Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 23,666
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	109
Held to Maturity, More than 12 Months, Estimated Fair Value	118,937
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,808
Held to Maturity, Total Estimated Fair Value	142,603
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2,917	$ 6,335
Available for Sale, Less than 12 Months, Estimated Fair Value	3,106,390
Available for Sale, Less than 12 Months, Unrealized Losses	46,530
Available for Sale, More than 12 Months, Estimated Fair Value	148,373
Available for Sale, More than 12 Months, Unrealized Losses	7,380
Available for Sale, Total Estimated Fair Value	3,254,763
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	53,910	108,372
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	2,191
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	24
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	2,191
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	24	0
Available for Sale, Less than 12 Months, Estimated Fair Value	67,596
Available for Sale, Less than 12 Months, Unrealized Losses	1,204
Available for Sale, More than 12 Months, Estimated Fair Value	5,866
Available for Sale, More than 12 Months, Unrealized Losses	232
Available for Sale, Total Estimated Fair Value	73,462
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,436	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	20,127
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	83
Held to Maturity, More than 12 Months, Estimated Fair Value	118,937
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,808
Held to Maturity, Total Estimated Fair Value	139,064
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2,891	6,335
Available for Sale, Less than 12 Months, Estimated Fair Value	1,450,982
Available for Sale, Less than 12 Months, Unrealized Losses	38,138
Available for Sale, More than 12 Months, Estimated Fair Value	142,507
Available for Sale, More than 12 Months, Unrealized Losses	7,148
Available for Sale, Total Estimated Fair Value	1,593,489
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	45,286	98,134
Other Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	1,348
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2
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	1,348
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
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	1,587,812
Available for Sale, Less than 12 Months, Unrealized Losses	7,188
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	1,587,812
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 7,188	$ 8,945