Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,909,415	$ 10,513,369
Total interest-bearing deposits	15,232,749	15,298,206
Total deposits	$ 26,142,164	$ 25,811,575
Total non-interest-bearing demand deposits, Percentage of Total	41.70%	40.80%
Total interest-bearing deposits, Percentage of Total	58.30%	59.20%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,220,181	$ 9,670,989
Total non-interest-bearing demand deposits, Percentage of Total	39.10%	37.50%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 264,543	$ 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	$ 6,545,178	$ 6,436,065
Money market accounts	7,489,565	7,486,431
Time accounts of $100,000 or More	446,809	460,028
Time accounts under $100,000	332,543	338,714
Total private accounts	$ 14,814,095	$ 14,721,238
Savings and interest checking, Percentage of Total	25.00%	24.90%
Money market accounts, Percentage of Total	28.70%	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	$ 424,691	$ 561,629
Savings and interest checking	302,202	446,872
Money market accounts	99,658	113,669
Time accounts of $100,000 or More	15,762	15,748
Time accounts under $100,000	1,032	679
Total public funds	$ 418,654	$ 576,968
Total non-interest-bearing demand deposits, Percentage of Total	1.60%	2.20%
Savings and interest checking, Percentage of Total	1.10%	1.70%
Money market accounts, Percentage of Total	0.40%	0.40%
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	1.60%	2.20%