Loans and Allowances for Credit Losses, Loans by Portfolio Segment (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Loans receivables disclosure [Abstract]
Loans receivables with fixed rate of interest		$ 4,169,034	$ 4,259,974
Loans receivables with variable rate of interest		11,624,992	9,867,292
Loan receivable non-accrual		147,128	80,771
Total		15,941,154	14,208,037
Accruing loans past due (90 days)	[1]	1,207	125
Foregone interest on nonaccrual loans		7,432	8,170
Credit Commitments [Abstract]
Outstanding commitments to extend credit		8,500,000
Outstanding standby letters of credit		508,000
Outstanding commercial letters of credit		7,900
Geographic Concentration Risk [Member] | Texas [Member]
Loans receivables disclosure [Abstract]
Total		$ 5,300,000	$ 4,900,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Texas		33.00%	34.00%
Geographic Concentration Risk [Member] | Oklahoma [Member]
Loans receivables disclosure [Abstract]
Total		$ 3,900,000	$ 3,400,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Oklahoma		24.00%	24.00%
Commercial [Member]
Loans receivables disclosure [Abstract]
Loans receivables with fixed rate of interest		$ 1,850,548	$ 1,736,976
Loans receivables with variable rate of interest		8,325,559	7,345,167
Loan receivable non-accrual		76,424	13,527
Total		10,252,531	9,095,670
Commercial [Member] | Geographic Concentration Risk [Member] | Texas [Member]
Loans receivables disclosure [Abstract]
Total		$ 3,500,000	$ 3,200,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Texas		34.00%	36.00%
Commercial [Member] | Geographic Concentration Risk [Member] | Oklahoma [Member]
Loans receivables disclosure [Abstract]
Total		$ 2,500,000	$ 2,000,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Oklahoma		24.00%	22.00%
Commercial [Member] | Energy [Member]
Loans receivables disclosure [Abstract]
Total		$ 3,097,328	$ 2,860,428
Commercial [Member] | Energy [Member] | Credit Concentration Risk [Member]
Loans receivables disclosure [Abstract]
Total		$ 3,100,000	$ 2,900,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans		19.00%	20.00%
Commercial [Member] | Energy Producers [Member] | Credit Concentration Risk [Member]
Loans receivables disclosure [Abstract]
Total		$ 2,500,000	$ 2,500,000
Loans receivable, other information [Abstract]
Percentage of committed energy production loans secured by oil		62.00%	59.00%
Percentage of committed energy production loans secured by natural gas		38.00%	41.00%
Commercial [Member] | Services [Member]
Loans receivables disclosure [Abstract]
Total		$ 2,784,276	$ 2,391,530
Commercial [Member] | Services [Member] | Credit Concentration Risk [Member]
Loans receivables disclosure [Abstract]
Total		$ 2,800,000	$ 2,400,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans		17.00%	17.00%
Amount of loans with individual balances less than $10 million		$ 1,200,000	$ 1,200,000
Maximum loan amount for certain individual loans in category		10,000	10,000
Commercial [Member] | Healthcare [Member]
Loans receivables disclosure [Abstract]
Total		1,883,380	1,454,969
Commercial [Member] | Healthcare [Member] | Credit Concentration Risk [Member]
Loans receivables disclosure [Abstract]
Total		$ 1,900,000	$ 1,500,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans		12.00%	10.00%
Commercial Real Estate [Member]
Loans receivables disclosure [Abstract]
Loans receivables with fixed rate of interest		$ 627,678	$ 721,513
Loans receivables with variable rate of interest		2,622,354	1,988,080
Loan receivable non-accrual		9,001	18,557
Total		$ 3,259,033	$ 2,728,150
Commercial Real Estate [Member] | Geographic Concentration Risk [Member] | Texas [Member]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Texas (in hundredths)		30.00%	34.00%
Commercial Real Estate [Member] | Geographic Concentration Risk [Member] | Oklahoma [Member]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Oklahoma (in hundredths)		13.00%	16.00%
Residential Mortgage [Member]
Loans receivables disclosure [Abstract]
Loans receivables with fixed rate of interest		$ 1,598,992	$ 1,698,620
Loans receivables with variable rate of interest		216,661	202,771
Loan receivable non-accrual		61,240	48,121
Total		$ 1,876,893	$ 1,949,512
Residential Mortgage [Member] | Geographic Concentration Risk [Member] | Texas [Member]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Texas (in hundredths)		29.00%	28.00%
Residential Mortgage [Member] | Geographic Concentration Risk [Member] | Oklahoma [Member]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Oklahoma (in hundredths)		37.00%	38.00%
Residential Mortgage [Member] | Geographic Concentration Risk [Member] | New Mexico [Memnber]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in New Mexico (in hundredths)		12.00%	12.00%
Residential Mortgage [Member] | Geographic Concentration Risk [Member] | Colorado [Member]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in New Mexico (in hundredths)			10.00%
Percentage of loan portfolio secured by property in Colorado (in hundredths)		9.00%
Residential Mortgage [Member] | Permanent mortgage [Member]
Loans receivables disclosure [Abstract]
Total		$ 945,336	$ 969,951
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
Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member]
Loans receivables disclosure [Abstract]
Total		$ 196,937	205,950
Residential Mortgage [Member] | Home equity [Member]
Loans receivables disclosure [Abstract]
Total		734,620	$ 773,611
Loans receivable, other information [Abstract]
Maximum loan amount for certain individual loans in category		$ 400
Percentage of home equity portfolio comprised of first lien loans		68.00%	69.00%
Percentage of home equity portfolio comprised of junior lien loans		32.00%	31.00%
Percentage of junior lien home equity loans that are amortizing term loans		65.00%	71.00%
Percentage of junior lien home equity loans that are revolving lines of credit		35.00%	29.00%
Home equity loans, description		Home equity loans generally require a minimum FICO score of 700 and a maximum DTI of 40%.
Home equity loans, revolving period		5 years
Home equity loans, amortization period following revolving period		15 years
Home equity loans, discretionary additional revolving period		5 years
Personal [Member]
Loans receivables disclosure [Abstract]
Loans receivables with fixed rate of interest		$ 91,816	$ 102,865
Loans receivables with variable rate of interest		460,418	331,274
Loan receivable non-accrual		463	566
Total		$ 552,697	$ 434,705

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