Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,128,908	$ 10,997,494
Total interest-bearing deposits	16,165,955	16,151,710
Total deposits	$ 26,294,863	$ 27,149,204
Total non-interest-bearing demand deposits, Percentage of Total	38.50%	40.50%
Total interest-bearing deposits, Percentage of Total	61.50%	59.50%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 9,621,070	$ 10,305,850
Total non-interest-bearing demand deposits, Percentage of Total	36.60%	37.90%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 197,784	$ 235,748
Total non-interest-bearing demand deposits, Percentage of Total	0.70%	0.90%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 6,975,228	$ 6,977,813
Money market accounts	7,755,793	7,777,470
Time accounts of $100,000 or More	592,708	526,789
Time accounts under $100,000	340,405	331,511
Total private accounts	$ 15,664,134	$ 15,613,583
Savings and interest checking, Percentage of Total	26.50%	25.70%
Money market accounts, Percentage of Total	29.50%	28.60%
Time accounts of $100,000 or more, Percentage of Total	2.30%	2.00%
Time accounts under $100,000, Percentage of Total	1.30%	1.20%
Total private accounts, Percentage of Total	59.60%	57.50%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 310,054	$ 455,896
Savings and interest checking	428,420	473,754
Money market accounts	69,245	59,953
Time accounts of $100,000 or More	4,068	4,332
Time accounts under $100,000	88	88
Total public funds	$ 501,821	$ 538,127
Total non-interest-bearing demand deposits, Percentage of Total	1.20%	1.70%
Savings and interest checking, Percentage of Total	1.60%	1.80%
Money market accounts, Percentage of Total	0.30%	0.20%
Time accounts of $100,000 or more, Percentage of Total	0.00%	0.00%
Time accounts under $100,000, Percentage of Total	0.00%	0.00%
Total public funds, Percentage of Total	1.90%	2.00%