Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,840,513	$ 11,197,093
Total interest-bearing deposits	15,508,341	15,675,296
Total deposits	$ 26,348,854	$ 26,872,389
Total non-interest-bearing demand deposits, Percentage of Total	41.10%	41.70%
Total interest-bearing deposits, Percentage of Total	58.90%	58.30%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,137,696	$ 10,412,882
Total non-interest-bearing demand deposits, Percentage of Total	38.40%	38.80%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 187,589	$ 222,648
Total non-interest-bearing demand deposits, Percentage of Total	0.70%	0.80%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 6,640,382	$ 6,788,766
Money market accounts	7,694,628	7,624,471
Time accounts of $100,000 or More	484,236	453,668
Time accounts under $100,000	327,790	324,636
Total private accounts	$ 15,147,036	$ 15,191,541
Savings and interest checking, Percentage of Total	25.20%	25.20%
Money market accounts, Percentage of Total	29.20%	28.40%
Time accounts of $100,000 or more, Percentage of Total	1.80%	1.70%
Time accounts under $100,000, Percentage of Total	1.30%	1.20%
Total private accounts, Percentage of Total	57.50%	56.50%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 515,228	$ 561,563
Savings and interest checking	303,150	410,140
Money market accounts	54,537	59,008
Time accounts of $100,000 or More	3,523	14,301
Time accounts under $100,000	95	306
Total public funds	$ 361,305	$ 483,755
Total non-interest-bearing demand deposits, Percentage of Total	2.00%	2.10%
Savings and interest checking, Percentage of Total	1.20%	1.50%
Money market accounts, Percentage of Total	0.20%	0.20%
Time accounts of $100,000 or more, Percentage of Total	0.00%	0.10%
Time accounts under $100,000, Percentage of Total	0.00%	0.00%
Total public funds, Percentage of Total	1.40%	1.80%