Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 458,343
Held to Maturity, Due after one year through five years, Amortized Cost	181,302
Held to Maturity, Due after five years through ten years, Amortized Cost	604,218
Held to Maturity, Due after ten years, Amortized Cost	515,565
Held to Maturity, Amortized Cost	1,759,428	$ 1,945,833
Held to Maturity, Due in one year or less, Estimated Fair Value	465,632
Held to Maturity, Due after one year through five years, Estimated Fair Value	187,495
Held to Maturity, Due after five years through ten years, Estimated Fair Value	629,437
Held to Maturity, Due after ten years, Estimated Fair Value	543,004
Held to Maturity, Estimated Fair Value	1,825,568	2,052,896
Available-for-sale Securities, Due in one year or less, Amortized Cost	582,966
Available for Sale, Due after one year through five years, Amortized Cost	1,943,087
Available for Sale, Due after five years through ten years, Amortized Cost	1,331,530
Available for Sale, Due after ten years, Amortized Cost	7,330,896
Debt Securities, Available-for-sale, Amortized Cost	11,230,837	9,727,401
Available for Sale, Due in one year or less, Estimated Fair Value	586,193
Available for Sale, Due after one year through five years, Estimated Fair Value	2,057,801
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,382,879
Available for Sale, Due after ten years, Estimated Fair Value	7,661,991
Securities available for sale, at estimated fair value	11,731,222	10,437,565
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	499,857
Available for Sale, Due after one year through five years, Amortized Cost	533,532
Available for Sale, Due after five years through ten years, Amortized Cost	445,697
Available for Sale, Due after ten years, Amortized Cost	191,606
Debt Securities, Available-for-sale, Amortized Cost	1,670,692	1,084,542
Available for Sale, Due in one year or less, Estimated Fair Value	501,489
Available for Sale, Due after one year through five years, Estimated Fair Value	548,216
Available for Sale, Due after five years through ten years, Estimated Fair Value	440,328
Available for Sale, Due after ten years, Estimated Fair Value	195,188
Securities available for sale, at estimated fair value	1,685,221	1,119,633
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	65
Held to Maturity, Due after one year through five years, Amortized Cost	0
Held to Maturity, Due after five years through ten years, Amortized Cost	515,356
Held to Maturity, Due after ten years, Amortized Cost	12,228
Held to Maturity, Amortized Cost	527,649	528,784
Held to Maturity, Due in one year or less, Estimated Fair Value	65
Held to Maturity, Due after one year through five years, Estimated Fair Value	0
Held to Maturity, Due after five years through ten years, Estimated Fair Value	539,142
Held to Maturity, Due after ten years, Estimated Fair Value	12,774
Held to Maturity, Estimated Fair Value	551,981	570,526
Available-for-sale Securities, Due in one year or less, Amortized Cost	743
Available for Sale, Due after one year through five years, Amortized Cost	16,678
Available for Sale, Due after five years through ten years, Amortized Cost	22,146
Available for Sale, Due after ten years, Amortized Cost	2,428,348
Debt Securities, Available-for-sale, Amortized Cost	2,467,915	1,916,581
Available for Sale, Due in one year or less, Estimated Fair Value	772
Available for Sale, Due after one year through five years, Estimated Fair Value	17,559
Available for Sale, Due after five years through ten years, Estimated Fair Value	22,743
Available for Sale, Due after ten years, Estimated Fair Value	2,450,080
Securities available for sale, at estimated fair value	2,491,154	1,987,679
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	456,778
Held to Maturity, Due after one year through five years, Amortized Cost	181,302
Held to Maturity, Due after five years through ten years, Amortized Cost	88,862
Held to Maturity, Due after ten years, Amortized Cost	503,337
Held to Maturity, Amortized Cost	1,230,279	1,415,549
Held to Maturity, Due in one year or less, Estimated Fair Value	464,067
Held to Maturity, Due after one year through five years, Estimated Fair Value	187,495
Held to Maturity, Due after five years through ten years, Estimated Fair Value	90,295
Held to Maturity, Due after ten years, Estimated Fair Value	530,230
Held to Maturity, Estimated Fair Value	1,272,087	1,480,870
Available-for-sale Securities, Due in one year or less, Amortized Cost	82,366
Available for Sale, Due after one year through five years, Amortized Cost	1,392,877
Available for Sale, Due after five years through ten years, Amortized Cost	863,687
Available for Sale, Due after ten years, Amortized Cost	4,710,942
Debt Securities, Available-for-sale, Amortized Cost	7,049,872	6,683,927
Available for Sale, Due in one year or less, Estimated Fair Value	83,932
Available for Sale, Due after one year through five years, Estimated Fair Value	1,492,026
Available for Sale, Due after five years through ten years, Estimated Fair Value	919,808
Available for Sale, Due after ten years, Estimated Fair Value	5,016,723
Securities available for sale, at estimated fair value	7,512,489	7,287,902
Other Investments [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	1,500
Held to Maturity, Due after one year through five years, Amortized Cost	0
Held to Maturity, Due after five years through ten years, Amortized Cost	0
Held to Maturity, Due after ten years, Amortized Cost	0
Held to Maturity, Amortized Cost	1,500	1,500
Held to Maturity, Due in one year or less, Estimated Fair Value	1,500
Held to Maturity, Due after one year through five years, Estimated Fair Value	0
Held to Maturity, Due after five years through ten years, Estimated Fair Value	0
Held to Maturity, Due after ten years, Estimated Fair Value	0
Held to Maturity, Estimated Fair Value	1,500	1,500
Available-for-sale Securities, Due in one year or less, Amortized Cost	0
Available for Sale, Due after one year through five years, Amortized Cost	0
Available for Sale, Due after five years through ten years, Amortized Cost	0
Available for Sale, Due after ten years, Amortized Cost	0
Debt Securities, Available-for-sale, Amortized Cost	42,358	42,351
Available for Sale, Due in one year or less, Estimated Fair Value	0
Available for Sale, Due after one year through five years, Estimated Fair Value	0
Available for Sale, Due after five years through ten years, Estimated Fair Value	0
Available for Sale, Due after ten years, Estimated Fair Value	0
Securities available for sale, at estimated fair value	$ 42,358	$ 42,351