Loans and Allowance for Credit Losses, Loans by Portfolio Type (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 4,540,923	$ 4,753,600	$ 4,302,800
Loans, variable rate of interest		12,397,283	12,005,076	11,856,765
Loans, non-accrual		245,439	230,984	247,184
Total		17,183,645	16,989,660	16,406,749
Accruing loans past due (90 days)	[1]	1,414	5	2,899
Credit Commitments [Abstract]
Outstanding commitments to extend credit		9,600,000
Outstanding standby letters of credit		615,000
Outstanding commercial letters of credit		3,200
Texas [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 5,700,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Texas (in hundredths)		33.00%
Oklahoma [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 3,400,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Oklahoma (in hundredths)		20.00%
Commercial
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 2,198,066	2,327,085	1,994,415
Loans, variable rate of interest		8,242,732	7,884,786	8,180,033
Loans, non-accrual		197,157	178,953	181,989
Total		10,637,955	10,390,824	10,356,437
Commercial | Texas [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 3,600,000
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		$ 2,100,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Oklahoma (in hundredths)		19.00%
Commercial | Energy [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,847,240	2,497,868	2,818,656
Commercial | 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 | Energy Producers [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,400,000
Loans receivable, other information [Abstract]
Percentage of committed energy production loans secured by oil		58.00%
Percentage of committed energy production loans secured by natural gas		42.00%
Commercial | Services [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,958,827	3,108,990	2,830,864
Commercial | Services [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 3,000,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans		17.00%
Amount of loans with individual balances less than $10 million		$ 1,500,000
Maximum loan amount for certain individual loans in category		10,000
Commercial | Healthcare [Member]
Loan receivables disclosure [Abstract]
Total		2,221,518	2,201,916	2,051,146
Commercial | Healthcare [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,200,000
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		$ 594,542	624,187	612,822
Loans, variable rate of interest		3,090,275	3,179,338	2,961,364
Loans, non-accrual		3,775	5,521	7,780
Total		$ 3,688,592	3,809,046	3,581,966
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)		32.00%
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		$ 1,597,587	1,647,357	1,586,116
Loans, variable rate of interest		297,376	256,255	237,746
Loans, non-accrual		44,235	46,220	57,061
Total		1,939,198	1,949,832	1,880,923
Residential mortgage | Permanent mortgage [Member]
Loan receivables disclosure [Abstract]
Total		$ 989,040	1,006,820	969,007
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		$ 191,729	199,387	192,732
Loans receivable, other information [Abstract]
Amount of loans guaranteed by US government and previously sold into GNMA mortgage pools		192,000
Residential mortgage | Home equity [Member]
Loan receivables disclosure [Abstract]
Total		$ 758,429	743,625	719,184
Loans receivable, other information [Abstract]
Percentage of home equity portfolio comprised of first lien loans		65.00%
Percentage of home equity portfolio comprised of junior lien loans		35.00%
Percentage of junior lien home equity loans that are amortizing term loans		49.00%
Percentage of junior lien home equity loans that are revolving lines of credit		51.00%
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		$ 150,728	154,971	109,447
Loans, variable rate of interest		766,900	684,697	477,622
Loans, non-accrual		272	290	354
Total		$ 917,900	$ 839,958	$ 587,423

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