Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 11,174,251	$ 10,513,369
Total interest-bearing deposits	15,229,018	15,298,206
Total deposits	$ 26,403,269	$ 25,811,575
Total non-interest-bearing demand deposits, Percentage of Total	42.30%	40.80%
Total interest-bearing deposits, Percentage of Total	57.70%	59.20%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,466,844	$ 9,670,989
Total non-interest-bearing demand deposits, Percentage of Total	39.60%	37.50%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 226,313	$ 280,751
Total non-interest-bearing demand deposits, Percentage of Total	0.90%	1.10%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 6,449,079	$ 6,436,065
Money market accounts	7,607,675	7,486,431
Time accounts of $100,000 or More	454,096	460,028
Time accounts under $100,000	323,748	338,714
Total private accounts	$ 14,834,598	$ 14,721,238
Savings and interest checking, Percentage of Total	24.40%	24.90%
Money market accounts, Percentage of Total	28.80%	29.00%
Time accounts of $100,000 or more, Percentage of Total	1.70%	1.80%
Time accounts under $100,000, Percentage of Total	1.30%	1.30%
Total private accounts, Percentage of Total	56.20%	57.00%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 481,094	$ 561,629
Savings and interest checking	312,430	446,872
Money market accounts	68,018	113,669
Time accounts of $100,000 or More	13,462	15,748
Time accounts under $100,000	510	679
Total public funds	$ 394,420	$ 576,968
Total non-interest-bearing demand deposits, Percentage of Total	1.80%	2.20%
Savings and interest checking, Percentage of Total	1.20%	1.70%
Money market accounts, Percentage of Total	0.30%	0.40%
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	1.50%	2.20%