Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 9,779,387	$ 10,270,233
Total interest-bearing deposits	14,508,064	14,073,362
Total deposits	$ 24,287,451	$ 24,343,595
Total non-interest-bearing demand deposits, Percentage of Total	40.30%	42.20%
Total interest-bearing deposits, Percentage of Total	59.70%	57.80%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 8,925,728	$ 9,251,463
Total non-interest-bearing demand deposits, Percentage of Total	36.80%	38.00%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 299,653	$ 378,930
Total non-interest-bearing demand deposits, Percentage of Total	1.20%	1.60%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 5,823,049	$ 5,149,905
Money market accounts	7,435,743	7,536,998
Time accounts of $100,000 or More	412,333	420,697
Time accounts under $100,000	394,882	405,726
Total private accounts	$ 14,066,007	$ 13,513,326
Savings and interest checking, Percentage of Total	24.00%	21.10%
Money market accounts, Percentage of Total	30.60%	31.00%
Time accounts of $100,000 or more, Percentage of Total	1.70%	1.70%
Time accounts under $100,000, Percentage of Total	1.60%	1.70%
Total private accounts, Percentage of Total	57.90%	55.50%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 554,006	$ 639,840
Savings and interest checking	320,571	420,324
Money market accounts	103,084	93,969
Time accounts of $100,000 or More	17,648	44,941
Time accounts under $100,000	754	802
Total public funds	$ 442,057	$ 560,036
Total non-interest-bearing demand deposits, Percentage of Total	2.30%	2.60%
Savings and interest checking, Percentage of Total	1.30%	1.70%
Money market accounts, Percentage of Total	0.40%	0.40%
Time accounts of $100,000 or more, Percentage of Total	0.10%	0.20%
Time accounts under $100,000, Percentage of Total	0.00%	0.00%
Total public funds, Percentage of Total	1.80%	2.30%