Securities (Year End Securities with Unrealized Losses Segregated by Length of Impairment) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 31,285	$ 425,896
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	96	2,596
Held to Maturity, More than 12 Months, Estimated Fair Value	74,113	183,245
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,688	3,739
Held to Maturity, Total Estimated Fair Value	105,398	609,141
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2,784	6,335
Available for Sale, Less than 12 Months, Estimated Fair Value	2,628,920	4,198,655
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	12,004	108,110
Available for Sale, More than 12 Months, Estimated Fair Value	1,401,340	6,413
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	22,773	262
Available for Sale, Total Estimated Fair Value	4,030,260	4,205,068
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	34,777	108,372
US Treasury Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Available for Sale, Less than 12 Months, Estimated Fair Value	2,336,081	1,421,216
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9,861	8,945
Available for Sale, More than 12 Months, Estimated Fair Value	517,575	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5,871	0
Available for Sale, Total Estimated Fair Value	2,853,656	1,421,216
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	15,732	8,945
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	2,694
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	38
Held to Maturity, More than 12 Months, Estimated Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Held to Maturity, Total Estimated Fair Value	2,694
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	38	0
Available for Sale, Less than 12 Months, Estimated Fair Value	144,264	81,442
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	949	1,031
Available for Sale, More than 12 Months, Estimated Fair Value	45,436	6,413
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,301	262
Available for Sale, Total Estimated Fair Value	189,700	87,855
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2,250	1,293
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	28,591	425,896
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	58	2,596
Held to Maturity, More than 12 Months, Estimated Fair Value	74,113	183,245
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,688	3,739
Held to Maturity, Total Estimated Fair Value	102,704	609,141
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2,746	6,335
Available for Sale, Less than 12 Months, Estimated Fair Value	148,575	2,695,997
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,194	98,134
Available for Sale, More than 12 Months, Estimated Fair Value	838,329	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	15,601	0
Available for Sale, Total Estimated Fair Value	986,904	2,695,997
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	16,795	98,134
Other Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 0	$ 0