Loans and Allowances for Credit Losses, Loans by Portfolio Segment (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Loans receivables disclosure [Abstract]
Loans, fixed rate of interest	$ 6,682,476	$ 6,134,362
Loans, variable rate of interest	16,090,351	15,435,674
Loans, non-accrual	234,693	180,951
Total	23,007,520	21,750,987
Past Due 90 Days or More and Accruing	88,718	7,680
Foregone interest on nonaccrual loans	22,870	17,409
Credit Commitments [Abstract]
Outstanding commitments to extend credit	11,000,000
Outstanding standby letters of credit	681,000
Geographic Concentration Risk [Member] \| Texas [Member]
Loans receivables disclosure [Abstract]
Total	$ 7,200,000	$ 6,800,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Texas (in hundredths)	31.00%	31.00%
Geographic Concentration Risk [Member] \| Oklahoma [Member]
Loans receivables disclosure [Abstract]
Total	$ 3,800,000	$ 3,500,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Oklahoma (in hundredths)	17.00%	16.00%
Geographic Concentration Risk [Member] \| Colorado [Member]
Loans receivables disclosure [Abstract]
Total	$ 2,800,000	$ 2,800,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Colorado (in hundredths)	12.00%	13.00%
Commercial [Member]
Loans receivables disclosure [Abstract]
Loans, fixed rate of interest	$ 1,805,286	$ 3,231,485
Loans, variable rate of interest	11,105,090	10,684,749
Loans, non-accrual	167,159	115,416
Total	13,077,535	14,031,650
Past Due 90 Days or More and Accruing	4,821
Commercial [Member] \| Geographic Concentration Risk [Member] \| Texas [Member]
Loans receivables disclosure [Abstract]
Total	$ 4,300,000	$ 4,700,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Texas (in hundredths)	33.00%	33.00%
Commercial [Member] \| Geographic Concentration Risk [Member] \| Oklahoma [Member]
Loans receivables disclosure [Abstract]
Total	$ 1,800,000	$ 2,000,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Oklahoma (in hundredths)	14.00%	14.00%
Commercial [Member] \| Geographic Concentration Risk [Member] \| Colorado [Member]
Loans receivables disclosure [Abstract]
Total	$ 1,700,000	$ 2,000,000
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Colorado (in hundredths)	13.00%	14.00%
Commercial [Member] \| Services [Member]
Loans receivables disclosure [Abstract]
Total	$ 3,508,583	$ 3,122,163
Past Due 90 Days or More and Accruing	326
Loans receivable, other information [Abstract]
Amount of loans with individual balances less than $10 million	1,800,000	1,500,000
Commercial [Member] \| Services [Member] \| Credit Concentration Risk [Member]
Loans receivables disclosure [Abstract]
Total	$ 3,500,000	$ 3,800,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans	15.00%	18.00%
Commercial [Member] \| Energy [Member]
Loans receivables disclosure [Abstract]
Total	$ 3,469,194	$ 3,973,377
Past Due 90 Days or More and Accruing	0
Commercial [Member] \| Energy [Member] \| Credit Concentration Risk [Member]
Loans receivables disclosure [Abstract]
Total	$ 3,500,000	$ 4,000,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans	15.00%	18.00%
Commercial [Member] \| Energy Producers [Member] \| Credit Concentration Risk [Member]
Loans receivables disclosure [Abstract]
Total	$ 2,600,000	$ 3,100,000
Loans receivable, other information [Abstract]
Percentage of committed energy production loans secured by oil	67.00%	58.00%
Percentage of committed energy production loans secured by natural gas	33.00%	42.00%
Commercial [Member] \| Healthcare [Member]
Loans receivables disclosure [Abstract]
Total	$ 3,305,990	$ 3,033,916
Past Due 90 Days or More and Accruing	0
Commercial [Member] \| Healthcare [Member] \| Credit Concentration Risk [Member]
Loans receivables disclosure [Abstract]
Total	$ 3,300,000	$ 3,000,000
Loans receivable, other information [Abstract]
Percentage of loan class to total loans	14.00%	14.00%
Commercial real estate [Member]
Loans receivables disclosure [Abstract]
Loans, fixed rate of interest	$ 1,021,443	$ 1,056,321
Loans, variable rate of interest	3,649,849	3,349,836
Loans, non-accrual	27,246	27,626
Total	4,698,538	$ 4,433,783
Past Due 90 Days or More and Accruing	$ 5,126
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)	27.00%	24.00%
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)	10.00%	12.00%
Commercial real estate [Member] \| Geographic Concentration Risk [Member] \| Colorado [Member]
Loans receivable, other information [Abstract]
Percentage of loan portfolio secured by property in Colorado (in hundredths)		11.00%
Paycheck Protection Program
Loans receivables disclosure [Abstract]
Loans, fixed rate of interest	$ 1,682,310
Loans, variable rate of interest	0
Loans, non-accrual	0
Total	1,682,310
Past Due 90 Days or More and Accruing	0
Loans to individuals [Member]
Loans receivables disclosure [Abstract]
Loans, fixed rate of interest	2,173,437
Loans, variable rate of interest	1,335,412
Loans, non-accrual	40,288
Total	3,549,137
Past Due 90 Days or More and Accruing	$ 78,771
Loans receivable, other information [Abstract]
Percentage of personal loans secured by collateral in our geographical footprint	91.00%
Loans to individuals [Member] \| Residential Mortgage [Member]
Loans receivables disclosure [Abstract]
Loans, fixed rate of interest		$ 1,652,653
Loans, variable rate of interest		393,897
Loans, non-accrual		37,622
Total		2,084,172
Loans to individuals [Member] \| Personal [Member]
Loans receivables disclosure [Abstract]
Loans, fixed rate of interest		193,903
Loans, variable rate of interest		1,007,192
Loans, non-accrual		287
Total	$ 1,277,447	$ 1,201,382
Past Due 90 Days or More and Accruing	$ 241

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Maximum dollar amount of loans in this category with individual loan amount less than specified threshold.

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Percentage of committed energy production loans secured by properties producing natural gas

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Reflects the percentage of the loan in portfolio secured by property in the state of Oklahoma.

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Reflects the percentage of the loan in portfolio secured by property in the state of Texas.

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Percentage Of Loan Portfolio Secured By Property in Colorado

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Percentage of personal loans secured by collateral in our geographical footprint

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Represents percentage of given loan class to total loans.

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Amount of financing receivable 90 days or more past due and still accruing.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

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Reflects the carrying amount of loans deemed to be questionable as to collection on which no interest is continuing to be recognized.

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Amount of loan with fixed rate of interest.

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Amount of loan with variable or adjustable rate of interest.

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Unused amount of commitments to extend credit to customers.

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