Securities (Year End Securities with Unrealized Losses Segregated by Length of Impairment) (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 425,896	$ 149,091
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,596	1,342
Held to Maturity, More than 12 Months, Estimated Fair Value	183,245	202,423
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,739	4,824
Held to Maturity, Total Estimated Fair Value	609,141	351,514
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	6,335	6,166
Available for Sale, Less than 12 Months, Estimated Fair Value	4,198,655	1,028,261
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	108,110	9,956
Available for Sale, More than 12 Months, Estimated Fair Value	6,413	36,266
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	262	744
Available for Sale, Total Estimated Fair Value	4,205,068	1,064,527
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	108,372	10,700
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	1,421,216	886,087
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	8,945	8,507
Available for Sale, More than 12 Months, Estimated Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available for Sale, Total Estimated Fair Value	1,421,216	886,087
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,945	8,507
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value		900
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		4
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		900
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	4
Available for Sale, Less than 12 Months, Estimated Fair Value	81,442	21,392
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,031	212
Available for Sale, More than 12 Months, Estimated Fair Value	6,413	17,781
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	262	522
Available for Sale, Total Estimated Fair Value	87,855	39,173
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,293	734
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	425,896	146,854
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,596	1,325
Held to Maturity, More than 12 Months, Estimated Fair Value	183,245	202,423
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,739	4,824
Held to Maturity, Total Estimated Fair Value	609,141	349,277
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	6,335	6,149
Available for Sale, Less than 12 Months, Estimated Fair Value	2,695,997	120,782
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	98,134	1,237
Available for Sale, More than 12 Months, Estimated Fair Value	0	18,485
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	222
Available for Sale, Total Estimated Fair Value	2,695,997	139,267
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	98,134	1,459
Other Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value		1,337
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		13
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		1,337
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	13
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 0	$ 0