held to Maturity, Estimated Fair Value	257,217	263,613
Available for Sale, Amortized Cost	3,980,986	3,783,899
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	22,041	30,594
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,507	3,241
Available for Sale, Estimated Fair Value	3,994,520	3,811,252
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	6,456	8,012
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	63	92
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	4	0
Held to Maturity, Estimated Fair Value	6,515	8,104
Available for Sale, Amortized Cost	1,000,024	1,331,114
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	42,142	68,027
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	734	417
Available for Sale, Estimated Fair Value	1,041,432	1,398,724
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	2,405,762	2,668,115
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	46,003	34,243
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	6,149	9,035
Held to Maturity, Estimated Fair Value	2,445,616	2,693,323
Available for Sale, Amortized Cost	3,996,113	3,104,563
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	133,305	104,500
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,459	156
Available for Sale, Estimated Fair Value	4,127,959	3,208,907
Other [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,350	1,350
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	13	0
Held to Maturity, Estimated Fair Value	1,337	1,350
Available for Sale, Amortized Cost	42,447	42,371
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Available for Sale, Estimated Fair Value	$ 42,447	$ 42,371

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
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-Section 25
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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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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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