Securities (Year End Securities with Unrealized Losses Segregated by Length of Impairment) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 149,091	$ 68,024
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,342	144
Held to Maturity, More than 12 Months, Estimated Fair Value	202,423	683,251
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4,824	8,891
Held to Maturity, Total Estimated Fair Value	351,514	751,275
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	6,166	9,035
Available for Sale, Less than 12 Months, Estimated Fair Value	1,028,261	1,093,531
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9,956	3,439
Available for Sale, More than 12 Months, Estimated Fair Value	36,266	16,944
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	744	375
Available for Sale, Total Estimated Fair Value	1,064,527	1,110,475
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	10,700	3,814
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	886,087	1,019,230
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	8,507	3,241
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	886,087	1,019,230
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,507	3,241
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	4	0
Available for Sale, Less than 12 Months, Estimated Fair Value	21,392	8,550
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	212	42
Available for Sale, More than 12 Months, Estimated Fair Value	17,781	16,944
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	522	375
Available for Sale, Total Estimated Fair Value	39,173	25,494
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	734	417
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	146,854	68,024
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,325	144
Held to Maturity, More than 12 Months, Estimated Fair Value	202,423	683,251
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4,824	8,891
Held to Maturity, Total Estimated Fair Value	349,277	751,275
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	6,149	9,035
Available for Sale, Less than 12 Months, Estimated Fair Value	120,782	65,751
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,237	156
Available for Sale, More than 12 Months, Estimated Fair Value	18,485	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	222	0
Available for Sale, Total Estimated Fair Value	139,267	65,751
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,459	156
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	13	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 0	$ 0