Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,934,162	$ 11,197,093
Total interest-bearing deposits	15,743,616	15,675,296
Total deposits	$ 26,677,778	$ 26,872,389
Total non-interest-bearing demand deposits, Percentage of Total	41.00%	41.70%
Total interest-bearing deposits, Percentage of Total	59.00%	58.30%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,294,977	$ 10,412,882
Total non-interest-bearing demand deposits, Percentage of Total	38.60%	38.80%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 212,199	$ 222,648
Total non-interest-bearing demand deposits, Percentage of Total	0.80%	0.80%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 6,869,783	$ 6,788,766
Money market accounts	7,706,531	7,624,471
Time accounts of $100,000 or More	458,103	453,668
Time accounts under $100,000	322,105	324,636
Total private accounts	$ 15,356,522	$ 15,191,541
Savings and interest checking, Percentage of Total	25.80%	25.20%
Money market accounts, Percentage of Total	28.90%	28.40%
Time accounts of $100,000 or more, Percentage of Total	1.70%	1.70%
Time accounts under $100,000, Percentage of Total	1.20%	1.20%
Total private accounts, Percentage of Total	57.60%	56.50%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 426,986	$ 561,563
Savings and interest checking	308,609	410,140
Money market accounts	63,155	59,008
Time accounts of $100,000 or More	14,892	14,301
Time accounts under $100,000	438	306
Total public funds	$ 387,094	$ 483,755
Total non-interest-bearing demand deposits, Percentage of Total	1.60%	2.10%
Savings and interest checking, Percentage of Total	1.10%	1.50%
Money market accounts, Percentage of Total	0.20%	0.20%
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	1.40%	1.80%