Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,750,244	$ 10,997,494
Total interest-bearing deposits	16,333,338	16,151,710
Total deposits	$ 27,083,582	$ 27,149,204
Total non-interest-bearing demand deposits, Percentage of Total	39.70%	40.50%
Total interest-bearing deposits, Percentage of Total	60.30%	59.50%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,070,621	$ 10,305,850
Total non-interest-bearing demand deposits, Percentage of Total	37.20%	37.90%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 258,981	$ 235,748
Total non-interest-bearing demand deposits, Percentage of Total	1.00%	0.90%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 6,741,340	$ 6,977,813
Money market accounts	7,960,223	7,777,470
Time accounts of $100,000 or More	707,657	526,789
Time accounts under $100,000	346,440	331,511
Total private accounts	$ 15,755,660	$ 15,613,583
Savings and interest checking, Percentage of Total	24.90%	25.70%
Money market accounts, Percentage of Total	29.40%	28.60%
Time accounts of $100,000 or more, Percentage of Total	2.60%	2.00%
Time accounts under $100,000, Percentage of Total	1.30%	1.20%
Total private accounts, Percentage of Total	58.20%	57.50%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 420,642	$ 455,896
Savings and interest checking	483,303	473,754
Money market accounts	68,321	59,953
Time accounts of $100,000 or More	25,922	4,332
Time accounts under $100,000	132	88
Total public funds	$ 577,678	$ 538,127
Total non-interest-bearing demand deposits, Percentage of Total	1.50%	1.70%
Savings and interest checking, Percentage of Total	1.80%	1.80%
Money market accounts, Percentage of Total	0.20%	0.20%
Time accounts of $100,000 or more, Percentage of Total	0.10%	0.00%
Time accounts under $100,000, Percentage of Total	0.00%	0.00%
Total public funds, Percentage of Total	2.10%	2.00%