Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 11,202,208	$ 10,873,629
Total interest-bearing deposits	16,938,659	16,765,935
Total deposits	$ 28,140,867	$ 27,639,564
Total non-interest-bearing demand deposits, Percentage of Total	39.80%	39.30%
Total interest-bearing deposits, Percentage of Total	60.20%	60.70%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,524,338	$ 10,212,265
Total non-interest-bearing demand deposits, Percentage of Total	37.40%	36.90%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 221,761	$ 246,181
Total non-interest-bearing demand deposits, Percentage of Total	0.80%	0.90%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 7,240,821	$ 7,147,327
Money market accounts	7,995,061	7,888,433
Time accounts of $100,000 or More	773,373	736,481
Time accounts under $100,000	347,349	347,418
Total private accounts	$ 16,356,604	$ 16,119,659
Savings and interest checking, Percentage of Total	25.70%	25.90%
Money market accounts, Percentage of Total	28.40%	28.50%
Time accounts of $100,000 or more, Percentage of Total	2.80%	2.70%
Time accounts under $100,000, Percentage of Total	1.20%	1.20%
Total private accounts, Percentage of Total	58.10%	58.30%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 456,109	$ 415,183
Savings and interest checking	479,629	548,399
Money market accounts	78,899	73,180
Time accounts of $100,000 or More	23,502	24,672
Time accounts under $100,000	25	25
Total public funds	$ 582,055	$ 646,276
Total non-interest-bearing demand deposits, Percentage of Total	1.60%	1.50%
Savings and interest checking, Percentage of Total	1.70%	2.00%
Money market accounts, Percentage of Total	0.30%	0.30%
Time accounts of $100,000 or more, Percentage of Total	0.10%	0.10%
Time accounts under $100,000, Percentage of Total	0.00%	0.00%
Total public funds, Percentage of Total	2.10%	2.40%