Deposits - Schedule of Deposits (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 7,675,466	$ 8,096,937	$ 6,784,112
Total interest-bearing deposits	11,368,438	11,400,429	10,124,909
Total deposits	19,043,904	19,497,366	16,909,021
Total non-interest-bearing demand deposits, Percentage of Total	40.30%	41.50%	40.10%
Total interest-bearing deposits, Percentage of Total	59.70%	58.50%	59.90%
Total deposits, Percentage of Total	100.00%	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	7,014,286	7,186,105	6,006,806
Total non-interest-bearing demand deposits, Percentage of Total	36.80%	36.90%	35.50%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	307,469	436,381	331,114
Total non-interest-bearing demand deposits, Percentage of Total	1.60%	2.20%	2.00%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	353,711	474,451	446,192
Savings and interest checking	223,295	287,391	208,760
Money market accounts	45,266	50,600	42,488
Time accounts of $100,000 or more	133,157	140,191	136,817
Time accounts under $100,000	3,881	3,292	3,376
Total public funds	405,599	481,474	391,441
Total non-interest-bearing demand deposits, Percentage of Total	1.90%	2.40%	2.60%
Savings and interest checking, Percentage of Total	1.20%	1.50%	1.20%
Money market accounts, Percentage of Total	0.20%	0.30%	0.30%
Time accounts of $100,000 or more, Percentage of Total	0.70%	0.70%	0.80%
Total public funds, Percentage of Total	2.10%	2.50%	2.30%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	3,571,010	3,812,712	2,950,197
Money market accounts	6,404,140	6,127,256	5,750,104
Time accounts of $100,000 or more	531,936	514,346	532,544
Time accounts under $100,000	455,753	464,641	500,623
Total private accounts	$ 10,962,839	$ 10,918,955	$ 9,733,468
Savings and interest checking, Percentage of Total	18.80%	19.60%	17.40%
Money market accounts, Percentage of Total	33.60%	31.40%	34.00%
Time accounts of $100,000 or more, Percentage of Total	2.80%	2.60%	3.20%
Time accounts under $100,000, Percentage of Total	2.40%	2.40%	3.00%
Total private accounts, Percentage of Total	57.60%	56.00%	57.60%