Loans and Allowance for Credit Losses, Loans by Portfolio Type (Details) (USD $)	9 Months Ended
	Sep. 30, 2014		Dec. 31, 2013		Sep. 30, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Document Period End Date	Sep. 30, 2014
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest	$ 4,301,697,000		$ 4,327,636,000		$ 4,102,943,000
Loans, variable rate of interest	9,285,491,000		8,363,479,000		8,134,253,000
Loans, non-accrual	96,551,000		101,149,000		112,904,000
Total	13,683,739,000		12,792,264,000		12,350,100,000
Accruing loans past due (90 days)	25,000	[1]	1,415,000	[1]	188,000	[1]
Credit Commitments [Abstract]
Outstanding commitments to extend credit	7,700,000,000
Outstanding standby letters of credit	451,000,000
Outstanding commercial letters of credit	5,100,000
Commercial [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest	1,714,251,000		1,637,620,000		1,468,198,000
Loans, variable rate of interest	6,841,383,000		6,288,841,000		6,083,355,000
Loans, non-accrual	16,404,000		16,760,000		19,522,000
Total	8,572,038,000		7,943,221,000		7,571,075,000
Energy [Member]
Loan receivables disclosure [Abstract]
Total	2,551,699,000		2,351,760,000		2,311,991,000
Services [Member]
Loan receivables disclosure [Abstract]
Total	2,487,817,000		2,282,210,000		2,148,551,000
Commercial Real Estate [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest	757,846,000		770,908,000		730,733,000
Loans, variable rate of interest	1,935,693,000		1,603,595,000		1,565,994,000
Loans, non-accrual	30,660,000		40,850,000		52,502,000
Total	2,724,199,000		2,415,353,000		2,349,229,000
Residential Mortgage [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest	1,722,864,000		1,783,614,000		1,766,818,000
Loans, variable rate of interest	207,892,000		226,092,000		228,691,000
Loans, non-accrual	48,907,000		42,320,000		39,256,000
Total	1,979,663,000		2,052,026,000		2,034,765,000
Permanent mortage [Member]
Loan receivables disclosure [Abstract]
Total	991,107,000		1,062,744,000		1,078,661,000
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	198,488,000		181,598,000		163,919,000
Loans receivable, other information [Abstract]
Amount of loans guaranteed by US government and previously sold into GNMA mortgage pools	198,000,000
Home equity {Member]
Loan receivables disclosure [Abstract]
Total	790,068,000		807,684,000		792,185,000
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	72.00%
Percentage of junior lien home equity loans that are revolving lines of credit	28.00%
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	106,736,000		135,494,000		137,194,000
Loans, variable rate of interest	300,523,000		244,951,000		256,213,000
Loans, non-accrual	580,000		1,219,000		1,624,000
Total	407,839,000		381,664,000		395,031,000
Credit Concentration Risk [Member] | Energy [Member]
Loan receivables disclosure [Abstract]
Total	2,600,000,000
Loans receivable, other information [Abstract]
Percentage of energy loans in loan portfolio	19.00%
Credit Concentration Risk [Member] | Energy Producers [Member]
Loan receivables disclosure [Abstract]
Total	2,200,000,000
Loans receivable, other information [Abstract]
Percentage of committed energy production loans secured by oil	59.00%
Percentage of committed energy production loans secured by natural gas	41.00%
Credit Concentration Risk [Member] | Services [Member]
Loan receivables disclosure [Abstract]
Total	2,500,000,000
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
Texas [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total	4,600,000,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Texas (in hundredths)	34.00%
Texas [Member] | Geographic Concentration Risk [Member] | Commercial [Member]
Loan receivables disclosure [Abstract]
Total	2,900,000,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Texas (in hundredths)	34.00%
Texas [Member] | Geographic Concentration Risk [Member] | Commercial Real Estate [Member]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Texas (in hundredths)	35.00%
Oklahoma [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total	3,400,000,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Oklahoma (in hundredths)	25.00%
Oklahoma [Member] | Geographic Concentration Risk [Member] | Commercial [Member]
Loan receivables disclosure [Abstract]
Total	$ 2,000,000,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Oklahoma (in hundredths)	24.00%
Oklahoma [Member] | Geographic Concentration Risk [Member] | Commercial Real Estate [Member]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Oklahoma (in hundredths)	16.00%

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