Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 405,872
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,082
Held to Maturity, More than 12 Months, Estimated Fair Value	1,850
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	75
Held to Maturity, Total Estimated Fair Value	407,722
Held-to-maturity Securities, Gross Unrealized Losses	2,157	$ 2,784
Available for Sale, Less than 12 Months, Estimated Fair Value	6,175,662
Available for Sale, Less than 12 Months, Unrealized Losses	105,875
Available for Sale, More than 12 Months, Estimated Fair Value	2,016,757
Available for Sale, More than 12 Months, Unrealized Losses	73,902
Available for Sale, Total Estimated Fair Value	8,192,419
Available-for-sale Securities, Gross Unrealized Loss, before Tax	179,777	34,777
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	413
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	19
Held to Maturity, More than 12 Months, Estimated Fair Value	1,850
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	75
Held to Maturity, Total Estimated Fair Value	2,263
Held-to-maturity Securities, Gross Unrealized Losses	94	38
Available for Sale, Less than 12 Months, Estimated Fair Value	294,839
Available for Sale, Less than 12 Months, Unrealized Losses	5,979
Available for Sale, More than 12 Months, Estimated Fair Value	98,757
Available for Sale, More than 12 Months, Unrealized Losses	5,553
Available for Sale, Total Estimated Fair Value	393,596
Available-for-sale Securities, Gross Unrealized Loss, before Tax	11,532	2,250
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	403,965
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,057
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	403,965
Held-to-maturity Securities, Gross Unrealized Losses	2,057	2,746
Available for Sale, Less than 12 Months, Estimated Fair Value	3,585,212
Available for Sale, Less than 12 Months, Unrealized Losses	65,581
Available for Sale, More than 12 Months, Estimated Fair Value	810,732
Available for Sale, More than 12 Months, Unrealized Losses	50,644
Available for Sale, Total Estimated Fair Value	4,395,944
Available-for-sale Securities, Gross Unrealized Loss, before Tax	116,225	16,795
Other Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	1,494
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6
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	1,494
Held-to-maturity Securities, Gross Unrealized Losses	6	0
Available-for-sale Securities, Gross Unrealized Loss, before Tax	0	0
US Treasury Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	2,295,611
Available for Sale, Less than 12 Months, Unrealized Losses	34,315
Available for Sale, More than 12 Months, Estimated Fair Value	1,107,268
Available for Sale, More than 12 Months, Unrealized Losses	17,705
Available for Sale, Total Estimated Fair Value	3,402,879
Available-for-sale Securities, Gross Unrealized Loss, before Tax	$ 52,020	$ 15,732