Loans and Allowance for Credit Losses, Loans by Portfolio Type (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 4,302,800	$ 4,169,034	$ 4,185,538
Loans, variable rate of interest		11,856,765	11,624,992	10,847,707
Loans, non-accrual		247,184	147,128	90,891
Total		16,406,749	15,941,154	15,124,136
Accruing loans past due (90 days)	[1]	2,899	1,207	99
Credit Commitments [Abstract]
Outstanding commitments to extend credit		8,500,000
Outstanding standby letters of credit		491,000
Outstanding commercial letters of credit		6,700
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		$ 3,800,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Oklahoma (in hundredths)		23.00%
Commercial [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 1,994,415	1,850,548	1,730,675
Loans, variable rate of interest		8,180,033	8,325,559	8,020,813
Loans, non-accrual		181,989	76,424	24,233
Total		10,356,437	10,252,531	9,775,721
Commercial [Member] | Texas [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 3,400,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Texas (in hundredths)		33.00%
Commercial [Member] | Oklahoma [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,400,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Oklahoma (in hundredths)		23.00%
Commercial [Member] | Energy [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,818,656	3,097,328	2,902,143
Commercial [Member] | Energy [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,800,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans		17.00%
Commercial [Member] | Energy Producers [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,200,000
Loans receivable, other information [Abstract]
Percentage of committed energy production loans secured by oil		60.00%
Percentage of committed energy production loans secured by natural gas		40.00%
Commercial [Member] | Services [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,830,864	2,784,276	2,681,126
Commercial [Member] | Services [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,800,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans		17.00%
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 [Member] | Healthcare [Member]
Loan receivables disclosure [Abstract]
Total		2,051,146	1,883,380	1,646,025
Commercial [Member] | 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 [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 612,822	627,678	715,062
Loans, variable rate of interest		2,961,364	2,622,354	2,298,296
Loans, non-accrual		7,780	9,001	20,139
Total		$ 3,581,966	3,259,033	3,033,497
Commercial Real Estate [Member] | 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 [Member] | Oklahoma [Member] | Geographic Concentration Risk [Member]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Oklahoma (in hundredths)		13.00%
Residential Mortgage [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 1,586,116	1,598,992	1,639,773
Loans, variable rate of interest		237,746	216,661	198,986
Loans, non-accrual		57,061	61,240	45,969
Total		1,880,923	1,876,893	1,884,728
Residential Mortgage [Member] | Permanent mortgage [Member]
Loan receivables disclosure [Abstract]
Total		$ 969,007	945,336	946,324
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 US government agencies [Member]
Loan receivables disclosure [Abstract]
Total		$ 192,732	196,937	190,839
Loans receivable, other information [Abstract]
Amount of loans guaranteed by US government and previously sold into GNMA mortgage pools		193,000
Residential Mortgage [Member] | Home equity {Member]
Loan receivables disclosure [Abstract]
Total		$ 719,184	734,620	747,565
Loans receivable, other information [Abstract]
Percentage of home equity portfolio comprised of first lien loans		67.00%
Percentage of home equity portfolio comprised of junior lien loans		33.00%
Percentage of junior lien home equity loans that are amortizing term loans		61.00%
Percentage of junior lien home equity loans that are revolving lines of credit		39.00%
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 [Member]
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 109,447	91,816	100,028
Loans, variable rate of interest		477,622	460,418	329,612
Loans, non-accrual		354	463	550
Total		$ 587,423	$ 552,697	$ 430,190

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