Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,157,812	$ 10,149,061
Total interest-bearing deposits	13,683,342	13,986,869
Total deposits	$ 23,841,154	$ 24,135,930
Total non-interest-bearing demand deposits, Percentage of Total	42.60%	42.00%
Total interest-bearing deposits, Percentage of Total	57.40%	58.00%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 9,341,295	$ 9,256,045
Total non-interest-bearing demand deposits, Percentage of Total	39.20%	38.30%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 386,369	$ 429,000
Total non-interest-bearing demand deposits, Percentage of Total	1.60%	1.80%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 4,753,351	$ 4,743,963
Money market accounts	7,648,184	7,860,403
Time accounts of $100,000 or more	438,197	490,209
Time accounts under $100,000	422,944	454,220
Total private accounts	$ 13,262,676	$ 13,548,795
Savings and interest checking, Percentage of Total	19.90%	19.70%
Money market accounts, Percentage of Total	32.10%	32.60%
Time accounts of $100,000 or more, Percentage of Total	1.80%	2.00%
Time accounts under $100,000, Percentage of Total	1.80%	1.90%
Total private accounts, Percentage of Total	55.60%	56.20%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 430,148	$ 464,016
Savings and interest checking	303,940	326,090
Money market accounts	67,696	57,145
Time accounts of $100,000 or more	48,015	53,684
Time accounts under $100,000	1,015	1,155
Total public funds	$ 420,666	$ 438,074
Total non-interest-bearing demand deposits, Percentage of Total	1.80%	1.90%
Savings and interest checking, Percentage of Total	1.30%	1.40%
Money market accounts, Percentage of Total	0.30%	0.20%
Time accounts of $100,000 or more, Percentage of Total	0.20%	0.20%
Time accounts under $100,000, Percentage of Total	0.00%	0.00%
Total public funds, Percentage of Total	1.80%	1.80%