Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 29,315
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	253
Held to Maturity, More than 12 Months, Estimated Fair Value	120,214
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,790
Held to Maturity, Total Estimated Fair Value	149,529
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	3,043	$ 6,335
Available for Sale, Less than 12 Months, Estimated Fair Value	3,865,281
Available for Sale, Less than 12 Months, Unrealized Losses	91,724
Available for Sale, More than 12 Months, Estimated Fair Value	6,200
Available for Sale, More than 12 Months, Unrealized Losses	260
Available for Sale, Total Estimated Fair Value	3,871,481
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	91,984	108,372
Residential mortgage-backed 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	61,669
Available for Sale, Less than 12 Months, Unrealized Losses	1,166
Available for Sale, More than 12 Months, Estimated Fair Value	6,200
Available for Sale, More than 12 Months, Unrealized Losses	260
Available for Sale, Total Estimated Fair Value	67,869
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,426	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	27,969
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	249
Held to Maturity, More than 12 Months, Estimated Fair Value	120,214
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,790
Held to Maturity, Total Estimated Fair Value	148,183
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	3,039	6,335
Available for Sale, Less than 12 Months, Estimated Fair Value	2,182,114
Available for Sale, Less than 12 Months, Unrealized Losses	82,860
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	2,182,114
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	82,860	98,134
Other Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	1,346
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	1,346
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	4	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	1,621,498
Available for Sale, Less than 12 Months, Unrealized Losses	7,698
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	1,621,498
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 7,698	$ 8,945