Loans and Allowance for Credit Losses Part 1 (Details) (USD $)	3 Months Ended
	Mar. 31, 2014		Dec. 31, 2013		Mar. 31, 2013
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest	$ 4,289,302,000		$ 4,327,637,000		$ 3,980,721,000
Loans, variable rate of interest	8,683,846,000		8,363,478,000		7,980,210,000
Loans, non-accrual	104,706,000		101,149,000		132,633,000
Total	13,077,854,000		12,792,264,000		12,093,564,000
Accruing loans past due (90 days)	1,991,000	[1]	1,415,000	[1]	4,229,000	[1]
Loans receivable, other information [Abstract]
Amount of total loan portfolio to businesses and individuals in Texas	4,400,000,000
Percentage of total loan portfolio to businesses and individuals in Texas (in hundredths)	34.00%
Amount of total loan portfolio to businesses and individuals in Oklahoma	3,200,000,000
Percentage of total loan portfolio to businesses and individuals in Oklahoma (in hundredths)	24.00%
Credit Commitments [Abstract]
Outstanding commitments to extend credit	7,200,000,000
Outstanding standby letters of credit	440,000,000
Outstanding commercial letters of credit	12,000,000
Commercial [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest	1,649,164,000		1,637,620,000		1,382,309,000
Loans, variable rate of interest	6,383,495,000		6,288,841,000		6,016,135,000
Loans, non-accrual	19,047,000		16,760,000		19,861,000
Total	8,051,706,000		7,943,221,000		7,418,305,000
Loans receivable, other information [Abstract]
Amount of loan portfolio to businesses in Texas	2,900,000,000
Percentage of loan portfolio to businesses in Texas (in hundredths)	36.00%
Amount of loan portfolio to businesses in Oklahoma	1,800,000,000
Percentage of loan portfolio to businesses in Oklahoma (in hundredths)	22.00%
Energy [Member]
Loan receivables disclosure [Abstract]
Total	2,344,072,000		2,351,760,000		2,349,432,000
Loans receivable, other information [Abstract]
Percentage of energy loans in loan portfolio	18.00%
Amount of loans to energy producers	2,100,000,000
Percentage of committed energy production loans secured by oil	59.00%
Percentage of committed energy production loans secured by natural gas	41.00%
Services [Member]
Loan receivables disclosure [Abstract]
Total	2,232,471,000		2,282,210,000		2,114,799,000
Loans receivable, other information [Abstract]
Amount of loans with individual balances less than $10 million	1,200,000,000
Maximum loan amount for certain individual loans in category	10,000,000
Commercial Real Estate [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest	764,688,000		770,908,000		712,275,000
Loans, variable rate of interest	1,827,414,000		1,603,595,000		1,507,710,000
Loans, non-accrual	39,305,000		40,850,000		65,175,000
Total	2,631,407,000		2,415,353,000		2,285,160,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Texas (in hundredths)	32.00%
Percentage of loan portfolio secured by property in Oklahoma (in hundredths)	17.00%
Residential Mortgage [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest	1,749,693,000		1,783,615,000		1,732,058,000
Loans, variable rate of interest	223,602,000		226,092,000		234,966,000
Loans, non-accrual	45,380,000		42,319,000		45,426,000
Total	2,018,675,000		2,052,026,000		2,012,450,000
Permanent mortage [Member]
Loan receivables disclosure [Abstract]
Total	1,033,572,000		1,062,744,000		1,091,575,000
Loans receivable, other information [Abstract]
Minimum FICO required for jumbo loan approval	720
Maximum debt-to-income ratio on jumbo loans (in hundredths)	38.00%
Minimum loan-to-value ratio on jumbo loans (in hundredths)	60.00%
Maximum loan-to-value ratio on jumbo loans (in hundredths)	100.00%
Minimum period for fixed rate on variable rate jumbo loans (in years)	3 years
Maximum period for fixed rate on variable rate jumbo loans (in years)	10 years
Permanent mortgages guaranteed by US govenment agencies [Member]
Loan receivables disclosure [Abstract]
Total	184,822,000		181,598,000		162,419,000
Loans receivable, other information [Abstract]
Amount of loans guaranteed by US government and previously sold into GNMA mortgage pools	185,000,000
Home equity {Member]
Loan receivables disclosure [Abstract]
Total	800,281,000		807,684,000		758,456,000
Loans receivable, other information [Abstract]
Percentage of home equity portfolio comprised of first lien loans	70.00%
Percentage of home equity portfolio comprised of junior lien loans	30.00%
Percentage of junior lien home equity loans that are amortizing term loans	74.00%
Percentage of junior lien home equity loans that are revolving lines of credit	26.00%
Home equity loans, description	Home equity loans generally require a minimum FICO score of 700 and a maximum DTI of 40%.
Home equity loans, maximum	400,000
Home equity loans, revolving period	5 years
Home equity loans, amortization period following revolving period	15 years
Home equity loans, discretionary additional revolving period	5 years
Consumer [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest	125,757,000		135,494,000		154,079,000
Loans, variable rate of interest	249,335,000		244,950,000		221,399,000
Loans, non-accrual	974,000		1,220,000		2,171,000
Total	$ 376,066,000		$ 381,664,000		$ 377,649,000

[1] Excludes residential mortgage loans guaranteed by agencies of the U.S. government