Securities (Year End Securities Held to Maturity and Available for Sale) (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 2,250,460	$ 2,663,009
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	18,622	53,842
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	6,335	6,166
Held to Maturity, Estimated Fair Value	2,262,747	2,710,685
Available for Sale, Amortized Cost	10,206,176	9,019,570
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	105,473	197,488
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	108,372	10,700
Available for Sale, Estimated Fair Value	10,203,277	9,206,358
US Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	249,889	249,441
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	1,762	7,776
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held to Maturity, Estimated Fair Value	251,651	257,217
Available for Sale, Amortized Cost	4,003,692	3,980,986
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	24,984	22,041
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,945	8,507
Available for Sale, Estimated Fair Value	4,019,731	3,994,520
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	4,511	6,456
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	39	63
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	4
Held to Maturity, Estimated Fair Value	4,550	6,515
Available for Sale, Amortized Cost	756,072	1,000,024
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	30,388	42,142
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,293	734
Available for Sale, Estimated Fair Value	785,167	1,041,432
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,994,710	2,405,762
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	16,821	46,003
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	6,335	6,149
Held to Maturity, Estimated Fair Value	2,005,196	2,445,616
Available for Sale, Amortized Cost	5,403,918	3,996,113
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	50,101	133,305
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	98,134	1,459
Available for Sale, Estimated Fair Value	5,355,885	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	42,494	42,447
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	$ 42,494	$ 42,447