Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 121,042
Held to Maturity, Due after one year through five years, Amortized Cost	31,034
Held to Maturity, Due after five years through ten years, Amortized Cost	524,201
Held to Maturity, Due after ten years, Amortized Cost	1,497,965
Held to Maturity, Amortized Cost	2,174,242	$ 1,749,337
Held to Maturity, Due in one year or less, Estimated Fair Value	120,786
Held to Maturity, Due after one year through five years, Estimated Fair Value	30,805
Held to Maturity, Due after five years through ten years, Estimated Fair Value	458,364
Held to Maturity, Due after ten years, Estimated Fair Value	1,307,332
Held to Maturity, Estimated Fair Value	1,917,287	1,809,143
Available-for-sale Securities, Due in one year or less, Amortized Cost	250,345
Available for Sale, Due after one year through five years, Amortized Cost	5,301,345
Available for Sale, Due after five years through ten years, Amortized Cost	2,395,444
Available for Sale, Due after ten years, Amortized Cost	11,307,492
Debt Securities, Available-for-sale, Amortized Cost	19,297,009	13,445,888
Available for Sale, Due in one year or less, Estimated Fair Value	250,902
Available for Sale, Due after one year through five years, Estimated Fair Value	5,119,877
Available for Sale, Due after five years through ten years, Estimated Fair Value	2,144,310
Available for Sale, Due after ten years, Estimated Fair Value	9,841,635
Securities available for sale, at estimated fair value	17,399,107	13,924,628
US Treasury Securities [Member]
Schedule of Investments [Line Items]
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	3,821,606
Available for Sale, Due after five years through ten years, Amortized Cost	1,433,912
Available for Sale, Due after ten years, Amortized Cost	191,960
Debt Securities, Available-for-sale, Amortized Cost	5,447,478	2,165,702
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	3,648,015
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,232,969
Available for Sale, Due after ten years, Estimated Fair Value	141,633
Securities available for sale, at estimated fair value	5,022,617	2,179,433
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	0
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	514,321
Held to Maturity, Due after ten years, Amortized Cost	12,076
Held to Maturity, Amortized Cost	526,397	527,264
Held to Maturity, Due in one year or less, Estimated Fair Value	0
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	448,514
Held to Maturity, Due after ten years, Estimated Fair Value	9,859
Held to Maturity, Estimated Fair Value	458,373	546,030
Available-for-sale Securities, Due in one year or less, Amortized Cost	28
Available for Sale, Due after one year through five years, Amortized Cost	9,355
Available for Sale, Due after five years through ten years, Amortized Cost	17,253
Available for Sale, Due after ten years, Amortized Cost	6,658,782
Debt Securities, Available-for-sale, Amortized Cost	6,685,418	4,059,692
Available for Sale, Due in one year or less, Estimated Fair Value	27
Available for Sale, Due after one year through five years, Estimated Fair Value	9,130
Available for Sale, Due after five years through ten years, Estimated Fair Value	17,167
Available for Sale, Due after ten years, Estimated Fair Value	5,672,485
Securities available for sale, at estimated fair value	5,698,809	4,066,265
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	121,042
Held to Maturity, Due after one year through five years, Amortized Cost	29,534
Held to Maturity, Due after five years through ten years, Amortized Cost	9,880
Held to Maturity, Due after ten years, Amortized Cost	1,485,889
Held to Maturity, Amortized Cost	1,646,345	1,220,573
Held to Maturity, Due in one year or less, Estimated Fair Value	120,786
Held to Maturity, Due after one year through five years, Estimated Fair Value	29,362
Held to Maturity, Due after five years through ten years, Estimated Fair Value	9,850
Held to Maturity, Due after ten years, Estimated Fair Value	1,297,473
Held to Maturity, Estimated Fair Value	1,457,471	1,261,613
Available-for-sale Securities, Due in one year or less, Amortized Cost	250,317
Available for Sale, Due after one year through five years, Amortized Cost	1,470,384
Available for Sale, Due after five years through ten years, Amortized Cost	944,279
Available for Sale, Due after ten years, Amortized Cost	4,456,750
Debt Securities, Available-for-sale, Amortized Cost	7,121,730	7,178,135
Available for Sale, Due in one year or less, Estimated Fair Value	250,875
Available for Sale, Due after one year through five years, Estimated Fair Value	1,462,732
Available for Sale, Due after five years through ten years, Estimated Fair Value	894,174
Available for Sale, Due after ten years, Estimated Fair Value	4,027,517
Securities available for sale, at estimated fair value	6,635,298	7,636,571
Other Investments [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	0
Held to Maturity, Due after one year through five years, Amortized Cost	1,500
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	0
Held to Maturity, Due after one year through five years, Estimated Fair Value	1,443
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	1,443	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	42,383	42,359
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	$ 42,383	$ 42,359