Loans and Allowance for Credit Losses, Loans by Portfolio Type (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 4,233,548	$ 4,169,034	$ 4,168,141
Loans, variable rate of interest		11,994,286	11,624,992	11,109,952
Loans, non-accrual		236,952	147,128	89,348
Total		16,464,786	15,941,154	15,367,441
Accruing loans past due (90 days)	[1]	3,839	1,207	101
Credit Commitments [Abstract]
Outstanding commitments to extend credit		8,700,000
Outstanding standby letters of credit		500,000
Outstanding commercial letters of credit		5,200
Texas [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 5,300,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Texas (in hundredths)		32.00%
Oklahoma [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 3,700,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Oklahoma (in hundredths)		22.00%
Commercial
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 1,991,423	1,850,548	1,854,163
Loans, variable rate of interest		7,952,276	8,325,559	7,909,461
Loans, non-accrual		176,464	76,424	33,798
Total		10,120,163	10,252,531	9,797,422
Commercial | Texas [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 3,300,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Texas (in hundredths)		33.00%
Commercial | Oklahoma [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,300,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Oklahoma (in hundredths)		23.00%
Commercial | Energy [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,520,804	3,097,328	2,838,167
Commercial | Energy [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,500,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans		15.00%
Commercial | Energy Producers [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,000,000
Loans receivable, other information [Abstract]
Percentage of committed energy production loans secured by oil		57.00%
Percentage of committed energy production loans secured by natural gas		43.00%
Commercial | Services [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,936,599	2,784,276	2,706,624
Commercial | Services [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,900,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans		18.00%
Amount of loans with individual balances less than $10 million		$ 1,300,000
Maximum loan amount for certain individual loans in category		10,000
Commercial | Healthcare [Member]
Loan receivables disclosure [Abstract]
Total		2,085,046	1,883,380	1,741,680
Commercial | Healthcare [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,100,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans		13.00%
Commercial real estate
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 565,429	627,678	588,604
Loans, variable rate of interest		3,220,819	2,622,354	2,635,507
Loans, non-accrual		7,350	9,001	10,956
Total		$ 3,793,598	3,259,033	3,235,067
Commercial real estate | Texas [Member] | Geographic Concentration Risk [Member]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Texas (in hundredths)		30.00%
Commercial real estate | Oklahoma [Member] | Geographic Concentration Risk [Member]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Oklahoma (in hundredths)		12.00%
Residential mortgage
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 1,572,288	1,598,992	1,624,759
Loans, variable rate of interest		248,053	216,661	200,136
Loans, non-accrual		52,452	61,240	44,100
Total		1,872,793	1,876,893	1,868,995
Residential mortgage | Permanent mortgage [Member]
Loan receivables disclosure [Abstract]
Total		$ 969,558	945,336	937,664
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 | Permanent mortgages guaranteed by US government agencies [Member]
Loan receivables disclosure [Abstract]
Total		$ 190,309	196,937	192,712
Loans receivable, other information [Abstract]
Amount of loans guaranteed by US government and previously sold into GNMA mortgage pools		190,000
Residential mortgage | Home equity {Member]
Loan receivables disclosure [Abstract]
Total		$ 712,926	734,620	738,619
Loans receivable, other information [Abstract]
Percentage of home equity portfolio comprised of first lien loans		66.00%
Percentage of home equity portfolio comprised of junior lien loans		34.00%
Percentage of junior lien home equity loans that are amortizing term loans		58.00%
Percentage of junior lien home equity loans that are revolving lines of credit		42.00%
Home equity loans, description		Home equity loans generally require a minimum FICO score of 700 and a maximum DTI of 40 percent.
Home equity loans, maximum		$ 400
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
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 104,408	91,816	100,615
Loans, variable rate of interest		573,138	460,418	364,848
Loans, non-accrual		686	463	494
Total		$ 678,232	$ 552,697	$ 465,957

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