Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 9,649,933	$ 10,270,233
Total interest-bearing deposits	14,507,042	14,073,362
Total deposits	$ 24,156,975	$ 24,343,595
Total non-interest-bearing demand deposits, Percentage of Total	39.90%	42.20%
Total interest-bearing deposits, Percentage of Total	60.10%	57.80%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 8,796,847	$ 9,251,463
Total non-interest-bearing demand deposits, Percentage of Total	36.40%	38.00%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 310,417	$ 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	$ 5,732,748	$ 5,149,905
Money market accounts	7,514,572	7,536,998
Time accounts of $100,000 or more	415,207	420,697
Time accounts under $100,000	397,091	405,726
Total private accounts	$ 14,059,618	$ 13,513,326
Savings and interest checking, Percentage of Total	23.70%	21.10%
Money market accounts, Percentage of Total	31.10%	31.00%
Time accounts of $100,000 or more, Percentage of Total	1.70%	1.70%
Time accounts under $100,000, Percentage of Total	1.70%	1.70%
Total private accounts, Percentage of Total	58.20%	55.50%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 542,669	$ 639,840
Savings and interest checking	304,500	420,324
Money market accounts	113,713	93,969
Time accounts of $100,000 or more	28,203	44,941
Time accounts under $100,000	1,008	802
Total public funds	$ 447,424	$ 560,036
Total non-interest-bearing demand deposits, Percentage of Total	2.20%	2.60%
Savings and interest checking, Percentage of Total	1.30%	1.70%
Money market accounts, Percentage of Total	0.50%	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	1.90%	2.30%