Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 16,103,649	$ 15,117,051
Interest-bearing Deposit Liabilities, Total	20,821,531	19,898,710
Total deposits	$ 36,925,180	$ 35,015,761
Total non-interest-bearing demand deposits, Percentage of Total	43.60%	43.20%
Percentage of Interest-bearing Domestic Deposits to Deposits	56.40%	56.80%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 15,002,423	$ 13,914,754
Total non-interest-bearing demand deposits, Percentage of Total	40.60%	39.70%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 213,554	$ 242,225
Total non-interest-bearing demand deposits, Percentage of Total	0.60%	0.70%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 9,648,088	$ 9,132,789
Money market accounts	9,416,210	8,977,585
Time Deposits	1,141,660	1,135,575
Interest-bearing Deposit Liabilities, Total	$ 20,205,958	$ 19,245,949
Savings and interest checking, Percentage of Total	26.10%	26.10%
Money market accounts, Percentage of Total	25.50%	25.60%
Percentage Of Time Accounts To Deposits	3.10%	3.30%
Percentage of Interest-bearing Domestic Deposits to Deposits	54.70%	55.00%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 887,672	$ 960,072
Savings and interest checking	556,523	597,503
Money market accounts	53,496	50,070
Time Deposits	5,554	5,188
Interest-bearing Deposit Liabilities, Total	$ 615,573	$ 652,761
Total non-interest-bearing demand deposits, Percentage of Total	2.40%	2.80%
Savings and interest checking, Percentage of Total	1.50%	1.70%
Money market accounts, Percentage of Total	0.20%	0.10%
Percentage Of Time Accounts To Deposits	0.00%	0.00%
Percentage of Interest-bearing Domestic Deposits to Deposits	1.70%	1.80%