Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,707,365	$ 10,513,369
Total interest-bearing deposits	14,906,208	15,298,206
Total deposits	$ 25,613,573	$ 25,811,575
Total non-interest-bearing demand deposits, Percentage of Total	41.80%	40.80%
Total interest-bearing deposits, Percentage of Total	58.20%	59.20%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,076,475	$ 9,670,989
Total non-interest-bearing demand deposits, Percentage of Total	39.30%	37.50%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 259,686	$ 280,751
Total non-interest-bearing demand deposits, Percentage of Total	1.00%	1.10%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 6,349,462	$ 6,436,065
Money market accounts	7,403,054	7,486,431
Time accounts of $100,000 or More	432,406	460,028
Time accounts under $100,000	327,228	338,714
Total private accounts	$ 14,512,150	$ 14,721,238
Savings and interest checking, Percentage of Total	24.80%	24.90%
Money market accounts, Percentage of Total	28.90%	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	56.70%	57.00%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 371,204	$ 561,629
Savings and interest checking	299,505	446,872
Money market accounts	77,361	113,669
Time accounts of $100,000 or More	16,426	15,748
Time accounts under $100,000	766	679
Total public funds	$ 394,058	$ 576,968
Total non-interest-bearing demand deposits, Percentage of Total	1.50%	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%