Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 17,151,583	$ 15,117,051
Interest-bearing Deposit Liabilities, Total	22,461,184	19,898,710
Total deposits	$ 39,612,767	$ 35,015,761
Total non-interest-bearing demand deposits, Percentage of Total	43.30%	43.20%
Percentage of Interest-bearing Domestic Deposits to Deposits	56.70%	56.80%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 15,997,472	$ 13,914,754
Total non-interest-bearing demand deposits, Percentage of Total	40.40%	39.70%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 223,466	$ 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	$ 10,326,315	$ 9,132,789
Money market accounts	10,381,703	8,977,585
Time Deposits	1,086,894	1,135,575
Interest-bearing Deposit Liabilities, Total	$ 21,794,912	$ 19,245,949
Savings and interest checking, Percentage of Total	26.10%	26.10%
Money market accounts, Percentage of Total	26.20%	25.60%
Percentage Of Time Accounts To Deposits	2.70%	3.30%
Percentage of Interest-bearing Domestic Deposits to Deposits	55.00%	55.00%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 930,645	$ 960,072
Savings and interest checking	580,174	597,503
Money market accounts	61,028	50,070
Time Deposits	25,070	5,188
Interest-bearing Deposit Liabilities, Total	$ 666,272	$ 652,761
Total non-interest-bearing demand deposits, Percentage of Total	2.30%	2.80%
Savings and interest checking, Percentage of Total	1.50%	1.70%
Money market accounts, Percentage of Total	0.10%	0.10%
Percentage Of Time Accounts To Deposits	0.10%	0.00%
Percentage of Interest-bearing Domestic Deposits to Deposits	1.70%	1.80%