Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 262,111
Held to Maturity, Due after one year through five years, Amortized Cost	337,335
Held to Maturity, Due after five years through ten years, Amortized Cost	631,698
Held to Maturity, Due after ten years, Amortized Cost	582,419
Held to Maturity, Amortized Cost	1,813,563	$ 1,945,833
Held to Maturity, Due in one year or less, Estimated Fair Value	265,690
Held to Maturity, Due after one year through five years, Estimated Fair Value	348,215
Held to Maturity, Due after five years through ten years, Estimated Fair Value	663,290
Held to Maturity, Due after ten years, Estimated Fair Value	619,040
Held to Maturity, Estimated Fair Value	1,896,235	2,052,896
Available-for-sale Securities, Due in one year or less, Amortized Cost	932,020
Available for Sale, Due after one year through five years, Amortized Cost	1,672,288
Available for Sale, Due after five years through ten years, Amortized Cost	1,037,740
Available for Sale, Due after ten years, Amortized Cost	6,575,069
Available for Sale, Amortized Cost	10,259,473	9,727,401
Available for Sale, Due in one year or less, Estimated Fair Value	938,579
Available for Sale, Due after one year through five years, Estimated Fair Value	1,796,638
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,093,251
Available for Sale, Due after ten years, Estimated Fair Value	6,969,702
Securities available for sale, estimated fair value	10,840,526	10,437,565
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	799,521
Available for Sale, Due after one year through five years, Amortized Cost	285,676
Available for Sale, Due after five years through ten years, Amortized Cost	197,086
Available for Sale, Due after ten years, Amortized Cost	191,517
Available for Sale, Amortized Cost	1,473,800	1,084,542
Available for Sale, Due in one year or less, Estimated Fair Value	804,722
Available for Sale, Due after one year through five years, Estimated Fair Value	302,656
Available for Sale, Due after five years through ten years, Estimated Fair Value	194,344
Available for Sale, Due after ten years, Estimated Fair Value	196,000
Securities available for sale, estimated fair value	1,497,722	1,119,633
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	48
Held to Maturity, Due after one year through five years, Amortized Cost	54
Held to Maturity, Due after five years through ten years, Amortized Cost	515,611
Held to Maturity, Due after ten years, Amortized Cost	12,245
Held to Maturity, Amortized Cost	527,958	528,784
Held to Maturity, Due in one year or less, Estimated Fair Value	48
Held to Maturity, Due after one year through five years, Estimated Fair Value	55
Held to Maturity, Due after five years through ten years, Estimated Fair Value	544,894
Held to Maturity, Due after ten years, Estimated Fair Value	13,089
Held to Maturity, Estimated Fair Value	558,086	570,526
Available-for-sale Securities, Due in one year or less, Amortized Cost	3,453
Available for Sale, Due after one year through five years, Amortized Cost	19,270
Available for Sale, Due after five years through ten years, Amortized Cost	22,748
Available for Sale, Due after ten years, Amortized Cost	1,683,727
Available for Sale, Amortized Cost	1,729,198	1,916,581
Available for Sale, Due in one year or less, Estimated Fair Value	3,592
Available for Sale, Due after one year through five years, Estimated Fair Value	20,327
Available for Sale, Due after five years through ten years, Estimated Fair Value	23,379
Available for Sale, Due after ten years, Estimated Fair Value	1,714,331
Securities available for sale, estimated fair value	1,761,629	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	260,563
Held to Maturity, Due after one year through five years, Amortized Cost	337,281
Held to Maturity, Due after five years through ten years, Amortized Cost	116,087
Held to Maturity, Due after ten years, Amortized Cost	570,174
Held to Maturity, Amortized Cost	1,284,105	1,415,549
Held to Maturity, Due in one year or less, Estimated Fair Value	264,142
Held to Maturity, Due after one year through five years, Estimated Fair Value	348,160
Held to Maturity, Due after five years through ten years, Estimated Fair Value	118,396
Held to Maturity, Due after ten years, Estimated Fair Value	605,951
Held to Maturity, Estimated Fair Value	1,336,649	1,480,870
Available-for-sale Securities, Due in one year or less, Amortized Cost	129,046
Available for Sale, Due after one year through five years, Amortized Cost	1,367,342
Available for Sale, Due after five years through ten years, Amortized Cost	817,906
Available for Sale, Due after ten years, Amortized Cost	4,699,825
Available for Sale, Amortized Cost	7,014,119	6,683,927
Available for Sale, Due in one year or less, Estimated Fair Value	130,265
Available for Sale, Due after one year through five years, Estimated Fair Value	1,473,655
Available for Sale, Due after five years through ten years, Estimated Fair Value	875,528
Available for Sale, Due after ten years, Estimated Fair Value	5,059,371
Securities available for sale, estimated fair value	7,538,819	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
Available for Sale, Amortized Cost	42,356	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, estimated fair value	$ 42,356	$ 42,351

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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