Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 14,671,989	$ 10,873,629
Total interest-bearing deposits	18,007,107	16,765,935
Total deposits	$ 32,679,096	$ 27,639,564
Total non-interest-bearing demand deposits, Percentage of Total	44.90%	39.30%
Total interest-bearing deposits, Percentage of Total	55.10%	60.70%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 13,866,774	$ 10,212,265
Total non-interest-bearing demand deposits, Percentage of Total	42.40%	36.90%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 283,893	$ 246,181
Total non-interest-bearing demand deposits, Percentage of Total	0.90%	0.90%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 7,937,394	$ 7,147,327
Money market accounts	8,431,539	7,888,433
Time accounts of $100,000 or More	779,859	736,481
Time accounts under $100,000	335,131	347,418
Total private accounts	$ 17,483,923	$ 16,119,659
Savings and interest checking, Percentage of Total	24.30%	25.90%
Money market accounts, Percentage of Total	25.80%	28.50%
Time accounts of $100,000 or more, Percentage of Total	2.40%	2.70%
Time accounts under $100,000, Percentage of Total	1.00%	1.20%
Total private accounts, Percentage of Total	53.50%	58.30%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 521,322	$ 415,183
Savings and interest checking	442,349	548,399
Money market accounts	76,437	73,180
Time accounts of $100,000 or More	4,315	24,672
Time accounts under $100,000	83	25
Total public funds	$ 523,184	$ 646,276
Total non-interest-bearing demand deposits, Percentage of Total	1.60%	1.50%
Savings and interest checking, Percentage of Total	1.40%	2.00%
Money market accounts, Percentage of Total	0.20%	0.30%
Time accounts of $100,000 or more, Percentage of Total	0.00%	0.10%
Time accounts under $100,000, Percentage of Total	0.00%	0.00%
Total public funds, Percentage of Total	1.60%	2.40%