Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 2,368,019	$ 2,663,009
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	62,105	53,842
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	5,090	6,166
Held to Maturity, Estimated Fair Value	2,425,034	2,710,685
Available for Sale, Amortized Cost	8,772,776	9,019,570
Available for Sale, Gross Unrealized Gains	295,545	197,488
Available for Sale, Gross Unrealized Losses	1,092	10,700
Available for Sale, Estimated Fair Value	9,067,229	9,206,358
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	249,550	249,441
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	6,895	7,776
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held to Maturity, Estimated Fair Value	256,445	257,217
Available for Sale, Amortized Cost	3,731,803	3,980,986
Available for Sale, Gross Unrealized Gains	93,031	22,041
Available for Sale, Gross Unrealized Losses	0	8,507
Available for Sale, Estimated Fair Value	3,824,834	3,994,520
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	5,756	6,456
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	71	63
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2	4
Held to Maturity, Estimated Fair Value	5,825	6,515
Available for Sale, Amortized Cost	930,927	1,000,024
Available for Sale, Gross Unrealized Gains	45,929	42,142
Available for Sale, Gross Unrealized Losses	216	734
Available for Sale, Estimated Fair Value	976,640	1,041,432
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	2,111,363	2,405,762
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	55,139	46,003
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	5,088	6,149
Held to Maturity, Estimated Fair Value	2,161,414	2,445,616
Available for Sale, Amortized Cost	4,067,577	3,996,113
Available for Sale, Gross Unrealized Gains	156,585	133,305
Available for Sale, Gross Unrealized Losses	876	1,459
Available for Sale, Estimated Fair Value	4,223,286	4,127,959
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	0	13
Held to Maturity, Estimated Fair Value	1,350	1,337
Available for Sale, Amortized Cost	42,469	42,447
Available for Sale, Gross Unrealized Gains	0	0
Available for Sale, Gross Unrealized Losses	0	0
Available for Sale, Estimated Fair Value	$ 42,469	$ 42,447

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
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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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