Loans and Allowance for Credit Losses Part 1 (Details) (USD $)	6 Months Ended
	Jun. 30, 2014		Dec. 31, 2013		Jun. 30, 2013
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest	$ 4,293,820,000		$ 4,327,637,000		$ 4,057,409,000
Loans, variable rate of interest	9,036,358,000		8,363,478,000		8,261,240,000
Loans, non-accrual	96,680,000		101,149,000		122,133,000
Total	13,426,858,000		12,792,264,000		12,440,782,000
Accruing loans past due (90 days)	67,000	[1]	1,415,000	[1]	2,460,000	[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)	33.00%
Amount of total loan portfolio to businesses and individuals in Oklahoma	3,400,000,000
Percentage of total loan portfolio to businesses and individuals in Oklahoma (in hundredths)	26.00%
Credit Commitments [Abstract]
Outstanding commitments to extend credit	7,500,000,000
Outstanding standby letters of credit	469,000,000
Outstanding commercial letters of credit	11,000,000
Commercial [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest	1,681,348,000		1,637,620,000		1,447,823,000
Loans, variable rate of interest	6,669,210,000		6,288,841,000		6,239,428,000
Loans, non-accrual	17,103,000		16,760,000		20,869,000
Total	8,367,661,000		7,943,221,000		7,708,120,000
Loans receivable, other information [Abstract]
Amount of loan portfolio to businesses in Texas	2,800,000,000
Percentage of loan portfolio to businesses in Texas (in hundredths)	34.00%
Amount of loan portfolio to businesses in Oklahoma	2,000,000,000
Percentage of loan portfolio to businesses in Oklahoma (in hundredths)	24.00%
Energy [Member]
Loan receivables disclosure [Abstract]
Total	2,419,788,000		2,351,760,000		2,384,746,000
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	2,377,065,000		2,282,210,000		2,204,253,000
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	744,101,000		770,908,000		698,242,000
Loans, variable rate of interest	1,876,405,000		1,603,595,000		1,560,161,000
Loans, non-accrual	34,472,000		40,850,000		58,693,000
Total	2,654,978,000		2,415,353,000		2,317,096,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Texas (in hundredths)	33.00%
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	1,753,186,000		1,783,615,000		1,768,607,000
Loans, variable rate of interest	210,689,000		226,092,000		230,644,000
Loans, non-accrual	44,340,000		42,319,000		40,534,000
Total	2,008,215,000		2,052,026,000		2,039,785,000
Permanent mortage [Member]
Loan receivables disclosure [Abstract]
Total	1,020,928,000		1,062,744,000		1,095,871,000
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	188,087,000		181,598,000		156,887,000
Loans receivable, other information [Abstract]
Amount of loans guaranteed by US government and previously sold into GNMA mortgage pools	188,000,000
Home equity {Member]
Loan receivables disclosure [Abstract]
Total	799,200,000		807,684,000		787,027,000
Loans receivable, other information [Abstract]
Percentage of home equity portfolio comprised of first lien loans	69.00%
Percentage of home equity portfolio comprised of junior lien loans	31.00%
Percentage of junior lien home equity loans that are amortizing term loans	73.00%
Percentage of junior lien home equity loans that are revolving lines of credit	27.00%
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	115,185,000		135,494,000		142,737,000
Loans, variable rate of interest	280,054,000		244,950,000		231,007,000
Loans, non-accrual	765,000		1,220,000		2,037,000
Total	$ 396,004,000		$ 381,664,000		$ 375,781,000

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