Deposits - Schedule of Deposits (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 7,744,676	$ 8,096,937
Total interest-bearing deposits	11,333,543	11,400,429
Total non-interest-bearing demand deposits, Percentage of Total	40.60%	41.50%
Total deposits	19,078,219	19,497,366
Total interest-bearing deposits, Percentage of Total	59.40%	58.50%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	7,057,425	7,186,105
Total non-interest-bearing demand deposits, Percentage of Total	37.00%	36.90%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	323,992	436,381
Total non-interest-bearing demand deposits, Percentage of Total	1.70%	2.20%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	363,259	474,451
Savings and interest checking	233,107	287,391
Money market accounts	42,548	50,600
Time accounts of $100,000 or more	146,300	140,191
Time accounts under $100,000	4,703	3,292
Total public funds	426,658	481,474
Total non-interest-bearing demand deposits, Percentage of Total	1.90%	2.40%
Savings and interest checking, Percentage of Total	1.20%	1.50%
Money market accounts, Percentage of Total	0.20%	0.30%
Time accounts of $100,000 or more, Percentage of Total	0.80%	0.70%
Time accounts under $100,000, Percentage of Total
Total public funds, Percentage of Total	2.20%	2.50%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	3,495,530	3,812,712
Money market accounts	6,438,506	6,127,256
Time accounts of $100,000 or more	523,944	514,346
Time accounts under $100,000	448,905	464,641
Total private accounts	$ 10,906,885	$ 10,918,955
Savings and interest checking, Percentage of Total	18.30%	19.60%
Money market accounts, Percentage of Total	33.80%	31.40%
Time accounts of $100,000 or more, Percentage of Total	2.70%	2.60%
Time accounts under $100,000, Percentage of Total	2.40%	2.40%
Total private accounts, Percentage of Total	57.20%	56.00%