Cullen/Frost Bankers (CFR) 10-Q 29 Apr 21 2021 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - $ / shares	3 Months Ended
	Mar. 31, 2021	Apr. 22, 2021	Dec. 31, 2020
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	001-13221
Entity Registrant Name	Cullen/Frost Bankers, Inc.
Entity Incorporation, State or Country Code	TX
Entity Tax Identification Number	74-1751768
Entity Address, Address Line One	111 W. Houston Street,
Entity Address, City or Town	San Antonio,
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	78205
City Area Code	(210)
Local Phone Number	220-4011
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		63,545,296
Common stock, par value	$ 0.01		$ 0.01
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Entity Central Index Key	0000039263
Current Fiscal Year End Date	--12-31
Common Stock [Member] | NEW YORK STOCK EXCHANGE, INC. [Member]
Entity Information [Line Items]
Title of 12(b) Security	Common Stock, $.01 Par Value
Trading Symbol	CFR
Security Exchange Name	NYSE
Series B Preferred Stock [Member] | NEW YORK STOCK EXCHANGE, INC. [Member]
Entity Information [Line Items]
Title of 12(b) Security	Depositary Shares, each representing a 1/40th interest in a share of 4.450% Non-Cumulative Perpetual Preferred Stock, Series B
Trading Symbol	CFR.PrB
Security Exchange Name	NYSE

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets:
Cash and due from banks	$ 536,604	$ 529,454
Interest-bearing deposits	11,257,662	9,758,624
Federal funds sold	100	775
Resell agreements	7,903	0
Total cash and cash equivalents	11,802,269	10,288,853
Securities held to maturity, net of allowance for credit losses of $158 at March 31, 2021 and $160 at December 31, 2020	1,820,567	1,945,673
Securities available-for-sale, at estimated fair value	10,377,444	10,437,565
Trading account securities	26,609	24,456
Loans, net of unearned discounts	17,889,651	17,481,309
Less: Allowance for credit losses on loans	(261,258)	(263,177)
Net loans	17,628,393	17,218,132
Premises and equipment, net	1,040,572	1,045,578
Goodwill	654,952	654,952
Other intangible assets, net	1,361	1,563
Cash surrender value of life insurance policies	189,756	189,984
Accrued interest receivable and other assets	504,864	584,561
Total assets	44,046,787	42,391,317
Liabilities:
Non-interest-bearing demand deposits	16,103,649	15,117,051
Interest-bearing deposits	20,821,531	19,898,710
Total deposits	36,925,180	35,015,761
Federal funds purchased	38,975	48,850
Repurchase agreements	1,884,449	2,068,147
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,371	136,357
Subordinated notes, net of unamortized issuance costs	99,061	99,021
Accrued interest payable and other liabilities	694,833	730,165
Total liabilities	39,778,869	38,098,301
Shareholders’ Equity:
Preferred stock, par value $0.01 per share; 10,000,000 shares authorized; 150,000 Series B shares ($1,000 liquidation preference) issued at March 31, 2021 and December 31, 2020	145,452	145,452
Common stock, par value $0.01 per share; 210,000,000 shares authorized; 64,236,306 shares issued at March 31, 2021 and December 31, 2020	642	642
Additional paid-in capital	999,694	997,168
Retained earnings	2,797,064	2,750,723
Accumulated other comprehensive income (loss), net of tax	387,748	512,970
Treasury stock, at cost; 704,312 shares at March 31, 2021 and 1,225,066 shares at December 31, 2020	(62,682)	(113,939)
Total shareholders’ equity	4,267,918	4,293,016
Total liabilities and shareholders’ equity	$ 44,046,787	$ 42,391,317

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Securities, held-to-maturity, allowance for credit loss	$ 158,000	$ 160,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	210,000,000	210,000,000
Common stock, shares issued	64,236,306	64,236,306
Treasury stock, shares	704,312	1,225,066
Series B Preferred Stock [Member]
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	150,000	150,000
Preferred stock liquidation preference value	$ 1,000	$ 1,000

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Interest income:
Loans, including fees	$ 167,483	$ 172,137
Securities:
Taxable	20,028	26,030
Tax-exempt	56,663	58,886
Interest-bearing deposits	2,433	8,125
Federal funds sold	3	683
Resell agreements	1	119
Total interest income	246,611	265,980
Interest expense:
Deposits	3,517	16,053
Federal funds purchased	8	79
Repurchase agreements	395	2,958
Junior subordinated deferrable interest debentures	646	1,205
Subordinated notes	1,164	1,164
Total interest expense	5,730	21,459
Net interest income	240,881	244,521
Credit loss expense	63	175,197
Net interest income after credit loss expense	240,818	69,324
Non-interest income:
Trust and investment management fees	35,314	34,473
Service charges on deposit accounts	19,993	22,651
Insurance commissions and fees	17,313	16,485
Interchange and card transaction fees	4,093	3,255
Other charges, commissions and fees	8,304	9,365
Net gain (loss) on securities transactions	0	108,989
Other	8,219	17,697
Total non-interest income	93,236	212,915
Non-interest expense:
Salaries and wages	93,458	98,812
Employee benefits	22,536	24,889
Net occupancy	26,051	25,384
Technology, furniture and equipment	28,016	25,240
Deposit insurance	2,928	2,624
Intangible amortization	202	257
Other	36,951	46,957
Total non-interest expense	210,142	224,163
Income before income taxes	123,912	58,076
Income taxes	7,897	3,323
Net income	116,015	54,753
Preferred stock dividends	2,151	2,016
Redemption of preferred stock	0	5,514
Net income available to common shareholders	$ 113,864	$ 47,223
Earnings per common share:
Basic	$ 1.78	$ 0.75
Diluted	$ 1.77	$ 0.75

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net income	$ 116,015	$ 54,753
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss on securities available for sale during the period	(159,779)	230,118
Change in net unrealized gain on securities transferred to held to maturity	(259)	(377)
Reclassification adjustment for net (gains) losses included in net income	0	(108,989)
Total securities available for sale and transferred securities, before tax amount	(160,038)	120,752
Defined-benefit post-retirement benefit plans:
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit)	1,529	1,330
Total defined-benefit post-retirement benefit plans	1,529	1,330
Other comprehensive income (loss), before tax	(158,509)	122,082
Deferred tax expense (benefit)	(33,287)	25,637
Other comprehensive income (loss), net of tax	(125,222)	96,445
Comprehensive income (loss)	$ (9,207)	$ 151,198

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Treasury Stock [Member]
Cumulative effect of accounting change	$ (29,252)				$ (29,252)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	3,882,416	$ 144,486	$ 642	$ 983,250	2,638,282	$ 267,370	$ (151,614)
Total shareholders’ equity at beginning of period at Dec. 31, 2019	3,911,668	144,486	642	983,250	2,667,534	267,370	(151,614)
Net income	54,753				54,753
Other comprehensive income (loss)	96,445					96,445
Stock option exercises/stock unit conversions (513,824 shares YTD in 2021 and 66,215 shares YTD in 2020)	1,463				(4,983)		6,446
Stock compensation expense recognized in earnings	2,512			2,512
Preferred stock, redemption amount (6,000,000 shares)	(150,000)	(144,486)			(5,514)
Purchase of treasury stock (11,625 shares YTD in 2021 and 181,825 shares YTD in 2020)	(13,973)						(13,973)
Treasury stock issued to the 401(k) stock purchase plan (18,555 shares YTD in 2021)	0
Cash dividends – preferred stock (approximately $14.34 per share which is equivalent to approximately $0.36 per depository share YTD in 2021 and $0.34 per share YTD in 2020)	(2,016)				(2,016)
Cash dividends – common stock ($0.72 per share YTD in 2021 and $0.71 per share YTD in 2020)	(44,934)				(44,934)
Total shareholders’ equity at end of period at Mar. 31, 2020	3,826,666	0	642	985,762	2,635,588	363,815	(159,141)
Total shareholders’ equity at beginning of period at Dec. 31, 2020	4,293,016	145,452	642	997,168	2,750,723	512,970	(113,939)
Net income	116,015				116,015
Other comprehensive income (loss)	(125,222)					(125,222)
Stock option exercises/stock unit conversions (513,824 shares YTD in 2021 and 66,215 shares YTD in 2020)	29,397				(21,342)		50,739
Stock compensation expense recognized in earnings	2,526			2,526
Purchase of treasury stock (11,625 shares YTD in 2021 and 181,825 shares YTD in 2020)	(1,288)						(1,288)
Treasury stock issued to the 401(k) stock purchase plan (18,555 shares YTD in 2021)	1,749				(57)		1,806
Cash dividends – preferred stock (approximately $14.34 per share which is equivalent to approximately $0.36 per depository share YTD in 2021 and $0.34 per share YTD in 2020)	(2,151)				(2,151)
Cash dividends – common stock ($0.72 per share YTD in 2021 and $0.71 per share YTD in 2020)	(46,124)				(46,124)
Total shareholders’ equity at end of period at Mar. 31, 2021	$ 4,267,918	$ 145,452	$ 642	$ 999,694	$ 2,797,064	$ 387,748	$ (62,682)

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Shareholders' Equity (Parenthetical) - $ / shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement of Stockholders' Equity [Abstract]
Stock option exercise/stock unit conversions (513,824 shares during 2021 and 66,215 shares during 2020)	513,824	66,215
Redemption of series A preferred stock (6,000,000 shares)		6,000,000
Purchase of treasury stock (11,625 shares during 2021 and 181,825 shares during 2020)	11,625	181,825
Treasury stock issued to the 401(k) stock purchase plan (18,555 shares)	18,555
Cash dividend - preferred stock (Approximately $14.34 per share during 2021 and $0.34 per share during 2020)	$ 14.34	$ 0.34
Cash dividend - Preferred Stock Dividends equivalent Per depositary Share Declared ($0.36 during 2021)	0.36
Cash dividends - common stock ($0.72 per share during 2021 and $0.71 per share during 2020)	$ 0.72	$ 0.71

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating Activities:
Net income	$ 116,015	$ 54,753
Adjustments to reconcile net income to net cash from operating activities:
Credit loss expense	63	175,197
Deferred tax expense (benefit)	4,172	(26,089)
Accretion of loan discounts	(3,425)	(4,060)
Securities premium amortization (discount accretion), net	30,598	31,760
Net (gain) loss on securities transactions	0	(108,989)
Depreciation and amortization	16,941	15,518
Net (gain) loss on sale/write-down of assets/foreclosed assets	(134)	397
Stock-based compensation	2,526	2,512
Net tax benefit from stock-based compensation	3,136	336
Earnings on life insurance policies	(740)	(922)
Net change in:
Trading account securities	(2,153)	(12,819)
Lease right-of-use assets	5,889	5,620
Accrued interest receivable and other assets	73,164	(81,100)
Accrued interest payable and other liabilities	(26,287)	196,814
Net cash from operating activities	219,765	248,928
Investing Activities:
Securities held-to-maturity - Purchases	0	(1,500)
Securities held to maturity: Maturities, calls and principal repayments	120,029	41,526
Securities available for sale:
Purchases	(455,091)	(687,759)
Sales	0	1,111,102
Maturities, calls and principal repayments	351,612	581,882
Proceeds from sale of loans	0	0
Net change in loans	(406,836)	(622,514)
Benefits received on life insurance policies	968	0
Proceeds from sales of premises and equipment	3	1
Purchases of premises and equipment	(12,814)	(31,749)
Proceeds from sales of repossessed properties	100	8
Net cash from investing activities	(402,029)	390,997
Financing Activities:
Net change in deposits	1,909,419	501,303
Net change in short-term borrowings	(193,573)	(524,364)
Redemption of Series A preferred stock	0	(150,000)
Proceeds from stock option exercises	29,397	1,463
Purchase of treasury stock	(1,288)	(13,973)
Cash dividends paid on preferred stock	(2,151)	(2,016)
Cash dividends paid on common stock	(46,124)	(44,934)
Net cash from financing activities	1,695,680	(232,521)
Net change in cash and cash equivalents	1,513,416	407,404
Cash and cash equivalent at beginning of period	10,288,853	3,788,181
Cash and cash equivalents at end of period	$ 11,802,269	$ 4,195,585

Significant Accounting Policies

Significant Accounting Policies	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Significant Accounting Policies	Significant Accounting Policies Nature of Operations. Cullen/Frost Bankers, Inc. (“Cullen/Frost”) is a financial holding company and a bank holding company headquartered in San Antonio, Texas that provides, through its subsidiaries, a broad array of products and services throughout numerous Texas markets. The terms “Cullen/Frost,” “the Corporation,” “we,” “us” and “our” mean Cullen/Frost Bankers, Inc. and its subsidiaries, when appropriate. In addition to general commercial and consumer banking, other products and services offered include trust and investment management, insurance, brokerage, mutual funds, leasing, treasury management, capital markets advisory and item processing. Basis of Presentation. The consolidated financial statements in this Quarterly Report on Form 10-Q include the accounts of Cullen/Frost and all other entities in which Cullen/Frost has a controlling financial interest. All significant intercompany balances and transactions have been eliminated in consolidation. The accounting and financial reporting policies we follow conform, in all material respects, to accounting principles generally accepted in the United States and to general practices within the financial services industry. The consolidated financial statements in this Quarterly Report on Form 10-Q have not been audited by an independent registered public accounting firm, but in the opinion of management, reflect all adjustments necessary for a fair presentation of our financial position and results of operations. All such adjustments were of a normal and recurring nature. The consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States for interim financial information and with the instructions to Form 10-Q adopted by the Securities and Exchange Commission (“SEC”). Accordingly, the financial statements do not include all of the information and footnotes required by accounting principles generally accepted in the United States for complete financial statements and should be read in conjunction with our consolidated financial statements, and notes thereto, for the year ended December 31, 2020, included in our Annual Report on Form 10-K filed with the SEC on February 5, 2021 (the “2020 Form 10-K”). Operating results for the interim periods disclosed herein are not necessarily indicative of the results that may be expected for a full year or any future period. Use of Estimates . The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements. Actual results could differ from those estimates. The allowance for loan losses and the fair values of financial instruments and the status of contingencies are particularly subject to change. Cash Flow Reporting . Additional cash flow information was as follows: Three Months Ended 2021 2020 Cash paid for interest $ 9,719 $ 23,168 Cash paid for income taxes — — Significant non-cash transactions: Unsettled securities transactions 21,957 11,196 Right-of-use lease assets obtained in exchange for lessee operating lease liabilities 1,326 9,892 Treasury stock issued to 401(k) stock purchase plan 1,749 — Accounting Changes, Reclassifications and Restatements. Certain items in prior financial statements have been reclassified to conform to the current presentation. On January 1, 2020, we adopted Accounting Standards Update (“ASU”) 2016-13, “Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments,” as subsequently updated for certain clarifications, targeted relief and codification improvements, using the modified retrospective method for all financial assets measured at amortized cost and off-balance-sheet credit exposures. Upon adoption, we recognized an after-tax cumulative effect reduction to retained earnings totaling $29.3 million, as detailed in our 2020 Form 10-K.

Securities

Securities	3 Months Ended
	Mar. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Securities	Securities Securities - Held to Maturity. A summary of the amortized cost, fair value and allowance for credit losses related to securities held to maturity as of March 31, 2021 and December 31, 2020 is presented below. Amortized Gross Gross Estimated Allowance Net March 31, 2021 Residential mortgage-backed securities $ 528,472 $ 13,458 $ — $ 541,930 $ — $ 528,472 States and political subdivisions 1,290,753 50,119 — 1,340,872 (158) 1,290,595 Other 1,500 — — 1,500 — 1,500 Total $ 1,820,725 $ 63,577 $ — $ 1,884,302 $ (158) $ 1,820,567 December 31, 2020 Residential mortgage-backed securities $ 528,784 $ 41,742 $ — $ 570,526 $ — $ 528,784 States and political subdivisions 1,415,549 65,321 — 1,480,870 (160) 1,415,389 Other 1,500 — — 1,500 — 1,500 Total $ 1,945,833 $ 107,063 $ — $ 2,052,896 $ (160) $ 1,945,673 All mortgage-backed securities included in the above table were issued by U.S. government agencies and corporations. The carrying value of held-to-maturity securities pledged to secure public funds, trust deposits, repurchase agreements and for other purposes, as required or permitted by law was $541.1 million and $659.2 million at March 31, 2021 and December 31, 2020, respectively. Accrued interest receivable on held-to-maturity securities totaled $11.2 million and $21.7 million at March 31, 2021 and December 31, 2020, respectively and is included in accrued interest receivable and other assets in the accompanying consolidated balance sheets. From time to time, we have reclassified certain securities from available for sale to held to maturity. The net unamortized, unrealized gain remaining on transferred securities included in accumulated other comprehensive income in the accompanying balance sheet totaled $3.2 million ($2.6 million, net of tax) at March 31, 2021 and $3.5 million ($2.8 million, net of tax) at December 31, 2020. This amount will be amortized out of accumulated other comprehensive income over the remaining life of the underlying securities as an adjustment of the yield on those securities. The following table summarizes Moody's and/or Standard & Poor's bond ratings for our portfolio of held-to-maturity securities issued by States and political subdivisions and other securities as of March 31, 2021 and December 31, 2020: States and Political Subdivisions Not Guaranteed or Pre-Refunded Guaranteed by the Texas PSF Pre-Refunded Total Other March 31, 2021 Aaa/AAA $ 104,425 $ 579,584 $ 437,406 $ 1,121,415 $ — Aa/AA 119,548 — — 119,548 — A 49,790 — — 49,790 — Not rated — — — — 1,500 Total $ 273,763 $ 579,584 $ 437,406 $ 1,290,753 $ 1,500 December 31, 2020 Aaa/AAA $ 115,016 $ 659,999 $ 470,745 $ 1,245,760 $ — Aa/AA 107,065 — — 107,065 — A 62,724 — — 62,724 — Not rated — — — — 1,500 Total $ 284,805 $ 659,999 $ 470,745 $ 1,415,549 $ 1,500 The following table details activity in the allowance for credit losses on held-to-maturity securities during the three months ended March 31, 2021 and 2020. Three Months Ended 2021 2020 Beginning balance $ 160 $ — Impact of adopting ASC 326 — 215 Credit loss expense (benefit) (2) (15) Ending balance $ 158 $ 200 Securities - Available for Sale. A summary of the amortized cost, fair value and allowance for credit losses related to securities available for sale as of March 31, 2021 and December 31, 2020 is presented below. Amortized Gross Gross Allowance Estimated March 31, 2021 U.S. Treasury $ 1,281,883 $ 28,350 $ 8,202 $ — $ 1,302,031 Residential mortgage-backed securities 1,860,427 51,614 17,236 — 1,894,805 States and political subdivisions 6,642,395 502,019 6,160 — 7,138,254 Other 42,354 — — — 42,354 Total $ 9,827,059 $ 581,983 $ 31,598 $ — $ 10,377,444 December 31, 2020 U.S. Treasury $ 1,084,542 $ 35,091 $ — $ — $ 1,119,633 Residential mortgage-backed securities 1,916,581 71,102 4 — 1,987,679 States and political subdivisions 6,683,927 603,975 — — 7,287,902 Other 42,351 — — — 42,351 Total $ 9,727,401 $ 710,168 $ 4 $ — $ 10,437,565 All mortgage-backed securities included in the above table were issued by U.S. government agencies and corporations. At March 31, 2021, all of the securities in our available for sale municipal bond portfolio were issued by the State of Texas or political subdivisions or agencies within the State of Texas, of which approximately 77.5% are either guaranteed by the PSF or have been pre-refunded. Securities with limited marketability, such as stock in the Federal Reserve Bank and the Federal Home Loan Bank, are carried at cost and are reported as other available for sale securities in the table above. The carrying value of available-for-sale securities pledged to secure public funds, trust deposits, repurchase agreements and for other purposes, as required or permitted by law was $4.3 billion and $4.4 billion at March 31, 2021 and December 31, 2020, respectively. Accrued interest receivable on available-for-sale securities totaled $65.6 million and $111.0 million at March 31, 2021 and December 31, 2020, respectively, and is included in accrued interest receivable and other assets in the accompanying consolidated balance sheets. The table below summarizes, as of March 31, 2021, securities available for sale in an unrealized loss position for which an allowance for credit losses has not been recorded, aggregated by type of security and length of time in a continuous unrealized loss position. Less than 12 Months More than 12 Months Total Estimated Unrealized Estimated Unrealized Estimated Unrealized U.S. Treasury $ 188,812 $ 8,202 $ — $ — $ 188,812 $ 8,202 Residential mortgage-backed securities 634,464 17,236 — — 634,464 17,236 States and political subdivisions 176,982 6,160 — — 176,982 6,160 Total $ 1,000,258 $ 31,598 $ — $ — $ 1,000,258 $ 31,598 As of March 31, 2021, no allowance for credit losses has been recognized on available for sale securities in an unrealized loss position as management does not believe any of the securities are impaired due to reasons of credit quality. This is based upon our analysis of the underlying risk characteristics, including credit ratings, and other qualitative factors related to our available for sale securities and in consideration of our historical credit loss experience and internal forecasts. The issuers of these securities continue to make timely principal and interest payments under the contractual terms of the securities. Furthermore, management does not have the intent to sell any of the securities classified as available for sale in the table above and believes that it is more likely than not that we will not have to sell any such securities before a recovery of cost. The unrealized losses are due to increases in market interest rates over the yields available at the time the underlying securities were purchased. The fair value is expected to recover as the securities approach their maturity date or repricing date or if market yields for such investments decline. Contractual Maturities. The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of March 31, 2021. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only. Within 1 Year 1 - 5 Years 5 - 10 Years After 10 Years Total Held To Maturity Amortized Cost Residential mortgage-backed securities $ 16 $ 129 $ 515,865 $ 12,462 $ 528,472 States and political subdivisions 129,873 380,885 196,400 583,595 1,290,753 Other 1,500 — — — 1,500 Total $ 131,389 $ 381,014 $ 712,265 $ 596,057 $ 1,820,725 Estimated Fair Value Residential mortgage-backed securities $ 16 $ 130 $ 529,086 $ 12,698 $ 541,930 States and political subdivisions 132,092 393,977 201,137 613,666 1,340,872 Other 1,500 — — — 1,500 Total $ 133,608 $ 394,107 $ 730,223 $ 626,364 $ 1,884,302 Available For Sale Amortized Cost U. S. Treasury $ 799,130 $ 285,739 $ 197,014 $ — $ 1,281,883 Residential mortgage-backed securities 7,147 22,151 22,162 1,808,967 1,860,427 States and political subdivisions 123,869 1,356,053 792,661 4,369,812 6,642,395 Other — — — — 42,354 Total $ 930,146 $ 1,663,943 $ 1,011,837 $ 6,178,779 $ 9,827,059 Estimated Fair Value U. S. Treasury $ 808,469 $ 304,750 $ 188,812 $ — $ 1,302,031 Residential mortgage-backed securities 7,468 23,421 22,634 1,841,282 1,894,805 States and political subdivisions 125,507 1,467,653 847,659 4,697,435 7,138,254 Other — — — — 42,354 Total $ 941,444 $ 1,795,824 $ 1,059,105 $ 6,538,717 $ 10,377,444 Sales of Securities. Sales of securities available for sale were as follows: Three Months Ended 2021 2020 Proceeds from sales $ — $ 1,111,102 Gross realized gains — 108,989 Gross realized losses — — Tax (expense) benefit of securities gains/losses — (22,888) Premiums and Discounts . Premium amortization and discount accretion included in interest income on securities was as follows: Three Months Ended 2021 2020 Premium amortization $ (31,235) $ (32,345) Discount accretion 637 585 Net (premium amortization) discount accretion $ (30,598) $ (31,760) Trading Account Securities. Trading account securities, at estimated fair value, were as follows: March 31, December 31, U.S. Treasury $ 24,038 $ 23,996 States and political subdivisions 2,571 460 Total $ 26,609 $ 24,456 Net gains and losses on trading account securities were as follows: Three Months Ended 2021 2020 Net gain on sales transactions $ 109 $ 470 Net mark-to-market gains (losses) (69) 115 Net gain (loss) on trading account securities $ 40 $ 585

Loans

Loans	3 Months Ended
	Mar. 31, 2021
Receivables [Abstract]
Loans	Loans Loans were as follows: March 31, Percentage December 31, Percentage Commercial and industrial $ 4,770,228 26.7 % $ 4,955,341 28.4 % Energy: Production 886,627 5.0 976,473 5.6 Service 90,510 0.5 116,825 0.7 Other 125,072 0.7 141,900 0.8 Total energy 1,102,209 6.2 1,235,198 7.1 Paycheck Protection Program 3,129,244 17.5 2,433,849 13.9 Commercial real estate: Commercial mortgages 5,566,367 31.1 5,478,806 31.3 Construction 1,236,994 6.9 1,223,814 7.0 Land 300,669 1.7 317,847 1.8 Total commercial real estate 7,104,030 39.7 7,020,467 40.1 Consumer real estate: Home equity loans 318,215 1.8 329,390 1.9 Home equity lines of credit 454,722 2.5 452,854 2.6 Other 549,217 3.0 548,530 3.1 Total consumer real estate 1,322,154 7.3 1,330,774 7.6 Total real estate 8,426,184 47.0 8,351,241 47.7 Consumer and other 461,786 2.6 505,680 2.9 Total loans $ 17,889,651 100.0 % $ 17,481,309 100.0 % Concentrations of Credit. Most of our lending activity occurs within the State of Texas, including the four largest metropolitan areas of Austin, Dallas/Ft. Worth, Houston and San Antonio, as well as other markets. The majority of our loan portfolio consists of commercial and industrial and commercial real estate loans. As of March 31, 2021, there were no concentrations of loans related to any single industry in excess of 10% of total loans. The largest industry concentration was related to the energy industry, which totaled 6.2% of total loans, or 7.5% excluding PPP loans. Unfunded commitments to extend credit and standby letters of credit issued to customers in the energy industry totaled $968.9 million and $70.4 million, respectively, as of March 31, 2021. Foreign Loans. We have U.S. dollar denominated loans and commitments to borrowers in Mexico. The outstanding balance of these loans and the unfunded amounts available under these commitments were not significant at March 31, 2021 or December 31, 2020. Related Party Loans . In the ordinary course of business, we have granted loans to certain directors, executive officers and their affiliates (collectively referred to as “related parties”). Such loans totaled $365.6 million at March 31, 2021 and $353.1 million at December 31, 2020. Accrued Interest Receivable. Accrued interest receivable on loans totaled $51.7 million and $48.7 million at March 31, 2021 and December 31, 2020, respectively and is included in accrued interest receivable and other assets in the accompany consolidated balance sheets. COVID-19 Loan Deferments. Certain borrowers are currently unable to meet their contractual payment obligations because of the adverse effects of COVID-19. To help mitigate these effects, loan customers may apply for a deferral of payments, or portions thereof, for up to 90 days. After 90 days, customers may apply for an additional deferral, and a small proportion of our customers have requested such an additional deferral. In the absence of other intervening factors, such short-term modifications made on a good faith basis are not categorized as troubled debt restructurings, nor are loans granted payment deferrals related to COVID-19 reported as past due or placed on non-accrual status (provided the loans were not past due or on non-accrual status prior to the deferral). At March 31, 2021, there were 22 loans in COVID-19 related deferment with an aggregate outstanding balance of approximately $11.4 million. Non-Accrual and Past Due Loans. Loans are considered past due if the required principal and interest payments have not been received as of the date such payments were due. Loans are placed on non-accrual status when, in management’s opinion, the borrower may be unable to meet payment obligations as they become due, as well as when required by regulatory provisions. Non-accrual loans, segregated by class of loans, were as follows: March 31, 2021 December 31, 2020 Total Non-Accrual Non-Accrual with No Credit Loss Allowance Total Non-Accrual Non-Accrual with No Credit Loss Allowance Commercial and industrial $ 18,509 $ 4,486 $ 19,849 $ 4,479 Energy 16,042 8,203 23,168 639 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 14,246 13,896 15,737 14,116 Construction 1,653 1,653 1,684 1,684 Consumer real estate 508 508 993 993 Consumer and other 18 — 18 — Total $ 50,976 $ 28,746 $ 61,449 $ 21,911 The following table presents non-accrual loans as of March 31, 2021 by class and year of origination. 2021 2020 2019 2018 2017 Prior Revolving Loans Revolving Loans Converted to Term Total Commercial and industrial $ — $ 10,344 $ 2,463 $ 921 $ 1,204 $ 114 $ 827 $ 2,636 $ 18,509 Energy — 900 6,314 1,901 — 347 6,466 114 16,042 Paycheck Protection Program — — — — — — — — — Commercial real estate: Buildings, land and other — 1,700 2,995 989 2,114 3,261 3,187 — 14,246 Construction — 1,653 — — — — — — 1,653 Consumer real estate — — — — — 464 — 44 508 Consumer and other — — — — — — 18 — 18 Total $ — $ 14,597 $ 11,772 $ 3,811 $ 3,318 $ 4,186 $ 10,498 $ 2,794 $ 50,976 Had non-accrual loans performed in accordance with their original contract terms, we would have recognized additional interest income, net of tax, of approximately $453 thousand for the three months ended March 31, 2021, and approximately $1.0 million for the three months ended March 31, 2020. An age analysis of past due loans (including both accruing and non-accruing loans), segregated by class of loans, as of March 31, 2021 was as follows: Loans Loans Total Current Total Accruing Commercial and industrial $ 24,542 $ 10,216 $ 34,758 $ 4,735,470 $ 4,770,228 $ 7,547 Energy 11,125 3,918 15,043 1,087,166 1,102,209 1,086 Paycheck Protection Program — — — 3,129,244 3,129,244 — Commercial real estate: Buildings, land and other 27,876 15,713 43,589 5,823,447 5,867,036 5,045 Construction 22,726 — 22,726 1,214,268 1,236,994 — Consumer real estate 5,973 3,254 9,227 1,312,927 1,322,154 2,826 Consumer and other 3,197 447 3,644 458,142 461,786 430 Total $ 95,439 $ 33,548 $ 128,987 $ 17,760,664 $ 17,889,651 $ 16,934 Troubled Debt Restructurings . Troubled debt restructurings during the three months ended March 31, 2021 and 2020 are set forth in the following table. Three Months Ended Three Months Ended Balance at Balance at Balance at Balance at Commercial and industrial $ — $ — $ 2,191 $ 2,198 Loan modifications are typically related to extending amortization periods, converting loans to interest only for a limited period of time, deferral of interest payments, waiver of certain covenants, consolidating notes and/or reducing collateral or interest rates. The modifications during the reported periods did not significantly impact our determination of the allowance for credit losses on loans. Additional information related to restructured loans as of or for the three months ended March 31, 2021 and 2020 is set forth in the following table. March 31, 2021 March 31, 2020 Restructured loans past due in excess of 90 days at period-end: Number of loans 2 3 Dollar amount of loans $ 1,392 $ 1,733 Restructured loans on non-accrual status at period end — 2,198 Charge-offs of restructured loans: Recognized on previously restructured loans 1,433 1,533 Credit Quality Indicators. As part of the on-going monitoring of the credit quality of our loan portfolio, management tracks certain credit quality indicators including trends related to (i) the weighted-average risk grade of commercial loans, (ii) the level of classified commercial loans, (iii) the delinquency status of consumer loans (iv) non-performing loans (see details above) and (vi) the general economic conditions in the State of Texas. We utilize a risk grading matrix to assign a risk grade to each of our commercial loans. Loans are graded on a scale of 1 to 14. A description of the general characteristics of the 14 risk grades is set forth in our 2020 Form 10-K. We monitor portfolio credit quality by the weighted-average risk grade of each class of commercial loan. Individual relationship managers, under the oversight of credit administration, review updated financial information for all pass grade loans to reassess the risk grade on at least an annual basis. When a loan has a risk grade of 9, it is still considered a pass grade loan; however, it is considered to be on management’s “watch list,” where a significant risk-modifying action is anticipated in the near term. When a loan has a risk grade of 10 or higher, a special assets officer monitors the loan on an on-going basis. The following tables present weighted-average risk grades for all commercial loans, by class and year of origination/renewal as of March 31, 2021. Paycheck Protection Program (“PPP”) loans are excluded as such loans are fully guaranteed by the Small Business Administration (“SBA”). 2021 2020 2019 2018 2017 Prior Revolving Loans Revolving Loans Converted to Term Total W/A Risk Grade Commercial and industrial Risk grades 1-8 $ 486,852 $ 874,754 $ 512,101 $ 260,536 $ 193,920 $ 198,420 $ 1,782,914 $ 58,427 $ 4,367,924 6.18 Risk grade 9 11,174 40,156 34,652 29,911 16,891 14,197 59,650 10,015 216,646 9.00 Risk grade 10 25,740 11,803 3,264 7,392 2,769 278 56,961 5,856 114,063 10.00 Risk grade 11 9 4,374 10,409 6,075 3,127 1,074 19,143 8,875 53,086 11.00 Risk grade 12 — 7,564 1,919 921 724 114 553 2,277 14,072 12.00 Risk grade 13 — 2,780 544 — 480 — 274 359 4,437 13.00 $ 523,775 $ 941,431 $ 562,889 $ 304,835 $ 217,911 $ 214,083 $ 1,919,495 $ 85,809 $ 4,770,228 6.48 W/A risk grade 6.33 6.33 6.88 7.17 6.32 6.03 6.37 7.70 6.48 Energy Risk grades 1-8 $ 344,718 $ 45,560 $ 16,767 $ 8,861 $ 7,330 $ 5,470 $ 453,529 $ 17,177 $ 899,412 6.29 Risk grade 9 61,308 8,400 2,247 1,481 — — 15,617 5,188 94,241 9.00 Risk grade 10 1,317 369 814 1,258 — 750 7,833 3,753 16,094 10.00 Risk grade 11 11,481 1,647 19,510 1,245 — 627 35,634 6,276 76,420 11.00 Risk grade 12 — 900 4,624 478 — 347 4,108 114 10,571 12.00 Risk grade 13 — — 1,690 1,423 — — 2,358 — 5,471 13.00 $ 418,824 $ 56,876 $ 45,652 $ 14,746 $ 7,330 $ 7,194 $ 519,079 $ 32,508 $ 1,102,209 6.99 W/A risk grade 6.93 6.94 9.70 8.67 7.42 7.83 6.64 8.63 6.99 Commercial real estate: Buildings, land, other Risk grades 1-8 $ 356,814 $ 1,497,610 $ 914,168 $ 666,186 $ 570,468 $ 969,182 $ 56,513 $ 66,100 $ 5,097,041 6.93 Risk grade 9 22,563 62,562 82,497 59,815 46,457 57,414 4,885 2,484 338,677 9.00 Risk grade 10 2,638 20,217 27,966 56,444 67,214 79,519 — 11,083 265,081 10.00 Risk grade 11 2,003 22,411 14,074 13,549 39,196 57,418 3,200 140 151,991 11.00 Risk grade 12 — 1,700 2,995 989 2,114 3,011 3,187 — 13,996 12.00 Risk grade 13 — — — — — 250 — — 250 13.00 $ 384,018 $ 1,604,500 $ 1,041,700 $ 796,983 $ 725,449 $ 1,166,794 $ 67,785 $ 79,807 $ 5,867,036 7.31 W/A risk grade 7.21 7.18 7.29 7.48 7.53 7.27 7.68 7.12 7.31 Construction Risk grades 1-8 $ 180,644 $ 277,006 $ 439,531 $ 124,821 $ 60 $ 2,848 $ 128,827 $ — $ 1,153,737 7.01 Risk grade 9 2,955 33,213 — — 2,813 — 13,793 — 52,774 9.00 Risk grade 10 — — — 27,720 — — — — 27,720 10.00 Risk grade 11 — 1,110 — — — — — — 1,110 11.00 Risk grade 12 — 1,653 — — — — — — 1,653 12.00 Risk grade 13 — — — — — — — — — 13.00 $ 183,599 $ 312,982 $ 439,531 $ 152,541 $ 2,873 $ 2,848 $ 142,620 $ — $ 1,236,994 7.17 W/A risk grade 6.84 6.85 7.13 8.24 8.96 6.79 7.23 — 7.17 Total commercial real estate $ 567,617 $ 1,917,482 $ 1,481,231 $ 949,524 $ 728,322 $ 1,169,642 $ 210,405 $ 79,807 $ 7,104,030 7.28 W/A risk grade 7.09 7.12 7.24 7.61 7.53 7.27 7.37 7.12 7.28 In the table above, certain loans are reported as 2021 originations and have risk grades of 11 or higher. These loans were, for the most part, first originated in various years prior to 2021 but were renewed in the current year. The following tables present weighted average risk grades for all commercial loans by class as of December 31, 2020. Refer to our 2020 Form 10-K for details of these loans by year of origination/renewal. Commercial and Industrial Energy Commercial Real Estate - Buildings, Land and Other Commercial Real Estate - Construction Total Commercial Real Estate W/A Risk Grade Loans W/A Risk Grade Loans W/A Risk Grade Loans W/A Risk Grade Loans W/A Risk Grade Loans Risk grades 1-8 6.13 $ 4,506,121 5.99 $ 968,144 6.97 $ 5,047,873 6.99 $ 1,110,025 6.98 $ 6,157,898 Risk grade 9 9.00 256,198 9.00 133,547 9.00 325,227 9.00 72,287 9.00 397,514 Risk grade 10 10.00 125,977 10.00 46,427 10.00 258,454 10.00 38,962 10.00 297,416 Risk grade 11 11.00 47,196 11.00 63,912 11.00 149,362 11.00 856 11.00 150,218 Risk grade 12 12.00 14,528 12.00 13,741 12.00 15,224 12.00 1,684 12.00 16,908 Risk grade 13 13.00 5,321 13.00 9,427 13.00 513 13.00 — 13.00 513 Total 6.45 $ 4,955,341 6.85 $ 1,235,198 7.34 $ 5,796,653 7.22 $ 1,223,814 7.32 $ 7,020,467 Information about the payment status of consumer loans, segregated by portfolio segment and year of origination, as of March 31, 2021 was as follows: 2021 2020 2019 2018 2017 Prior Revolving Loans Revolving Loans Converted to Term Total Consumer real estate: Past due 30-89 days $ — $ 230 $ 255 $ 1,418 $ 716 $ 1,464 $ 994 $ 896 $ 5,973 Past due 90 or more days — — 470 — 404 1,558 492 330 3,254 Total past due — 230 725 1,418 1,120 3,022 1,486 1,226 9,227 Current loans 62,908 331,440 144,445 81,559 71,346 169,122 437,555 14,552 1,312,927 Total $ 62,908 $ 331,670 $ 145,170 $ 82,977 $ 72,466 $ 172,144 $ 439,041 $ 15,778 $ 1,322,154 Consumer and other: Past due 30-89 days $ 4 $ 1,266 $ 131 $ 144 $ 83 $ 24 $ 695 $ 850 $ 3,197 Past due 90 or more days — 110 — 113 — — 45 179 447 Total past due 4 1,376 131 257 83 24 740 1,029 3,644 Current loans 20,060 31,370 18,966 4,669 2,408 1,914 349,628 29,127 458,142 Total $ 20,064 $ 32,746 $ 19,097 $ 4,926 $ 2,491 $ 1,938 $ 350,368 $ 30,156 $ 461,786 Revolving loans that converted to term during the three months ended March 31, 2021 and 2020 were as follows: Three months ended March 31, 2021 2020 Commercial and industrial $ 11,843 $ 11,346 Energy 5,928 15,353 Commercial real estate: Buildings, land and other 23,303 7,639 Construction — — Consumer real estate 793 1,071 Consumer and other 3,861 — Total $ 45,728 $ 35,409 In assessing the general economic conditions in the State of Texas, management monitors and tracks the Texas Leading Index (“TLI”), which is produced by the Federal Reserve Bank of Dallas. The TLI, the components of which are more fully described in our 2020 Form 10-K, totaled 121.4 at March 31, 2021 and 118.6 at December 31, 2020. A higher TLI value implies more favorable economic conditions. Allowance For Credit Losses - Loans. The allowance for credit losses on loans is a contra-asset valuation account, calculated in accordance with ASC 326, that is deducted from the amortized cost basis of loans to present the net amount expected to be collected. The amount of the allowance represents management's best estimate of current expected credit losses on loans considering available information, from internal and external sources, relevant to assessing collectibility over the loans' contractual terms, adjusted for expected prepayments when appropriate. Credit loss expense related to loans reflects the totality of actions taken on all loans for a particular period including any necessary increases or decreases in the allowance related to changes in credit loss expectations associated with specific loans or pools of loans. Portions of the allowance may be allocated for specific credits; however, the entire allowance is available for any credit that, in management’s judgment, should be charged off. While management utilizes its best judgment and information available, the ultimate appropriateness of the allowance is dependent upon a variety of factors beyond our control, including the performance of our loan portfolio, the economy, changes in interest rates and the view of the regulatory authorities toward loan classifications. Our allowance methodology is more fully described in our 2020 Form 10-K. The following table presents details of the allowance for credit losses on loans segregated by loan portfolio segment as of March 31, 2021 and December 31, 2020. No allowance for credit losses has been recognized for PPP loans as such loans are fully guaranteed by the SBA. March 31, 2021 Commercial Energy Commercial Consumer Consumer Total Modeled expected credit losses $ 54,849 $ 11,560 $ 40,831 $ 5,608 $ 6,794 $ 119,642 Q-Factor and other qualitative adjustments 10,964 15,047 103,359 28 6 129,404 Specific allocations 5,079 6,865 250 — 18 12,212 Total $ 70,892 $ 33,472 $ 144,440 $ 5,636 $ 6,818 $ 261,258 December 31, 2020 Modeled expected credit losses $ 65,645 $ 8,910 $ 125,126 $ 7,926 $ 6,945 $ 214,552 Q-Factor and other qualitative adjustments 2,877 21,216 9,253 — — 33,346 Specific allocations 5,321 9,427 513 — 18 15,279 Total $ 73,843 $ 39,553 $ 134,892 $ 7,926 $ 6,963 $ 263,177 The following table details activity in the allowance for credit losses on loans by portfolio segment for the three months ended March 31, 2021 and 2020. Allocation of a portion of the allowance to one category of loans does not preclude its availability to absorb losses in other categories. No allowance for credit losses has been recognized for PPP loans as such loans are fully guaranteed by the SBA. Commercial Energy Commercial Consumer Consumer Total Three months ended: March 31, 2021 Beginning balance $ 73,843 $ 39,553 $ 134,892 $ 7,926 $ 6,963 $ 263,177 Credit loss expense (benefit) (1,965) (5,801) 8,922 (2,722) 1,566 — Charge-offs (2,189) (1,433) — (284) (4,060) (7,966) Recoveries 1,203 1,153 626 716 2,349 6,047 Net (charge-offs) recoveries (986) (280) 626 432 (1,711) (1,919) Ending balance $ 70,892 $ 33,472 $ 144,440 $ 5,636 $ 6,818 $ 261,258 March 31, 2020 Beginning balance $ 51,593 $ 37,382 $ 31,037 $ 4,113 $ 8,042 $ 132,167 Impact of adopting ASC 326 21,263 (10,453) (13,519) 2,392 (2,248) (2,565) Credit loss expense 21,950 110,006 34,761 1,570 4,638 172,925 Charge-offs (4,369) (33,800) (73) (285) (5,005) (43,532) Recoveries 1,715 66 113 380 2,612 4,886 Net (charge-offs) recoveries (2,654) (33,734) 40 95 (2,393) (38,646) Ending balance $ 92,152 $ 103,201 $ 52,319 $ 8,170 $ 8,039 $ 263,881 The following table presents loans that were evaluated for expected credit losses on an individual basis and the related specific allocations, by loan portfolio segment as of March 31, 2021 and December 31, 2020. March 31, 2021 December 31, 2020 Loan Specific Allocations Loan Specific Allocations Commercial and industrial $ 21,858 $ 5,079 $ 21,287 $ 5,321 Energy 22,496 6,865 22,888 9,427 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 32,345 250 34,057 513 Construction 1,653 — 1,684 — Consumer real estate 302 — 561 — Consumer and other 18 18 18 18 Total $ 78,672 $ 12,212 $ 80,495 $ 15,279

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	3 Months Ended
	Mar. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets Goodwill and other intangible assets are presented in the table below. March 31, December 31, Goodwill $ 654,952 $ 654,952 Other intangible assets: Core deposits $ 1,127 $ 1,296 Customer relationships 234 267 $ 1,361 $ 1,563 The estimated aggregate future amortization expense for intangible assets remaining as of March 31, 2021 is as follows: Remainder of 2021 $ 495 2022 481 2023 282 2024 87 2025 11 Thereafter 5 $ 1,361

Deposits

Deposits	3 Months Ended
	Mar. 31, 2021
Disclosure Text Block [Abstract]
Deposits	Deposits Deposits were as follows: March 31, Percentage December 31, Percentage Non-interest-bearing demand deposits: Commercial and individual $ 15,002,423 40.6 % $ 13,914,754 39.7 % Correspondent banks 213,554 0.6 242,225 0.7 Public funds 887,672 2.4 960,072 2.8 Total non-interest-bearing demand deposits 16,103,649 43.6 15,117,051 43.2 Interest-bearing deposits: Private accounts: Savings and interest checking 9,648,088 26.1 9,132,789 26.1 Money market accounts 9,416,210 25.5 8,977,585 25.6 Time accounts 1,141,660 3.1 1,135,575 3.3 Total private accounts 20,205,958 54.7 19,245,949 55.0 Public funds: Savings and interest checking 556,523 1.5 597,503 1.7 Money market accounts 53,496 0.2 50,070 0.1 Time accounts 5,554 — 5,188 — Total public funds 615,573 1.7 652,761 1.8 Total interest-bearing deposits 20,821,531 56.4 19,898,710 56.8 Total deposits $ 36,925,180 100.0 % $ 35,015,761 100.0 % The following table presents additional information about our deposits: March 31, December 31, Deposits from the Certificate of Deposit Account Registry Service (CDARS) deposits $ 374 $ 372 Deposits from foreign sources (primarily Mexico) 910,652 884,169 Total deposits not covered by deposit insurance 19,766,784 18,694,320 Time deposits not covered by deposit insurance 251,639 237,298

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Off-Balance-Sheet Arrangements, Commitments, Guarantees and Contingencies Financial Instruments with Off-Balance-Sheet Risk . In the normal course of business, we enter into various transactions, which, in accordance with generally accepted accounting principles are not included in our consolidated balance sheets. We enter into these transactions to meet the financing needs of our customers. As more fully discussed in our 2020 Form 10-K, these transactions include commitments to extend credit and standby letters of credit, which involve, to varying degrees, elements of credit risk and interest rate risk in excess of the amounts recognized in the consolidated balance sheets. We minimize our exposure to loss under these commitments by subjecting them to credit approval and monitoring procedures. Financial instruments with off-balance-sheet risk were as follows: March 31, December 31, Commitments to extend credit $ 9,853,069 $ 9,814,475 Standby letters of credit 261,341 241,345 Deferred standby letter of credit fees 1,711 1,723 Allowance For Credit Losses - Off-Balance-Sheet Credit Exposures. The allowance for credit losses on off-balance-sheet credit exposures is a liability account, calculated in accordance with ASC 326, representing expected credit losses over the contractual period for which we are exposed to credit risk resulting from a contractual obligation to extend credit. No allowance is recognized if we have the unconditional right to cancel the obligation. Off-balance-sheet credit exposures primarily consist of amounts available under outstanding lines of credit and letters of credit detailed in the table above. The amount of the allowance represents management's best estimate of expected credit losses on commitments expected to be funded over the contractual life of the commitment. Our allowance methodology is more fully described in our 2020 Form 10-K. The following table details activity in the allowance for credit losses on off-balance-sheet credit exposures. Three Months Ended 2021 2020 Beginning balance $ 44,152 $ 500 Impact of adopting ASC 326 — 39,377 Credit loss expense 65 2,287 Ending balance $ 44,217 $ 42,164 Lease Commitments . We lease certain office facilities and office equipment under operating leases. The components of total lease expense were as follows: Three Months Ended 2021 2020 Amortization of lease right-of-use assets $ 8,283 $ 7,874 Short-term lease expense 204 561 Non-lease components (including taxes, insurance, common maintenance, etc.) 3,004 2,927 Total $ 11,491 $ 11,362 Right-of-use lease assets totaled $287.5 million at March 31, 2021 and $292.1 million at December 31, 2020 and are reported as a component of premises and equipment on our accompanying consolidated balance sheets. The related lease liabilities totaled $318.7 million at March 31, 2021 and $323.0 million at December 31, 2020 and are reported as a component of accrued interest payable and other liabilities in the accompanying consolidated balance sheets. Lease payments under operating leases that were applied to our operating lease liability totaled $8.1 million during the three months ended March 31, 2021 and $7.7 million during the three months ended March 31, 2020. There has been no significant change in our expected future minimum lease payments since December 31, 2020. See the 2020 Form 10-K for information regarding these commitments.

Capital and Regulatory Matters

Capital and Regulatory Matters	3 Months Ended
	Mar. 31, 2021
Banking Regulation, Common Equity Tier One Risk-Based Capital [Abstract]
Capital and Regulatory Matters	Capital and Regulatory Matters Banks and bank holding companies are subject to various regulatory capital requirements administered by state and federal banking agencies. Capital adequacy guidelines and, additionally for banks, prompt corrective action regulations, involve quantitative measures of assets, liabilities, and certain off-balance-sheet items calculated under regulatory accounting practices. Capital amounts and classifications are also subject to qualitative judgments by regulators about components, risk weighting and other factors. Cullen/Frost’s and Frost Bank’s Common Equity Tier 1 capital (“CET1”) includes common stock and related paid-in capital, net of treasury stock, and retained earnings. In connection with the adoption of the Basel III Capital Rules, we elected to opt-out of the requirement to include most components of accumulated other comprehensive income in CET1. We also elected to exclude the effects of credit loss accounting under CECL from CET1 for a five-year transitional period, as further discussed in our 2020 Form 10-K. This CECL transitional adjustment totaled $63.2 million and $63.7 million at March 31, 2021 and December 31, 2020, respectively. CET1 is reduced by goodwill and other intangible assets, net of associated deferred tax liabilities. Frost Bank's CET1 is also reduced by its equity investment in its financial subsidiary, Frost Insurance Agency (“FIA”). Tier 1 capital includes CET1 and additional Tier 1 capital. For Cullen/Frost, additional Tier 1 capital included $145.5 million of 4.450% non-cumulative perpetual preferred stock at March 31, 2021 and December 31, 2020, the details of which are further discussed below. Frost Bank did not have any additional Tier 1 capital beyond Common Equity Tier 1 at March 31, 2021 or December 31, 2020. Total capital includes Tier 1 capital and Tier 2 capital. Tier 2 capital for both Cullen/Frost and Frost Bank includes a permissible portion of the allowances for credit losses on securities, loans and off-balance-sheet credit exposures. Tier 2 capital for Cullen/Frost also includes $100.0 million of qualified subordinated debt and $133.0 million of trust preferred securities at both March 31, 2021 and December 31, 2020. The following table presents actual and required capital ratios as of March 31, 2021 and December 31, 2020 for Cullen/Frost and Frost Bank under the Basel III Capital Rules. Capital levels required to be considered well capitalized are based upon prompt corrective action regulations, as amended to reflect the changes under the Basel III Capital Rules. See the 2020 Form 10-K for a more detailed discussion of the Basel III Capital Rules. Actual Minimum Capital Required - Basel III Required to be Capital Ratio Capital Ratio Capital Ratio March 31, 2021 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 3,158,758 13.45 % $ 1,643,402 7.00 % $ 1,526,017 6.50 % Frost Bank 3,104,492 13.25 1,640,032 7.00 1,522,886 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 3,304,210 14.07 1,995,560 8.50 1,878,174 8.00 Frost Bank 3,104,492 13.25 1,991,467 8.50 1,874,322 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,771,830 16.07 2,465,104 10.50 2,347,718 10.00 Frost Bank 3,339,112 14.25 2,460,047 10.50 2,342,902 10.00 Leverage Ratio Cullen/Frost 3,304,210 7.97 1,658,592 4.00 2,073,240 5.00 Frost Bank 3,104,492 7.49 1,657,640 4.00 2,072,050 5.00 December 31, 2020 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 3,058,447 12.86 % $ 1,664,867 7.00 % $ 1,545,948 6.50 % Frost Bank 3,030,093 12.77 1,661,620 7.00 1,542,933 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 3,203,899 13.47 2,021,624 8.50 1,902,705 8.00 Frost Bank 3,030,093 12.77 2,017,682 8.50 1,898,995 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,672,912 15.44 2,497,300 10.50 2,378,381 10.00 Frost Bank 3,266,106 13.76 2,492,430 10.50 2,373,743 10.00 Leverage Ratio Cullen/Frost 3,203,899 8.07 1,589,004 4.00 1,986,255 5.00 Frost Bank 3,030,093 7.63 1,588,200 4.00 1,985,250 5.00 As of March 31, 2021, capital levels at Cullen/Frost and Frost Bank exceed all capital adequacy requirements under the Basel III Capital Rules. Based on the ratios presented above, capital levels as of March 31, 2021 at Cullen/Frost and Frost Bank exceed the minimum levels necessary to be considered “well capitalized.” Cullen/Frost and Frost Bank are subject to the regulatory capital requirements administered by the Federal Reserve Board and, for Frost Bank, the Federal Deposit Insurance Corporation (“FDIC”). Regulatory authorities can initiate certain mandatory actions if Cullen/Frost or Frost Bank fail to meet the minimum capital requirements, which could have a direct material effect on our financial statements. Management believes, as of March 31, 2021, that Cullen/Frost and Frost Bank meet all capital adequacy requirements to which they are subject. Preferred Stock. On March 16, 2020, we redeemed all 6,000,000 shares of our 5.375% Non-Cumulative Perpetual Preferred Stock, Series A, (“Series A Preferred Stock”) at a redemption price of $25 per share, or an aggregate redemption of $150.0 million. When issued, the net proceeds of the Series A Preferred Stock totaled $144.5 million after deducting $5.5 million of issuance costs including the underwriting discount and professional service fees, among other things. Upon redemption, these issuance costs were reclassified to retained earnings and reported as a reduction of net income available to common shareholders. On November 19, 2020, we issued 150,000 shares, or $150.0 million in aggregate liquidation preference, of our 4.450% Non-Cumulative Perpetual Preferred Stock, Series B, par value $0.01 and liquidation preference $1,000 per share (“Series B Preferred Stock”). Each share of Series B Preferred Stock issued and outstanding is represented by 40 depositary shares, each representing a 1/40th ownership interest in a share of the Series B Preferred Stock (equivalent to a liquidation preference of $25 per share). Dividends on the Series B Preferred Stock will be non-cumulative and, if declared, accrue and are payable quarterly, in arrears, at a rate of 4.450% per annum. The Series B Preferred Stock qualifies as Tier 1 capital for the purposes of the regulatory capital calculations. The net proceeds from the issuance and sale of the Series B Preferred Stock, after deducting $4.5 million of issuance costs including the underwriting discount and professional service fees, among other things, were approximately $145.5 million. Stock Repurchase Plans. From time to time, our board of directors has authorized stock repurchase plans. In general, stock repurchase plans allow us to proactively manage our capital position and return excess capital to shareholders. Shares purchased under such plans also provide us with shares of common stock necessary to satisfy obligations related to stock compensation awards. On January 27, 2021, our board of directors authorized a $100.0 million stock repurchase program, allowing us to repurchase shares of our common stock over a one Dividend Restrictions . In the ordinary course of business, Cullen/Frost is dependent upon dividends from Frost Bank to provide funds for the payment of dividends to shareholders and to provide for other cash requirements, including to repurchase its common stock. Banking regulations may limit the amount of dividends that may be paid. Approval by regulatory authorities is required if the effect of dividends declared would cause the regulatory capital of Frost Bank to fall below specified minimum levels. Approval is also required if dividends declared exceed the net profits for that year combined with the retained net profits for the preceding two years. Under the foregoing dividend restrictions and while maintaining its “well capitalized” status, at March 31, 2021, Frost Bank could pay aggregate dividends of up to $334.0 million to Cullen/Frost without prior regulatory approval. Under the terms of the junior subordinated deferrable interest debentures that Cullen/Frost has issued to Cullen/Frost Capital Trust II and WNB Capital Trust I, Cullen/Frost has the right at any time during the term of the debentures to defer the payment of interest at any time or from time to time for an extension period not exceeding 20 consecutive quarterly periods with respect to each extension period. In the event that we have elected to defer interest on the debentures, we may not, with certain exceptions, declare or pay any dividends or distributions on our capital stock or purchase or acquire any of our capital stock.

Derivative Financial Instrument

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Financial Instruments	Derivative Financial Instruments The fair value of derivative positions outstanding is included in accrued interest receivable and other assets and accrued interest payable and other liabilities in the accompanying consolidated balance sheets and in the net change in each of these financial statement line items in the accompanying consolidated statements of cash flows. Interest Rate Derivatives. We utilize interest rate swaps, caps and floors to mitigate exposure to interest rate risk and to facilitate the needs of our customers. Our objectives for utilizing these derivative instruments are described in our 2020 Form 10-K. The notional amounts and estimated fair values of interest rate derivative contracts are presented in the following table. The fair values of interest rate derivative contracts are estimated utilizing internal valuation models with observable market data inputs, or as determined by the Chicago Mercantile Exchange (“CME”) for centrally cleared derivative contracts. CME rules legally characterize variation margin payments for centrally cleared derivatives as settlements of the derivatives' exposure rather than collateral. As a result, the variation margin payment and the related derivative instruments are considered a single unit of account for accounting and financial reporting purposes. Variation margin, as determined by the CME, is settled daily. As a result, derivative contracts that clear through the CME have an estimated fair value of zero as of March 31, 2021 and December 31, 2020. March 31, 2021 December 31, 2020 Notional Estimated Notional Estimated Derivatives designated as hedges of fair value: Financial institution counterparties: Loan/lease interest rate swaps – liabilities 3,407 $ (104) 3,724 $ (134) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets 164,768 2,560 — — Loan/lease interest rate swaps – liabilities 906,227 (23,479) 1,173,173 (33,812) Loan/lease interest rate caps – assets 382,086 3,670 356,601 1,241 Customer counterparties: Loan/lease interest rate swaps – assets 906,227 52,590 1,173,173 84,424 Loan/lease interest rate swaps – liabilities 164,768 (4,736) — — Loan/lease interest rate caps – liabilities 382,086 (3,670) 356,601 (1,241) The weighted-average rates paid and received for interest rate swaps outstanding at March 31, 2021 were as follows: Weighted-Average Interest Interest Interest rate swaps: Fair value hedge loan/lease interest rate swaps 3.14 % 0.11 % Non-hedging interest rate swaps – financial institution counterparties 3.81 1.85 Non-hedging interest rate swaps – customer counterparties 1.85 3.81 The weighted-average strike rate for outstanding interest rate caps was 3.29% at March 31, 2021. Commodity Derivatives. We enter into commodity swaps and option contracts that are not designated as hedging instruments primarily to accommodate the business needs of our customers. Upon the origination of a commodity swap or option contract with a customer, we simultaneously enter into an offsetting contract with a third party financial institution to mitigate the exposure to fluctuations in commodity prices. The notional amounts and estimated fair values of non-hedging commodity swap and option derivative positions outstanding are presented in the following table. We obtain dealer quotations and use internal valuation models with observable market data inputs to value our commodity derivative positions. March 31, 2021 December 31, 2020 Notional Notional Estimated Notional Estimated Financial institution counterparties: Oil – assets Barrels 4,620 $ 9,176 3,056 $ 8,341 Oil – liabilities Barrels 8,906 (74,039) 6,391 (32,112) Natural gas – assets MMBTUs 2,422 448 9,281 1,529 Natural gas – liabilities MMBTUs 17,857 (3,835) 15,079 (3,265) Customer counterparties: Oil – assets Barrels 8,906 74,932 6,391 32,670 Oil – liabilities Barrels 4,620 (8,897) 3,056 (8,264) Natural gas – assets MMBTUs 17,857 4,048 17,636 3,451 Natural gas – liabilities MMBTUs 2,422 (444) 6,724 (1,458) Foreign Currency Derivatives . We enter into foreign currency forward contracts that are not designated as hedging instruments primarily to accommodate the business needs of our customers. Upon the origination of a foreign currency denominated transaction with a customer, we simultaneously enter into an offsetting contract with a third party financial institution to negate the exposure to fluctuations in foreign currency exchange rates. We also utilize foreign currency forward contracts that are not designated as hedging instruments to mitigate the economic effect of fluctuations in foreign currency exchange rates on foreign currency holdings and certain short-term, non-U.S. dollar denominated loans. The notional amounts and fair values of open foreign currency forward contracts were as follows: March 31, 2021 December 31, 2020 Notional Notional Estimated Notional Estimated Financial institution counterparties: Forward contracts – assets EUR 6,311 $ 144 — $ — Customer counterparties: Forward contracts – liabilities EUR 6,299 (132) — — Gains, Losses and Derivative Cash Flows . For fair value hedges, the changes in the fair value of both the derivative hedging instrument and the hedged item are included in other non-interest income or other non-interest expense. The extent that such changes in fair value do not offset represents hedge ineffectiveness. Net cash flows from interest rate swaps on commercial loans/leases designated as hedging instruments in effective hedges of fair value are included in interest income on loans. For non-hedging derivative instruments, gains and losses due to changes in fair value and all cash flows are included in other non-interest income and other non-interest expense. Amounts included in the consolidated statements of income related to interest rate derivatives designated as hedges of fair value were as follows: Three Months Ended 2021 2020 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ (28) $ (8) Amount of (gain) loss included in other non-interest expense 3 1 As stated above, we enter into non-hedge related derivative positions primarily to accommodate the business needs of our customers. Upon the origination of a derivative contract with a customer, we simultaneously enter into an offsetting derivative contract with a third party financial institution. We recognize immediate income based upon the difference in the bid/ask spread of the underlying transactions with our customers and the third party. Because we act only as an intermediary for our customer, subsequent changes in the fair value of the underlying derivative contracts for the most part offset each other and do not significantly impact our results of operations. During the first three months of 2020, we sold certain non-hedge related, short-term put options on U.S. Treasury securities with an aggregate notional amount of $500 million and realized gains totaling approximately $6.0 million in connection with the sales. The put options were not exercised and expired in March 2020. Amounts included in the consolidated statements of income related to non-hedge related derivative instruments are presented in the table below. Three Months Ended 2021 2020 Non-hedging interest rate derivatives: Other non-interest income $ 1,587 $ 2,039 Other non-interest expense (1) — Non-hedging commodity derivatives: Other non-interest income 1,154 139 Non-hedging foreign currency derivatives: Other non-interest income 30 7 Non-hedging put options: Other non-interest income — 5,980 Counterparty Credit Risk. Our credit exposure relating to interest rate swaps, commodity swaps/options and foreign currency forward contracts with bank customers was approximately $122.1 million at March 31, 2021. This credit exposure is partly mitigated as transactions with customers are generally secured by the collateral, if any, securing the underlying transaction being hedged. Our credit exposure, net of collateral pledged, relating to interest rate swaps, commodity swaps/options and foreign currency forward contracts with upstream financial institution counterparties was approximately $24.8 million at March 31, 2021. This amount was primarily related to initial margin payments to the CME and excess collateral we posted to counterparties. Collateral levels for upstream financial institution counterparties are monitored and adjusted as necessary. See Note 9 – Balance Sheet Offsetting and Repurchase Agreements for additional information regarding our credit exposure with upstream financial institution counterparties. At March 31, 2021, we had $110.2 million in cash collateral related to derivative contracts on deposit with other financial institution counterparties.

Balance Sheet Offsetting

Balance Sheet Offsetting	3 Months Ended
	Mar. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Balance Sheet Offsetting	Balance Sheet Offsetting and Repurchase Agreements Balance Sheet Offsetting. Certain financial instruments, including resell and repurchase agreements and derivatives, may be eligible for offset in the consolidated balance sheet and/or subject to master netting arrangements or similar agreements. Our derivative transactions with upstream financial institution counterparties are generally executed under International Swaps and Derivative Association (“ISDA”) master agreements which include “right of set-off” provisions. In such cases there is generally a legally enforceable right to offset recognized amounts and there may be an intention to settle such amounts on a net basis. Nonetheless, we do not generally offset such financial instruments for financial reporting purposes. Information about financial instruments that are eligible for offset in the consolidated balance sheet as of March 31, 2021 is presented in the following tables. Gross Amount Gross Amount Net Amount March 31, 2021 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 6,230 $ — $ 6,230 Commodity swaps and options 9,624 — 9,624 Foreign currency forward contracts 144 — 144 Total derivatives 15,998 — 15,998 Resell agreements 7,903 — 7,903 Total $ 23,901 $ — $ 23,901 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 23,583 $ — $ 23,583 Commodity swaps and options 77,874 — 77,874 Total derivatives 101,457 — 101,457 Repurchase agreements 1,884,449 — 1,884,449 Total $ 1,985,906 $ — $ 1,985,906 Gross Amounts Not Offset Net Amount Financial Collateral Net March 31, 2021 Financial assets: Derivatives: Counterparty A $ 5 $ (5) $ — $ — Counterparty B 8,250 (8,250) — — Other counterparties 7,743 (7,743) — — Total derivatives 15,998 (15,998) — — Resell agreements 7,903 — (7,903) — Total $ 23,901 $ (15,998) $ (7,903) $ — Financial liabilities: Derivatives: Counterparty A $ 5,349 $ (5) $ (5,344) $ — Counterparty B 32,181 (8,250) (23,931) — Counterparty C 50 — (50) — Other counterparties 63,877 (7,743) (56,134) — Total derivatives 101,457 (15,998) (85,459) — Repurchase agreements 1,884,449 — (1,884,449) — Total $ 1,985,906 $ (15,998) $ (1,969,908) $ — Information about financial instruments that are eligible for offset in the consolidated balance sheet as of December 31, 2020 is presented in the following tables. Gross Amount Gross Amount Net Amount December 31, 2020 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 1,241 $ — $ 1,241 Commodity swaps and options 9,870 — 9,870 Total derivatives 11,111 — 11,111 Resell agreements — — — Total $ 11,111 $ — $ 11,111 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 33,946 $ — $ 33,946 Commodity swaps and options 35,377 — 35,377 Total derivatives 69,323 — 69,323 Repurchase agreements 2,068,147 — 2,068,147 Total $ 2,137,470 $ — $ 2,137,470 Gross Amounts Not Offset Net Amount Financial Collateral Net December 31, 2020 Financial assets: Derivatives: Counterparty A $ 2 $ (2) $ — $ — Counterparty B 5,838 (5,838) — — Other counterparties 5,271 (5,271) — — Total derivatives 11,111 (11,111) — — Resell agreements — — — — Total $ 11,111 $ (11,111) $ — $ — Financial liabilities: Derivatives: Counterparty A $ 6,430 $ (2) $ (6,428) $ — Counterparty B 20,722 (5,838) (14,700) 184 Counterparty C 71 — (71) — Other counterparties 42,100 (5,271) (35,832) 997 Total derivatives 69,323 (11,111) (57,031) 1,181 Repurchase agreements 2,068,147 — (2,068,147) — Total $ 2,137,470 $ (11,111) $ (2,125,178) $ 1,181 Repurchase Agreements. We utilize securities sold under agreements to repurchase to facilitate the needs of our customers and to facilitate secured short-term funding needs. Securities sold under agreements to repurchase are stated at the amount of cash received in connection with the transaction. We monitor collateral levels on a continuous basis. We may be required to provide additional collateral based on the fair value of the underlying securities. Securities pledged as collateral under repurchase agreements are maintained with our safekeeping agents. The remaining contractual maturity of repurchase agreements in the consolidated balance sheets as of March 31, 2021 and December 31, 2020 is presented in the following tables. Remaining Contractual Maturity of the Agreements Overnight and Continuous Up to 30 Days 30-90 Days Greater than 90 Days Total March 31, 2021 Repurchase agreements: U.S. Treasury $ 730,620 $ — $ — $ — $ 730,620 Residential mortgage-backed securities 1,153,829 — — — 1,153,829 Total borrowings $ 1,884,449 $ — $ — $ — $ 1,884,449 Gross amount of recognized liabilities for repurchase agreements $ 1,884,449 Amounts related to agreements not included in offsetting disclosures above $ — December 31, 2020 Repurchase agreements: U.S. Treasury $ 692,860 $ — $ — $ — $ 692,860 Residential mortgage-backed securities 1,375,287 — — — 1,375,287 Total borrowings $ 2,068,147 $ — $ — $ — $ 2,068,147 Gross amount of recognized liabilities for repurchase agreements $ 2,068,147 Amounts related to agreements not included in offsetting disclosures above $ —

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2021
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	Stock-Based Compensation A combined summary of activity in our active stock plans is presented in the table. Performance stock units outstanding are presented assuming attainment of the maximum payout rate as set forth by the performance criteria. As of March 31, 2021, there were 895,295 shares remaining available for grant for future stock-based compensation awards. Director Deferred Non-Vested Stock Performance Stock Units Outstanding Stock Options Number of Units Weighted- Number Weighted- Number of Units Weighted- Number Weighted- Balance, January 1, 2021 52,860 $ 75.47 470,359 $ 86.24 201,257 $ 77.18 1,739,559 $ 66.11 Authorized — — — — — — — — Granted — — — — — — — — Exercised/vested — — — — (35,131) 92.27 (478,693) 61.41 Forfeited/expired — — (6,991) 91.57 (1,115) 92.27 — — Balance, March 31, 2021 52,860 $ 75.47 463,368 $ 86.16 165,011 $ 73.86 1,260,866 $ 67.89 Shares issued in connection with stock compensation awards are issued from available treasury shares. If no treasury shares are available, new shares are issued from available authorized shares. Shares issued in connection with stock compensation awards along with other related information were as follows: Three Months Ended 2021 2020 New shares issued from available authorized shares — — Shares issued from available treasury stock 513,824 66,215 Total 513,824 66,215 Proceeds from stock option exercises $ 29,397 $ 1,463 Stock-based compensation expense is recognized ratably over the requisite service period for all awards. For most stock option awards, the service period generally matches the vesting period. For stock options granted to certain executive officers and for non-vested stock units granted to all participants, the service period does not extend past the date the participant reaches 65 years of age. Deferred stock units granted to non-employee directors generally have immediate vesting and the related expense is fully recognized on the date of grant. For performance stock units, the service period generally matches the three-year performance period specified by the award, however, the service period does not extend past the date the participant reaches 65 years of age. Expense recognized each period is dependent upon our estimate of the number of shares that will ultimately be issued. Stock-based compensation expense and the related income tax benefit is presented in the following table. Three Months Ended 2021 2020 Non-vested stock awards/stock units $ 1,883 $ 2,264 Performance stock units 643 248 Total $ 2,526 $ 2,512 Income tax benefit $ 367 $ 465 Unrecognized stock-based compensation expense at March 31, 2021 is presented in the table below. Unrecognized stock-based compensation expense related to performance stock units is presented assuming attainment of the maximum payout rate as set forth by the performance criteria. Non-vested stock awards/stock units $ 15,907 Performance stock units 6,288 Total $ 22,195

Earnings Per Common Share

Earnings Per Common Share	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Earnings Per Common Share	Earnings Per Common Share Earnings per common share is computed using the two-class method as more fully described in our 2020 Form 10-K. The following table presents a reconciliation of net income available to common shareholders, net earnings allocated to common stock and the number of shares used in the calculation of basic and diluted earnings per common share. Three Months Ended 2021 2020 Net income $ 116,015 $ 54,753 Less: Preferred stock dividends 2,151 2,016 Redemption of preferred stock — 5,514 Net income available to common shareholders 113,864 47,223 Less: Earnings allocated to participating securities 1,162 445 Net earnings allocated to common stock $ 112,702 $ 46,778 Distributed earnings allocated to common stock $ 45,660 $ 44,512 Undistributed earnings allocated to common stock 67,042 2,266 Net earnings allocated to common stock $ 112,702 $ 46,778 Weighted-average shares outstanding for basic earnings per common share 63,306,493 62,642,682 Dilutive effect of stock compensation 509,290 407,154 Weighted-average shares outstanding for diluted earnings per common share 63,815,783 63,049,836

Defined Benefit Plans

Defined Benefit Plans	3 Months Ended
	Mar. 31, 2021
Retirement Benefits [Abstract]
Defined Benefit Plans	Defined Benefit Plans The components of the combined net periodic expense (benefit) for our defined benefit pension plans are presented in the table below. Three Months Ended 2021 2020 Expected return on plan assets, net of expenses $ (3,210) $ (3,072) Interest cost on projected benefit obligation 835 1,252 Net amortization and deferral 1,529 1,330 Net periodic expense (benefit) $ (846) $ (490) Our non-qualified defined benefit pension plan is not funded. No contributions to the qualified defined benefit pension plan were made during the three months ended March 31, 2021. We do not expect to make any contributions to the qualified defined benefit plan during the remainder of 2021.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Income tax expense was as follows: Three Months Ended 2021 2020 Current income tax expense (benefit) $ 3,725 $ 29,412 Deferred income tax expense (benefit) 4,172 (26,089) Income tax expense, as reported $ 7,897 $ 3,323 Effective tax rate 6.4 % 5.7 % We had a net deferred tax liability totaling $88.4 million at March 31, 2021 and $117.5 million at December 31, 2020. No valuation allowance for deferred tax assets was recorded at March 31, 2021 as management believes it is more likely than not that all of the deferred tax assets will be realized against deferred tax liabilities and projected future taxable income. The effective income tax rates differed from the U.S. statutory federal income tax rates of 21% during the comparable periods primarily due to the effect of tax-exempt income from loans, securities and life insurance policies and the income tax effects associated with stock-based compensation. There were no unrecognized tax benefits during any of the reported periods. Interest and/or penalties related to income taxes are reported as a component of income tax expense. Such amounts were not significant during the reported periods. We file income tax returns in the U.S. federal jurisdiction. We are no longer subject to U.S. federal income tax examinations by tax authorities for years before 2017.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	3 Months Ended
	Mar. 31, 2021
Equity [Abstract]
Comprehensive Income (Loss) Note	Other Comprehensive Income (Loss) The before and after tax amounts allocated to each component of other comprehensive income (loss) are presented in the following table. Reclassification adjustments related to securities available for sale are included in net gain (loss) on securities transactions in the accompanying consolidated statements of income. Reclassification adjustments related to defined-benefit post-retirement benefit plans are included in the computation of net periodic pension expense (see Note 12 – Defined Benefit Plans). Three Months Ended Three Months Ended Before Tax Tax Expense, Net of Tax Before Tax Tax Expense, Net of Tax Securities available for sale and transferred securities: Change in net unrealized gain/loss during the period $ (159,779) $ (33,554) $ (126,225) $ 230,118 $ 48,325 $ 181,793 Change in net unrealized gain on securities transferred to held to maturity (259) (54) (205) (377) (79) (298) Reclassification adjustment for net (gains) losses included in net income — — — (108,989) (22,888) (86,101) Total securities available for sale and transferred securities (160,038) (33,608) (126,430) 120,752 25,358 95,394 Defined-benefit post-retirement benefit plans: Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) 1,529 321 1,208 1,330 279 1,051 Total defined-benefit post-retirement benefit plans 1,529 321 1,208 1,330 279 1,051 Total other comprehensive income (loss) $ (158,509) $ (33,287) $ (125,222) $ 122,082 $ 25,637 $ 96,445 Activity in accumulated other comprehensive income (loss), net of tax, was as follows: Securities Defined Accumulated Balance January 1, 2021 $ 563,801 $ (50,831) $ 512,970 Other comprehensive income (loss) before reclassifications (126,430) — (126,430) Reclassification of amounts included in net income — 1,208 1,208 Net other comprehensive income (loss) during period (126,430) 1,208 (125,222) Balance at March 31, 2021 $ 437,371 $ (49,623) $ 387,748 Balance January 1, 2020 $ 313,304 $ (45,934) $ 267,370 Other comprehensive income (loss) before reclassifications 181,495 — 181,495 Reclassification of amounts included in net income (86,101) 1,051 (85,050) Net other comprehensive income (loss) during period 95,394 1,051 96,445 Balance at March 31, 2020 $ 408,698 $ (44,883) $ 363,815

Operating Segments

Operating Segments	3 Months Ended
	Mar. 31, 2021
Segment Reporting [Abstract]
Operating Segments	Operating Segments We are managed under a matrix organizational structure whereby our two primary operating segments, Banking and Frost Wealth Advisors, overlap a regional reporting structure. See our 2020 Form 10-K for additional information regarding our operating segments. Summarized operating results by segment were as follows: Banking Frost Wealth Non-Banks Consolidated Three months ended: March 31, 2021 Net interest income (expense) $ 242,205 $ 486 $ (1,810) $ 240,881 Credit loss expense 63 — — 63 Non-interest income 53,780 39,609 (153) 93,236 Non-interest expense 179,151 29,937 1,054 210,142 Income (loss) before income taxes 116,771 10,158 (3,017) 123,912 Income tax expense (benefit) 7,177 2,133 (1,413) 7,897 Net income (loss) 109,594 8,025 (1,604) 116,015 Preferred stock dividends — — 2,151 2,151 Net income (loss) available to common shareholders $ 109,594 $ 8,025 $ (3,755) $ 113,864 Revenues from (expenses to) external customers $ 295,985 $ 40,095 $ (1,963) $ 334,117 March 31, 2020 Net interest income (expense) $ 246,013 $ 877 $ (2,369) $ 244,521 Credit loss expense (benefit) 175,198 (1) — 175,197 Non-interest income 173,416 39,730 (231) 212,915 Non-interest expense 188,972 33,536 1,655 224,163 Income (loss) before income taxes 55,259 7,072 (4,255) 58,076 Income tax expense (benefit) 3,235 1,485 (1,397) 3,323 Net income (loss) 52,024 5,587 (2,858) 54,753 Preferred stock dividends — — 2,016 2,016 Redemption of preferred stock — — 5,514 5,514 Net income (loss) available to common shareholders $ 52,024 $ 5,587 $ (10,388) $ 47,223 Revenues from (expenses to) external customers $ 419,429 $ 40,607 $ (2,600) $ 457,436

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The fair value of an asset or liability is the price that would be received to sell that asset or paid to transfer that liability in an orderly transaction occurring in the principal market (or most advantageous market in the absence of a principal market) for such asset or liability. In estimating fair value, we utilize valuation techniques that are consistent with the market approach, the income approach and/or the cost approach. Such valuation techniques are consistently applied. Inputs to valuation techniques include the assumptions that market participants would use in pricing an asset or liability. ASC Topic 820 establishes a three-level fair value hierarchy for valuation inputs that gives the highest priority to quoted prices in active markets for identical assets or liabilities and the lowest priority to unobservable inputs. See our 2020 Form 10-K for additional information regarding the fair value hierarchy and a description of our valuation techniques. Financial Assets and Financial Liabilities. The tables below summarize financial assets and financial liabilities measured at fair value on a recurring basis as of March 31, 2021 and December 31, 2020, segregated by the level of the valuation inputs within the fair value hierarchy of ASC Topic 820 utilized to measure fair value. Level 1 Inputs Level 2 Inputs Level 3 Inputs Total Fair Value March 31, 2021 Securities available for sale: U.S. Treasury $ 1,302,031 $ — $ — $ 1,302,031 Residential mortgage-backed securities — 1,894,805 — 1,894,805 States and political subdivisions — 7,138,254 — 7,138,254 Other — 42,354 — 42,354 Trading account securities: U.S. Treasury 24,038 — — 24,038 States and political subdivisions — 2,571 — 2,571 Derivative assets: Interest rate swaps, caps and floors — 58,820 — 58,820 Commodity swaps and options — 88,604 — 88,604 Foreign currency forward contracts 144 — — 144 Derivative liabilities: Interest rate swaps, caps and floors — 31,989 — 31,989 Commodity swaps and options — 87,215 — 87,215 Foreign currency forward contracts 132 — — 132 December 31, 2020 Securities available for sale: U.S. Treasury $ 1,119,633 $ — $ — $ 1,119,633 Residential mortgage-backed securities — 1,987,679 — 1,987,679 States and political subdivisions — 7,287,902 — 7,287,902 Other — 42,351 — 42,351 Trading account securities: U.S. Treasury 23,996 — — 23,996 States and political subdivisions — 460 — 460 Derivative assets: Interest rate swaps, caps and floors — 85,665 — 85,665 Commodity swaps and options — 45,535 456 45,991 Foreign currency forward contracts — — — — Derivative liabilities: Interest rate swaps, caps and floors — 35,187 — 35,187 Commodity swaps and options — 45,099 — 45,099 Foreign currency forward contracts — — — — Certain financial assets and financial liabilities are measured at fair value on a nonrecurring basis; that is, the instruments are not measured at fair value on an ongoing basis but are subject to fair value adjustments in certain circumstances. Financial assets measured at fair value on a non-recurring basis during the reported periods include certain collateral dependent loans reported at the fair value of the underlying collateral if repayment is expected solely from the collateral. The following table presents collateral dependent loans that were remeasured and reported at fair value through a specific allocation of the allowance for credit losses on loans based upon the fair value of the underlying collateral during the reported periods. Three Months Ended Three Months Ended Level 2 Level 3 Level 2 Level 3 Carrying value before allocations $ 100 $ 15,013 $ 1,919 $ 3,867 Specific (allocations) reversals of prior allocations — 600 70 (244) Fair value $ 100 $ 15,613 $ 1,989 $ 3,623 Non-Financial Assets and Non-Financial Liabilities. We do not have any non-financial assets or non-financial liabilities measured at fair value on a recurring basis. Non-financial assets measured at fair value on a non-recurring basis during the reported periods may include certain foreclosed assets which, upon initial recognition, were remeasured and reported at fair value through a charge-off to the allowance for loan losses and certain foreclosed assets which, subsequent to their initial recognition, were remeasured at fair value through a write-down included in other non-interest expense. The following table presents foreclosed assets that were remeasured and reported at fair value during the reported periods: Three Months Ended 2021 2020 Foreclosed assets remeasured subsequent to initial recognition: Carrying value of foreclosed assets prior to remeasurement $ — $ 328 Write-downs included in other non-interest expense — (231) Fair value $ — $ 97 Financial Instruments Reported at Amortized Cost. The estimated fair values of financial instruments that are reported at amortized cost in our consolidated balance sheets, segregated by the level of the valuation inputs within the fair value hierarchy utilized to measure fair value, were as follows: March 31, 2021 December 31, 2020 Carrying Estimated Carrying Estimated Financial assets: Level 2 inputs: Cash and cash equivalents $ 11,802,269 $ 11,802,269 $ 10,288,853 $ 10,288,853 Securities held to maturity 1,820,567 1,884,302 1,945,673 2,052,896 Cash surrender value of life insurance policies 189,756 189,756 189,984 189,984 Accrued interest receivable 128,725 128,725 181,432 181,432 Level 3 inputs: Loans, net 17,628,393 17,738,315 17,218,132 17,390,683 Financial liabilities: Level 2 inputs: Deposits 36,925,180 36,926,759 35,015,761 35,018,185 Federal funds purchased 38,975 38,975 48,850 48,850 Repurchase agreements 1,884,449 1,884,449 2,068,147 2,068,147 Junior subordinated deferrable interest debentures 136,371 137,115 136,357 137,115 Subordinated notes 99,061 114,818 99,021 115,717 Accrued interest payable 4,138 4,138 8,127 8,127 Under ASC Topic 825, entities may choose to measure eligible financial instruments at fair value at specified election dates. The fair value measurement option (i) may be applied instrument by instrument, with certain exceptions, (ii) is generally irrevocable and (iii) is applied only to entire instruments and not to portions of instruments. Unrealized gains and losses on items for which the fair value measurement option has been elected must be reported in earnings at each subsequent reporting date. During the reported periods, we had no financial instruments measured at fair value under the fair value measurement option.

Accounting Standards Updates

Accounting Standards Updates	3 Months Ended
	Mar. 31, 2021
Accounting Changes and Error Corrections [Abstract]
Accounting Standards Updates	Accounting Standards Updates Information about certain recently issued accounting standards updates is presented below. Also refer to Note 20 - Accounting Standards Updates in our 2020 Form 10-K for additional information related to previously issued accounting standards updates. ASU 2020-04, “Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting.” ASU 2020-04 provides optional expedients and exceptions for accounting related to contracts, hedging relationships and other transactions affected by reference rate reform if certain criteria are met. ASU 2020-04 applies only to contracts, hedging relationships, and other transactions that reference LIBOR or another reference rate expected to be discontinued because of reference rate reform and do not apply to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022, except for hedging relationships existing as of December 31, 2022, that an entity has elected certain optional expedients for and that are retained through the end of the hedging relationship. ASU 2020-04 was effective upon issuance and generally can be applied through December 31, 2022. The adoption of ASU 2020-04 did not significantly impact our financial statements. ASU 2020-08, “Codification Improvements to Subtopic 310-20, Receivables - Nonrefundable Fees and Other Costs.” ASU 2020-08 clarifies the accounting for the amortization of purchase premiums for callable debt securities with multiple call dates. ASU 2020-08 became effective for us on January 1, 2021 and did not have a significant impact on our financial statements. ASU 2020-09, “Debt (Topic 470): Amendments to SEC Paragraphs Pursuant to SEC Release No. 33-10762.” ASU 2020-09 amends the ASC to reflect the issuance of an SEC rule related to financial disclosure requirements for subsidiary issuers and guarantors of registered debt securities and affiliates whose securities are pledged as collateral for registered securities. ASU 2020-09 became effective for us on January 4, 2021, concurrent with the effective date of the SEC release, and did not have a significant impact on our financial statements. ASU 2021-01, “Reference Rate Reform (Topic 848): Scope.” ASU 2021-01 clarifies that certain optional expedients and exceptions in ASC 848 for contract modifications and hedge accounting apply to derivatives that are affected by the discounting transition. ASU 2021-01 also amends the expedients and exceptions in ASC 848 to capture the incremental consequences of the scope clarification and to tailor the existing guidance to derivative instruments affected by the discounting transition. ASU 2021-01 was effective upon issuance and generally can be applied through December 31, 2022. The adoption of ASU 2021-01 did not significantly impact our financial statements.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Nature of Operations	Nature of Operations. Cullen/Frost Bankers, Inc. (“Cullen/Frost”) is a financial holding company and a bank holding company headquartered in San Antonio, Texas that provides, through its subsidiaries, a broad array of products and services throughout numerous Texas markets. The terms “Cullen/Frost,” “the Corporation,” “we,” “us” and “our” mean Cullen/Frost Bankers, Inc. and its subsidiaries, when appropriate. In addition to general commercial and consumer banking, other products and services offered include trust and investment management, insurance, brokerage, mutual funds, leasing, treasury management, capital markets advisory and item processing.
Basis of Presentation	Basis of Presentation. The consolidated financial statements in this Quarterly Report on Form 10-Q include the accounts of Cullen/Frost and all other entities in which Cullen/Frost has a controlling financial interest. All significant intercompany balances and transactions have been eliminated in consolidation. The accounting and financial reporting policies we follow conform, in all material respects, to accounting principles generally accepted in the United States and to general practices within the financial services industry. The consolidated financial statements in this Quarterly Report on Form 10-Q have not been audited by an independent registered public accounting firm, but in the opinion of management, reflect all adjustments necessary for a fair presentation of our financial position and results of operations. All such adjustments were of a normal and recurring nature. The consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States for interim financial information and with the instructions to Form 10-Q adopted by the Securities and Exchange Commission (“SEC”). Accordingly, the financial statements do not include all of the information and footnotes required by accounting principles generally accepted in the United States for complete financial statements and should be read in conjunction with our consolidated financial statements, and notes thereto, for the year ended December 31, 2020, included in our Annual Report on Form 10-K filed with the SEC on February 5, 2021 (the “2020 Form 10-K”). Operating results for the interim periods disclosed herein are not necessarily indicative of the results that may be expected for a full year or any future period.
Use of Estimates	Use of Estimates . The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements. Actual results could differ from those estimates. The allowance for loan losses and the fair values of financial instruments and the status of contingencies are particularly subject to change.
Reclassification, Policy [Policy Text Block]	Certain items in prior financial statements have been reclassified to conform to the current presentation.
New Accounting Pronouncements, Policy [Policy Text Block]	On January 1, 2020, we adopted Accounting Standards Update (“ASU”) 2016-13, “Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments,” as subsequently updated for certain clarifications, targeted relief and codification improvements, using the modified retrospective method for all financial assets measured at amortized cost and off-balance-sheet credit exposures. Upon adoption, we recognized an after-tax cumulative effect reduction to retained earnings totaling $29.3 million, as detailed in our 2020 Form 10-K.

Loans Loans - Allowance For Cre

Loans Loans - Allowance For Credit Losses (Policies)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Loans and Leases Receivable, Allowance for Loan Losses Policy [Policy Text Block]	Allowance For Credit Losses - Loans. The allowance for credit losses on loans is a contra-asset valuation account, calculated in accordance with ASC 326, that is deducted from the amortized cost basis of loans to present the net amount expected to be collected. The amount of the allowance represents management's best estimate of current expected credit losses on loans considering available information, from internal and external sources, relevant to assessing collectibility over the loans' contractual terms, adjusted for expected prepayments when appropriate. Credit loss expense related to loans reflects the totality of actions taken on all loans for a particular period including any necessary increases or decreases in the allowance related to changes in credit loss expectations associated with specific loans or pools of loans. Portions of the allowance may be allocated for specific credits; however, the entire allowance is available for any credit that, in management’s judgment, should be charged off. While management utilizes its best judgment and information available, the ultimate appropriateness of the allowance is dependent upon a variety of factors beyond our control, including the performance of our loan portfolio, the economy, changes in interest rates and the view of the regulatory authorities toward loan classifications. Our allowance methodology is more fully described in our 2020 Form 10-K.

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Additional Cash Flow Information	Additional cash flow information was as follows: Three Months Ended 2021 2020 Cash paid for interest $ 9,719 $ 23,168 Cash paid for income taxes — — Significant non-cash transactions: Unsettled securities transactions 21,957 11,196 Right-of-use lease assets obtained in exchange for lessee operating lease liabilities 1,326 9,892 Treasury stock issued to 401(k) stock purchase plan 1,749 —

Securities (Tables)

Securities (Tables)	3 Months Ended
	Mar. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity [Table Text Block]	A summary of the amortized cost, fair value and allowance for credit losses related to securities held to maturity as of March 31, 2021 and December 31, 2020 is presented below. Amortized Gross Gross Estimated Allowance Net March 31, 2021 Residential mortgage-backed securities $ 528,472 $ 13,458 $ — $ 541,930 $ — $ 528,472 States and political subdivisions 1,290,753 50,119 — 1,340,872 (158) 1,290,595 Other 1,500 — — 1,500 — 1,500 Total $ 1,820,725 $ 63,577 $ — $ 1,884,302 $ (158) $ 1,820,567 December 31, 2020 Residential mortgage-backed securities $ 528,784 $ 41,742 $ — $ 570,526 $ — $ 528,784 States and political subdivisions 1,415,549 65,321 — 1,480,870 (160) 1,415,389 Other 1,500 — — 1,500 — 1,500 Total $ 1,945,833 $ 107,063 $ — $ 2,052,896 $ (160) $ 1,945,673
Moody's and Standard & Poor's Bond Ratings [Table Text Block]	The following table summarizes Moody's and/or Standard & Poor's bond ratings for our portfolio of held-to-maturity securities issued by States and political subdivisions and other securities as of March 31, 2021 and December 31, 2020: States and Political Subdivisions Not Guaranteed or Pre-Refunded Guaranteed by the Texas PSF Pre-Refunded Total Other March 31, 2021 Aaa/AAA $ 104,425 $ 579,584 $ 437,406 $ 1,121,415 $ — Aa/AA 119,548 — — 119,548 — A 49,790 — — 49,790 — Not rated — — — — 1,500 Total $ 273,763 $ 579,584 $ 437,406 $ 1,290,753 $ 1,500 December 31, 2020 Aaa/AAA $ 115,016 $ 659,999 $ 470,745 $ 1,245,760 $ — Aa/AA 107,065 — — 107,065 — A 62,724 — — 62,724 — Not rated — — — — 1,500 Total $ 284,805 $ 659,999 $ 470,745 $ 1,415,549 $ 1,500
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Table Text Block]	The following table details activity in the allowance for credit losses on held-to-maturity securities during the three months ended March 31, 2021 and 2020. Three Months Ended 2021 2020 Beginning balance $ 160 $ — Impact of adopting ASC 326 — 215 Credit loss expense (benefit) (2) (15) Ending balance $ 158 $ 200
Amortized Cost, Estimated Fair Value and Allowance for Credit Losses of Securities Available for Sale	A summary of the amortized cost, fair value and allowance for credit losses related to securities available for sale as of March 31, 2021 and December 31, 2020 is presented below. Amortized Gross Gross Allowance Estimated March 31, 2021 U.S. Treasury $ 1,281,883 $ 28,350 $ 8,202 $ — $ 1,302,031 Residential mortgage-backed securities 1,860,427 51,614 17,236 — 1,894,805 States and political subdivisions 6,642,395 502,019 6,160 — 7,138,254 Other 42,354 — — — 42,354 Total $ 9,827,059 $ 581,983 $ 31,598 $ — $ 10,377,444 December 31, 2020 U.S. Treasury $ 1,084,542 $ 35,091 $ — $ — $ 1,119,633 Residential mortgage-backed securities 1,916,581 71,102 4 — 1,987,679 States and political subdivisions 6,683,927 603,975 — — 7,287,902 Other 42,351 — — — 42,351 Total $ 9,727,401 $ 710,168 $ 4 $ — $ 10,437,565
Securities, with Unrealized Losses Segregated by Length of Impairment	The table below summarizes, as of March 31, 2021, securities available for sale in an unrealized loss position for which an allowance for credit losses has not been recorded, aggregated by type of security and length of time in a continuous unrealized loss position. Less than 12 Months More than 12 Months Total Estimated Unrealized Estimated Unrealized Estimated Unrealized U.S. Treasury $ 188,812 $ 8,202 $ — $ — $ 188,812 $ 8,202 Residential mortgage-backed securities 634,464 17,236 — — 634,464 17,236 States and political subdivisions 176,982 6,160 — — 176,982 6,160 Total $ 1,000,258 $ 31,598 $ — $ — $ 1,000,258 $ 31,598
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity	The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of March 31, 2021. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only. Within 1 Year 1 - 5 Years 5 - 10 Years After 10 Years Total Held To Maturity Amortized Cost Residential mortgage-backed securities $ 16 $ 129 $ 515,865 $ 12,462 $ 528,472 States and political subdivisions 129,873 380,885 196,400 583,595 1,290,753 Other 1,500 — — — 1,500 Total $ 131,389 $ 381,014 $ 712,265 $ 596,057 $ 1,820,725 Estimated Fair Value Residential mortgage-backed securities $ 16 $ 130 $ 529,086 $ 12,698 $ 541,930 States and political subdivisions 132,092 393,977 201,137 613,666 1,340,872 Other 1,500 — — — 1,500 Total $ 133,608 $ 394,107 $ 730,223 $ 626,364 $ 1,884,302 Available For Sale Amortized Cost U. S. Treasury $ 799,130 $ 285,739 $ 197,014 $ — $ 1,281,883 Residential mortgage-backed securities 7,147 22,151 22,162 1,808,967 1,860,427 States and political subdivisions 123,869 1,356,053 792,661 4,369,812 6,642,395 Other — — — — 42,354 Total $ 930,146 $ 1,663,943 $ 1,011,837 $ 6,178,779 $ 9,827,059 Estimated Fair Value U. S. Treasury $ 808,469 $ 304,750 $ 188,812 $ — $ 1,302,031 Residential mortgage-backed securities 7,468 23,421 22,634 1,841,282 1,894,805 States and political subdivisions 125,507 1,467,653 847,659 4,697,435 7,138,254 Other — — — — 42,354 Total $ 941,444 $ 1,795,824 $ 1,059,105 $ 6,538,717 $ 10,377,444
Sales of Securities Available for Sale	Sales of securities available for sale were as follows: Three Months Ended 2021 2020 Proceeds from sales $ — $ 1,111,102 Gross realized gains — 108,989 Gross realized losses — — Tax (expense) benefit of securities gains/losses — (22,888)
Premium Amortization and Discount Accretion Included in Income on Securities [Table Text Block]	Premium amortization and discount accretion included in interest income on securities was as follows: Three Months Ended 2021 2020 Premium amortization $ (31,235) $ (32,345) Discount accretion 637 585 Net (premium amortization) discount accretion $ (30,598) $ (31,760)
Trading Account Securities, at Estimated Fair Value	Trading account securities, at estimated fair value, were as follows: March 31, December 31, U.S. Treasury $ 24,038 $ 23,996 States and political subdivisions 2,571 460 Total $ 26,609 $ 24,456
Net Gains and Losses on Trading Account Securities	Net gains and losses on trading account securities were as follows: Three Months Ended 2021 2020 Net gain on sales transactions $ 109 $ 470 Net mark-to-market gains (losses) (69) 115 Net gain (loss) on trading account securities $ 40 $ 585

Loans (Tables)

Loans (Tables)	3 Months Ended
	Mar. 31, 2021
Receivables [Abstract]
Loans	Loans were as follows: March 31, Percentage December 31, Percentage Commercial and industrial $ 4,770,228 26.7 % $ 4,955,341 28.4 % Energy: Production 886,627 5.0 976,473 5.6 Service 90,510 0.5 116,825 0.7 Other 125,072 0.7 141,900 0.8 Total energy 1,102,209 6.2 1,235,198 7.1 Paycheck Protection Program 3,129,244 17.5 2,433,849 13.9 Commercial real estate: Commercial mortgages 5,566,367 31.1 5,478,806 31.3 Construction 1,236,994 6.9 1,223,814 7.0 Land 300,669 1.7 317,847 1.8 Total commercial real estate 7,104,030 39.7 7,020,467 40.1 Consumer real estate: Home equity loans 318,215 1.8 329,390 1.9 Home equity lines of credit 454,722 2.5 452,854 2.6 Other 549,217 3.0 548,530 3.1 Total consumer real estate 1,322,154 7.3 1,330,774 7.6 Total real estate 8,426,184 47.0 8,351,241 47.7 Consumer and other 461,786 2.6 505,680 2.9 Total loans $ 17,889,651 100.0 % $ 17,481,309 100.0 %
Non-Accrual Loans, Segregated by Class of Loans	Non-accrual loans, segregated by class of loans, were as follows: March 31, 2021 December 31, 2020 Total Non-Accrual Non-Accrual with No Credit Loss Allowance Total Non-Accrual Non-Accrual with No Credit Loss Allowance Commercial and industrial $ 18,509 $ 4,486 $ 19,849 $ 4,479 Energy 16,042 8,203 23,168 639 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 14,246 13,896 15,737 14,116 Construction 1,653 1,653 1,684 1,684 Consumer real estate 508 508 993 993 Consumer and other 18 — 18 — Total $ 50,976 $ 28,746 $ 61,449 $ 21,911
Financing Receivable, Non-Accrual Loans, Segregated by Class of Loan and Year of Origination	The following table presents non-accrual loans as of March 31, 2021 by class and year of origination. 2021 2020 2019 2018 2017 Prior Revolving Loans Revolving Loans Converted to Term Total Commercial and industrial $ — $ 10,344 $ 2,463 $ 921 $ 1,204 $ 114 $ 827 $ 2,636 $ 18,509 Energy — 900 6,314 1,901 — 347 6,466 114 16,042 Paycheck Protection Program — — — — — — — — — Commercial real estate: Buildings, land and other — 1,700 2,995 989 2,114 3,261 3,187 — 14,246 Construction — 1,653 — — — — — — 1,653 Consumer real estate — — — — — 464 — 44 508 Consumer and other — — — — — — 18 — 18 Total $ — $ 14,597 $ 11,772 $ 3,811 $ 3,318 $ 4,186 $ 10,498 $ 2,794 $ 50,976
Age Analysis of Past Due Loans, Segregated by Class of Loans	An age analysis of past due loans (including both accruing and non-accruing loans), segregated by class of loans, as of March 31, 2021 was as follows: Loans Loans Total Current Total Accruing Commercial and industrial $ 24,542 $ 10,216 $ 34,758 $ 4,735,470 $ 4,770,228 $ 7,547 Energy 11,125 3,918 15,043 1,087,166 1,102,209 1,086 Paycheck Protection Program — — — 3,129,244 3,129,244 — Commercial real estate: Buildings, land and other 27,876 15,713 43,589 5,823,447 5,867,036 5,045 Construction 22,726 — 22,726 1,214,268 1,236,994 — Consumer real estate 5,973 3,254 9,227 1,312,927 1,322,154 2,826 Consumer and other 3,197 447 3,644 458,142 461,786 430 Total $ 95,439 $ 33,548 $ 128,987 $ 17,760,664 $ 17,889,651 $ 16,934
Troubled Debt Restructurings	Troubled debt restructurings during the three months ended March 31, 2021 and 2020 are set forth in the following table. Three Months Ended Three Months Ended Balance at Balance at Balance at Balance at Commercial and industrial $ — $ — $ 2,191 $ 2,198
Restructured Loans Past Due in Excess of 90 Days on Financing Receivables	Additional information related to restructured loans as of or for the three months ended March 31, 2021 and 2020 is set forth in the following table. March 31, 2021 March 31, 2020 Restructured loans past due in excess of 90 days at period-end: Number of loans 2 3 Dollar amount of loans $ 1,392 $ 1,733 Restructured loans on non-accrual status at period end — 2,198 Charge-offs of restructured loans: Recognized on previously restructured loans 1,433 1,533
Weighted Average Risk Grades for All Commercial Loans by Class and Year of Origination	The following tables present weighted-average risk grades for all commercial loans, by class and year of origination/renewal as of March 31, 2021. Paycheck Protection Program (“PPP”) loans are excluded as such loans are fully guaranteed by the Small Business Administration (“SBA”). 2021 2020 2019 2018 2017 Prior Revolving Loans Revolving Loans Converted to Term Total W/A Risk Grade Commercial and industrial Risk grades 1-8 $ 486,852 $ 874,754 $ 512,101 $ 260,536 $ 193,920 $ 198,420 $ 1,782,914 $ 58,427 $ 4,367,924 6.18 Risk grade 9 11,174 40,156 34,652 29,911 16,891 14,197 59,650 10,015 216,646 9.00 Risk grade 10 25,740 11,803 3,264 7,392 2,769 278 56,961 5,856 114,063 10.00 Risk grade 11 9 4,374 10,409 6,075 3,127 1,074 19,143 8,875 53,086 11.00 Risk grade 12 — 7,564 1,919 921 724 114 553 2,277 14,072 12.00 Risk grade 13 — 2,780 544 — 480 — 274 359 4,437 13.00 $ 523,775 $ 941,431 $ 562,889 $ 304,835 $ 217,911 $ 214,083 $ 1,919,495 $ 85,809 $ 4,770,228 6.48 W/A risk grade 6.33 6.33 6.88 7.17 6.32 6.03 6.37 7.70 6.48 Energy Risk grades 1-8 $ 344,718 $ 45,560 $ 16,767 $ 8,861 $ 7,330 $ 5,470 $ 453,529 $ 17,177 $ 899,412 6.29 Risk grade 9 61,308 8,400 2,247 1,481 — — 15,617 5,188 94,241 9.00 Risk grade 10 1,317 369 814 1,258 — 750 7,833 3,753 16,094 10.00 Risk grade 11 11,481 1,647 19,510 1,245 — 627 35,634 6,276 76,420 11.00 Risk grade 12 — 900 4,624 478 — 347 4,108 114 10,571 12.00 Risk grade 13 — — 1,690 1,423 — — 2,358 — 5,471 13.00 $ 418,824 $ 56,876 $ 45,652 $ 14,746 $ 7,330 $ 7,194 $ 519,079 $ 32,508 $ 1,102,209 6.99 W/A risk grade 6.93 6.94 9.70 8.67 7.42 7.83 6.64 8.63 6.99 Commercial real estate: Buildings, land, other Risk grades 1-8 $ 356,814 $ 1,497,610 $ 914,168 $ 666,186 $ 570,468 $ 969,182 $ 56,513 $ 66,100 $ 5,097,041 6.93 Risk grade 9 22,563 62,562 82,497 59,815 46,457 57,414 4,885 2,484 338,677 9.00 Risk grade 10 2,638 20,217 27,966 56,444 67,214 79,519 — 11,083 265,081 10.00 Risk grade 11 2,003 22,411 14,074 13,549 39,196 57,418 3,200 140 151,991 11.00 Risk grade 12 — 1,700 2,995 989 2,114 3,011 3,187 — 13,996 12.00 Risk grade 13 — — — — — 250 — — 250 13.00 $ 384,018 $ 1,604,500 $ 1,041,700 $ 796,983 $ 725,449 $ 1,166,794 $ 67,785 $ 79,807 $ 5,867,036 7.31 W/A risk grade 7.21 7.18 7.29 7.48 7.53 7.27 7.68 7.12 7.31 Construction Risk grades 1-8 $ 180,644 $ 277,006 $ 439,531 $ 124,821 $ 60 $ 2,848 $ 128,827 $ — $ 1,153,737 7.01 Risk grade 9 2,955 33,213 — — 2,813 — 13,793 — 52,774 9.00 Risk grade 10 — — — 27,720 — — — — 27,720 10.00 Risk grade 11 — 1,110 — — — — — — 1,110 11.00 Risk grade 12 — 1,653 — — — — — — 1,653 12.00 Risk grade 13 — — — — — — — — — 13.00 $ 183,599 $ 312,982 $ 439,531 $ 152,541 $ 2,873 $ 2,848 $ 142,620 $ — $ 1,236,994 7.17 W/A risk grade 6.84 6.85 7.13 8.24 8.96 6.79 7.23 — 7.17 Total commercial real estate $ 567,617 $ 1,917,482 $ 1,481,231 $ 949,524 $ 728,322 $ 1,169,642 $ 210,405 $ 79,807 $ 7,104,030 7.28 W/A risk grade 7.09 7.12 7.24 7.61 7.53 7.27 7.37 7.12 7.28
Weighted Average Risk Grades for All Commercial Loans by Class	The following tables present weighted average risk grades for all commercial loans by class as of December 31, 2020. Refer to our 2020 Form 10-K for details of these loans by year of origination/renewal. Commercial and Industrial Energy Commercial Real Estate - Buildings, Land and Other Commercial Real Estate - Construction Total Commercial Real Estate W/A Risk Grade Loans W/A Risk Grade Loans W/A Risk Grade Loans W/A Risk Grade Loans W/A Risk Grade Loans Risk grades 1-8 6.13 $ 4,506,121 5.99 $ 968,144 6.97 $ 5,047,873 6.99 $ 1,110,025 6.98 $ 6,157,898 Risk grade 9 9.00 256,198 9.00 133,547 9.00 325,227 9.00 72,287 9.00 397,514 Risk grade 10 10.00 125,977 10.00 46,427 10.00 258,454 10.00 38,962 10.00 297,416 Risk grade 11 11.00 47,196 11.00 63,912 11.00 149,362 11.00 856 11.00 150,218 Risk grade 12 12.00 14,528 12.00 13,741 12.00 15,224 12.00 1,684 12.00 16,908 Risk grade 13 13.00 5,321 13.00 9,427 13.00 513 13.00 — 13.00 513 Total 6.45 $ 4,955,341 6.85 $ 1,235,198 7.34 $ 5,796,653 7.22 $ 1,223,814 7.32 $ 7,020,467
Age analysis of Past Due Consumer Loans by Class and Year of Origination	Information about the payment status of consumer loans, segregated by portfolio segment and year of origination, as of March 31, 2021 was as follows: 2021 2020 2019 2018 2017 Prior Revolving Loans Revolving Loans Converted to Term Total Consumer real estate: Past due 30-89 days $ — $ 230 $ 255 $ 1,418 $ 716 $ 1,464 $ 994 $ 896 $ 5,973 Past due 90 or more days — — 470 — 404 1,558 492 330 3,254 Total past due — 230 725 1,418 1,120 3,022 1,486 1,226 9,227 Current loans 62,908 331,440 144,445 81,559 71,346 169,122 437,555 14,552 1,312,927 Total $ 62,908 $ 331,670 $ 145,170 $ 82,977 $ 72,466 $ 172,144 $ 439,041 $ 15,778 $ 1,322,154 Consumer and other: Past due 30-89 days $ 4 $ 1,266 $ 131 $ 144 $ 83 $ 24 $ 695 $ 850 $ 3,197 Past due 90 or more days — 110 — 113 — — 45 179 447 Total past due 4 1,376 131 257 83 24 740 1,029 3,644 Current loans 20,060 31,370 18,966 4,669 2,408 1,914 349,628 29,127 458,142 Total $ 20,064 $ 32,746 $ 19,097 $ 4,926 $ 2,491 $ 1,938 $ 350,368 $ 30,156 $ 461,786
Revolving Loans Converted to Term [Table Text Block]	Revolving loans that converted to term during the three months ended March 31, 2021 and 2020 were as follows: Three months ended March 31, 2021 2020 Commercial and industrial $ 11,843 $ 11,346 Energy 5,928 15,353 Commercial real estate: Buildings, land and other 23,303 7,639 Construction — — Consumer real estate 793 1,071 Consumer and other 3,861 — Total $ 45,728 $ 35,409
Financing Receivable, Allowance for Credit Loss By Loan Class Calculated in Accordance With CECL Methodology	The following table presents details of the allowance for credit losses on loans segregated by loan portfolio segment as of March 31, 2021 and December 31, 2020. No allowance for credit losses has been recognized for PPP loans as such loans are fully guaranteed by the SBA. March 31, 2021 Commercial Energy Commercial Consumer Consumer Total Modeled expected credit losses $ 54,849 $ 11,560 $ 40,831 $ 5,608 $ 6,794 $ 119,642 Q-Factor and other qualitative adjustments 10,964 15,047 103,359 28 6 129,404 Specific allocations 5,079 6,865 250 — 18 12,212 Total $ 70,892 $ 33,472 $ 144,440 $ 5,636 $ 6,818 $ 261,258 December 31, 2020 Modeled expected credit losses $ 65,645 $ 8,910 $ 125,126 $ 7,926 $ 6,945 $ 214,552 Q-Factor and other qualitative adjustments 2,877 21,216 9,253 — — 33,346 Specific allocations 5,321 9,427 513 — 18 15,279 Total $ 73,843 $ 39,553 $ 134,892 $ 7,926 $ 6,963 $ 263,177
Activity in Allowance for Loan Losses by Portfolio Segment	The following table details activity in the allowance for credit losses on loans by portfolio segment for the three months ended March 31, 2021 and 2020. Allocation of a portion of the allowance to one category of loans does not preclude its availability to absorb losses in other categories. No allowance for credit losses has been recognized for PPP loans as such loans are fully guaranteed by the SBA. Commercial Energy Commercial Consumer Consumer Total Three months ended: March 31, 2021 Beginning balance $ 73,843 $ 39,553 $ 134,892 $ 7,926 $ 6,963 $ 263,177 Credit loss expense (benefit) (1,965) (5,801) 8,922 (2,722) 1,566 — Charge-offs (2,189) (1,433) — (284) (4,060) (7,966) Recoveries 1,203 1,153 626 716 2,349 6,047 Net (charge-offs) recoveries (986) (280) 626 432 (1,711) (1,919) Ending balance $ 70,892 $ 33,472 $ 144,440 $ 5,636 $ 6,818 $ 261,258 March 31, 2020 Beginning balance $ 51,593 $ 37,382 $ 31,037 $ 4,113 $ 8,042 $ 132,167 Impact of adopting ASC 326 21,263 (10,453) (13,519) 2,392 (2,248) (2,565) Credit loss expense 21,950 110,006 34,761 1,570 4,638 172,925 Charge-offs (4,369) (33,800) (73) (285) (5,005) (43,532) Recoveries 1,715 66 113 380 2,612 4,886 Net (charge-offs) recoveries (2,654) (33,734) 40 95 (2,393) (38,646) Ending balance $ 92,152 $ 103,201 $ 52,319 $ 8,170 $ 8,039 $ 263,881
Investment in Loans Related to Allowance for Loan Losses by Portfolio Segment Disaggregated Based on Impairment Methodology	The following table presents loans that were evaluated for expected credit losses on an individual basis and the related specific allocations, by loan portfolio segment as of March 31, 2021 and December 31, 2020. March 31, 2021 December 31, 2020 Loan Specific Allocations Loan Specific Allocations Commercial and industrial $ 21,858 $ 5,079 $ 21,287 $ 5,321 Energy 22,496 6,865 22,888 9,427 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 32,345 250 34,057 513 Construction 1,653 — 1,684 — Consumer real estate 302 — 561 — Consumer and other 18 18 18 18 Total $ 78,672 $ 12,212 $ 80,495 $ 15,279

Goodwill and Other Intangible_2

Goodwill and Other Intangible Assets (Tables)	3 Months Ended
	Mar. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets and Goodwill	Goodwill and other intangible assets are presented in the table below. March 31, December 31, Goodwill $ 654,952 $ 654,952 Other intangible assets: Core deposits $ 1,127 $ 1,296 Customer relationships 234 267 $ 1,361 $ 1,563
Estimated Aggregate Future Amortization Expense for Intangible Assets	The estimated aggregate future amortization expense for intangible assets remaining as of March 31, 2021 is as follows: Remainder of 2021 $ 495 2022 481 2023 282 2024 87 2025 11 Thereafter 5 $ 1,361

Deposits (Tables)

Deposits (Tables)	3 Months Ended
	Mar. 31, 2021
Disclosure Text Block [Abstract]
Deposit Liabilities, Type	Deposits were as follows: March 31, Percentage December 31, Percentage Non-interest-bearing demand deposits: Commercial and individual $ 15,002,423 40.6 % $ 13,914,754 39.7 % Correspondent banks 213,554 0.6 242,225 0.7 Public funds 887,672 2.4 960,072 2.8 Total non-interest-bearing demand deposits 16,103,649 43.6 15,117,051 43.2 Interest-bearing deposits: Private accounts: Savings and interest checking 9,648,088 26.1 9,132,789 26.1 Money market accounts 9,416,210 25.5 8,977,585 25.6 Time accounts 1,141,660 3.1 1,135,575 3.3 Total private accounts 20,205,958 54.7 19,245,949 55.0 Public funds: Savings and interest checking 556,523 1.5 597,503 1.7 Money market accounts 53,496 0.2 50,070 0.1 Time accounts 5,554 — 5,188 — Total public funds 615,573 1.7 652,761 1.8 Total interest-bearing deposits 20,821,531 56.4 19,898,710 56.8 Total deposits $ 36,925,180 100.0 % $ 35,015,761 100.0 %
Additional Information About Corporation's Deposits	The following table presents additional information about our deposits: March 31, December 31, Deposits from the Certificate of Deposit Account Registry Service (CDARS) deposits $ 374 $ 372 Deposits from foreign sources (primarily Mexico) 910,652 884,169 Total deposits not covered by deposit insurance 19,766,784 18,694,320 Time deposits not covered by deposit insurance 251,639 237,298

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Financial Instruments with Off-Balance-Sheet Risk	Financial instruments with off-balance-sheet risk were as follows: March 31, December 31, Commitments to extend credit $ 9,853,069 $ 9,814,475 Standby letters of credit 261,341 241,345 Deferred standby letter of credit fees 1,711 1,723
Commitments, Off-Balance-Sheet, Allowance for Credit Losses [Table Text Block]	The following table details activity in the allowance for credit losses on off-balance-sheet credit exposures. Three Months Ended 2021 2020 Beginning balance $ 44,152 $ 500 Impact of adopting ASC 326 — 39,377 Credit loss expense 65 2,287 Ending balance $ 44,217 $ 42,164
Lease Commitments, Components of Lease Expense [Table Text Block]	We lease certain office facilities and office equipment under operating leases. The components of total lease expense were as follows: Three Months Ended 2021 2020 Amortization of lease right-of-use assets $ 8,283 $ 7,874 Short-term lease expense 204 561 Non-lease components (including taxes, insurance, common maintenance, etc.) 3,004 2,927 Total $ 11,491 $ 11,362

Capital and Regulatory Matters

Capital and Regulatory Matters (Tables)	3 Months Ended
	Mar. 31, 2021
Banking Regulation, Common Equity Tier One Risk-Based Capital [Abstract]
Actual and Required Capital Ratios	The following table presents actual and required capital ratios as of March 31, 2021 and December 31, 2020 for Cullen/Frost and Frost Bank under the Basel III Capital Rules. Capital levels required to be considered well capitalized are based upon prompt corrective action regulations, as amended to reflect the changes under the Basel III Capital Rules. See the 2020 Form 10-K for a more detailed discussion of the Basel III Capital Rules. Actual Minimum Capital Required - Basel III Required to be Capital Ratio Capital Ratio Capital Ratio March 31, 2021 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 3,158,758 13.45 % $ 1,643,402 7.00 % $ 1,526,017 6.50 % Frost Bank 3,104,492 13.25 1,640,032 7.00 1,522,886 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 3,304,210 14.07 1,995,560 8.50 1,878,174 8.00 Frost Bank 3,104,492 13.25 1,991,467 8.50 1,874,322 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,771,830 16.07 2,465,104 10.50 2,347,718 10.00 Frost Bank 3,339,112 14.25 2,460,047 10.50 2,342,902 10.00 Leverage Ratio Cullen/Frost 3,304,210 7.97 1,658,592 4.00 2,073,240 5.00 Frost Bank 3,104,492 7.49 1,657,640 4.00 2,072,050 5.00 December 31, 2020 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 3,058,447 12.86 % $ 1,664,867 7.00 % $ 1,545,948 6.50 % Frost Bank 3,030,093 12.77 1,661,620 7.00 1,542,933 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 3,203,899 13.47 2,021,624 8.50 1,902,705 8.00 Frost Bank 3,030,093 12.77 2,017,682 8.50 1,898,995 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,672,912 15.44 2,497,300 10.50 2,378,381 10.00 Frost Bank 3,266,106 13.76 2,492,430 10.50 2,373,743 10.00 Leverage Ratio Cullen/Frost 3,203,899 8.07 1,589,004 4.00 1,986,255 5.00 Frost Bank 3,030,093 7.63 1,588,200 4.00 1,985,250 5.00

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts and Estimated Fair Values of Interest Rate Derivative Contracts Outstanding	The notional amounts and estimated fair values of interest rate derivative contracts are presented in the following table. The fair values of interest rate derivative contracts are estimated utilizing internal valuation models with observable market data inputs, or as determined by the Chicago Mercantile Exchange (“CME”) for centrally cleared derivative contracts. CME rules legally characterize variation margin payments for centrally cleared derivatives as settlements of the derivatives' exposure rather than collateral. As a result, the variation margin payment and the related derivative instruments are considered a single unit of account for accounting and financial reporting purposes. Variation margin, as determined by the CME, is settled daily. As a result, derivative contracts that clear through the CME have an estimated fair value of zero as of March 31, 2021 and December 31, 2020. March 31, 2021 December 31, 2020 Notional Estimated Notional Estimated Derivatives designated as hedges of fair value: Financial institution counterparties: Loan/lease interest rate swaps – liabilities 3,407 $ (104) 3,724 $ (134) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets 164,768 2,560 — — Loan/lease interest rate swaps – liabilities 906,227 (23,479) 1,173,173 (33,812) Loan/lease interest rate caps – assets 382,086 3,670 356,601 1,241 Customer counterparties: Loan/lease interest rate swaps – assets 906,227 52,590 1,173,173 84,424 Loan/lease interest rate swaps – liabilities 164,768 (4,736) — — Loan/lease interest rate caps – liabilities 382,086 (3,670) 356,601 (1,241)
Schedule of Weighted-Average Rates Paid and Received for Interest Rate Swaps Outstanding	The weighted-average rates paid and received for interest rate swaps outstanding at March 31, 2021 were as follows: Weighted-Average Interest Interest Interest rate swaps: Fair value hedge loan/lease interest rate swaps 3.14 % 0.11 % Non-hedging interest rate swaps – financial institution counterparties 3.81 1.85 Non-hedging interest rate swaps – customer counterparties 1.85 3.81
Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions	The notional amounts and estimated fair values of non-hedging commodity swap and option derivative positions outstanding are presented in the following table. We obtain dealer quotations and use internal valuation models with observable market data inputs to value our commodity derivative positions. March 31, 2021 December 31, 2020 Notional Notional Estimated Notional Estimated Financial institution counterparties: Oil – assets Barrels 4,620 $ 9,176 3,056 $ 8,341 Oil – liabilities Barrels 8,906 (74,039) 6,391 (32,112) Natural gas – assets MMBTUs 2,422 448 9,281 1,529 Natural gas – liabilities MMBTUs 17,857 (3,835) 15,079 (3,265) Customer counterparties: Oil – assets Barrels 8,906 74,932 6,391 32,670 Oil – liabilities Barrels 4,620 (8,897) 3,056 (8,264) Natural gas – assets MMBTUs 17,857 4,048 17,636 3,451 Natural gas – liabilities MMBTUs 2,422 (444) 6,724 (1,458)
Notional Amounts and Fair Values of Open Foreign Currency Forward Contracts	The notional amounts and fair values of open foreign currency forward contracts were as follows: March 31, 2021 December 31, 2020 Notional Notional Estimated Notional Estimated Financial institution counterparties: Forward contracts – assets EUR 6,311 $ 144 — $ — Customer counterparties: Forward contracts – liabilities EUR 6,299 (132) — —
Schedule of Amounts Related to Interest Rate Derivatives Designated as Hedges of Fair Value	Amounts included in the consolidated statements of income related to interest rate derivatives designated as hedges of fair value were as follows: Three Months Ended 2021 2020 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ (28) $ (8) Amount of (gain) loss included in other non-interest expense 3 1
Schedule of Amounts Related to Non-Hedging Interest Rate and Commodity Derivatives	Amounts included in the consolidated statements of income related to non-hedge related derivative instruments are presented in the table below. Three Months Ended 2021 2020 Non-hedging interest rate derivatives: Other non-interest income $ 1,587 $ 2,039 Other non-interest expense (1) — Non-hedging commodity derivatives: Other non-interest income 1,154 139 Non-hedging foreign currency derivatives: Other non-interest income 30 7 Non-hedging put options: Other non-interest income — 5,980

Balance Sheet Offsetting (Table

Balance Sheet Offsetting (Tables)	3 Months Ended
	Mar. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Financial Instruments Eligible for Offset Consolidated Balance Sheet	Information about financial instruments that are eligible for offset in the consolidated balance sheet as of March 31, 2021 is presented in the following tables. Gross Amount Gross Amount Net Amount March 31, 2021 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 6,230 $ — $ 6,230 Commodity swaps and options 9,624 — 9,624 Foreign currency forward contracts 144 — 144 Total derivatives 15,998 — 15,998 Resell agreements 7,903 — 7,903 Total $ 23,901 $ — $ 23,901 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 23,583 $ — $ 23,583 Commodity swaps and options 77,874 — 77,874 Total derivatives 101,457 — 101,457 Repurchase agreements 1,884,449 — 1,884,449 Total $ 1,985,906 $ — $ 1,985,906 Information about financial instruments that are eligible for offset in the consolidated balance sheet as of December 31, 2020 is presented in the following tables. Gross Amount Gross Amount Net Amount December 31, 2020 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 1,241 $ — $ 1,241 Commodity swaps and options 9,870 — 9,870 Total derivatives 11,111 — 11,111 Resell agreements — — — Total $ 11,111 $ — $ 11,111 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 33,946 $ — $ 33,946 Commodity swaps and options 35,377 — 35,377 Total derivatives 69,323 — 69,323 Repurchase agreements 2,068,147 — 2,068,147 Total $ 2,137,470 $ — $ 2,137,470
Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset	Gross Amounts Not Offset Net Amount Financial Collateral Net March 31, 2021 Financial assets: Derivatives: Counterparty A $ 5 $ (5) $ — $ — Counterparty B 8,250 (8,250) — — Other counterparties 7,743 (7,743) — — Total derivatives 15,998 (15,998) — — Resell agreements 7,903 — (7,903) — Total $ 23,901 $ (15,998) $ (7,903) $ — Financial liabilities: Derivatives: Counterparty A $ 5,349 $ (5) $ (5,344) $ — Counterparty B 32,181 (8,250) (23,931) — Counterparty C 50 — (50) — Other counterparties 63,877 (7,743) (56,134) — Total derivatives 101,457 (15,998) (85,459) — Repurchase agreements 1,884,449 — (1,884,449) — Total $ 1,985,906 $ (15,998) $ (1,969,908) $ — Gross Amounts Not Offset Net Amount Financial Collateral Net December 31, 2020 Financial assets: Derivatives: Counterparty A $ 2 $ (2) $ — $ — Counterparty B 5,838 (5,838) — — Other counterparties 5,271 (5,271) — — Total derivatives 11,111 (11,111) — — Resell agreements — — — — Total $ 11,111 $ (11,111) $ — $ — Financial liabilities: Derivatives: Counterparty A $ 6,430 $ (2) $ (6,428) $ — Counterparty B 20,722 (5,838) (14,700) 184 Counterparty C 71 — (71) — Other counterparties 42,100 (5,271) (35,832) 997 Total derivatives 69,323 (11,111) (57,031) 1,181 Repurchase agreements 2,068,147 — (2,068,147) — Total $ 2,137,470 $ (11,111) $ (2,125,178) $ 1,181
Remaining Contractual Maturity of the Securities Sold Under Agreement [Table Text Block]	The remaining contractual maturity of repurchase agreements in the consolidated balance sheets as of March 31, 2021 and December 31, 2020 is presented in the following tables. Remaining Contractual Maturity of the Agreements Overnight and Continuous Up to 30 Days 30-90 Days Greater than 90 Days Total March 31, 2021 Repurchase agreements: U.S. Treasury $ 730,620 $ — $ — $ — $ 730,620 Residential mortgage-backed securities 1,153,829 — — — 1,153,829 Total borrowings $ 1,884,449 $ — $ — $ — $ 1,884,449 Gross amount of recognized liabilities for repurchase agreements $ 1,884,449 Amounts related to agreements not included in offsetting disclosures above $ — December 31, 2020 Repurchase agreements: U.S. Treasury $ 692,860 $ — $ — $ — $ 692,860 Residential mortgage-backed securities 1,375,287 — — — 1,375,287 Total borrowings $ 2,068,147 $ — $ — $ — $ 2,068,147 Gross amount of recognized liabilities for repurchase agreements $ 2,068,147 Amounts related to agreements not included in offsetting disclosures above $ —

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2021
Share-based Payment Arrangement [Abstract]
Summary of Activity in Corporation's Active Stock Plans	A combined summary of activity in our active stock plans is presented in the table. Performance stock units outstanding are presented assuming attainment of the maximum payout rate as set forth by the performance criteria. As of March 31, 2021, there were 895,295 shares remaining available for grant for future stock-based compensation awards. Director Deferred Non-Vested Stock Performance Stock Units Outstanding Stock Options Number of Units Weighted- Number Weighted- Number of Units Weighted- Number Weighted- Balance, January 1, 2021 52,860 $ 75.47 470,359 $ 86.24 201,257 $ 77.18 1,739,559 $ 66.11 Authorized — — — — — — — — Granted — — — — — — — — Exercised/vested — — — — (35,131) 92.27 (478,693) 61.41 Forfeited/expired — — (6,991) 91.57 (1,115) 92.27 — — Balance, March 31, 2021 52,860 $ 75.47 463,368 $ 86.16 165,011 $ 73.86 1,260,866 $ 67.89
Shares Issued in Connection with Stock Compensation Awards	Shares issued in connection with stock compensation awards along with other related information were as follows: Three Months Ended 2021 2020 New shares issued from available authorized shares — — Shares issued from available treasury stock 513,824 66,215 Total 513,824 66,215 Proceeds from stock option exercises $ 29,397 $ 1,463
Share-based Payment Arrangement, Cost by Plan	Stock-based compensation expense and the related income tax benefit is presented in the following table. Three Months Ended 2021 2020 Non-vested stock awards/stock units $ 1,883 $ 2,264 Performance stock units 643 248 Total $ 2,526 $ 2,512 Income tax benefit $ 367 $ 465
Unrecognized Stock-Based Compensation Expense	Unrecognized stock-based compensation expense at March 31, 2021 is presented in the table below. Unrecognized stock-based compensation expense related to performance stock units is presented assuming attainment of the maximum payout rate as set forth by the performance criteria. Non-vested stock awards/stock units $ 15,907 Performance stock units 6,288 Total $ 22,195

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Basic and Diluted Earnings Per Common Share	The following table presents a reconciliation of net income available to common shareholders, net earnings allocated to common stock and the number of shares used in the calculation of basic and diluted earnings per common share. Three Months Ended 2021 2020 Net income $ 116,015 $ 54,753 Less: Preferred stock dividends 2,151 2,016 Redemption of preferred stock — 5,514 Net income available to common shareholders 113,864 47,223 Less: Earnings allocated to participating securities 1,162 445 Net earnings allocated to common stock $ 112,702 $ 46,778 Distributed earnings allocated to common stock $ 45,660 $ 44,512 Undistributed earnings allocated to common stock 67,042 2,266 Net earnings allocated to common stock $ 112,702 $ 46,778 Weighted-average shares outstanding for basic earnings per common share 63,306,493 62,642,682 Dilutive effect of stock compensation 509,290 407,154 Weighted-average shares outstanding for diluted earnings per common share 63,815,783 63,049,836

Defined Benefit Plans (Tables)

Defined Benefit Plans (Tables)	3 Months Ended
	Mar. 31, 2021
Retirement Benefits [Abstract]
Net Periodic Cost (Benefit)	The components of the combined net periodic expense (benefit) for our defined benefit pension plans are presented in the table below. Three Months Ended 2021 2020 Expected return on plan assets, net of expenses $ (3,210) $ (3,072) Interest cost on projected benefit obligation 835 1,252 Net amortization and deferral 1,529 1,330 Net periodic expense (benefit) $ (846) $ (490)

Income Taxes (Tables)

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Tax Expense	Income tax expense was as follows: Three Months Ended 2021 2020 Current income tax expense (benefit) $ 3,725 $ 29,412 Deferred income tax expense (benefit) 4,172 (26,089) Income tax expense, as reported $ 7,897 $ 3,323 Effective tax rate 6.4 % 5.7 %

Other Comprehensive Income (L_2

Other Comprehensive Income (Loss) (Tables)	3 Months Ended
	Mar. 31, 2021
Equity [Abstract]
Comprehensive Income (Loss)	The before and after tax amounts allocated to each component of other comprehensive income (loss) are presented in the following table. Reclassification adjustments related to securities available for sale are included in net gain (loss) on securities transactions in the accompanying consolidated statements of income. Reclassification adjustments related to defined-benefit post-retirement benefit plans are included in the computation of net periodic pension expense (see Note 12 – Defined Benefit Plans). Three Months Ended Three Months Ended Before Tax Tax Expense, Net of Tax Before Tax Tax Expense, Net of Tax Securities available for sale and transferred securities: Change in net unrealized gain/loss during the period $ (159,779) $ (33,554) $ (126,225) $ 230,118 $ 48,325 $ 181,793 Change in net unrealized gain on securities transferred to held to maturity (259) (54) (205) (377) (79) (298) Reclassification adjustment for net (gains) losses included in net income — — — (108,989) (22,888) (86,101) Total securities available for sale and transferred securities (160,038) (33,608) (126,430) 120,752 25,358 95,394 Defined-benefit post-retirement benefit plans: Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) 1,529 321 1,208 1,330 279 1,051 Total defined-benefit post-retirement benefit plans 1,529 321 1,208 1,330 279 1,051 Total other comprehensive income (loss) $ (158,509) $ (33,287) $ (125,222) $ 122,082 $ 25,637 $ 96,445
Schedule of Accumulated Other Comprehensive Income (Loss)	Activity in accumulated other comprehensive income (loss), net of tax, was as follows: Securities Defined Accumulated Balance January 1, 2021 $ 563,801 $ (50,831) $ 512,970 Other comprehensive income (loss) before reclassifications (126,430) — (126,430) Reclassification of amounts included in net income — 1,208 1,208 Net other comprehensive income (loss) during period (126,430) 1,208 (125,222) Balance at March 31, 2021 $ 437,371 $ (49,623) $ 387,748 Balance January 1, 2020 $ 313,304 $ (45,934) $ 267,370 Other comprehensive income (loss) before reclassifications 181,495 — 181,495 Reclassification of amounts included in net income (86,101) 1,051 (85,050) Net other comprehensive income (loss) during period 95,394 1,051 96,445 Balance at March 31, 2020 $ 408,698 $ (44,883) $ 363,815

Operating Segments (Tables)

Operating Segments (Tables)	3 Months Ended
	Mar. 31, 2021
Segment Reporting [Abstract]
Summary of Operating Results by Segment	Summarized operating results by segment were as follows: Banking Frost Wealth Non-Banks Consolidated Three months ended: March 31, 2021 Net interest income (expense) $ 242,205 $ 486 $ (1,810) $ 240,881 Credit loss expense 63 — — 63 Non-interest income 53,780 39,609 (153) 93,236 Non-interest expense 179,151 29,937 1,054 210,142 Income (loss) before income taxes 116,771 10,158 (3,017) 123,912 Income tax expense (benefit) 7,177 2,133 (1,413) 7,897 Net income (loss) 109,594 8,025 (1,604) 116,015 Preferred stock dividends — — 2,151 2,151 Net income (loss) available to common shareholders $ 109,594 $ 8,025 $ (3,755) $ 113,864 Revenues from (expenses to) external customers $ 295,985 $ 40,095 $ (1,963) $ 334,117 March 31, 2020 Net interest income (expense) $ 246,013 $ 877 $ (2,369) $ 244,521 Credit loss expense (benefit) 175,198 (1) — 175,197 Non-interest income 173,416 39,730 (231) 212,915 Non-interest expense 188,972 33,536 1,655 224,163 Income (loss) before income taxes 55,259 7,072 (4,255) 58,076 Income tax expense (benefit) 3,235 1,485 (1,397) 3,323 Net income (loss) 52,024 5,587 (2,858) 54,753 Preferred stock dividends — — 2,016 2,016 Redemption of preferred stock — — 5,514 5,514 Net income (loss) available to common shareholders $ 52,024 $ 5,587 $ (10,388) $ 47,223 Revenues from (expenses to) external customers $ 419,429 $ 40,607 $ (2,600) $ 457,436

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis	The tables below summarize financial assets and financial liabilities measured at fair value on a recurring basis as of March 31, 2021 and December 31, 2020, segregated by the level of the valuation inputs within the fair value hierarchy of ASC Topic 820 utilized to measure fair value. Level 1 Inputs Level 2 Inputs Level 3 Inputs Total Fair Value March 31, 2021 Securities available for sale: U.S. Treasury $ 1,302,031 $ — $ — $ 1,302,031 Residential mortgage-backed securities — 1,894,805 — 1,894,805 States and political subdivisions — 7,138,254 — 7,138,254 Other — 42,354 — 42,354 Trading account securities: U.S. Treasury 24,038 — — 24,038 States and political subdivisions — 2,571 — 2,571 Derivative assets: Interest rate swaps, caps and floors — 58,820 — 58,820 Commodity swaps and options — 88,604 — 88,604 Foreign currency forward contracts 144 — — 144 Derivative liabilities: Interest rate swaps, caps and floors — 31,989 — 31,989 Commodity swaps and options — 87,215 — 87,215 Foreign currency forward contracts 132 — — 132 December 31, 2020 Securities available for sale: U.S. Treasury $ 1,119,633 $ — $ — $ 1,119,633 Residential mortgage-backed securities — 1,987,679 — 1,987,679 States and political subdivisions — 7,287,902 — 7,287,902 Other — 42,351 — 42,351 Trading account securities: U.S. Treasury 23,996 — — 23,996 States and political subdivisions — 460 — 460 Derivative assets: Interest rate swaps, caps and floors — 85,665 — 85,665 Commodity swaps and options — 45,535 456 45,991 Foreign currency forward contracts — — — — Derivative liabilities: Interest rate swaps, caps and floors — 35,187 — 35,187 Commodity swaps and options — 45,099 — 45,099 Foreign currency forward contracts — — — —
Impaired Loans Remeasured And Reported At Fair Value Specific Valuation Allowance Allocation Method Of Underlying Collateral [Table Text Block]	The following table presents collateral dependent loans that were remeasured and reported at fair value through a specific allocation of the allowance for credit losses on loans based upon the fair value of the underlying collateral during the reported periods. Three Months Ended Three Months Ended Level 2 Level 3 Level 2 Level 3 Carrying value before allocations $ 100 $ 15,013 $ 1,919 $ 3,867 Specific (allocations) reversals of prior allocations — 600 70 (244) Fair value $ 100 $ 15,613 $ 1,989 $ 3,623
Foreclosed Assets Remeasured and Reported at Fair Value	The following table presents foreclosed assets that were remeasured and reported at fair value during the reported periods: Three Months Ended 2021 2020 Foreclosed assets remeasured subsequent to initial recognition: Carrying value of foreclosed assets prior to remeasurement $ — $ 328 Write-downs included in other non-interest expense — (231) Fair value $ — $ 97
Estimated Fair Values of Financial Instruments	The estimated fair values of financial instruments that are reported at amortized cost in our consolidated balance sheets, segregated by the level of the valuation inputs within the fair value hierarchy utilized to measure fair value, were as follows: March 31, 2021 December 31, 2020 Carrying Estimated Carrying Estimated Financial assets: Level 2 inputs: Cash and cash equivalents $ 11,802,269 $ 11,802,269 $ 10,288,853 $ 10,288,853 Securities held to maturity 1,820,567 1,884,302 1,945,673 2,052,896 Cash surrender value of life insurance policies 189,756 189,756 189,984 189,984 Accrued interest receivable 128,725 128,725 181,432 181,432 Level 3 inputs: Loans, net 17,628,393 17,738,315 17,218,132 17,390,683 Financial liabilities: Level 2 inputs: Deposits 36,925,180 36,926,759 35,015,761 35,018,185 Federal funds purchased 38,975 38,975 48,850 48,850 Repurchase agreements 1,884,449 1,884,449 2,068,147 2,068,147 Junior subordinated deferrable interest debentures 136,371 137,115 136,357 137,115 Subordinated notes 99,061 114,818 99,021 115,717 Accrued interest payable 4,138 4,138 8,127 8,127

Significant Accounting Polici_4

Significant Accounting Policies Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change		$ (29,252)
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change	$ (29,300)	$ (29,252)

Significant Accounting Polici_5

Significant Accounting Policies - Additional Cash Flow Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accounting Policies [Abstract]
Cash paid for interest	$ 9,719	$ 23,168
Cash paid for income taxes	0	0
Unsettled securities transactions	21,957	11,196
Right-of-use lease assets obtained in exchange for lessee operating lease liabilities	1,326	9,892
Treasury stock issued to 401(k) stock purchase plan	$ 1,749	$ 0

Securities - Additional Informa

Securities - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Marketable Securities [Line Items]
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income Pre Tax	$ 3,200	$ 3,500
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	$ 2,600	2,800
Percent Municipal Bond Portfolio Issued By Political Subdivision Or Agencies Within State Of Texas Guaranteed By Texas Permanent School Fund Or Have Been Pre-Refunded	77.50%
Debt Securities, Held-to-maturity, Allowance for Credit Loss	$ 0	0
Held-to-maturity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	541,100	659,200
Accrued interest receivable	11,200	21,700
Available-for-sale Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	4,300,000	4,400,000
Accrued interest receivable	$ 65,600	$ 111,000

Securities Securities - Amortiz

Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Held to Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 1,820,725	$ 1,945,833
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	63,577	107,063
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	1,884,302	2,052,896
Securities, held-to-maturity, allowance for credit loss	(158)	(160)
Debt Securities, Held-to-Maturity Net Carrying Amount	1,820,567	1,945,673
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	528,472	528,784
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	13,458	41,742
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	541,930	570,526
Securities, held-to-maturity, allowance for credit loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	528,472	528,784
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,290,753	1,415,549
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	50,119	65,321
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	1,340,872	1,480,870
Securities, held-to-maturity, allowance for credit loss	(158)	(160)
Debt Securities, Held-to-Maturity Net Carrying Amount	1,290,595	1,415,389
Other Investments [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,500	1,500
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	1,500	1,500
Securities, held-to-maturity, allowance for credit loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	$ 1,500	$ 1,500

Securities Securities - Moody's

Securities Securities - Moody's and/or Standard & Poor's Bond Rating (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	$ 1,820,725	$ 1,945,833
US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,290,753	1,415,549
Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,500	1,500
Aaa/AAA [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,121,415	1,245,760
Aaa/AAA [Member] | Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Aa/AA [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	119,548	107,065
Aa/AA [Member] | Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
A [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	49,790	62,724
A [Member] | Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Not rated [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Not rated [Member] | Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,500	1,500
Not Guaranteed or Pre-Refunded [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	273,763	284,805
Not Guaranteed or Pre-Refunded [Member] | Aaa/AAA [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	104,425	115,016
Not Guaranteed or Pre-Refunded [Member] | Aa/AA [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	119,548	107,065
Not Guaranteed or Pre-Refunded [Member] | A [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	49,790	62,724
Not Guaranteed or Pre-Refunded [Member] | Not rated [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Guaranteed by the Texas PSF [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	579,584	659,999
Guaranteed by the Texas PSF [Member] | Aaa/AAA [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	579,584	659,999
Guaranteed by the Texas PSF [Member] | Aa/AA [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Guaranteed by the Texas PSF [Member] | A [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Guaranteed by the Texas PSF [Member] | Not rated [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Pre-Refunded [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	437,406	470,745
Pre-Refunded [Member] | Aaa/AAA [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	437,406	470,745
Pre-Refunded [Member] | Aa/AA [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Pre-Refunded [Member] | A [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Pre-Refunded [Member] | Not rated [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	$ 0	$ 0

Securities Securities - Allowan

Securities Securities - Allowance for Credit Losses on Held to Maturity Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Securities, held-to-maturity, allowance for credit loss	$ 158		$ 160
Impact of adopting ASC 326	0	$ 39,377
Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities, held-to-maturity, allowance for credit loss	158	200	$ 160	$ 0
Impact of adopting ASC 326	0	215
Credit loss expense (benefit)	$ (2)	$ (15)

Securities Securities - Amort_2

Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 9,827,059	$ 9,727,401
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	581,983	710,168
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	31,598	4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	10,377,444	10,437,565
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,281,883	1,084,542
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	28,350	35,091
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,202	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	1,302,031	1,119,633
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,860,427	1,916,581
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	51,614	71,102
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	17,236	4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	1,894,805	1,987,679
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	6,642,395	6,683,927
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	502,019	603,975
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	6,160	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	7,138,254	7,287,902
Other Investments [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	42,354	42,351
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	$ 42,354	$ 42,351

Securities - Securities Availab

Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	$ 1,000,258
Available for Sale, Less than 12 Months, Unrealized Losses	31,598
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	1,000,258
Available-for-sale Securities, Gross Unrealized Loss, before Tax	31,598	$ 4
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	188,812
Available for Sale, Less than 12 Months, Unrealized Losses	8,202
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	188,812
Available-for-sale Securities, Gross Unrealized Loss, before Tax	8,202	0
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	634,464
Available for Sale, Less than 12 Months, Unrealized Losses	17,236
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	634,464
Available-for-sale Securities, Gross Unrealized Loss, before Tax	17,236	4
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	176,982
Available for Sale, Less than 12 Months, Unrealized Losses	6,160
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	176,982
Available-for-sale Securities, Gross Unrealized Loss, before Tax	$ 6,160	$ 0

Securities - Amortized Cost and

Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 131,389
Held to Maturity, Due after one year through five years, Amortized Cost	381,014
Held to Maturity, Due after five years through ten years, Amortized Cost	712,265
Held to Maturity, Due after ten years, Amortized Cost	596,057
Held to Maturity, Amortized Cost	1,820,725	$ 1,945,833
Held to Maturity, Due in one year or less, Estimated Fair Value	133,608
Held to Maturity, Due after one year through five years, Estimated Fair Value	394,107
Held to Maturity, Due after five years through ten years, Estimated Fair Value	730,223
Held to Maturity, Due after ten years, Estimated Fair Value	626,364
Held to Maturity, Estimated Fair Value	1,884,302	2,052,896
Available-for-sale Securities, Due in one year or less, Amortized Cost	930,146
Available for Sale, Due after one year through five years, Amortized Cost	1,663,943
Available for Sale, Due after five years through ten years, Amortized Cost	1,011,837
Available for Sale, Due after ten years, Amortized Cost	6,178,779
Available for Sale, Amortized Cost	9,827,059	9,727,401
Available for Sale, Due in one year or less, Estimated Fair Value	941,444
Available for Sale, Due after one year through five years, Estimated Fair Value	1,795,824
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,059,105
Available for Sale, Due after ten years, Estimated Fair Value	6,538,717
Securities available for sale, estimated fair value	10,377,444	10,437,565
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	799,130
Available for Sale, Due after one year through five years, Amortized Cost	285,739
Available for Sale, Due after five years through ten years, Amortized Cost	197,014
Available for Sale, Due after ten years, Amortized Cost	0
Available for Sale, Amortized Cost	1,281,883	1,084,542
Available for Sale, Due in one year or less, Estimated Fair Value	808,469
Available for Sale, Due after one year through five years, Estimated Fair Value	304,750
Available for Sale, Due after five years through ten years, Estimated Fair Value	188,812
Available for Sale, Due after ten years, Estimated Fair Value	0
Securities available for sale, estimated fair value	1,302,031	1,119,633
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	16
Held to Maturity, Due after one year through five years, Amortized Cost	129
Held to Maturity, Due after five years through ten years, Amortized Cost	515,865
Held to Maturity, Due after ten years, Amortized Cost	12,462
Held to Maturity, Amortized Cost	528,472	528,784
Held to Maturity, Due in one year or less, Estimated Fair Value	16
Held to Maturity, Due after one year through five years, Estimated Fair Value	130
Held to Maturity, Due after five years through ten years, Estimated Fair Value	529,086
Held to Maturity, Due after ten years, Estimated Fair Value	12,698
Held to Maturity, Estimated Fair Value	541,930	570,526
Available-for-sale Securities, Due in one year or less, Amortized Cost	7,147
Available for Sale, Due after one year through five years, Amortized Cost	22,151
Available for Sale, Due after five years through ten years, Amortized Cost	22,162
Available for Sale, Due after ten years, Amortized Cost	1,808,967
Available for Sale, Amortized Cost	1,860,427	1,916,581
Available for Sale, Due in one year or less, Estimated Fair Value	7,468
Available for Sale, Due after one year through five years, Estimated Fair Value	23,421
Available for Sale, Due after five years through ten years, Estimated Fair Value	22,634
Available for Sale, Due after ten years, Estimated Fair Value	1,841,282
Securities available for sale, estimated fair value	1,894,805	1,987,679
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	129,873
Held to Maturity, Due after one year through five years, Amortized Cost	380,885
Held to Maturity, Due after five years through ten years, Amortized Cost	196,400
Held to Maturity, Due after ten years, Amortized Cost	583,595
Held to Maturity, Amortized Cost	1,290,753	1,415,549
Held to Maturity, Due in one year or less, Estimated Fair Value	132,092
Held to Maturity, Due after one year through five years, Estimated Fair Value	393,977
Held to Maturity, Due after five years through ten years, Estimated Fair Value	201,137
Held to Maturity, Due after ten years, Estimated Fair Value	613,666
Held to Maturity, Estimated Fair Value	1,340,872	1,480,870
Available-for-sale Securities, Due in one year or less, Amortized Cost	123,869
Available for Sale, Due after one year through five years, Amortized Cost	1,356,053
Available for Sale, Due after five years through ten years, Amortized Cost	792,661
Available for Sale, Due after ten years, Amortized Cost	4,369,812
Available for Sale, Amortized Cost	6,642,395	6,683,927
Available for Sale, Due in one year or less, Estimated Fair Value	125,507
Available for Sale, Due after one year through five years, Estimated Fair Value	1,467,653
Available for Sale, Due after five years through ten years, Estimated Fair Value	847,659
Available for Sale, Due after ten years, Estimated Fair Value	4,697,435
Securities available for sale, estimated fair value	7,138,254	7,287,902
Other Investments [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	1,500
Held to Maturity, Due after one year through five years, Amortized Cost	0
Held to Maturity, Due after five years through ten years, Amortized Cost	0
Held to Maturity, Due after ten years, Amortized Cost	0
Held to Maturity, Amortized Cost	1,500	1,500
Held to Maturity, Due in one year or less, Estimated Fair Value	1,500
Held to Maturity, Due after one year through five years, Estimated Fair Value	0
Held to Maturity, Due after five years through ten years, Estimated Fair Value	0
Held to Maturity, Due after ten years, Estimated Fair Value	0
Held to Maturity, Estimated Fair Value	1,500	1,500
Available-for-sale Securities, Due in one year or less, Amortized Cost	0
Available for Sale, Due after one year through five years, Amortized Cost	0
Available for Sale, Due after five years through ten years, Amortized Cost	0
Available for Sale, Due after ten years, Amortized Cost	0
Available for Sale, Amortized Cost	42,354	42,351
Available for Sale, Due in one year or less, Estimated Fair Value	0
Available for Sale, Due after one year through five years, Estimated Fair Value	0
Available for Sale, Due after five years through ten years, Estimated Fair Value	0
Available for Sale, Due after ten years, Estimated Fair Value	0
Securities available for sale, estimated fair value	$ 42,354	$ 42,351

Securities - Sales of Securitie

Securities - Sales of Securities Available for Sale (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 0	$ 1,111,102
Gross realized gains	0	108,989
Gross realized losses	0	0
Tax (expense)benefit of securities gains/losses	$ 0	$ (22,888)

Securities Securities - Premium

Securities Securities - Premium Amortization and Discount Accretion Included in Income on Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Premium amortization	$ (31,235)	$ (32,345)
Discount accretion	637	585
Net (premium amortization) discount accretion	$ (30,598)	$ (31,760)

Securities - Trading Account Se

Securities - Trading Account Securities, at Estimated Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	$ 26,609	$ 24,456
U.S. Treasury [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	24,038	23,996
States and political subdivisions [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	$ 2,571	$ 460

Securities - Net Gains and Loss

Securities - Net Gains and Losses on Trading Account Securities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Net gain on sales transactions	$ 109	$ 470
Net mark-to-market gains (losses)	(69)	115
Net gain (loss) on trading account securities	$ 40	$ 585

Loans - Additional Information

Loans - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)Item	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration of loans in a single industry in excess of ten percent	0.00%
Maximum percentage loan related to single industry	10.00%
Loans and Leases Receivable, Related Parties	$ 365,600		$ 353,100
Number Of COVID-19 Loan Deferments At Period-End | Item	22
COVID 19 Loan Deferment	$ 11,400
Texas Leading Index economic condition indicator	121.4		118.6
Financing Receivable, Allowance for Credit Loss	$ 261,258		$ 263,177
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Additional interest that would have been recognized on non-accrual loans, net of tax	$ 453	$ 1,000
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total energy loans to total loans	6.20%		7.10%
Percentage of total energy loans to total loans excluding PPP Loans	7.50%
Financing Receivable, Allowance for Credit Loss	$ 33,472	$ 103,201	$ 39,553	$ 37,382
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss	0		0
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	968,900
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	70,400
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest Receivable	$ 51,700		$ 48,700

Loans - Loans (Detail)

Loans - Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 17,889,651	$ 17,481,309
Total loans percentage	100.00%	100.00%
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 4,770,228	$ 4,955,341
Total commercial and industrial percentage of total loans	26.70%	28.40%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 886,627	$ 976,473
Percentage of energy production loans to total loans	5.00%	5.60%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 90,510	$ 116,825
Percentage of energy service loans to total loans	0.50%	0.70%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 125,072	$ 141,900
Percentage of other energy loans to total loans	0.70%	0.80%
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,102,209	$ 1,235,198
Percentage of total energy loans to total loans	6.20%	7.10%
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 3,129,244	$ 2,433,849
Percentage Of Payment Protection Program Loans To Loans	17.50%	13.90%
Commercial mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 5,566,367	$ 5,478,806
Commercial mortgages percentage of total loans	31.10%	31.30%
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,236,994	$ 1,223,814
Construction percentage of total loans	6.90%	7.00%
Commercial land loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 300,669	$ 317,847
Land percentage of total loans	1.70%	1.80%
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 7,104,030	$ 7,020,467
Total commercial real estate percentage of total loans	39.70%	40.10%
Home Equity Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 318,215	$ 329,390
Home equity loans percentage of total loans	1.80%	1.90%
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 454,722	$ 452,854
Home equity lines of credit percentage of total loans	2.50%	2.60%
Other consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 549,217	$ 548,530
Other percentage of total loans	3.00%	3.10%
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,322,154	$ 1,330,774
Total consumer real estate percentage of total loans	7.30%	7.60%
Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 8,426,184	$ 8,351,241
Total real estate percentage of total loans	47.00%	47.70%
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 461,786	$ 505,680
Total consumer and other percentage of total loans	2.60%	2.90%

Loans - Non-Accrual Loans, Segr

Loans - Non-Accrual Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 50,976	$ 61,449
Non-accrual with no credit loss allowance	28,746	21,911
Commercial Portfolio Segment [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	18,509	19,849
Non-accrual with no credit loss allowance	4,486	4,479
Commercial and Industrial Total Energy Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	16,042	23,168
Non-accrual with no credit loss allowance	8,203	639
Paycheck Protection Program [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	0	0
Non-accrual with no credit loss allowance	0	0
Commercial Real Estate Buildings Land And Other Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	14,246	15,737
Non-accrual with no credit loss allowance	13,896	14,116
Construction Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	1,653	1,684
Non-accrual with no credit loss allowance	1,653	1,684
Total consumer real estate loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	508	993
Non-accrual with no credit loss allowance	508	993
Consumer Loan [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	18	18
Non-accrual with no credit loss allowance	$ 0	$ 0

Loans Loans - Non-Accrual Loans

Loans Loans - Non-Accrual Loans, Segregated by Class of Loans and Year of Origination (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	$ 50,976	$ 61,449
Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	14,597
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	11,772
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	3,811
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	3,318
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	4,186
Financing Receivable, Revolving	10,498
Financing Receivable, Revolving, Converted to Term Loan	2,794
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	523,775
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	941,431
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	562,889
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	304,835
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	217,911
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	214,083
Financing Receivable, Revolving	1,919,495
Non-accrual loans	18,509	19,849
Commercial Portfolio Segment [Member] | Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	10,344
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	2,463
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	921
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	1,204
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	114
Financing Receivable, Revolving	827
Financing Receivable, Revolving, Converted to Term Loan	2,636
Commercial and Industrial Total Energy Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	418,824
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	56,876
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	45,652
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	14,746
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	7,330
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	7,194
Financing Receivable, Revolving	519,079
Non-accrual loans	16,042	23,168
Commercial and Industrial Total Energy Loans [Member] | Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	900
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	6,314
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,901
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	347
Financing Receivable, Revolving	6,466
Financing Receivable, Revolving, Converted to Term Loan	114
Paycheck Protection Program [Member]
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	0	0
Paycheck Protection Program [Member] | Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	0
Financing Receivable, Revolving, Converted to Term Loan	0
Commercial Real Estate Buildings Land And Other Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	384,018
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,604,500
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,041,700
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	796,983
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	725,449
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,166,794
Financing Receivable, Revolving	67,785
Non-accrual loans	14,246	15,737
Commercial Real Estate Buildings Land And Other Loans [Member] | Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,700
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	2,995
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	989
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,114
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	3,261
Financing Receivable, Revolving	3,187
Financing Receivable, Revolving, Converted to Term Loan	0
Construction Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	183,599
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	312,982
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	439,531
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	152,541
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,873
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,848
Financing Receivable, Revolving	142,620
Non-accrual loans	1,653	1,684
Construction Loans [Member] | Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,653
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	0
Financing Receivable, Revolving, Converted to Term Loan	0
Total consumer real estate loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	508	993
Total consumer real estate loans [Member] | Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	464
Financing Receivable, Revolving	0
Financing Receivable, Revolving, Converted to Term Loan	44
Consumer Loan [Member]
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	18	$ 18
Consumer Loan [Member] | Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	18
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	85,809
Revolving Loans Converted to Term [Member] | Commercial and Industrial Total Energy Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	32,508
Revolving Loans Converted to Term [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	79,807
Revolving Loans Converted to Term [Member] | Construction Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	$ 0

Loans - Age Analysis of Past Du

Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	$ 128,987
Current Loans	17,760,664
Total loans	17,889,651	$ 17,481,309
Accruing Loans 90 or More Days Past Due	16,934
Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	34,758
Current Loans	4,735,470
Total loans	4,770,228	4,955,341
Accruing Loans 90 or More Days Past Due	7,547
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	15,043
Current Loans	1,087,166
Total loans	1,102,209	1,235,198
Accruing Loans 90 or More Days Past Due	1,086
Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	3,129,244
Total loans	3,129,244	2,433,849
Accruing Loans 90 or More Days Past Due	0
Commercial Real Estate Buildings Land And Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	43,589
Current Loans	5,823,447
Total loans	5,867,036	5,796,653
Accruing Loans 90 or More Days Past Due	5,045
Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	22,726
Current Loans	1,214,268
Total loans	1,236,994	1,223,814
Accruing Loans 90 or More Days Past Due	0
Total consumer real estate loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	9,227
Current Loans	1,312,927
Total loans	1,322,154	1,330,774
Accruing Loans 90 or More Days Past Due	2,826
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,644
Current Loans	458,142
Total loans	461,786	$ 505,680
Accruing Loans 90 or More Days Past Due	430
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	95,439
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	24,542
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	11,125
Financing Receivables, 30 to 89 Days Past Due [Member] | Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	27,876
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	22,726
Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	5,973
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,197
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	33,548
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	10,216
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,918
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	15,713
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Total consumer real estate loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,254
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	$ 447

Loans - Troubled Debt Restructu

Loans - Troubled Debt Restructurings (Detail) - Commercial Portfolio Segment [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	$ 0	$ 2,191
Balance at period end	$ 0	$ 2,198

Loans Loans - Restructured Loan

Loans Loans - Restructured Loans Past Due in Excess of 90 Days on Financing Receivables (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)Item	Mar. 31, 2020USD ($)Item
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number Of Restructured Loans Past Due In Excess of 90 Days At Period-End | Item	2	3
Dollar amount of previously restructured loans past due	$ 1,392	$ 1,733
Restructured loans on non-accrual status at period end	0	2,198
Charge-offs of restructured loans recognized on previously restructured loans	$ 1,433	$ 1,533

Loans Loans - Weighted Average

Loans Loans - Weighted Average Risk Grade for All Commercial Loans by Class and Year of Origination (Details) $ in Thousands	Mar. 31, 2021USD ($)Grade	Dec. 31, 2020USD ($)Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 17,889,651	$ 17,481,309
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 4,770,228	$ 4,955,341
Weighted Average Risk Grade For Loans | Grade	6.48	6.45
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 523,775
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	941,431
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	562,889
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	304,835
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	217,911
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	214,083
Financing Receivable, Revolving	1,919,495
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 1,102,209	$ 1,235,198
Weighted Average Risk Grade For Loans | Grade	6.99	6.85
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 418,824
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	56,876
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	45,652
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	14,746
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	7,330
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	7,194
Financing Receivable, Revolving	519,079
Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 5,867,036	$ 5,796,653
Weighted Average Risk Grade For Loans | Grade	7.31	7.34
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 384,018
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,604,500
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,041,700
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	796,983
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	725,449
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,166,794
Financing Receivable, Revolving	67,785
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 1,236,994	$ 1,223,814
Weighted Average Risk Grade For Loans | Grade	7.17	7.22
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 183,599
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	312,982
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	439,531
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	152,541
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,873
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,848
Financing Receivable, Revolving	142,620
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 7,104,030	$ 7,020,467
Weighted Average Risk Grade For Loans | Grade	7.28	7.32
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 567,617
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,917,482
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,481,231
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	949,524
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	728,322
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,169,642
Financing Receivable, Revolving	210,405
Risk Grade One To Eight [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 4,367,924	$ 4,506,121
Weighted Average Risk Grade For Loans | Grade	6.18	6.13
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 486,852
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	874,754
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	512,101
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	260,536
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	193,920
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	198,420
Financing Receivable, Revolving	1,782,914
Risk Grade One To Eight [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 899,412	$ 968,144
Weighted Average Risk Grade For Loans | Grade	6.29	5.99
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 344,718
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	45,560
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	16,767
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	8,861
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	7,330
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	5,470
Financing Receivable, Revolving	453,529
Risk Grade One To Eight [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 5,097,041	$ 5,047,873
Weighted Average Risk Grade For Loans | Grade	6.93	6.97
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 356,814
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,497,610
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	914,168
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	666,186
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	570,468
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	969,182
Financing Receivable, Revolving	56,513
Risk Grade One To Eight [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 1,153,737	$ 1,110,025
Weighted Average Risk Grade For Loans | Grade	7.01	6.99
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 180,644
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	277,006
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	439,531
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	124,821
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	60
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,848
Financing Receivable, Revolving	128,827
Risk Grade One To Eight [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income		$ 6,157,898
Weighted Average Risk Grade For Loans | Grade		6.98
Risk Grade Nine [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 216,646	$ 256,198
Weighted Average Risk Grade For Loans | Grade	9	9
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 11,174
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	40,156
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	34,652
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	29,911
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	16,891
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	14,197
Financing Receivable, Revolving	59,650
Risk Grade Nine [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 94,241	$ 133,547
Weighted Average Risk Grade For Loans | Grade	9	9
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 61,308
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	8,400
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	2,247
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,481
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	15,617
Risk Grade Nine [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 338,677	$ 325,227
Weighted Average Risk Grade For Loans | Grade	9	9
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 22,563
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	62,562
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	82,497
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	59,815
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	46,457
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	57,414
Financing Receivable, Revolving	4,885
Risk Grade Nine [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 52,774	$ 72,287
Weighted Average Risk Grade For Loans | Grade	9	9
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 2,955
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	33,213
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,813
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	13,793
Risk Grade Nine [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income		$ 397,514
Weighted Average Risk Grade For Loans | Grade		9
Risk Grade Ten [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 114,063	$ 125,977
Weighted Average Risk Grade For Loans | Grade	10	10
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 25,740
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	11,803
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	3,264
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	7,392
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,769
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	278
Financing Receivable, Revolving	56,961
Risk Grade Ten [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 16,094	$ 46,427
Weighted Average Risk Grade For Loans | Grade	10	10
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,317
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	369
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	814
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,258
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	750
Financing Receivable, Revolving	7,833
Risk Grade Ten [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 265,081	$ 258,454
Weighted Average Risk Grade For Loans | Grade	10	10
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 2,638
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	20,217
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	27,966
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	56,444
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	67,214
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	79,519
Financing Receivable, Revolving	0
Risk Grade Ten [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 27,720	$ 38,962
Weighted Average Risk Grade For Loans | Grade	10	10
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	27,720
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	0
Risk Grade Ten [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income		$ 297,416
Weighted Average Risk Grade For Loans | Grade		10
Risk Grade Eleven [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 53,086	$ 47,196
Weighted Average Risk Grade For Loans | Grade	11	11
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 9
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	4,374
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	10,409
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	6,075
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	3,127
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,074
Financing Receivable, Revolving	19,143
Risk Grade Eleven [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 76,420	$ 63,912
Weighted Average Risk Grade For Loans | Grade	11	11
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 11,481
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,647
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	19,510
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,245
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	627
Financing Receivable, Revolving	35,634
Risk Grade Eleven [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 151,991	$ 149,362
Weighted Average Risk Grade For Loans | Grade	11	11
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 2,003
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	22,411
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	14,074
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	13,549
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	39,196
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	57,418
Financing Receivable, Revolving	3,200
Risk Grade Eleven [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 1,110	$ 856
Weighted Average Risk Grade For Loans | Grade	11	11
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,110
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	0
Risk Grade Eleven [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income		$ 150,218
Weighted Average Risk Grade For Loans | Grade		11
Risk Grade Twelve [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 14,072	$ 14,528
Weighted Average Risk Grade For Loans | Grade	12	12
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	7,564
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,919
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	921
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	724
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	114
Financing Receivable, Revolving	553
Risk Grade Twelve [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 10,571	$ 13,741
Weighted Average Risk Grade For Loans | Grade	12	12
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	900
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	4,624
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	478
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	347
Financing Receivable, Revolving	4,108
Risk Grade Twelve [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 13,996	$ 15,224
Weighted Average Risk Grade For Loans | Grade	12	12
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,700
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	2,995
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	989
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,114
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	3,011
Financing Receivable, Revolving	3,187
Risk Grade Twelve [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 1,653	$ 1,684
Weighted Average Risk Grade For Loans | Grade	12	12
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,653
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	0
Risk Grade Twelve [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income		$ 16,908
Weighted Average Risk Grade For Loans | Grade		12
Risk Grade Thirteen [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 4,437	$ 5,321
Weighted Average Risk Grade For Loans | Grade	13	13
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	2,780
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	544
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	480
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	274
Risk Grade Thirteen [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 5,471	$ 9,427
Weighted Average Risk Grade For Loans | Grade	13	13
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,690
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,423
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	2,358
Risk Grade Thirteen [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 250	$ 513
Weighted Average Risk Grade For Loans | Grade	13	13
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	250
Financing Receivable, Revolving	0
Risk Grade Thirteen [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 0	$ 0
Weighted Average Risk Grade For Loans | Grade	13	13
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	$ 0
Risk Grade Thirteen [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income		$ 513
Weighted Average Risk Grade For Loans | Grade		13
Revolving Credit Facility [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.37
Revolving Credit Facility [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.64
Revolving Credit Facility [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.68
Revolving Credit Facility [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.23
Revolving Credit Facility [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.37
Revolving Loans Converted to Term [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.70
Financing Receivable, Revolving, Converted to Term Loan	$ 85,809
Revolving Loans Converted to Term [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	8.63
Financing Receivable, Revolving, Converted to Term Loan	$ 32,508
Revolving Loans Converted to Term [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.12
Financing Receivable, Revolving, Converted to Term Loan	$ 79,807
Revolving Loans Converted to Term [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	0
Financing Receivable, Revolving, Converted to Term Loan	$ 0
Revolving Loans Converted to Term [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.12
Financing Receivable, Revolving, Converted to Term Loan	$ 79,807
Revolving Loans Converted to Term [Member] | Risk Grade One To Eight [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	58,427
Revolving Loans Converted to Term [Member] | Risk Grade One To Eight [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	17,177
Revolving Loans Converted to Term [Member] | Risk Grade One To Eight [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	66,100
Revolving Loans Converted to Term [Member] | Risk Grade One To Eight [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Risk Grade Nine [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	10,015
Revolving Loans Converted to Term [Member] | Risk Grade Nine [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	5,188
Revolving Loans Converted to Term [Member] | Risk Grade Nine [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	2,484
Revolving Loans Converted to Term [Member] | Risk Grade Nine [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Risk Grade Ten [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	5,856
Revolving Loans Converted to Term [Member] | Risk Grade Ten [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	3,753
Revolving Loans Converted to Term [Member] | Risk Grade Ten [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	11,083
Revolving Loans Converted to Term [Member] | Risk Grade Ten [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Risk Grade Eleven [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	8,875
Revolving Loans Converted to Term [Member] | Risk Grade Eleven [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	6,276
Revolving Loans Converted to Term [Member] | Risk Grade Eleven [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	140
Revolving Loans Converted to Term [Member] | Risk Grade Eleven [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Risk Grade Twelve [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	2,277
Revolving Loans Converted to Term [Member] | Risk Grade Twelve [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	114
Revolving Loans Converted to Term [Member] | Risk Grade Twelve [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Risk Grade Twelve [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Risk Grade Thirteen [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	359
Revolving Loans Converted to Term [Member] | Risk Grade Thirteen [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Risk Grade Thirteen [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Risk Grade Thirteen [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	$ 0
2021 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.33
2021 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.93
2021 [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.21
2021 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.84
2021 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.09
2020 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.33
2020 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.94
2020 [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.18
2020 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.85
2020 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.12
2019 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.88
2019 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	9.70
2019 [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.29
2019 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.13
2019 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.24
2018 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.17
2018 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	8.67
2018 [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.48
2018 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	8.24
2018 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.61
2017 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.32
2017 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.42
2017 [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.53
2017 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	8.96
2017 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.53
Prior Years [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.03
Prior Years [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.83
Prior Years [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.27
Prior Years [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.79
Prior Years [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.27

Loans - Weighted Average Risk G

Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) $ in Thousands	Mar. 31, 2021USD ($)Grade	Dec. 31, 2020USD ($)Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 17,889,651	$ 17,481,309
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.48	6.45
Loans and Leases Receivable, Net of Deferred Income	$ 4,770,228	$ 4,955,341
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.99	6.85
Loans and Leases Receivable, Net of Deferred Income	$ 1,102,209	$ 1,235,198
Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.31	7.34
Loans and Leases Receivable, Net of Deferred Income	$ 5,867,036	$ 5,796,653
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.17	7.22
Loans and Leases Receivable, Net of Deferred Income	$ 1,236,994	$ 1,223,814
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.28	7.32
Loans and Leases Receivable, Net of Deferred Income	$ 7,104,030	$ 7,020,467
Risk Grade One To Eight [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.18	6.13
Loans and Leases Receivable, Net of Deferred Income	$ 4,367,924	$ 4,506,121
Risk Grade One To Eight [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.29	5.99
Loans and Leases Receivable, Net of Deferred Income	$ 899,412	$ 968,144
Risk Grade One To Eight [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.93	6.97
Loans and Leases Receivable, Net of Deferred Income	$ 5,097,041	$ 5,047,873
Risk Grade One To Eight [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.01	6.99
Loans and Leases Receivable, Net of Deferred Income	$ 1,153,737	$ 1,110,025
Risk Grade One To Eight [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		6.98
Loans and Leases Receivable, Net of Deferred Income		$ 6,157,898
Risk Grade Nine [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9	9
Loans and Leases Receivable, Net of Deferred Income	$ 216,646	$ 256,198
Risk Grade Nine [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9	9
Loans and Leases Receivable, Net of Deferred Income	$ 94,241	$ 133,547
Risk Grade Nine [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9	9
Loans and Leases Receivable, Net of Deferred Income	$ 338,677	$ 325,227
Risk Grade Nine [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9	9
Loans and Leases Receivable, Net of Deferred Income	$ 52,774	$ 72,287
Risk Grade Nine [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		9
Loans and Leases Receivable, Net of Deferred Income		$ 397,514
Risk Grade Ten [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10	10
Loans and Leases Receivable, Net of Deferred Income	$ 114,063	$ 125,977
Risk Grade Ten [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10	10
Loans and Leases Receivable, Net of Deferred Income	$ 16,094	$ 46,427
Risk Grade Ten [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10	10
Loans and Leases Receivable, Net of Deferred Income	$ 265,081	$ 258,454
Risk Grade Ten [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10	10
Loans and Leases Receivable, Net of Deferred Income	$ 27,720	$ 38,962
Risk Grade Ten [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		10
Loans and Leases Receivable, Net of Deferred Income		$ 297,416
Risk Grade Eleven [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11	11
Loans and Leases Receivable, Net of Deferred Income	$ 53,086	$ 47,196
Risk Grade Eleven [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11	11
Loans and Leases Receivable, Net of Deferred Income	$ 76,420	$ 63,912
Risk Grade Eleven [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11	11
Loans and Leases Receivable, Net of Deferred Income	$ 151,991	$ 149,362
Risk Grade Eleven [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11	11
Loans and Leases Receivable, Net of Deferred Income	$ 1,110	$ 856
Risk Grade Eleven [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		11
Loans and Leases Receivable, Net of Deferred Income		$ 150,218
Risk Grade Twelve [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12	12
Loans and Leases Receivable, Net of Deferred Income	$ 14,072	$ 14,528
Risk Grade Twelve [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12	12
Loans and Leases Receivable, Net of Deferred Income	$ 10,571	$ 13,741
Risk Grade Twelve [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12	12
Loans and Leases Receivable, Net of Deferred Income	$ 13,996	$ 15,224
Risk Grade Twelve [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12	12
Loans and Leases Receivable, Net of Deferred Income	$ 1,653	$ 1,684
Risk Grade Twelve [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		12
Loans and Leases Receivable, Net of Deferred Income		$ 16,908
Risk Grade Thirteen [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13	13
Loans and Leases Receivable, Net of Deferred Income	$ 4,437	$ 5,321
Risk Grade Thirteen [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13	13
Loans and Leases Receivable, Net of Deferred Income	$ 5,471	$ 9,427
Risk Grade Thirteen [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13	13
Loans and Leases Receivable, Net of Deferred Income	$ 250	$ 513
Risk Grade Thirteen [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13	13
Loans and Leases Receivable, Net of Deferred Income	$ 0	$ 0
Risk Grade Thirteen [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		13
Loans and Leases Receivable, Net of Deferred Income		$ 513

Loans Loans - Age Analysis of P

Loans Loans - Age Analysis of Past Due Consumer Loans,Segregated by Class and Year of Origination (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	$ 128,987
Current Loans	17,760,664
Loans and Leases Receivable, Net of Deferred Income	17,889,651	$ 17,481,309
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	9,227
Current Loans	1,312,927
Loans and Leases Receivable, Net of Deferred Income	1,322,154	1,330,774
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	3,644
Current Loans	458,142
Loans and Leases Receivable, Net of Deferred Income	461,786	$ 505,680
Financing Receivables, 30 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	95,439
Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	5,973
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	3,197
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	33,548
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	3,254
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	447
Revolving Credit Facility [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,486
Current Loans	437,555
Loans and Leases Receivable, Net of Deferred Income	439,041
Revolving Credit Facility [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	740
Current Loans	349,628
Loans and Leases Receivable, Net of Deferred Income	350,368
Revolving Credit Facility [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	994
Revolving Credit Facility [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	695
Revolving Credit Facility [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	492
Revolving Credit Facility [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	45
Revolving Loans Converted to Term [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,226
Current Loans	14,552
Loans and Leases Receivable, Net of Deferred Income	15,778
Revolving Loans Converted to Term [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,029
Current Loans	29,127
Loans and Leases Receivable, Net of Deferred Income	30,156
Revolving Loans Converted to Term [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	896
Revolving Loans Converted to Term [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	850
Revolving Loans Converted to Term [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	330
Revolving Loans Converted to Term [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	179
2021 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	0
Current Loans	62,908
Loans and Leases Receivable, Net of Deferred Income	62,908
2021 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	4
Current Loans	20,060
Loans and Leases Receivable, Net of Deferred Income	20,064
2021 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	0
2021 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	4
2021 [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	0
2021 [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	0
2020 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	230
Current Loans	331,440
Loans and Leases Receivable, Net of Deferred Income	331,670
2020 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,376
Current Loans	31,370
Loans and Leases Receivable, Net of Deferred Income	32,746
2020 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	230
2020 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,266
2020 [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	0
2020 [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	110
2019 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	725
Current Loans	144,445
Loans and Leases Receivable, Net of Deferred Income	145,170
2019 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	131
Current Loans	18,966
Loans and Leases Receivable, Net of Deferred Income	19,097
2019 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	255
2019 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	131
2019 [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	470
2019 [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	0
2018 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,418
Current Loans	81,559
Loans and Leases Receivable, Net of Deferred Income	82,977
2018 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	257
Current Loans	4,669
Loans and Leases Receivable, Net of Deferred Income	4,926
2018 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,418
2018 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	144
2018 [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	0
2018 [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	113
2017 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,120
Current Loans	71,346
Loans and Leases Receivable, Net of Deferred Income	72,466
2017 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	83
Current Loans	2,408
Loans and Leases Receivable, Net of Deferred Income	2,491
2017 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	716
2017 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	83
2017 [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	404
2017 [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	0
Prior Years [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	3,022
Current Loans	169,122
Loans and Leases Receivable, Net of Deferred Income	172,144
Prior Years [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	24
Current Loans	1,914
Loans and Leases Receivable, Net of Deferred Income	1,938
Prior Years [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,464
Prior Years [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	24
Prior Years [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,558
Prior Years [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	$ 0

Loans Loans - Revolving Loans C

Loans Loans - Revolving Loans Converted to Term (Details) - Revolving Loans Converted to Term [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	$ 45,728	$ 35,409
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	11,843	11,346
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	5,928	15,353
Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	23,303	7,639
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	0	0
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	793	1,071
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	$ 3,861	$ 0

Loans Loans - Allowance for C_2

Loans Loans - Allowance for Credit Loss By Loan Class Calculated in Accordance With CECL Methodology (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	$ 261,258	$ 263,177
Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	70,892	73,843	$ 92,152	$ 51,593
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	33,472	39,553	103,201	37,382
Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	144,440	134,892	52,319	31,037
Total consumer real estate loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	5,636	7,926	8,170	4,113
Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	6,818	6,963	8,039	8,042
Loans Receivable [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	261,258	263,177	$ 263,881	$ 132,167
Modeled Expected Credit Losses [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	119,642	214,552
Modeled Expected Credit Losses [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	54,849	65,645
Modeled Expected Credit Losses [Member] | Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	11,560	8,910
Modeled Expected Credit Losses [Member] | Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	40,831	125,126
Modeled Expected Credit Losses [Member] | Total consumer real estate loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	5,608	7,926
Modeled Expected Credit Losses [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	6,794	6,945
Q-factor adjustments [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	129,404	33,346
Q-factor adjustments [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	10,964	2,877
Q-factor adjustments [Member] | Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	15,047	21,216
Q-factor adjustments [Member] | Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	103,359	9,253
Q-factor adjustments [Member] | Total consumer real estate loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	28	0
Q-factor adjustments [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	6	0
Specific Valuation Allowances [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	12,212	15,279
Specific Valuation Allowances [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	5,079	5,321
Specific Valuation Allowances [Member] | Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	6,865	9,427
Specific Valuation Allowances [Member] | Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	250	513
Specific Valuation Allowances [Member] | Total consumer real estate loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	0	0
Specific Valuation Allowances [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	$ 18	$ 18

Loans - Activity in the Allowan

Loans - Activity in the Allowance for Credit Losses on Loans by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 263,177
Impact of adopting ASC 326	0	$ 39,377
Ending balance	261,258
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	73,843	51,593
Impact of adopting ASC 326		21,263
Credit loss, expense (reversal)	(1,965)	21,950
Charge-offs	(2,189)	(4,369)
Recoveries	1,203	1,715
Net charge-offs	(986)	(2,654)
Ending balance	70,892	92,152
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	39,553	37,382
Impact of adopting ASC 326		(10,453)
Credit loss, expense (reversal)	(5,801)	110,006
Charge-offs	(1,433)	(33,800)
Recoveries	1,153	66
Net charge-offs	(280)	(33,734)
Ending balance	33,472	103,201
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	134,892	31,037
Impact of adopting ASC 326		(13,519)
Credit loss, expense (reversal)	8,922	34,761
Charge-offs	0	(73)
Recoveries	626	113
Net charge-offs	626	40
Ending balance	144,440	52,319
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	7,926	4,113
Impact of adopting ASC 326		2,392
Credit loss, expense (reversal)	(2,722)	1,570
Charge-offs	(284)	(285)
Recoveries	716	380
Net charge-offs	432	95
Ending balance	5,636	8,170
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	6,963	8,042
Impact of adopting ASC 326		(2,248)
Credit loss, expense (reversal)	1,566	4,638
Charge-offs	(4,060)	(5,005)
Recoveries	2,349	2,612
Net charge-offs	(1,711)	(2,393)
Ending balance	6,818	8,039
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	263,177	132,167
Impact of adopting ASC 326		(2,565)
Credit loss, expense (reversal)	0	172,925
Charge-offs	(7,966)	(43,532)
Recoveries	6,047	4,886
Net charge-offs	(1,919)	(38,646)
Ending balance	$ 261,258	$ 263,881

Loans - Investment in Loans Rel

Loans - Investment in Loans Related to Allowance for Loan Losses by Portfolio Segment Disaggregated Based on Impairment Methodology (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 17,889,651	$ 17,481,309
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	4,770,228	4,955,341
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,102,209	1,235,198
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,129,244	2,433,849
Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	5,867,036	5,796,653
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,236,994	1,223,814
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,322,154	1,330,774
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	461,786	505,680
Specific Valuation Allowances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	12,212	15,279
Specific Valuation Allowances [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	5,079	5,321
Specific Valuation Allowances [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	6,865	9,427
Specific Valuation Allowances [Member] | Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	0	0
Specific Valuation Allowances [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	250	513
Specific Valuation Allowances [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	0	0
Specific Valuation Allowances [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	0	0
Specific Valuation Allowances [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	18	18
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	78,672	80,495
Loans Individually Evaluated for Impairment [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	21,858	21,287
Loans Individually Evaluated for Impairment [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	22,496	22,888
Loans Individually Evaluated for Impairment [Member] | Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	0	0
Loans Individually Evaluated for Impairment [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	32,345	34,057
Loans Individually Evaluated for Impairment [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,653	1,684
Loans Individually Evaluated for Impairment [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	302	561
Loans Individually Evaluated for Impairment [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 18	$ 18

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Schedule of Goodwill (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	$ 654,952	$ 654,952

Goodwill and Other Intangible_4

Goodwill and Other Intangible Assets - Schedule of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, net	$ 1,361	$ 1,563
Core deposits [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, net	1,127	1,296
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, net	$ 234	$ 267

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Estimated Aggregate Future Amortization Expense for Intangible Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Remainder of 2021	$ 495
2022	481
2023	282
2024	87
2025	11
Thereafter	5
Future Amortization Expense, Total	$ 1,361	$ 1,563

Deposits - Schedule of Deposits

Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 16,103,649	$ 15,117,051
Interest-bearing Deposit Liabilities, Total	20,821,531	19,898,710
Total deposits	$ 36,925,180	$ 35,015,761
Total non-interest-bearing demand deposits, Percentage of Total	43.60%	43.20%
Percentage of Interest-bearing Domestic Deposits to Deposits	56.40%	56.80%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 15,002,423	$ 13,914,754
Total non-interest-bearing demand deposits, Percentage of Total	40.60%	39.70%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 213,554	$ 242,225
Total non-interest-bearing demand deposits, Percentage of Total	0.60%	0.70%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 9,648,088	$ 9,132,789
Money market accounts	9,416,210	8,977,585
Time Deposits	1,141,660	1,135,575
Interest-bearing Deposit Liabilities, Total	$ 20,205,958	$ 19,245,949
Savings and interest checking, Percentage of Total	26.10%	26.10%
Money market accounts, Percentage of Total	25.50%	25.60%
Percentage Of Time Accounts To Deposits	3.10%	3.30%
Percentage of Interest-bearing Domestic Deposits to Deposits	54.70%	55.00%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 887,672	$ 960,072
Savings and interest checking	556,523	597,503
Money market accounts	53,496	50,070
Time Deposits	5,554	5,188
Interest-bearing Deposit Liabilities, Total	$ 615,573	$ 652,761
Total non-interest-bearing demand deposits, Percentage of Total	2.40%	2.80%
Savings and interest checking, Percentage of Total	1.50%	1.70%
Money market accounts, Percentage of Total	0.20%	0.10%
Percentage Of Time Accounts To Deposits	0.00%	0.00%
Percentage of Interest-bearing Domestic Deposits to Deposits	1.70%	1.80%

Deposits - Additional Informati

Deposits - Additional Information About Corporation's Deposits (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Disclosure Text Block [Abstract]
Deposits from the Certificate of Deposit Account Registry Service (CDARS) deposits	$ 374	$ 372
Deposits from foreign sources (primarily Mexico)	910,652	884,169
Total deposits not covered by deposit insurance	19,766,784	18,694,320
Time deposits not covered by deposit insurance	$ 251,639	$ 237,298

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Operating Lease, Right-of-Use Asset	$ 287,500		$ 292,100
Operating Lease, Liability	318,700		$ 323,000
Lessee, Operating Lease, Liability, Payments, Due	8,100	$ 7,700
Significant change in expected future minimum lease payments since year end	$ 0

Commitments and Contingencies_2

Commitments and Contingencies - Financial Instruments with Off-Balance-Sheet Risk (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Standby Letters of Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 261,341	$ 241,345
Deferred Standby Letter Of Credit Fees [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	1,711	1,723
Commitments to Extend Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 9,853,069	$ 9,814,475

Commitments and Contingencies C

Commitments and Contingencies Commitments and Contingencies - Credit Losses on Off-Balance-Sheet Commitments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Off-Balance Sheet, Credit Loss, Liability	$ 44,152	$ 500
Impact of adopting ASC 326	0	39,377
Credit loss expense	65	2,287
Off-Balance Sheet, Credit Loss, Liability	$ 44,217	$ 42,164

Commitments and Contingencies_3

Commitments and Contingencies Commitments and Contingencies - Components of Lease Commitments Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Amortization of lease right-of-use assets	$ 8,283	$ 7,874
Short-term lease expense	204	561
Non-lease components (including taxes, insurance, common maintenance, etc.)	3,004	2,927
Total operating lease expense, excluding rent	$ 11,491	$ 11,362

Capital and Regulatory Matter_2

Capital and Regulatory Matters - Additional Information (Detail)	Jan. 27, 2021USD ($)	Nov. 19, 2020USD ($)$ / sharesshares	Mar. 16, 2020USD ($)shares	Feb. 15, 2013USD ($)	Mar. 31, 2021USD ($)quarter$ / sharesshares	Mar. 31, 2020USD ($)shares	Dec. 31, 2020USD ($)$ / sharesshares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
CECL Transition Amount					$ 63,200,000		$ 63,700,000
Redemption of series A preferred stock (6,000,000 shares) | shares			6,000,000			6,000,000
Preferred stock dividend rate percentage			5.375%
Preferred stock, redemption amount			$ 150,000,000			$ 150,000,000
Proceeds from Issuance of Preferred Stock and Preference Stock				$ 5,500,000
Purchase of treasury stock (11,625 shares during 2021 and 181,825 shares during 2020) | shares					11,625	181,825
Treasury Stock, Value, Acquired, Cost Method					$ (1,288,000)	$ (13,973,000)
Maximum dividends available without prior regulatory approval					$ 334,000,000
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter					20
Series A Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock liquidation preference value			25
Series B Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock dividend rate percentage		4.45%
Preferred stock liquidation preference value		$ 150,000,000			$ 1,000		$ 1,000
Preferred stock, shares issued | shares		150,000			150,000		150,000
Preferred stock, par value | $ / shares		$ 0.01			$ 0.01		$ 0.01
Preferred Stock, Liquidation Preference Per Share | $ / shares		$ 1,000
Number of Preferred Stock equivalent depositary shares | shares		40
Payments of Stock Issuance Costs		$ 4,500,000
Proceeds from Issuance of Preferred Stock and Preference Stock		$ 145,500,000
Series B Preferred Stock Equivalent Depositary Shares Member
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred Stock, Liquidation Preference Per Share | $ / shares		$ 25
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1					$ 145,500,000		$ 145,500,000
Preferred stock dividend rate percentage					4.45%		4.45%
Cullen/Frost [Member] | Trust Preferred Securities Tier Two Capital Allowable Portion [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities					$ 133,000,000		$ 133,000,000
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1					$ 0		0
stock repurchase 2019 plan [Member] | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Purchase of treasury stock (11,625 shares during 2021 and 181,825 shares during 2020) | shares						177,834
Treasury Stock, Value, Acquired, Cost Method						$ (13,700,000)
stock repurchase 2021 plan
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Authorized Amount	$ 100,000,000
Stock Repurchase Program, Period in Force	1 year
stock repurchase 2021 plan | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Purchase of treasury stock (11,625 shares during 2021 and 181,825 shares during 2020) | shares					0
Subordinated Debt [Member] | Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Permissible portion of subordinated note included in capital					$ 100,000,000		$ 100,000,000
Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, redemption amount			144,500,000			144,486,000
Proceeds from Issuance of Preferred Stock and Preference Stock				$ 144,500,000
Retained Earnings [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, redemption amount			$ 5,500,000			$ 5,514,000

Capital and Regulatory Matter_3

Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 3,158,758	$ 3,058,447
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	13.45%	12.86%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,643,402	$ 1,664,867
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio	7.00%	7.00%
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount	$ 1,526,017	$ 1,545,948
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Ratio	6.50%	6.50%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 3,304,210	$ 3,203,899
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.07%	13.47%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,995,560	$ 2,021,624
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	8.50%	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,878,174	$ 1,902,705
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	8.00%	8.00%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 3,771,830	$ 3,672,912
Total Capital to Risk-Weighted Assets, Actual, Ratio	16.07%	15.44%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,465,104	$ 2,497,300
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	10.50%	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 2,347,718	$ 2,378,381
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 3,304,210	$ 3,203,899
Leverage Ratio, Actual, Ratio	7.97%	8.07%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,658,592	$ 1,589,004
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 2,073,240	$ 1,986,255
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 3,104,492	$ 3,030,093
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	13.25%	12.77%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,640,032	$ 1,661,620
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio	7.00%	7.00%
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount	$ 1,522,886	$ 1,542,933
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Ratio	6.50%	6.50%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 3,104,492	$ 3,030,093
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.25%	12.77%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,991,467	$ 2,017,682
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	8.50%	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,874,322	$ 1,898,995
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	8.00%	8.00%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 3,339,112	$ 3,266,106
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.25%	13.76%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,460,047	$ 2,492,430
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	10.50%	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 2,342,902	$ 2,373,743
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 3,104,492	$ 3,030,093
Leverage Ratio, Actual, Ratio	7.49%	7.63%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,657,640	$ 1,588,200
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 2,072,050	$ 1,985,250
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%

Derivative Financial Instrume_3

Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Derivative [Line Items]
Weighted-average strike rate for outstanding interest rate caps	3.29%
Approximate credit exposure related to swaps with bank customers	$ 122,100
Cash collateral for borrowed securities deposited with other financial institutions	110,200
Interest rate swaps with upstream financial institution counterparties [Member]
Derivative [Line Items]
Approximate credit exposure related to swaps with bank customers	24,800
Not Designated as Hedging Instrument [Member] | Financial Institution Counterparties Loan Lease Interest Rate Swaps Liabilities [Member]
Derivative [Line Items]
Estimated fair value of derivative contracts cleared through Chicago Mercantile Exchange	0		$ 0
Other Income [Member] | Not Designated as Hedging Instrument [Member] | Put Option [Member]
Derivative [Line Items]
Amount of gain (loss) included in income	$ 0	$ 5,980
US Treasury Securities [Member] | Not Designated as Hedging Instrument [Member] | Put Option [Member]
Derivative [Line Items]
Notional Amount		$ 500,000

Derivative Financial Instrume_4

Derivative Financial Instruments - Schedule of Notional Amounts and Estimated Fair Values of Interest Rate Derivative Contracts Outstanding (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Forward Contracts – Liability Notional Amount	$ 3,407	$ 3,724
Estimated Fair Value	(104)	(134)
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Forward Contracts – Liability Notional Amount	906,227	1,173,173
Estimated Fair Value	(23,479)	(33,812)
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Forward Contracts – Assets Notional Amount	164,768	0
Estimated Fair Value	2,560	0
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest-rate caps assets [Member]
Derivatives, Fair Value [Line Items]
Forward Contracts – Assets Notional Amount	382,086	356,601
Estimated Fair Value	3,670	1,241
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Forward Contracts – Assets Notional Amount	906,227	1,173,173
Estimated Fair Value	52,590	84,424
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Forward Contracts – Liability Notional Amount	164,768	0
Estimated Fair Value	(4,736)	0
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate caps liabilities [Member]
Derivatives, Fair Value [Line Items]
Forward Contracts – Liability Notional Amount	382,086	356,601
Estimated Fair Value	$ (3,670)	$ (1,241)

Derivative Financial Instrume_5

Derivative Financial Instruments - Schedule of Weighted-Average Rates Paid and Received for Interest Rate Swaps Outstanding (Detail)	Mar. 31, 2021
Weighted Average Interest Rate Paid [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Fair value hedge loan/lease interest rate swaps	3.14%
Weighted-Average Interest Rate Received [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Fair value hedge loan/lease interest rate swaps	0.11%
Financial Institution Counterparties [Member] | Weighted Average Interest Rate Paid [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Non-hedging interest rate swaps – financial institution counterparties	3.81%
Financial Institution Counterparties [Member] | Weighted-Average Interest Rate Received [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Non-hedging interest rate swaps – financial institution counterparties	1.85%
Customer Counterparties [Member] | Weighted Average Interest Rate Paid [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Non-hedging interest rate swaps – customer counterparties	1.85%
Customer Counterparties [Member] | Weighted-Average Interest Rate Received [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Non-hedging interest rate swaps – customer counterparties	3.81%

Derivative Financial Instrume_6

Derivative Financial Instruments - Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions (Detail) bbl in Thousands, MMBTU in Thousands, $ in Thousands	Mar. 31, 2021USD ($)bblMMBTU	Dec. 31, 2020USD ($)bblMMBTU
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	4,620	3,056
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	8,906	6,391
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 9,176	$ 8,341
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (74,039)	$ (32,112)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	2,422	9,281
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	17,857	15,079
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 448	$ 1,529
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (3,835)	$ (3,265)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	8,906	6,391
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	4,620	3,056
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 74,932	$ 32,670
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (8,897)	$ (8,264)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	17,857	17,636
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	2,422	6,724
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 4,048	$ 3,451
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (444)	$ (1,458)

Derivative Financial Instrume_7

Derivative Financial Instruments - Notional Amount and Fair Value of Foreign Currency Forward Contract (Detail) € in Thousands, $ in Thousands	Mar. 31, 2021USD ($)	Mar. 31, 2021EUR (€)	Dec. 31, 2020USD ($)	Dec. 31, 2020EUR (€)
Financial Institution Counterparties [Member] | Other Assets [Member] | Non Designated as Hedging Instrument [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Notional Amount | €		€ 6,311		€ 0
Customer Counterparties [Member] | Other Liabilities [Member] | Non Designated as Hedging Instrument [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Notional Amount | €		€ 6,299		€ 0
Eurodollar Future | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value | $	$ 144		$ 0
Eurodollar Future | Customer Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Estimated Fair Value | $	$ (132)		$ 0

Derivative Financial Instrume_8

Derivative Financial Instruments - Schedule of Amounts Related to Interest Rate Derivatives Designated as Hedges of Fair Value (Detail) - Designated as Hedging Instrument [Member] - Commercial Loan/Lease Interest Rate Swaps [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Interest Income on Loans [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ (28)	$ (8)
Other Non-Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ 3	$ 1

Derivative Financial Instrume_9

Derivative Financial Instruments - Schedule of Amounts Related to Non-Hedging Interest Rate and Commodity Derivatives (Detail) - Non Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Interest Rate Contract [Member] | Other Non-Interest Income [Member]
Other Non Interest Income Non Hedging Interest Rate Derivatives [Line Items]
Amount of gain (loss) included in income (expense)	$ 1,587	$ 2,039
Interest Rate Contract [Member] | Other Non-Interest Expense [Member]
Other Non Interest Income Non Hedging Interest Rate Derivatives [Line Items]
Amount of gain (loss) included in income (expense)	(1)	0
Commodity Contract [Member] | Other Non-Interest Income [Member]
Other Non Interest Income Non Hedging Interest Rate Derivatives [Line Items]
Amount of gain (loss) included in income (expense)	1,154	139
Foreign Currency Derivative Contracts [Member] | Other Non-Interest Income [Member]
Other Non Interest Income Non Hedging Interest Rate Derivatives [Line Items]
Amount of gain (loss) included in income (expense)	30	7
Put Option [Member] | Other Non-Interest Income [Member]
Other Non Interest Income Non Hedging Interest Rate Derivatives [Line Items]
Amount of gain (loss) included in income (expense)	$ 0	$ 5,980

Balance Sheet Offsetting - Fina

Balance Sheet Offsetting - Financial Instruments Eligible for Offset Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	$ 15,998	$ 11,111
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	15,998	11,111
Gross Amount Recognized, Resell agreements, Financial assets	7,903	0
Gross Amount Offset, Resell agreements, Financial assets	0	0
Net Amount Recognized, Resell agreements, Financial assets	7,903	0
Gross Amount Recognized, Financial assets	23,901	11,111
Gross Amount Offset, Financial assets	0	0
Net Amount Recognized, Financial assets	23,901	11,111
Gross Amount Recognized, Derivatives, Financial Liabilities	101,457	69,323
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	101,457	69,323
Gross Amount Recognized, Repurchase agreements, Financial liabilities	1,884,449	2,068,147
Gross Amount Offset, Repurchase agreements, Financial liabilities	0	0
Net Amount Recognized, Repurchase agreements, Financial liabilities	1,884,449	2,068,147
Gross Amount Recognized, Financial liabilities	1,985,906	2,137,470
Gross Amount Offset, Financial liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,985,906	2,137,470
Loan/lease interest rate swaps and caps [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	6,230	1,241
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	6,230	1,241
Commodity swaps and options [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	9,624	9,870
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	9,624	9,870
Gross Amount Recognized, Derivatives, Financial Liabilities	77,874	35,377
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	77,874	35,377
Foreign currency forward contracts [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	144
Gross Amount Offset, Derivatives, Financial assets	0
Net Amount Recognized, Derivatives, Financial assets	144
Loan/lease interest rate swaps [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Liabilities	23,583	33,946
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	$ 23,583	$ 33,946

Balance Sheet Offsetting - Fi_2

Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 15,998	$ 11,111
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(15,998)	(11,111)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Resell agreements, Financial Assets	7,903	0
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(7,903)	0
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	23,901	11,111
Gross Amounts Not Offset Collateral, Financial Assets	(7,903)	0
Net Amount, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	101,457	69,323
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(15,998)	(11,111)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(85,459)	(57,031)
Net Amount, Derivatives, Financial Liabilities	0	1,181
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	1,884,449	2,068,147
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(1,884,449)	(2,068,147)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,985,906	2,137,470
Gross Amounts Not Offset Collateral, Financial Liabilities	(1,969,908)	(2,125,178)
Net Amount, Financial Liabilities	0	1,181
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	5	2
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(5)	(2)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	5,349	6,430
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(5)	(2)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(5,344)	(6,428)
Net Amount, Derivatives, Financial Liabilities	0	0
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	8,250	5,838
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(8,250)	(5,838)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	32,181	20,722
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(8,250)	(5,838)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(23,931)	(14,700)
Net Amount, Derivatives, Financial Liabilities	0	184
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Liabilities	50	71
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(50)	(71)
Net Amount, Derivatives, Financial Liabilities	0	0
Other Counterparties [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	7,743	5,271
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(7,743)	(5,271)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	63,877	42,100
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(7,743)	(5,271)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(56,134)	(35,832)
Net Amount, Derivatives, Financial Liabilities	$ 0	$ 997

Balance Sheet Offsetting Balanc

Balance Sheet Offsetting Balance Sheet Offsetting - Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase [Line Items]
Remaining contractual maturity of the agreements overnight and continuous	$ 1,884,449	$ 2,068,147
Remaining contractual maturity of the agreements up to 30 Days	0	0
Remaining contractual maturity of the agreements 30 to 90 Days	0	0
Remaining contractual maturity of the agreements greater than 90 Days	0	0
Remaining contractual maturity of the agreements total	1,884,449	2,068,147
Securities Sold under Agreements to Repurchase, Gross	1,884,449	2,068,147
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	0	0
US Treasury Securities [Member]
Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase [Line Items]
Remaining contractual maturity of the agreements overnight and continuous	730,620	692,860
Remaining contractual maturity of the agreements up to 30 Days	0	0
Remaining contractual maturity of the agreements 30 to 90 Days	0	0
Remaining contractual maturity of the agreements greater than 90 Days	0	0
Remaining contractual maturity of the agreements total	730,620	692,860
Residential Mortgage Backed Securities [Member]
Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase [Line Items]
Remaining contractual maturity of the agreements overnight and continuous	1,153,829	1,375,287
Remaining contractual maturity of the agreements up to 30 Days	0	0
Remaining contractual maturity of the agreements 30 to 90 Days	0	0
Remaining contractual maturity of the agreements greater than 90 Days	0	0
Remaining contractual maturity of the agreements total	$ 1,153,829	$ 1,375,287

Stock-Based Compensation Stock-

Stock-Based Compensation Stock-Based Compensation - Narrative (Details)	Mar. 31, 2021shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	895,295

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Activity in Corporation's Active Stock Plans (Detail) - $ / shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options exercised, Number of Shares	(513,824)	(66,215)
Director deferred stock units outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares/Units	52,860
Ending Balance, Number of Shares/Units	52,860
Beginning Balance, Weighted-Average Grant-Date Fair Value	$ 75.47
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 75.47
Non-Vested Stock Awards/Stock Units Outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares/Units	470,359
Forfeited/expired, Number of Shares/Units	(6,991)
Ending Balance, Number of Shares/Units	463,368
Beginning Balance, Weighted-Average Grant-Date Fair Value	$ 86.24
Forfeited/Expired, Weighted-Average Price	91.57
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 86.16
Stock Options Outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares	1,739,559
Stock options exercised, Number of Shares	(478,693)
Ending Balance, Number of Shares	1,260,866
Beginning Balance, Weighted-Average Exercise Price	$ 66.11
Stock options exercised, Weighted-Average Exercise Price	61.41
Ending Balance, Weighted-Average Exercise Price	$ 67.89
Maximum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares/Units	201,257
Stock Awards exercised/vested, Number of Shares	(35,131)
Forfeited/expired, Number of Shares/Units	(1,115)
Ending Balance, Number of Shares/Units	165,011
Beginning Balance, Weighted-Average Grant-Date Fair Value	$ 77.18
Stock Award Exercised/Vested, Weighted Average Grant Date Fair Value	92.27
Forfeited/Expired, Weighted-Average Price	92.27
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 73.86

Stock-Based Compensation - Shar

Stock-Based Compensation - Shares Issued in Connection with Stock Compensation Awards (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Payment Arrangement [Abstract]
New shares issued from available authorized shares	0	0
Shares issued from available treasury stock	513,824	66,215
Total	513,824	66,215
Proceeds from stock option exercises	$ 29,397	$ 1,463

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	$ 2,526	$ 2,512
Income tax benefit	367	465
Non-Vested Stock Awards/Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	1,883	2,264
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	$ 643	$ 248

Stock-Based Compensation - Unre

Stock-Based Compensation - Unrecognized Stock-Based Compensation Expense (Detail) $ in Thousands	Mar. 31, 2021USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 22,195
Non-Vested Stock Awards/Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested stock awards/stock units and Performance stock units	15,907
Maximum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested stock awards/stock units and Performance stock units	$ 6,288

Earnings Per Common Share - Bas

Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net income	$ 116,015	$ 54,753
Less: Preferred stock dividends	2,151	2,016
Redemption of preferred stock	0	5,514
Net income available to common shareholders	113,864	47,223
Less: Earnings allocated to participating securities	1,162	445
Distributed earnings allocated to common stock	45,660	44,512
Undistributed earnings allocated to common stock	67,042	2,266
Net earnings allocated to common stock	$ 112,702	$ 46,778
Weighted-average shares outstanding for basic earnings per common share	63,306,493	62,642,682
Dilutive effect of stock compensation	509,290	407,154
Weighted-average shares outstanding for diluted earnings per common share	63,815,783	63,049,836

Defined Benefit Plans Additiona

Defined Benefit Plans Additional Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Retirement Benefits [Abstract]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 0
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year	$ 0

Defined Benefit Plans - Net Per

Defined Benefit Plans - Net Periodic Cost (Benefit) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Retirement Benefits [Abstract]
Expected return on plan assets, net of expenses	$ (3,210)	$ (3,072)
Interest cost on projected benefit obligation	835	1,252
Net amortization and deferral	1,529	1,330
Net periodic expense (benefit)	$ (846)	$ (490)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Deferred Tax Liabilities, Net	$ 88.4		$ 117.5
Valuation allowance	$ 0
U.S. federal statutory income tax rate, percent	21.00%	21.00%
Unrecognized tax benefits	$ 0	$ 0

Income Taxes - Income Tax Expen

Income Taxes - Income Tax Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Current income tax expense (benefit)	$ 3,725	$ 29,412
Deferred income tax expense (benefit)	4,172	(26,089)
Income tax expense, as reported	$ 7,897	$ 3,323
Effective tax rate	6.40%	5.70%

Other Comprehensive Income (L_3

Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (159,779)	$ 230,118
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(259)	(377)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	(108,989)
Total securities available for sale and transferred securities, before tax amount	(160,038)	120,752
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	1,529	1,330
Total defined-benefit post-retirement benefit plans	1,529	1,330
Total other comprehensive income (loss), before tax	(158,509)	122,082
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(33,554)	48,325
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(54)	(79)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	(22,888)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(33,608)	25,358
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	321	279
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	321	279
Total other comprehensive income (loss), Tax Expense (Benefit)	(33,287)	25,637
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(126,225)	181,793
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(205)	(298)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	(86,101)
Total securities available for sale and transferred securities, Net of Tax	(126,430)	95,394
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	1,208	1,051
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	1,208	1,051
Other comprehensive income (loss), net of tax	$ (125,222)	$ 96,445

Other Comprehensive Income - Sc

Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 512,970
Other comprehensive income (loss), net of tax	(125,222)	$ 96,445
Ending Balance	387,748
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	563,801	313,304
Other comprehensive income (loss) before reclassifications	(126,430)	181,495
Reclassification of amounts included in net income	0	(86,101)
Other comprehensive income (loss), net of tax	(126,430)	95,394
Ending Balance	437,371	408,698
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(50,831)	(45,934)
Other comprehensive income (loss) before reclassifications	0	0
Reclassification of amounts included in net income	1,208	1,051
Other comprehensive income (loss), net of tax	1,208	1,051
Ending Balance	(49,623)	(44,883)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	512,970	267,370
Other comprehensive income (loss) before reclassifications	(126,430)	181,495
Reclassification of amounts included in net income	1,208	(85,050)
Other comprehensive income (loss), net of tax	(125,222)	96,445
Ending Balance	$ 387,748	$ 363,815

Operating Segments - Additional

Operating Segments - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2021Segment
Segment Reporting [Abstract]
Number of primary operating segments	2

Operating Segments - Summary of

Operating Segments - Summary of Operating Results by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 240,881	$ 244,521
Credit loss expense	63	175,197
Total non-interest income	93,236	212,915
Total non-interest expense	210,142	224,163
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	123,912	58,076
Income taxes	7,897	3,323
Net income	116,015	54,753
Less: Preferred stock dividends	2,151	2,016
Redemption of preferred stock	0	5,514
Net Income (Loss) Available to Common Stockholders, Basic	113,864	47,223
Revenues from (expenses to) external customers	334,117	457,436
Bank
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	242,205	246,013
Credit loss expense	63	175,198
Total non-interest income	53,780	173,416
Total non-interest expense	179,151	188,972
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	116,771	55,259
Income taxes	7,177	3,235
Net income	109,594	52,024
Less: Preferred stock dividends	0	0
Redemption of preferred stock		0
Net Income (Loss) Available to Common Stockholders, Basic	109,594	52,024
Revenues from (expenses to) external customers	295,985	419,429
Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	486	877
Credit loss expense	0	(1)
Total non-interest income	39,609	39,730
Total non-interest expense	29,937	33,536
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	10,158	7,072
Income taxes	2,133	1,485
Net income	8,025	5,587
Less: Preferred stock dividends	0	0
Redemption of preferred stock		0
Net Income (Loss) Available to Common Stockholders, Basic	8,025	5,587
Revenues from (expenses to) external customers	40,095	40,607
Non-Banks [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	(1,810)	(2,369)
Credit loss expense	0	0
Total non-interest income	(153)	(231)
Total non-interest expense	1,054	1,655
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(3,017)	(4,255)
Income taxes	(1,413)	(1,397)
Net income	(1,604)	(2,858)
Less: Preferred stock dividends	2,151	2,016
Redemption of preferred stock		5,514
Net Income (Loss) Available to Common Stockholders, Basic	(3,755)	(10,388)
Revenues from (expenses to) external customers	$ (1,963)	$ (2,600)

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments using fair value measurement option	$ 0	$ 0

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	$ 10,377,444	$ 10,437,565
Trading account securities	26,609	24,456
Derivative assets	15,998	11,111
Derivative liabilities	101,457	69,323
Interest rate swaps, caps and floors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	58,820	85,665
Derivative liabilities	31,989	35,187
Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	88,604	45,991
Derivative liabilities	87,215	45,099
Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	144	0
Derivative liabilities	132	0
U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	1,302,031	1,119,633
Trading account securities	24,038	23,996
Residential Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	1,894,805	1,987,679
States and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	7,138,254	7,287,902
Trading account securities	2,571	460
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	42,354	42,351
Level 1 [Member] | Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	144	0
Derivative liabilities	132	0
Level 1 [Member] | U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	1,302,031	1,119,633
Trading account securities	24,038	23,996
Level 2 [Member] | Interest rate swaps, caps and floors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	58,820	85,665
Derivative liabilities	31,989	35,187
Level 2 [Member] | Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	88,604	45,535
Derivative liabilities	87,215	45,099
Level 2 [Member] | Residential Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	1,894,805	1,987,679
Level 2 [Member] | States and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	7,138,254	7,287,902
Trading account securities	2,571	460
Level 2 [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	42,354	42,351
Level 3 [Member] | Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 0	$ 456

Fair Value Measurements - Impai

Fair Value Measurements - Impaired Loans Remeasured and Reported at Fair Value of Underlying Collateral (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Mar. 31, 2020
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	$ 100	$ 1,919
Impaired Financing Receivable, Specific Valuation Allowance Allocation	0
Impaired Financing Receivable, Reversal of Specific Valuation Allowance Allocations		70
Loans receivable, fair value disclosure	100	1,989
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	15,013	3,867
Impaired Financing Receivable, Specific Valuation Allowance Allocation		(244)
Impaired Financing Receivable, Reversal of Specific Valuation Allowance Allocations	600
Loans receivable, fair value disclosure	$ 15,613	$ 3,623

Fair Value Measurements - Forec

Fair Value Measurements - Foreclosed Assets Remeasured and Reported at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Mar. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed Assets Remeasured Subsequent to initial Recognition Carrying value of foreclosed assets prior to remeasurement	$ 0	$ 328
Foreclosed Assets Remeasured Subsequent to Initial Recognition Write-downs included in other non-interest expense	0	(231)
Fair Value of Foreclosed Assets remeasured subsequent to initial recognition	$ 0	$ 97

Fair Value Measurements - Estim

Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 11,802,269	$ 10,288,853	$ 4,195,585	$ 3,788,181
Securities held to maturity	1,820,725	1,945,833
Loans, net	17,628,393	17,218,132
Deposits	36,925,180	35,015,761
Federal funds purchased	38,975	48,850
Repurchase agreements	1,884,449	2,068,147
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,371	136,357
Subordinated notes	99,061	99,021
Carrying Amount [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	11,802,269	10,288,853
Securities held to maturity	1,820,567	1,945,673
Cash surrender value of life insurance policies	189,756	189,984
Accrued interest receivable	128,725	181,432
Deposits	36,925,180	35,015,761
Federal funds purchased	38,975	48,850
Repurchase agreements	1,884,449	2,068,147
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,371	136,357
Subordinated notes	99,061	99,021
Accrued interest payable	4,138	8,127
Carrying Amount [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	17,628,393	17,218,132
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	11,802,269	10,288,853
Securities held to maturity	1,884,302	2,052,896
Cash surrender value of life insurance policies	189,756	189,984
Accrued interest receivable	128,725	181,432
Deposits	36,926,759	35,018,185
Federal funds purchased	38,975	48,850
Repurchase agreements	1,884,449	2,068,147
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	137,115	137,115
Subordinated notes	114,818	115,717
Accrued interest payable	4,138	8,127
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 17,738,315	$ 17,390,683