Loans and Allowance for Credit Losses, Loans by Portfolio Type (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Loan receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 4,525,904	$ 4,529,296	$ 4,540,923
Loans, variable rate of interest		13,312,135	12,436,254	12,397,283
Loans, non-accrual		165,657	187,874	245,439
Total		18,003,696	17,153,424	17,183,645
Accruing loans past due (90 days)	[1]	879	633	1,414
Credit Commitments [Abstract]
Outstanding commitments to extend credit		10,300,000
Outstanding standby letters of credit		660,000
Texas [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 6,000,000
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		$ 3,500,000
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		$ 2,206,735	2,217,432	2,198,066
Loans, variable rate of interest		9,021,326	8,379,240	8,242,732
Loans, non-accrual		120,978	137,303	197,157
Total		11,349,039	10,733,975	10,637,955
Commercial | Texas [Member] | Geographic Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 3,800,000
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		$ 2,200,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio attributed to Oklahoma (in hundredths)		19.00%
Commercial | Energy [Member]
Loan receivables disclosure [Abstract]
Total		$ 3,147,219	2,930,156	2,847,240
Commercial | Energy [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 3,100,000
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		$ 2,600,000
Loans receivable, other information [Abstract]
Percentage of committed energy production loans secured by oil		56.00%
Percentage of committed energy production loans secured by natural gas		44.00%
Commercial | Services [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,944,499	2,986,949	2,958,827
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		16.00%
Amount of loans with individual balances less than $10 million		$ 1,400,000
Maximum loan amount for certain individual loans in category		10,000
Commercial | Healthcare [Member]
Loan receivables disclosure [Abstract]
Total		2,353,722	2,314,753	2,221,518
Commercial | Healthcare [Member] | Credit Concentration Risk [Member]
Loan receivables disclosure [Abstract]
Total		$ 2,400,000
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		$ 583,782	548,692	594,542
Loans, variable rate of interest		3,126,442	2,928,440	3,090,275
Loans, non-accrual		1,996	2,855	3,775
Total		$ 3,712,220	3,479,987	3,688,592
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)		33.00%
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		$ 1,567,216	1,608,655	1,597,587
Loans, variable rate of interest		332,691	317,584	297,376
Loans, non-accrual		42,343	47,447	44,235
Total		1,942,250	1,973,686	1,939,198
Residential mortgage | Permanent mortgage [Member]
Loan receivables disclosure [Abstract]
Total		$ 1,068,412	1,043,435	989,040
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		$ 169,653	197,506	191,729
Residential mortgage | Home equity [Member]
Loan receivables disclosure [Abstract]
Total		$ 704,185	732,745	758,429
Loans receivable, other information [Abstract]
Percentage of home equity portfolio comprised of first lien loans		62.00%
Percentage of home equity portfolio comprised of junior lien loans		38.00%
Percentage of junior lien home equity loans that are amortizing term loans		45.00%
Percentage of junior lien home equity loans that are revolving lines of credit		55.00%
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		$ 168,171	154,517	150,728
Loans, variable rate of interest		831,676	810,990	766,900
Loans, non-accrual		340	269	272
Total		$ 1,000,187	$ 965,776	$ 917,900

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