Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 234,519
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	930
Held to Maturity, More than 12 Months, Estimated Fair Value	46,385
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,260
Held to Maturity, Total Estimated Fair Value	280,904
Held-to-maturity Securities, Gross Unrealized Losses	3,190	$ 2,784
Available for Sale, Less than 12 Months, Estimated Fair Value	4,839,766
Available for Sale, Less than 12 Months, Unrealized Losses	58,771
Available for Sale, More than 12 Months, Estimated Fair Value	1,375,871
Available for Sale, More than 12 Months, Unrealized Losses	48,801
Available for Sale, Total Estimated Fair Value	6,215,637
Available-for-sale Securities, Gross Unrealized Loss, before Tax	107,572	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	1,120
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	23
Held to Maturity, More than 12 Months, Estimated Fair Value	1,516
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	34
Held to Maturity, Total Estimated Fair Value	2,636
Held-to-maturity Securities, Gross Unrealized Losses	57	38
Available for Sale, Less than 12 Months, Estimated Fair Value	219,091
Available for Sale, Less than 12 Months, Unrealized Losses	4,508
Available for Sale, More than 12 Months, Estimated Fair Value	45,424
Available for Sale, More than 12 Months, Unrealized Losses	2,307
Available for Sale, Total Estimated Fair Value	264,515
Available-for-sale Securities, Gross Unrealized Loss, before Tax	6,815	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	231,906
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	900
Held to Maturity, More than 12 Months, Estimated Fair Value	44,869
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,226
Held to Maturity, Total Estimated Fair Value	276,775
Held-to-maturity Securities, Gross Unrealized Losses	3,126	2,746
Available for Sale, Less than 12 Months, Estimated Fair Value	2,004,045
Available for Sale, Less than 12 Months, Unrealized Losses	26,466
Available for Sale, More than 12 Months, Estimated Fair Value	815,756
Available for Sale, More than 12 Months, Unrealized Losses	38,178
Available for Sale, Total Estimated Fair Value	2,819,801
Available-for-sale Securities, Gross Unrealized Loss, before Tax	64,644	16,795
Other Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	1,493
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7
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	1,493
Held-to-maturity Securities, Gross Unrealized Losses	7	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	2,616,630
Available for Sale, Less than 12 Months, Unrealized Losses	27,797
Available for Sale, More than 12 Months, Estimated Fair Value	514,691
Available for Sale, More than 12 Months, Unrealized Losses	8,316
Available for Sale, Total Estimated Fair Value	3,131,321
Available-for-sale Securities, Gross Unrealized Loss, before Tax	$ 36,113	$ 15,732