Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,280,060	$ 10,270,233
Total interest-bearing deposits	14,827,488	14,073,362
Total deposits	$ 25,107,548	$ 24,343,595
Total non-interest-bearing demand deposits, Percentage of Total	40.90%	42.20%
Total interest-bearing deposits, Percentage of Total	59.10%	57.80%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 9,422,496	$ 9,251,463
Total non-interest-bearing demand deposits, Percentage of Total	37.50%	38.00%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 316,037	$ 378,930
Total non-interest-bearing demand deposits, Percentage of Total	1.30%	1.60%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 6,094,960	$ 5,149,905
Money market accounts	7,510,102	7,536,998
Time accounts of $100,000 or More	457,229	463,740
Time accounts under $100,000	343,593	362,683
Total private accounts	$ 14,405,884	$ 13,513,326
Savings and interest checking, Percentage of Total	24.30%	21.10%
Money market accounts, Percentage of Total	29.90%	31.00%
Time accounts of $100,000 or more, Percentage of Total	1.80%	1.90%
Time accounts under $100,000, Percentage of Total	1.40%	1.50%
Total private accounts, Percentage of Total	57.40%	55.50%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 541,527	$ 639,840
Savings and interest checking	314,998	420,324
Money market accounts	88,553	93,969
Time accounts of $100,000 or More	17,379	44,941
Time accounts under $100,000	674	802
Total public funds	$ 421,604	$ 560,036
Total non-interest-bearing demand deposits, Percentage of Total	2.10%	2.60%
Savings and interest checking, Percentage of Total	1.30%	1.70%
Money market accounts, Percentage of Total	0.30%	0.40%
Time accounts of $100,000 or more, Percentage of Total	0.10%	0.20%
Time accounts under $100,000, Percentage of Total	0.00%	0.00%
Total public funds, Percentage of Total	1.70%	2.30%