Securities (Year End Securities Held to Maturity and Available for Sale) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,432,098	$ 2,250,460
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	26,477	18,622
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2,784	6,335
Held to Maturity, Estimated Fair Value	1,455,791	2,262,747
Available for Sale, Amortized Cost	10,329,951	10,206,176
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	193,835	105,473
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	34,777	108,372
Available for Sale, Estimated Fair Value	10,489,009	10,203,277
US Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	0	249,889
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0	1,762
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held to Maturity, Estimated Fair Value	0	251,651
Available for Sale, Amortized Cost	3,453,391	4,003,692
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	7,494	24,984
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	15,732	8,945
Available for Sale, Estimated Fair Value	3,445,153	4,019,731
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	3,610	4,511
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	15	39
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	38	0
Held to Maturity, Estimated Fair Value	3,587	4,550
Available for Sale, Amortized Cost	648,288	756,072
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	19,048	30,388
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2,250	1,293
Available for Sale, Estimated Fair Value	665,086	785,167
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,428,488	1,994,710
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	26,462	16,821
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2,746	6,335
Held to Maturity, Estimated Fair Value	1,452,204	2,005,196
Available for Sale, Amortized Cost	6,185,711	5,403,918
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	167,293	50,101
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	16,795	98,134
Available for Sale, Estimated Fair Value	6,336,209	5,355,885
Other [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	0	1,350
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held to Maturity, Estimated Fair Value	0	1,350
Available for Sale, Amortized Cost	42,561	42,494
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	$ 42,561	$ 42,494