Securities (Amortized Cost and Estimated Fair Value of Securities Excluding Trading Securities Presented by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 465,649
Held to Maturity, Due after one year through five years, Amortized Cost	180,373
Held to Maturity, Due after five years through ten years, Amortized Cost	588,908
Held to Maturity, Due after ten years, Amortized Cost	514,407
Held to Maturity, Amortized Cost	1,749,337	$ 1,945,833
Held to Maturity, Due in one year or less, Estimated Fair Value	470,222
Held to Maturity, Due after one year through five years, Estimated Fair Value	185,458
Held to Maturity, Due after five years through ten years, Estimated Fair Value	608,183
Held to Maturity, Due after ten years, Estimated Fair Value	545,280
Held to Maturity, Estimated Fair Value	1,809,143	2,052,896
Available for Sale, Due in one year or less, Amortized Cost	86,548
Available for Sale, Due after one year through five years, Amortized Cost	2,656,909
Available for Sale, Due after five years through ten years, Amortized Cost	1,770,915
Available for Sale, Due after ten years, Amortized Cost	8,889,157
Debt Securities, Available-for-sale, Amortized Cost, Total	13,445,888	9,727,401
Available for Sale, Due in one year or less, Estimated Fair Value	87,557
Available for Sale, Due after one year through five years, Estimated Fair Value	2,769,753
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,820,222
Available for Sale, Due after ten years, Estimated Fair Value	9,204,737
Debt Securities, Available-for-sale	13,924,628	10,437,565
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	37
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	515,100
Held to Maturity, Due after ten years, Amortized Cost	12,127
Held to Maturity, Amortized Cost	527,264	528,784
Held to Maturity, Due in one year or less, Estimated Fair Value	37
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	533,454
Held to Maturity, Due after ten years, Estimated Fair Value	12,539
Held to Maturity, Estimated Fair Value	546,030	570,526
Available for Sale, Due in one year or less, Amortized Cost	63
Available for Sale, Due after one year through five years, Amortized Cost	15,278
Available for Sale, Due after five years through ten years, Amortized Cost	19,175
Available for Sale, Due after ten years, Amortized Cost	4,025,176
Debt Securities, Available-for-sale, Amortized Cost, Total	4,059,692	1,916,581
Available for Sale, Due in one year or less, Estimated Fair Value	64
Available for Sale, Due after one year through five years, Estimated Fair Value	15,954
Available for Sale, Due after five years through ten years, Estimated Fair Value	19,624
Available for Sale, Due after ten years, Estimated Fair Value	4,030,623
Debt Securities, Available-for-sale	4,066,265	1,987,679
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	464,112
Held to Maturity, Due after one year through five years, Amortized Cost	180,373
Held to Maturity, Due after five years through ten years, Amortized Cost	73,808
Held to Maturity, Due after ten years, Amortized Cost	502,280
Held to Maturity, Amortized Cost	1,220,573	1,415,549
Held to Maturity, Due in one year or less, Estimated Fair Value	468,685
Held to Maturity, Due after one year through five years, Estimated Fair Value	185,458
Held to Maturity, Due after five years through ten years, Estimated Fair Value	74,729
Held to Maturity, Due after ten years, Estimated Fair Value	532,741
Held to Maturity, Estimated Fair Value	1,261,613	1,480,870
Available for Sale, Due in one year or less, Amortized Cost	86,485
Available for Sale, Due after one year through five years, Amortized Cost	1,611,040
Available for Sale, Due after five years through ten years, Amortized Cost	808,323
Available for Sale, Due after ten years, Amortized Cost	4,672,287
Debt Securities, Available-for-sale, Amortized Cost, Total	7,178,135	6,683,927
Available for Sale, Due in one year or less, Estimated Fair Value	87,493
Available for Sale, Due after one year through five years, Estimated Fair Value	1,715,065
Available for Sale, Due after five years through ten years, Estimated Fair Value	858,485
Available for Sale, Due after ten years, Estimated Fair Value	4,975,528
Debt Securities, Available-for-sale	7,636,571	7,287,902
Other [Member]
Schedule of Available-for-Sale Securities [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, 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, Total	42,359	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
Debt Securities, Available-for-sale	42,359	42,351
US Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Due in one year or less, Amortized Cost	0
Available for Sale, Due after one year through five years, Amortized Cost	1,030,591
Available for Sale, Due after five years through ten years, Amortized Cost	943,417
Available for Sale, Due after ten years, Amortized Cost	191,694
Debt Securities, Available-for-sale, Amortized Cost, Total	2,165,702	1,084,542
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	1,038,734
Available for Sale, Due after five years through ten years, Estimated Fair Value	942,113
Available for Sale, Due after ten years, Estimated Fair Value	198,586
Debt Securities, Available-for-sale	$ 2,179,433	$ 1,119,633