Cullen/Frost Bankers (CFR) 10-Q 28 Oct 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - $ / shares	9 Months Ended
	Sep. 30, 2021	Oct. 21, 2021	Dec. 31, 2020
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 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		63,715,008
Common stock, par value	$ 0.01		$ 0.01
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
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	Sep. 30, 2021	Dec. 31, 2020
Assets:
Cash and due from banks	$ 638,296	$ 529,454
Interest-bearing deposits	15,601,647	9,758,624
Federal funds sold	6,125	775
Resell agreements	7,903	0
Total cash and cash equivalents	16,253,971	10,288,853
Securities held to maturity, net of allowance for credit losses of $158 at September 30, 2021 and $160 at December 31, 2020	1,759,270	1,945,673
Securities available for sale, at estimated fair value	11,731,222	10,437,565
Trading account securities	25,292	24,456
Loans, net of unearned discounts	15,832,670	17,481,309
Less: Allowance for credit losses on loans	(250,150)	(263,177)
Net loans	15,582,520	17,218,132
Premises and equipment, net	1,023,170	1,045,578
Goodwill	654,952	654,952
Other intangible assets, net	1,019	1,563
Cash surrender value of life insurance policies	189,941	189,984
Accrued interest receivable and other assets	638,968	584,561
Total assets	47,860,325	42,391,317
Liabilities:
Non-interest-bearing demand deposits	17,151,583	15,117,051
Interest-bearing deposits	22,461,184	19,898,710
Total deposits	39,612,767	35,015,761
Federal funds purchased	27,200	48,850
Repurchase agreements	2,200,029	2,068,147
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,400	136,357
Subordinated notes, net of unamortized issuance costs	99,139	99,021
Accrued interest payable and other liabilities	1,412,428	730,165
Total liabilities	43,487,963	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 September 30, 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 September 30, 2021 and December 31, 2020	642	642
Additional paid-in capital	1,004,924	997,168
Retained earnings	2,920,074	2,750,723
Accumulated other comprehensive income (loss), net of tax	350,283	512,970
Treasury stock, at cost; 568,570 shares at September 30, 2021 and 1,225,066 shares at December 31, 2020	(49,013)	(113,939)
Total shareholders’ equity	4,372,362	4,293,016
Total liabilities and shareholders’ equity	$ 47,860,325	$ 42,391,317

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 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	568,570	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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Interest income:
Loans, including fees	$ 168,779	$ 169,002	$ 518,957	$ 512,444
Securities:
Taxable	20,736	22,560	61,366	71,950
Tax-exempt	56,882	58,073	169,656	175,968
Interest-bearing deposits	5,830	1,512	11,877	10,894
Federal funds sold	2	5	13	721
Resell agreements	6	14	11	163
Total interest income	252,235	251,166	761,880	772,140
Interest expense:
Deposits	3,677	5,397	10,693	27,677
Federal funds purchased	9	7	24	92
Repurchase agreements	632	475	1,597	3,913
Junior subordinated deferrable interest debentures	631	700	1,915	2,893
Subordinated notes	1,164	1,164	3,492	3,492
Federal Home Loan Bank advances	0	0	0	318
Total interest expense	6,113	7,743	17,721	38,385
Net interest income	246,122	243,423	744,159	733,755
Credit loss expense	0	20,302	63	227,474
Net interest income after credit loss expense	246,122	223,121	744,096	506,281
Non-interest income:
Trust and investment management fees	37,381	31,469	110,569	97,002
Service charges on deposit accounts	21,216	19,812	61,058	60,043
Insurance commissions and fees	11,748	11,456	39,834	38,609
Interchange and card transaction fees	4,490	3,503	13,224	9,724
Other charges, commissions and fees	9,785	8,370	26,729	25,398
Net gain (loss) on securities transactions	0	0	0	108,989
Other	8,569	8,991	26,258	34,352
Total non-interest income	93,189	83,601	277,672	374,117
Non-interest expense:
Salaries and wages	99,463	93,323	289,956	282,485
Employee benefits	21,576	16,074	62,840	59,824
Net occupancy	27,208	25,466	79,909	76,116
Technology, furniture and equipment	28,494	26,482	84,508	77,768
Deposit insurance	3,088	2,372	8,893	7,796
Intangible amortization	157	212	544	710
Other	38,017	38,221	116,749	121,293
Total non-interest expense	218,003	202,150	643,399	625,992
Income before income taxes	121,308	104,572	378,369	254,406
Income taxes	13,333	9,516	36,311	11,525
Net income	107,975	95,056	342,058	242,881
Preferred stock dividends	1,668	0	5,488	2,016
Redemption of preferred stock	0	0	0	5,514
Net income available to common shareholders	$ 106,307	$ 95,056	$ 336,570	$ 235,351
Earnings per common share:
Basic	$ 1.66	$ 1.50	$ 5.25	$ 3.72
Diluted	$ 1.65	$ 1.50	$ 5.22	$ 3.71

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net income	$ 107,975	$ 95,056	$ 342,058	$ 242,881
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss on securities available for sale during the period	(80,668)	20,332	(209,779)	406,959
Change in net unrealized gain on securities transferred to held to maturity	(237)	(294)	(741)	(979)
Reclassification adjustment for net (gains) losses included in net income	0	0	0	(108,989)
Total securities available for sale and transferred securities, before tax amount	(80,905)	20,038	(210,520)	296,991
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	4,587	3,989
Total defined-benefit post-retirement benefit plans	1,529	1,330	4,587	3,989
Other comprehensive income (loss), before tax	(79,376)	21,368	(205,933)	300,980
Deferred tax expense (benefit)	(16,669)	4,487	(43,246)	63,205
Other comprehensive income (loss), net of tax	(62,707)	16,881	(162,687)	237,775
Comprehensive income (loss)	$ 45,268	$ 111,937	$ 179,371	$ 480,656

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	242,881				242,881
Other comprehensive income (loss)	237,775					237,775
Stock option exercises/stock unit conversions (21,968 shares QTD and 650,044 shares YTD in 2021 and 53,425 shares QTD and 196,948 shares YTD in 2020)	7,544				(11,627)		19,171
Stock compensation expense recognized in earnings	8,227			8,227
Preferred stock, redemption amount (6,000,000 shares)	(150,000)	(144,486)			(5,514)
Purchase of treasury stock (478 shares QTD and 12,103 shares YTD in 2021 and 400 shares QTD and 182,225 shares YTD in 2020)	(14,000)						(14,000)
Treasury stock issued to the 401(k) stock purchase plan (18,555 shares YTD in 2021 and 58,900 shares QTD and 97,843 shares YTD in 2020)	6,961				(2,563)		9,524
Cash dividends – preferred stock (approximately $11.13 per share which is equivalent to approximately $0.28 per depository share QTD and $36.59 per share which is equivalent to approximately $0.91 per depository share YTD in 2021 and $0.34 per share YTD in 2020)	(2,016)				(2,016)
Cash dividends – common stock ($0.75 per share QTD and $2.19 per share YTD in 2021 and $0.71 per share QTD and $2.13 per share YTD in 2020)	(134,762)				(134,762)
Total shareholders’ equity at end of period at Sep. 30, 2020	$ 4,085,026	0	642	991,477	2,724,681	505,145	(136,919)
Purchase of treasury stock (478 shares QTD and 12,103 shares during 2021 and 400 shares QTD and 182,225 shares during 2020)	(182,225)
Total shareholders’ equity at beginning of period at Jun. 30, 2020	$ 4,008,801	0	642	989,034	2,678,686	488,264	(147,825)
Net income	95,056				95,056
Other comprehensive income (loss)	16,881					16,881
Stock option exercises/stock unit conversions (21,968 shares QTD and 650,044 shares YTD in 2021 and 53,425 shares QTD and 196,948 shares YTD in 2020)	2,698				(2,502)		5,200
Stock compensation expense recognized in earnings	2,443			2,443
Purchase of treasury stock (478 shares QTD and 12,103 shares YTD in 2021 and 400 shares QTD and 182,225 shares YTD in 2020)	(27)						(27)
Treasury stock issued to the 401(k) stock purchase plan (18,555 shares YTD in 2021 and 58,900 shares QTD and 97,843 shares YTD in 2020)	4,131				(1,602)		5,733
Cash dividends – common stock ($0.75 per share QTD and $2.19 per share YTD in 2021 and $0.71 per share QTD and $2.13 per share YTD in 2020)	(44,957)				(44,957)
Total shareholders’ equity at end of period at Sep. 30, 2020	$ 4,085,026	0	642	991,477	2,724,681	505,145	(136,919)
Purchase of treasury stock (478 shares QTD and 12,103 shares during 2021 and 400 shares QTD and 182,225 shares during 2020)	(400)
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	342,058				342,058
Other comprehensive income (loss)	(162,687)					(162,687)
Stock option exercises/stock unit conversions (21,968 shares QTD and 650,044 shares YTD in 2021 and 53,425 shares QTD and 196,948 shares YTD in 2020)	37,810				(26,652)		64,462
Stock compensation expense recognized in earnings	7,756			7,756
Purchase of treasury stock (478 shares QTD and 12,103 shares YTD in 2021 and 400 shares QTD and 182,225 shares YTD in 2020)	(1,342)						(1,342)
Treasury stock issued to the 401(k) stock purchase plan (18,555 shares YTD in 2021 and 58,900 shares QTD and 97,843 shares YTD in 2020)	1,749				(57)		1,806
Cash dividends – preferred stock (approximately $11.13 per share which is equivalent to approximately $0.28 per depository share QTD and $36.59 per share which is equivalent to approximately $0.91 per depository share YTD in 2021 and $0.34 per share YTD in 2020)	(5,488)				(5,488)
Cash dividends – common stock ($0.75 per share QTD and $2.19 per share YTD in 2021 and $0.71 per share QTD and $2.13 per share YTD in 2020)	(140,510)				(140,510)
Total shareholders’ equity at end of period at Sep. 30, 2021	$ 4,372,362	145,452	642	1,004,924	2,920,074	350,283	(49,013)
Purchase of treasury stock (478 shares QTD and 12,103 shares during 2021 and 400 shares QTD and 182,225 shares during 2020)	(12,103)
Total shareholders’ equity at beginning of period at Jun. 30, 2021	$ 4,373,817	145,452	642	1,002,939	2,862,966	412,990	(51,172)
Net income	107,975				107,975
Other comprehensive income (loss)	(62,707)					(62,707)
Stock option exercises/stock unit conversions (21,968 shares QTD and 650,044 shares YTD in 2021 and 53,425 shares QTD and 196,948 shares YTD in 2020)	1,128				(1,085)		2,213
Stock compensation expense recognized in earnings	1,985			1,985
Purchase of treasury stock (478 shares QTD and 12,103 shares YTD in 2021 and 400 shares QTD and 182,225 shares YTD in 2020)	(54)						(54)
Cash dividends – preferred stock (approximately $11.13 per share which is equivalent to approximately $0.28 per depository share QTD and $36.59 per share which is equivalent to approximately $0.91 per depository share YTD in 2021 and $0.34 per share YTD in 2020)	(1,668)				(1,668)
Cash dividends – common stock ($0.75 per share QTD and $2.19 per share YTD in 2021 and $0.71 per share QTD and $2.13 per share YTD in 2020)	(48,114)				(48,114)
Total shareholders’ equity at end of period at Sep. 30, 2021	$ 4,372,362	$ 145,452	$ 642	$ 1,004,924	$ 2,920,074	$ 350,283	$ (49,013)
Purchase of treasury stock (478 shares QTD and 12,103 shares during 2021 and 400 shares QTD and 182,225 shares during 2020)	(478)

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Shareholders' Equity (Parenthetical) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Stockholders' Equity [Abstract]
Stock option exercise/stock unit conversions (21,968 shares QTD and 650,044 shares YTD during 2021 and 53,425 shares QTD and 196,948 shares YTD during 2020)	21,968	53,425	650,044	196,948
Redemption of series A preferred stock (6,000,000 shares)				6,000,000
Purchase of treasury stock (478 shares QTD and 12,103 shares during 2021 and 400 shares QTD and 182,225 shares during 2020)	478	400	12,103	182,225
Treasury stock issued to the 401(k) stock purchase plan (18,555 shares YTD in 2021 and 58,900 shares QTD and 97,843 shares YTD in 2020)		58,900	18,555	97,843
Cash dividend - preferred stock (Approximately $11.13 per share QTD and $36.59 per share YTD in 2021 and $0.34 per share YTD during 2020)	$ 11.13		$ 36.59	$ 0.34
Cash dividend - Preferred Stock Dividends equivalent Per depositary Share Declared ($0.28 QTD and $0.91 YTD during 2021)	0.28		0.91
Cash dividends - common stock ($0.75 per share QTD and $2.19 per share YTD during 2021 and $0.71 per share QTD and $2.13 per share YTD during 2020)	$ 0.75	$ 0.71	$ 2.19	$ 2.13

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Operating Activities:
Net income	$ 342,058	$ 242,881
Adjustments to reconcile net income to net cash from operating activities:
Credit loss expense	63	227,474
Deferred tax expense (benefit)	4,672	(8,803)
Accretion of loan discounts	(9,879)	(11,905)
Securities premium amortization (discount accretion), net	90,017	93,080
Net (gain) loss on securities transactions	0	(108,989)
Depreciation and amortization	51,550	47,752
Net (gain) loss on sale/write-down of assets/foreclosed assets	(1,885)	(6)
Stock-based compensation	7,756	8,227
Net tax benefit from stock-based compensation	4,505	694
Earnings on life insurance policies	(1,901)	(2,895)
Net change in:
Trading account securities	(421)	(197)
Lease right-of-use assets	17,566	17,419
Accrued interest receivable and other assets	(65,849)	(52,842)
Accrued interest payable and other liabilities	114,361	23,208
Net cash from operating activities	552,613	475,098
Investing Activities:
Securities held-to-maturity - Purchases	0	(1,500)
Securities held to maturity: Maturities, calls and principal repayments	171,969	63,333
Securities available for sale:
Purchases	4,105,085	8,324,364
Sales	0	1,162,352
Maturities, calls and principal repayments	3,125,786	8,162,036
Net change in loans	1,649,429	(3,551,650)
Benefits received on life insurance policies	1,944	903
Proceeds from sales of premises and equipment	7,033	3,054
Purchases of premises and equipment	(37,088)	(81,866)
Proceeds from sales of repossessed properties	809	73
Net cash from investing activities	814,797	(2,567,629)
Financing Activities:
Net change in deposits	4,597,006	5,859,939
Net change in short-term borrowings	110,232	(86,675)
Proceeds from Federal Home Loan Bank advances	0	1,250,000
Principal payments on Federal Home Loan Bank advances	0	(1,250,000)
Redemption of Series A preferred stock	0	(150,000)
Proceeds from stock option exercises	37,810	7,544
Purchase of treasury stock	(1,342)	(14,000)
Cash dividends paid on preferred stock	(5,488)	(2,016)
Cash dividends paid on common stock	(140,510)	(134,762)
Net cash from financing activities	4,597,708	5,480,030
Net change in cash and cash equivalents	5,965,118	3,387,499
Cash and cash equivalent at beginning of period	10,288,853	3,788,181
Cash and cash equivalents at end of period	$ 16,253,971	$ 7,175,680

Significant Accounting Policies

Significant Accounting Policies	9 Months Ended
	Sep. 30, 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: Nine Months Ended 2021 2020 Cash paid for interest $ 23,722 $ 41,634 Cash paid for income taxes 28,043 44,140 Significant non-cash transactions: Unsettled securities transactions 600,874 6,698 Loans foreclosed and transferred to other real estate owned and foreclosed assets 3,464 140 Right-of-use lease assets obtained in exchange for lessee operating lease liabilities 4,045 18,284 Treasury stock issued to 401(k) stock purchase plan 1,749 6,961 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	9 Months Ended
	Sep. 30, 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 September 30, 2021 and December 31, 2020 is presented below. Amortized Gross Gross Estimated Allowance Net September 30, 2021 Residential mortgage-backed securities $ 527,649 $ 24,332 $ — $ 551,981 $ — $ 527,649 States and political subdivisions 1,230,279 41,840 32 1,272,087 (158) 1,230,121 Other 1,500 — — 1,500 — 1,500 Total $ 1,759,428 $ 66,172 $ 32 $ 1,825,568 $ (158) $ 1,759,270 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 $553.1 million and $659.2 million at September 30, 2021 and December 31, 2020, respectively. Accrued interest receivable on held-to-maturity securities totaled $10.8 million and $21.7 million at September 30, 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 $2.8 million ($2.2 million, net of tax) at September 30, 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 September 30, 2021 and December 31, 2020: States and Political Subdivisions Not Guaranteed or Pre-Refunded Guaranteed by the Texas PSF Pre-Refunded Total Other September 30, 2021 Aaa/AAA $ 95,820 $ 466,028 $ 562,320 $ 1,124,168 $ — Aa/AA 106,111 — — 106,111 — A — — — — — Not rated — — — — 1,500 Total $ 201,931 $ 466,028 $ 562,320 $ 1,230,279 $ 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 and nine months ended September 30, 2021 and 2020. Three Months Ended Nine Months Ended 2021 2020 2021 2020 Beginning balance $ 158 $ 172 $ 160 $ — Impact of adopting ASC 326 — — — 215 Credit loss expense (benefit) — (12) (2) (55) Ending balance $ 158 $ 160 $ 158 $ 160 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 September 30, 2021 and December 31, 2020 is presented below. Amortized Gross Gross Allowance Estimated September 30, 2021 U.S. Treasury $ 1,670,692 $ 20,609 $ 6,080 $ — $ 1,685,221 Residential mortgage-backed securities 2,467,915 37,910 14,671 — 2,491,154 States and political subdivisions 7,049,872 471,883 9,266 — 7,512,489 Other 42,358 — — — 42,358 Total $ 11,230,837 $ 530,402 $ 30,017 $ — $ 11,731,222 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 September 30, 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 76.9% 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 $4.9 billion and $4.4 billion at September 30, 2021 and December 31, 2020, respectively. Accrued interest receivable on available-for-sale securities totaled $69.0 million and $111.0 million at September 30, 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 September 30, 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 $ 687,535 $ 6,080 $ — $ — $ 687,535 $ 6,080 Residential mortgage-backed securities 984,236 14,671 — — 984,236 14,671 States and political subdivisions 636,776 9,266 — — 636,776 9,266 Total $ 2,308,547 $ 30,017 $ — $ — $ 2,308,547 $ 30,017 As of September 30, 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 September 30, 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 $ 65 $ — $ 515,356 $ 12,228 $ 527,649 States and political subdivisions 456,778 181,302 88,862 503,337 1,230,279 Other 1,500 — — — 1,500 Total $ 458,343 $ 181,302 $ 604,218 $ 515,565 $ 1,759,428 Estimated Fair Value Residential mortgage-backed securities $ 65 $ — $ 539,142 $ 12,774 $ 551,981 States and political subdivisions 464,067 187,495 90,295 530,230 1,272,087 Other 1,500 — — — 1,500 Total $ 465,632 $ 187,495 $ 629,437 $ 543,004 $ 1,825,568 Available For Sale Amortized Cost U. S. Treasury $ 499,857 $ 533,532 $ 445,697 $ 191,606 $ 1,670,692 Residential mortgage-backed securities 743 16,678 22,146 2,428,348 2,467,915 States and political subdivisions 82,366 1,392,877 863,687 4,710,942 7,049,872 Other — — — — 42,358 Total $ 582,966 $ 1,943,087 $ 1,331,530 $ 7,330,896 $ 11,230,837 Estimated Fair Value U. S. Treasury $ 501,489 $ 548,216 $ 440,328 $ 195,188 $ 1,685,221 Residential mortgage-backed securities 772 17,559 22,743 2,450,080 2,491,154 States and political subdivisions 83,932 1,492,026 919,808 5,016,723 7,512,489 Other — — — — 42,358 Total $ 586,193 $ 2,057,801 $ 1,382,879 $ 7,661,991 $ 11,731,222 Sales of Securities. Sales of securities available for sale were as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Proceeds from sales $ — $ — $ — $ 1,162,352 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 Nine Months Ended 2021 2020 2021 2020 Premium amortization $ (30,240) $ (31,540) $ (92,016) $ (94,873) Discount accretion 657 637 1,999 1,793 Net (premium amortization) discount accretion $ (29,583) $ (30,903) $ (90,017) $ (93,080) Trading Account Securities. Trading account securities, at estimated fair value, were as follows: September 30, December 31, U.S. Treasury $ 24,087 $ 23,996 States and political subdivisions 1,205 460 Total $ 25,292 $ 24,456 Net gains and losses on trading account securities were as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Net gain on sales transactions $ 314 $ 159 $ 716 $ 933 Net mark-to-market gains (losses) (2) 1 (71) 94 Net gain (loss) on trading account securities $ 312 $ 160 $ 645 $ 1,027

Loans

Loans	9 Months Ended
	Sep. 30, 2021
Receivables [Abstract]
Loans	Loans Loans were as follows: September 30, Percentage December 31, Percentage Commercial and industrial $ 4,929,656 31.1 % $ 4,955,341 28.4 % Energy: Production 774,753 4.9 976,473 5.6 Service 107,585 0.7 116,825 0.7 Other 95,873 0.6 141,900 0.8 Total energy 978,211 6.2 1,235,198 7.1 Paycheck Protection Program 827,820 5.2 2,433,849 13.9 Commercial real estate: Commercial mortgages 5,648,541 35.7 5,478,806 31.3 Construction 1,266,633 8.0 1,223,814 7.0 Land 335,849 2.1 317,847 1.8 Total commercial real estate 7,251,023 45.8 7,020,467 40.1 Consumer real estate: Home equity loans 320,597 2.0 329,390 1.9 Home equity lines of credit 478,978 3.0 452,854 2.6 Other 563,464 3.6 548,530 3.1 Total consumer real estate 1,363,039 8.6 1,330,774 7.6 Total real estate 8,614,062 54.4 8,351,241 47.7 Consumer and other 482,921 3.1 505,680 2.9 Total loans $ 15,832,670 100.0 % $ 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 September 30, 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 6.5% excluding PPP loans. Unfunded commitments to extend credit and standby letters of credit issued to customers in the energy industry totaled $1.0 billion and $63.7 million, respectively, as of September 30, 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 September 30, 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 $318.8 million at September 30, 2021 and $353.1 million at December 31, 2020. Accrued Interest Receivable. Accrued interest receivable on loans totaled $39.0 million and $48.7 million at September 30, 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 have been unable to meet their contractual payment obligations because of the adverse effects of COVID-19. To help mitigate these effects, loan customers were able to apply for a deferral of payments, or portions thereof, for up to 90 days. After 90 days, customers were able to apply for an additional deferral, and a small proportion of our customers requested such an additional deferral. In the absence of other intervening factors, such short-term modifications made on a good faith basis have not been categorized as troubled debt restructurings, nor have loans granted payment deferrals related to COVID-19 been 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). As of September 30, 2021, the amount of loans remaining in COVID-19 related deferment was not significant. 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: September 30, 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 $ 19,349 $ 11,611 $ 19,849 $ 4,479 Energy 23,347 7,224 23,168 639 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 12,247 8,770 15,737 14,116 Construction 1,632 664 1,684 1,684 Consumer real estate 480 178 993 993 Consumer and other — — 18 — Total $ 57,055 $ 28,447 $ 61,449 $ 21,911 The following table presents non-accrual loans as of September 30, 2021 by class and year of origination. 2021 2020 2019 2018 2017 Prior Revolving Loans Revolving Loans Converted to Term Total Commercial and industrial $ 315 $ 7,922 $ 4,211 $ 676 $ 882 $ 370 $ 2,067 $ 2,906 $ 19,349 Energy — 477 6,467 1,513 — 323 12,170 2,397 23,347 Paycheck Protection Program — — — — — — — — — Commercial real estate: Buildings, land and other 2,096 1,723 2,811 824 2,051 2,742 — — 12,247 Construction — 1,632 — — — — — — 1,632 Consumer real estate — — — — — 443 — 37 480 Consumer and other — — — — — — — — — Total $ 2,411 $ 11,754 $ 13,489 $ 3,013 $ 2,933 $ 3,878 $ 14,237 $ 5,340 $ 57,055 Had non-accrual loans performed in accordance with their original contract terms, we would have recognized additional interest income, net of tax, of approximately $474 thousand and $1.4 million for the three and nine months ended September 30, 2021, and approximately $670 thousand and $2.3 million for the three and nine months ended September 30, 2020. An age analysis of past due loans (including both accruing and non-accruing loans), segregated by class of loans, as of September 30, 2021 was as follows: Loans Loans Total Current Total Accruing Commercial and industrial $ 42,705 $ 8,634 $ 51,339 $ 4,878,317 $ 4,929,656 $ 7,522 Energy 1,237 13,233 14,470 963,741 978,211 965 Paycheck Protection Program — — — 827,820 827,820 — Commercial real estate: Buildings, land and other 20,910 19,190 40,100 5,944,290 5,984,390 12,224 Construction — — — 1,266,633 1,266,633 — Consumer real estate 4,148 2,101 6,249 1,356,790 1,363,039 1,985 Consumer and other 3,678 1,029 4,707 478,214 482,921 1,029 Total $ 72,678 $ 44,187 $ 116,865 $ 15,715,805 $ 15,832,670 $ 23,725 Troubled Debt Restructurings . Troubled debt restructurings during the nine months ended September 30, 2021 and 2020 are set forth in the following table. Nine Months Ended Nine Months Ended Balance at Balance at Balance at Balance at Commercial and industrial $ 1,312 $ 1,237 $ 3,660 $ 1,415 Energy 3,817 3,566 — — Commercial real estate: Buildings, land and other 1,888 1,875 6,606 6,585 Construction — — 1,192 1,181 Consumer and other — — 1,104 104 $ 7,017 $ 6,678 $ 12,562 $ 9,285 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 nine months ended September 30, 2021 and 2020 is set forth in the following table. September 30, 2021 September 30, 2020 Restructured loans past due in excess of 90 days at period-end: Number of loans 1 2 Dollar amount of loans $ 306 $ 3,682 Restructured loans on non-accrual status at period end 5,072 5,353 Charge-offs of restructured loans: Recognized in connection with restructuring — — Recognized on previously restructured loans 1,433 2,188 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 September 30, 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 $ 1,009,752 $ 730,976 $ 405,216 $ 201,578 $ 160,431 $ 149,611 $ 1,875,496 $ 65,603 $ 4,598,663 6.04 Risk grade 9 9,569 35,274 30,953 26,216 12,258 18,329 70,412 5,272 208,283 9.00 Risk grade 10 28,117 6,546 1,871 4,653 1,358 112 12,954 1,652 57,263 10.00 Risk grade 11 1,326 8,603 9,644 4,160 3,098 279 7,189 11,799 46,098 11.00 Risk grade 12 135 7,922 3,263 676 411 370 962 1,974 15,713 12.00 Risk grade 13 180 — 948 — 471 — 1,105 932 3,636 13.00 $ 1,049,079 $ 789,321 $ 451,895 $ 237,283 $ 178,027 $ 168,701 $ 1,968,118 $ 87,232 $ 4,929,656 6.29 W/A risk grade 5.97 6.35 6.91 7.04 6.20 5.94 6.17 7.60 6.29 Energy Risk grades 1-8 $ 350,757 $ 10,562 $ 10,339 $ 7,060 $ 3,958 $ 4,518 $ 392,527 $ 68,437 $ 848,158 5.94 Risk grade 9 32,434 712 1,831 270 — 737 10,136 2,574 48,694 9.00 Risk grade 10 — 115 703 668 — — 860 478 2,824 10.00 Risk grade 11 11,517 1,168 17,711 1,607 — 573 19,133 3,479 55,188 11.00 Risk grade 12 — 34 4,777 90 — 323 4,641 99 9,964 12.00 Risk grade 13 — 443 1,690 1,423 — — 7,529 2,298 13,383 13.00 $ 394,708 $ 13,034 $ 37,051 $ 11,118 $ 3,958 $ 6,151 $ 434,826 $ 77,365 $ 978,211 6.55 W/A risk grade 6.37 8.13 9.98 8.77 7.19 7.84 6.26 6.76 6.55 Commercial real estate: Buildings, land, other Risk grades 1-8 $ 1,153,181 $ 1,146,764 $ 862,629 $ 578,768 $ 517,822 $ 842,409 $ 44,525 $ 75,360 $ 5,221,458 6.90 Risk grade 9 13,873 143,504 70,218 48,372 29,691 34,746 4,669 6,858 351,931 9.00 Risk grade 10 14,376 16,956 69,882 51,903 65,363 55,732 — — 274,212 10.00 Risk grade 11 3,995 19,766 11,351 7,615 32,698 40,245 3,000 5,872 124,542 11.00 Risk grade 12 1,896 1,723 2,511 824 2,051 2,492 — — 11,497 12.00 Risk grade 13 200 — 300 — — 250 — — 750 13.00 $ 1,187,521 $ 1,328,713 $ 1,016,891 $ 687,482 $ 647,625 $ 975,874 $ 52,194 $ 88,090 $ 5,984,390 7.27 W/A risk grade 7.21 7.27 7.40 7.37 7.38 7.04 7.30 7.15 7.27 Construction Risk grades 1-8 $ 429,990 $ 284,149 $ 299,927 $ 20,457 $ — $ 2,593 $ 144,950 $ — $ 1,182,066 7.03 Risk grade 9 37,113 32,002 — — 513 — 13,307 — 82,935 9.00 Risk grade 10 — — — — — — — — — 10.00 Risk grade 11 — — — — — — — — — 11.00 Risk grade 12 — 1,432 — — — — — — 1,432 12.00 Risk grade 13 — 200 — — — — — — 200 13.00 $ 467,103 $ 317,783 $ 299,927 $ 20,457 $ 513 $ 2,593 $ 158,257 $ — $ 1,266,633 7.16 W/A risk grade 7.13 6.87 7.46 7.45 9.00 6.79 7.24 — 7.16 Total commercial real estate $ 1,654,624 $ 1,646,496 $ 1,316,818 $ 707,939 $ 648,138 $ 978,467 $ 210,451 $ 88,090 $ 7,251,023 7.25 W/A risk grade 7.19 7.19 7.41 7.38 7.38 7.04 7.26 7.15 7.25 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 September 30, 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 $ 396 $ 34 $ 277 $ 370 $ 326 $ 1,187 $ 174 $ 1,384 $ 4,148 Past due 90 or more days — — 110 11 226 645 967 142 2,101 Total past due 396 34 387 381 552 1,832 1,141 1,526 6,249 Current loans 235,725 283,853 111,449 62,416 54,827 132,108 464,440 11,972 1,356,790 Total $ 236,121 $ 283,887 $ 111,836 $ 62,797 $ 55,379 $ 133,940 $ 465,581 $ 13,498 $ 1,363,039 Consumer and other: Past due 30-89 days $ 1,527 $ 283 $ 182 $ 89 $ 63 $ 18 $ 481 $ 1,035 $ 3,678 Past due 90 or more days 576 10 — 90 — — — 353 1,029 Total past due 2,103 293 182 179 63 18 481 1,388 4,707 Current loans 39,156 20,924 10,442 3,268 1,844 1,391 374,993 26,196 478,214 Total $ 41,259 $ 21,217 $ 10,624 $ 3,447 $ 1,907 $ 1,409 $ 375,474 $ 27,584 $ 482,921 Revolving loans that converted to term during the three and nine months ended September 30, 2021 and 2020 were as follows: Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Commercial and industrial $ 12,009 $ 10,224 $ 39,551 $ 25,340 Energy 52,055 7,144 57,871 38,642 Commercial real estate: Buildings, land and other 6,012 637 37,093 8,094 Construction — — — — Consumer real estate 330 421 1,172 2,132 Consumer and other 1,523 5,494 7,017 15,338 Total $ 71,929 $ 23,920 $ 142,704 $ 89,546 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 130.4 at August 31, 2021 (most recent date available) and 118.1 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 September 30, 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. September 30, 2021 Commercial Energy Commercial Consumer Consumer Total Modeled expected credit losses $ 45,395 $ 6,962 $ 17,033 $ 5,934 $ 6,412 $ 81,736 Q-Factor and other qualitative adjustments 14,081 104 134,071 59 1,440 149,755 Specific allocations 4,290 13,383 950 36 — 18,659 Total $ 63,766 $ 20,449 $ 152,054 $ 6,029 $ 7,852 $ 250,150 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 and nine months ended September 30, 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: September 30, 2021 Beginning balance $ 65,271 $ 28,010 $ 147,993 $ 6,154 $ 7,860 $ 255,288 Credit loss expense (benefit) (1,020) (7,720) 4,007 (982) 2,692 (3,023) Charge-offs (1,611) — — — (5,073) (6,684) Recoveries 1,126 159 54 857 2,373 4,569 Net charge-offs (485) 159 54 857 (2,700) (2,115) Ending balance $ 63,766 $ 20,449 $ 152,054 $ 6,029 $ 7,852 $ 250,150 September 30, 2020 Beginning balance $ 98,536 $ 40,817 $ 93,425 $ 8,998 $ 8,285 $ 250,061 Credit loss expense (benefit) (18,547) 13,814 25,368 1,794 1,161 23,590 Charge-offs (8,605) — (242) (1,088) (4,219) (14,154) Recoveries 1,121 — 42 573 2,242 3,978 Net charge-offs (7,484) — (200) (515) (1,977) (10,176) Ending balance $ 72,505 $ 54,631 $ 118,593 $ 10,277 $ 7,469 $ 263,475 Commercial Energy Commercial Consumer Consumer Total Nine months ended: September 30, 2021 Beginning balance $ 73,843 $ 39,553 $ 134,892 $ 7,926 $ 6,963 $ 263,177 Credit loss expense (benefit) (8,886) (19,048) 16,583 (3,093) 7,042 (7,402) Charge-offs (4,485) (1,433) (137) (672) (13,015) (19,742) Recoveries 3,294 1,377 716 1,868 6,862 14,117 Net (charge-offs) recoveries (1,191) (56) 579 1,196 (6,153) (5,625) Ending balance $ 63,766 $ 20,449 $ 152,054 $ 6,029 $ 7,852 $ 250,150 September 30, 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 (benefit) 10,737 96,478 104,716 3,716 8,096 223,743 Charge-offs (14,815) (68,842) (3,826) (1,508) (13,402) (102,393) Recoveries 3,727 66 185 1,564 6,981 12,523 Net (charge-offs) recoveries (11,088) (68,776) (3,641) 56 (6,421) (89,870) Ending balance $ 72,505 $ 54,631 $ 118,593 $ 10,277 $ 7,469 $ 263,475 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 September 30, 2021 and December 31, 2020. September 30, 2021 December 31, 2020 Loan Specific Allocations Loan Specific Allocations Commercial and industrial $ 22,717 $ 4,290 $ 21,287 $ 5,321 Energy 25,671 13,383 22,888 9,427 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 28,619 750 34,057 513 Construction 1,632 200 1,684 — Consumer real estate 302 36 561 — Consumer and other — — 18 18 Total $ 78,941 $ 18,659 $ 80,495 $ 15,279

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	9 Months Ended
	Sep. 30, 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. September 30, December 31, Goodwill $ 654,952 $ 654,952 Other intangible assets: Core deposits $ 845 $ 1,296 Customer relationships 174 267 $ 1,019 $ 1,563 The estimated aggregate future amortization expense for intangible assets remaining as of September 30, 2021 is as follows: Remainder of 2021 $ 153 2022 481 2023 282 2024 87 2025 11 Thereafter 5 $ 1,019

Deposits

Deposits	9 Months Ended
	Sep. 30, 2021
Disclosure Text Block [Abstract]
Deposits	Deposits Deposits were as follows: September 30, Percentage December 31, Percentage Non-interest-bearing demand deposits: Commercial and individual $ 15,997,472 40.4 % $ 13,914,754 39.7 % Correspondent banks 223,466 0.6 242,225 0.7 Public funds 930,645 2.3 960,072 2.8 Total non-interest-bearing demand deposits 17,151,583 43.3 15,117,051 43.2 Interest-bearing deposits: Private accounts: Savings and interest checking 10,326,315 26.1 9,132,789 26.1 Money market accounts 10,381,703 26.2 8,977,585 25.6 Time accounts 1,086,894 2.7 1,135,575 3.3 Total private accounts 21,794,912 55.0 19,245,949 55.0 Public funds: Savings and interest checking 580,174 1.5 597,503 1.7 Money market accounts 61,028 0.1 50,070 0.1 Time accounts 25,070 0.1 5,188 — Total public funds 666,272 1.7 652,761 1.8 Total interest-bearing deposits 22,461,184 56.7 19,898,710 56.8 Total deposits $ 39,612,767 100.0 % $ 35,015,761 100.0 % The following table presents additional information about our deposits: September 30, December 31, Deposits from the Certificate of Deposit Account Registry Service (CDARS) deposits $ — $ 372 Deposits from foreign sources (primarily Mexico) 1,007,416 884,169 Total deposits not covered by deposit insurance 21,419,285 18,694,320 Time deposits not covered by deposit insurance 239,598 237,298

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 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: September 30, December 31, Commitments to extend credit $ 10,262,077 $ 9,814,475 Standby letters of credit 225,311 241,345 Deferred standby letter of credit fees 1,935 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 Nine Months Ended 2021 2020 2021 2020 Beginning balance $ 48,596 $ 46,939 $ 44,152 $ 500 Impact of adopting ASC 326 — — — 39,377 Credit loss expense 3,023 (3,276) 7,467 3,786 Ending balance $ 51,619 $ 43,663 $ 51,619 $ 43,663 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 Nine Months Ended 2021 2020 2021 2020 Amortization of lease right-of-use assets $ 8,172 $ 8,521 $ 24,584 $ 24,545 Short-term lease expense 384 172 1,118 1,343 Non-lease components (including taxes, insurance, common maintenance, etc.) 2,864 2,586 8,205 8,410 Total $ 11,420 $ 11,279 $ 33,907 $ 34,298 Right-of-use lease assets totaled $278.6 million at September 30, 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 $310.3 million at September 30, 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 $7.9 million and $24.0 million during the three and nine months ended September 30, 2021 and $8.0 million and $23.8 million during the three and nine months ended September 30, 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	9 Months Ended
	Sep. 30, 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 $62.3 million and $63.7 million at September 30, 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 September 30, 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 September 30, 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 September 30, 2021 and December 31, 2020. All $13.0 million of the trust preferred securities issued by WNB Capital Trust I were redeemed in October 2021. The following table presents actual and required capital ratios as of September 30, 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 September 30, 2021 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 3,300,979 13.42 % $ 1,722,459 7.00 % $ 1,599,426 6.50 % Frost Bank 3,215,116 13.09 1,719,451 7.00 1,596,633 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 3,446,431 14.01 2,091,558 8.50 1,968,525 8.00 Frost Bank 3,215,116 13.09 2,087,905 8.50 1,965,087 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,911,272 15.90 2,583,689 10.50 2,460,656 10.00 Frost Bank 3,446,957 14.03 2,579,177 10.50 2,456,359 10.00 Leverage Ratio Cullen/Frost 3,446,431 7.52 1,833,156 4.00 2,291,445 5.00 Frost Bank 3,215,116 7.02 1,832,498 4.00 2,290,623 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 September 30, 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 September 30, 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 September 30, 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 September 30, 2021, Frost Bank could pay aggregate dividends of up to $445.8 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, 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	9 Months Ended
	Sep. 30, 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 methods 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 September 30, 2021 and December 31, 2020. September 30, 2021 December 31, 2020 Notional Estimated Notional Estimated Derivatives designated as hedges of fair value: Financial institution counterparties: Loan/lease interest rate swaps – liabilities 2,758 $ (59) 3,724 $ (134) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets 127,420 641 — — Loan/lease interest rate swaps – liabilities 997,411 (22,874) 1,173,173 (33,812) Loan/lease interest rate caps – assets 405,163 2,915 356,601 1,241 Customer counterparties: Loan/lease interest rate swaps – assets 997,411 49,630 1,173,173 84,424 Loan/lease interest rate swaps – liabilities 127,420 (1,761) — — Loan/lease interest rate caps – liabilities 405,163 (2,915) 356,601 (1,241) The weighted-average rates paid and received for interest rate swaps outstanding at September 30, 2021 were as follows: Weighted-Average Interest Interest Interest rate swaps: Fair value hedge loan/lease interest rate swaps 2.75 % 0.09 % Non-hedging interest rate swaps – financial institution counterparties 3.78 1.87 Non-hedging interest rate swaps – customer counterparties 1.87 3.78 The weighted-average strike rate for outstanding interest rate caps was 3.12% at September 30, 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 methods with observable market data inputs to value our commodity derivative positions. September 30, 2021 December 31, 2020 Notional Notional Estimated Notional Estimated Financial institution counterparties: Oil – assets Barrels 5,632 $ 21,271 3,056 $ 8,341 Oil – liabilities Barrels 12,228 (147,576) 6,391 (32,112) Natural gas – assets MMBTUs 18,908 3,312 9,281 1,529 Natural gas – liabilities MMBTUs 39,901 (53,923) 15,079 (3,265) Customer counterparties: Oil – assets Barrels 12,276 148,742 6,391 32,670 Oil – liabilities Barrels 5,584 (21,245) 3,056 (8,264) Natural gas – assets MMBTUs 39,901 54,202 17,636 3,451 Natural gas – liabilities MMBTUs 18,908 (3,310) 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: September 30, 2021 December 31, 2020 Notional Notional Estimated Notional Estimated Financial institution counterparties: Forward contracts – assets CAD 658 $ 2 — — Customer counterparties: Forward contracts – assets CAD 658 2 — — 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 Nine Months Ended 2021 2020 2021 2020 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ (21) $ (36) $ (74) $ (79) Amount of (gain) loss included in other non-interest expense 3 2 8 6 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 Nine Months Ended 2021 2020 2021 2020 Non-hedging interest rate derivatives: Other non-interest income $ 946 $ 280 $ 2,674 $ 2,615 Other non-interest expense — 1 (1) 1 Non-hedging commodity derivatives: Other non-interest income 1,143 379 3,266 1,057 Non-hedging foreign currency derivatives: Other non-interest income 4 10 34 28 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 $227.4 million at September 30, 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 $12.5 million at September 30, 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 September 30, 2021, we had $200.9 million in cash collateral related to derivative contracts on deposit with other financial institution counterparties.

Balance Sheet Offsetting

Balance Sheet Offsetting	9 Months Ended
	Sep. 30, 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 September 30, 2021 is presented in the following tables. Gross Amount Gross Amount Net Amount September 30, 2021 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 3,556 $ — $ 3,556 Commodity swaps and options 24,583 — 24,583 Foreign currency forward contracts 2 — 2 Total derivatives 28,141 — 28,141 Resell agreements 7,903 — 7,903 Total $ 36,044 $ — $ 36,044 Financial liabilities: Derivatives: Loan/lease interest rate swaps and caps $ 22,933 $ — $ 22,933 Commodity swaps and options 201,499 — 201,499 Total derivatives 224,432 — 224,432 Repurchase agreements 2,200,029 — 2,200,029 Total $ 2,424,461 $ — $ 2,424,461 Gross Amounts Not Offset Net Amount Financial Collateral Net September 30, 2021 Financial assets: Derivatives: Counterparty A $ 4 $ (4) $ — $ — Counterparty B 12,537 (12,537) — — Other counterparties 15,600 (15,600) — — Total derivatives 28,141 (28,141) — — Resell agreements 7,903 — (7,903) — Total $ 36,044 $ (28,141) $ (7,903) $ — Financial liabilities: Derivatives: Counterparty A $ 4,577 $ (4) $ (4,573) $ — Counterparty B 77,110 (12,537) (60,860) 3,713 Counterparty C 19 — (19) — Other counterparties 142,726 (15,600) (122,984) 4,142 Total derivatives 224,432 (28,141) (188,436) 7,855 Repurchase agreements 2,200,029 — (2,200,029) — Total $ 2,424,461 $ (28,141) $ (2,388,465) $ 7,855 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 and caps $ 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 September 30, 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 September 30, 2021 Repurchase agreements: U.S. Treasury $ 1,122,968 $ — $ — $ — $ 1,122,968 Residential mortgage-backed securities 1,077,061 — — — 1,077,061 Total borrowings $ 2,200,029 $ — $ — $ — $ 2,200,029 Gross amount of recognized liabilities for repurchase agreements $ 2,200,029 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	9 Months Ended
	Sep. 30, 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 September 30, 2021, there were 917,440 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 5,940 117.90 — — — — — — Exercised/vested (2,499) 92.03 (1,213) 98.90 (35,131) 92.27 (611,201) 61.86 Forfeited/expired — — (26,439) 87.08 (9,752) 75.70 — — Balance, September 30, 2021 56,301 $ 79.21 442,707 $ 86.16 156,374 $ 73.88 1,128,358 $ 68.41 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 Nine Months Ended 2021 2020 2021 2020 New shares issued from available authorized shares — — — — Shares issued from available treasury stock 21,968 53,425 650,044 196,948 Total 21,968 53,425 650,044 196,948 Proceeds from stock option exercises $ 1,128 $ 2,698 $ 37,810 $ 7,544 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 Nine Months Ended 2021 2020 2021 2020 Non-vested stock awards/stock units $ 1,559 $ 1,999 $ 5,210 $ 6,325 Director deferred stock units — — 700 770 Performance stock units 426 444 1,846 1,132 Total $ 1,985 $ 2,443 $ 7,756 $ 8,227 Income tax benefit $ 281 $ 450 $ 1,162 $ 1,539 Unrecognized stock-based compensation expense at September 30, 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 $ 10,918 Performance stock units 4,450 Total $ 15,368

Earnings Per Common Share

Earnings Per Common Share	9 Months Ended
	Sep. 30, 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 Nine Months Ended 2021 2020 2021 2020 Net income $ 107,975 $ 95,056 $ 342,058 $ 242,881 Less: Preferred stock dividends 1,668 — 5,488 2,016 Redemption of preferred stock — — — 5,514 Net income available to common shareholders 106,307 95,056 336,570 235,351 Less: Earnings allocated to participating securities 932 877 3,228 2,236 Net earnings allocated to common stock $ 105,375 $ 94,179 $ 333,342 $ 233,115 Distributed earnings allocated to common stock $ 47,740 $ 44,546 $ 139,224 $ 133,511 Undistributed earnings allocated to common stock 57,635 49,633 194,118 99,604 Net earnings allocated to common stock $ 105,375 $ 94,179 $ 333,342 $ 233,115 Weighted-average shares outstanding for basic earnings per common share 63,652,020 62,726,542 63,522,884 62,655,393 Dilutive effect of stock compensation 444,636 193,116 489,219 263,028 Weighted-average shares outstanding for diluted earnings per common share 64,096,656 62,919,658 64,012,103 62,918,421

Defined Benefit Plans

Defined Benefit Plans	9 Months Ended
	Sep. 30, 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 Nine Months Ended 2021 2020 2021 2020 Expected return on plan assets, net of expenses $ (3,210) $ (3,073) $ (9,630) $ (9,217) Interest cost on projected benefit obligation 835 1,252 2,506 3,757 Net amortization and deferral 1,529 1,330 4,587 3,989 Net periodic expense (benefit) $ (846) $ (491) $ (2,537) $ (1,471) Our non-qualified defined benefit pension plan is not funded. No contributions to the qualified defined benefit pension plan were made during the nine months ended September 30, 2021. We do not expect to make any contributions to the qualified defined benefit plan during the remainder of 2021.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Income tax expense was as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Current income tax expense (benefit) $ 13,849 $ (33,614) $ 31,639 $ 20,328 Deferred income tax expense (benefit) (516) 43,130 4,672 (8,803) Income tax expense, as reported $ 13,333 $ 9,516 $ 36,311 $ 11,525 Effective tax rate 11.0 % 9.1 % 9.6 % 4.5 % We had a net deferred tax liability totaling $78.9 million at September 30, 2021 and $117.5 million at December 31, 2020. No valuation allowance for deferred tax assets was recorded at September 30, 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. The effective tax rates during 2020 were also impacted by a one-time, discrete tax benefit associated with an asset contribution to a charitable trust during the second quarter. 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)	9 Months Ended
	Sep. 30, 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 $ (80,668) $ (16,941) $ (63,727) $ 20,332 $ 4,269 $ 16,063 Change in net unrealized gain on securities transferred to held to maturity (237) (49) (188) (294) (62) (232) Reclassification adjustment for net (gains) losses included in net income — — — — — — Total securities available for sale and transferred securities (80,905) (16,990) (63,915) 20,038 4,207 15,831 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 1,330 280 1,050 Total defined-benefit post-retirement benefit plans 1,529 321 1,208 1,330 280 1,050 Total other comprehensive income (loss) $ (79,376) $ (16,669) $ (62,707) $ 21,368 $ 4,487 $ 16,881 Nine Months Ended Nine 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 $ (209,779) $ (44,054) $ (165,725) $ 406,959 $ 85,462 $ 321,497 Change in net unrealized gain on securities transferred to held to maturity (741) (155) (586) (979) (206) (773) Reclassification adjustment for net (gains) losses included in net income — — — (108,989) (22,888) (86,101) Total securities available for sale and transferred securities (210,520) (44,209) (166,311) 296,991 62,368 234,623 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) 4,587 963 3,624 3,989 837 3,152 Total defined-benefit post-retirement benefit plans 4,587 963 3,624 3,989 837 3,152 Total other comprehensive income (loss) $ (205,933) $ (43,246) $ (162,687) $ 300,980 $ 63,205 $ 237,775 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 (166,311) — (166,311) Reclassification of amounts included in net income — 3,624 3,624 Net other comprehensive income (loss) during period (166,311) 3,624 (162,687) Balance at September 30, 2021 $ 397,490 $ (47,207) $ 350,283 Balance January 1, 2020 $ 313,304 $ (45,934) $ 267,370 Other comprehensive income (loss) before reclassifications 320,724 — 320,724 Reclassification of amounts included in net income (86,101) 3,152 (82,949) Net other comprehensive income (loss) during period 234,623 3,152 237,775 Balance at September 30, 2020 $ 547,927 $ (42,782) $ 505,145

Operating Segments

Operating Segments	9 Months Ended
	Sep. 30, 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: September 30, 2021 Net interest income (expense) $ 247,381 $ 536 $ (1,795) $ 246,122 Credit loss expense (6) 6 — — Non-interest income 51,279 42,369 (459) 93,189 Non-interest expense 185,891 31,086 1,026 218,003 Income (loss) before income taxes 112,775 11,813 (3,280) 121,308 Income tax expense (benefit) 12,029 2,481 (1,177) 13,333 Net income (loss) 100,746 9,332 (2,103) 107,975 Preferred stock dividends — — 1,668 1,668 Net income (loss) available to common shareholders $ 100,746 $ 9,332 $ (3,771) $ 106,307 Revenues from (expenses to) external customers $ 298,660 $ 42,905 $ (2,254) $ 339,311 September 30, 2020 Net interest income (expense) $ 244,685 $ 602 $ (1,864) $ 243,423 Credit loss expense (benefit) 20,300 2 — 20,302 Non-interest income 49,170 34,680 (249) 83,601 Non-interest expense 170,290 30,052 1,808 202,150 Income (loss) before income taxes 103,265 5,228 (3,921) 104,572 Income tax expense (benefit) 9,758 1,098 (1,340) 9,516 Net income (loss) available to common shareholders $ 93,507 $ 4,130 $ (2,581) $ 95,056 Revenues from (expenses to) external customers $ 293,855 $ 35,282 $ (2,113) $ 327,024 Nine months ended: September 30, 2021 Net interest income (expense) $ 748,036 $ 1,530 $ (5,407) $ 744,159 Credit loss expense 57 6 — 63 Non-interest income 154,003 124,512 (843) 277,672 Non-interest expense 548,151 91,037 4,211 643,399 Income (loss) before income taxes 353,831 34,999 (10,461) 378,369 Income tax expense (benefit) 32,811 7,350 (3,850) 36,311 Net income (loss) 321,020 27,649 (6,611) 342,058 Preferred stock dividends — — 5,488 5,488 Net income (loss) available to common shareholders $ 321,020 $ 27,649 $ (12,099) $ 336,570 Revenues from (expenses to) external customers $ 902,039 $ 126,042 $ (6,250) $ 1,021,831 September 30, 2020 Net interest income (expense) $ 737,977 $ 2,163 $ (6,385) $ 733,755 Credit loss expense 227,474 — — 227,474 Non-interest income 265,492 109,330 (705) 374,117 Non-interest expense 525,882 94,225 5,885 625,992 Income (loss) before income taxes 250,113 17,268 (12,975) 254,406 Income tax expense (benefit) 12,090 3,626 (4,191) 11,525 Net income (loss) 238,023 13,642 (8,784) 242,881 Preferred stock dividends — — 2,016 2,016 Redemption of preferred stock — — 5,514 5,514 Net income (loss) available to common shareholders $ 238,023 $ 13,642 $ (16,314) $ 235,351 Revenues from (expenses to) external customers $ 1,003,469 $ 111,493 $ (7,090) $ 1,107,872

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 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 September 30, 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 September 30, 2021 Securities available for sale: U.S. Treasury $ 1,685,221 $ — $ — $ 1,685,221 Residential mortgage-backed securities — 2,491,154 — 2,491,154 States and political subdivisions — 7,512,489 — 7,512,489 Other — 42,358 — 42,358 Trading account securities: U.S. Treasury 24,087 — — 24,087 States and political subdivisions — 1,205 — 1,205 Derivative assets: Interest rate swaps, caps and floors — 53,186 — 53,186 Commodity swaps and options — 227,527 — 227,527 Foreign currency forward contracts 4 — — 4 Derivative liabilities: Interest rate swaps, caps and floors — 27,609 — 27,609 Commodity swaps and options — 226,054 — 226,054 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 Derivative liabilities: Interest rate swaps, caps and floors — 35,187 — 35,187 Commodity swaps and options — 45,099 — 45,099 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. Nine Months Ended Nine Months Ended Level 2 Level 3 Level 2 Level 3 Carrying value before allocations $ 3,329 $ 15,274 $ 6,388 $ 8,660 Specific (allocations) reversals of prior allocations (336) (4,791) (930) 14,928 Fair value $ 2,993 $ 10,483 $ 5,458 $ 23,588 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: Nine 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: September 30, 2021 December 31, 2020 Carrying Estimated Carrying Estimated Financial assets: Level 2 inputs: Cash and cash equivalents $ 16,253,971 $ 16,253,971 $ 10,288,853 $ 10,288,853 Securities held to maturity 1,759,270 1,825,568 1,945,673 2,052,896 Cash surrender value of life insurance policies 189,941 189,941 189,984 189,984 Accrued interest receivable 119,232 119,232 181,432 181,432 Level 3 inputs: Loans, net 15,582,520 15,656,269 17,218,132 17,390,683 Financial liabilities: Level 2 inputs: Deposits 39,612,767 39,613,343 35,015,761 35,018,185 Federal funds purchased 27,200 27,200 48,850 48,850 Repurchase agreements 2,200,029 2,200,029 2,068,147 2,068,147 Junior subordinated deferrable interest debentures 136,400 137,115 136,357 137,115 Subordinated notes 99,139 113,223 99,021 115,717 Accrued interest payable 2,126 2,126 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	9 Months Ended
	Sep. 30, 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)	9 Months Ended
	Sep. 30, 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)	9 Months Ended
	Sep. 30, 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)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Additional Cash Flow Information	Additional cash flow information was as follows: Nine Months Ended 2021 2020 Cash paid for interest $ 23,722 $ 41,634 Cash paid for income taxes 28,043 44,140 Significant non-cash transactions: Unsettled securities transactions 600,874 6,698 Loans foreclosed and transferred to other real estate owned and foreclosed assets 3,464 140 Right-of-use lease assets obtained in exchange for lessee operating lease liabilities 4,045 18,284 Treasury stock issued to 401(k) stock purchase plan 1,749 6,961

Securities (Tables)

Securities (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2021 and December 31, 2020 is presented below. Amortized Gross Gross Estimated Allowance Net September 30, 2021 Residential mortgage-backed securities $ 527,649 $ 24,332 $ — $ 551,981 $ — $ 527,649 States and political subdivisions 1,230,279 41,840 32 1,272,087 (158) 1,230,121 Other 1,500 — — 1,500 — 1,500 Total $ 1,759,428 $ 66,172 $ 32 $ 1,825,568 $ (158) $ 1,759,270 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 September 30, 2021 and December 31, 2020: States and Political Subdivisions Not Guaranteed or Pre-Refunded Guaranteed by the Texas PSF Pre-Refunded Total Other September 30, 2021 Aaa/AAA $ 95,820 $ 466,028 $ 562,320 $ 1,124,168 $ — Aa/AA 106,111 — — 106,111 — A — — — — — Not rated — — — — 1,500 Total $ 201,931 $ 466,028 $ 562,320 $ 1,230,279 $ 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 and nine months ended September 30, 2021 and 2020. Three Months Ended Nine Months Ended 2021 2020 2021 2020 Beginning balance $ 158 $ 172 $ 160 $ — Impact of adopting ASC 326 — — — 215 Credit loss expense (benefit) — (12) (2) (55) Ending balance $ 158 $ 160 $ 158 $ 160
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 September 30, 2021 and December 31, 2020 is presented below. Amortized Gross Gross Allowance Estimated September 30, 2021 U.S. Treasury $ 1,670,692 $ 20,609 $ 6,080 $ — $ 1,685,221 Residential mortgage-backed securities 2,467,915 37,910 14,671 — 2,491,154 States and political subdivisions 7,049,872 471,883 9,266 — 7,512,489 Other 42,358 — — — 42,358 Total $ 11,230,837 $ 530,402 $ 30,017 $ — $ 11,731,222 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 September 30, 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 $ 687,535 $ 6,080 $ — $ — $ 687,535 $ 6,080 Residential mortgage-backed securities 984,236 14,671 — — 984,236 14,671 States and political subdivisions 636,776 9,266 — — 636,776 9,266 Total $ 2,308,547 $ 30,017 $ — $ — $ 2,308,547 $ 30,017
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 September 30, 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 $ 65 $ — $ 515,356 $ 12,228 $ 527,649 States and political subdivisions 456,778 181,302 88,862 503,337 1,230,279 Other 1,500 — — — 1,500 Total $ 458,343 $ 181,302 $ 604,218 $ 515,565 $ 1,759,428 Estimated Fair Value Residential mortgage-backed securities $ 65 $ — $ 539,142 $ 12,774 $ 551,981 States and political subdivisions 464,067 187,495 90,295 530,230 1,272,087 Other 1,500 — — — 1,500 Total $ 465,632 $ 187,495 $ 629,437 $ 543,004 $ 1,825,568 Available For Sale Amortized Cost U. S. Treasury $ 499,857 $ 533,532 $ 445,697 $ 191,606 $ 1,670,692 Residential mortgage-backed securities 743 16,678 22,146 2,428,348 2,467,915 States and political subdivisions 82,366 1,392,877 863,687 4,710,942 7,049,872 Other — — — — 42,358 Total $ 582,966 $ 1,943,087 $ 1,331,530 $ 7,330,896 $ 11,230,837 Estimated Fair Value U. S. Treasury $ 501,489 $ 548,216 $ 440,328 $ 195,188 $ 1,685,221 Residential mortgage-backed securities 772 17,559 22,743 2,450,080 2,491,154 States and political subdivisions 83,932 1,492,026 919,808 5,016,723 7,512,489 Other — — — — 42,358 Total $ 586,193 $ 2,057,801 $ 1,382,879 $ 7,661,991 $ 11,731,222
Schedule of Realized Gain (Loss)	Sales of securities available for sale were as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Proceeds from sales $ — $ — $ — $ 1,162,352 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 Nine Months Ended 2021 2020 2021 2020 Premium amortization $ (30,240) $ (31,540) $ (92,016) $ (94,873) Discount accretion 657 637 1,999 1,793 Net (premium amortization) discount accretion $ (29,583) $ (30,903) $ (90,017) $ (93,080)
Trading Account Securities, at Estimated Fair Value	Trading account securities, at estimated fair value, were as follows: September 30, December 31, U.S. Treasury $ 24,087 $ 23,996 States and political subdivisions 1,205 460 Total $ 25,292 $ 24,456
Net Gains and Losses on Trading Account Securities	Net gains and losses on trading account securities were as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Net gain on sales transactions $ 314 $ 159 $ 716 $ 933 Net mark-to-market gains (losses) (2) 1 (71) 94 Net gain (loss) on trading account securities $ 312 $ 160 $ 645 $ 1,027

Loans (Tables)

Loans (Tables)	9 Months Ended
	Sep. 30, 2021
Receivables [Abstract]
Loans	Loans were as follows: September 30, Percentage December 31, Percentage Commercial and industrial $ 4,929,656 31.1 % $ 4,955,341 28.4 % Energy: Production 774,753 4.9 976,473 5.6 Service 107,585 0.7 116,825 0.7 Other 95,873 0.6 141,900 0.8 Total energy 978,211 6.2 1,235,198 7.1 Paycheck Protection Program 827,820 5.2 2,433,849 13.9 Commercial real estate: Commercial mortgages 5,648,541 35.7 5,478,806 31.3 Construction 1,266,633 8.0 1,223,814 7.0 Land 335,849 2.1 317,847 1.8 Total commercial real estate 7,251,023 45.8 7,020,467 40.1 Consumer real estate: Home equity loans 320,597 2.0 329,390 1.9 Home equity lines of credit 478,978 3.0 452,854 2.6 Other 563,464 3.6 548,530 3.1 Total consumer real estate 1,363,039 8.6 1,330,774 7.6 Total real estate 8,614,062 54.4 8,351,241 47.7 Consumer and other 482,921 3.1 505,680 2.9 Total loans $ 15,832,670 100.0 % $ 17,481,309 100.0 %
Non-Accrual Loans, Segregated by Class of Loans	Non-accrual loans, segregated by class of loans, were as follows: September 30, 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 $ 19,349 $ 11,611 $ 19,849 $ 4,479 Energy 23,347 7,224 23,168 639 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 12,247 8,770 15,737 14,116 Construction 1,632 664 1,684 1,684 Consumer real estate 480 178 993 993 Consumer and other — — 18 — Total $ 57,055 $ 28,447 $ 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 September 30, 2021 by class and year of origination. 2021 2020 2019 2018 2017 Prior Revolving Loans Revolving Loans Converted to Term Total Commercial and industrial $ 315 $ 7,922 $ 4,211 $ 676 $ 882 $ 370 $ 2,067 $ 2,906 $ 19,349 Energy — 477 6,467 1,513 — 323 12,170 2,397 23,347 Paycheck Protection Program — — — — — — — — — Commercial real estate: Buildings, land and other 2,096 1,723 2,811 824 2,051 2,742 — — 12,247 Construction — 1,632 — — — — — — 1,632 Consumer real estate — — — — — 443 — 37 480 Consumer and other — — — — — — — — — Total $ 2,411 $ 11,754 $ 13,489 $ 3,013 $ 2,933 $ 3,878 $ 14,237 $ 5,340 $ 57,055
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 September 30, 2021 was as follows: Loans Loans Total Current Total Accruing Commercial and industrial $ 42,705 $ 8,634 $ 51,339 $ 4,878,317 $ 4,929,656 $ 7,522 Energy 1,237 13,233 14,470 963,741 978,211 965 Paycheck Protection Program — — — 827,820 827,820 — Commercial real estate: Buildings, land and other 20,910 19,190 40,100 5,944,290 5,984,390 12,224 Construction — — — 1,266,633 1,266,633 — Consumer real estate 4,148 2,101 6,249 1,356,790 1,363,039 1,985 Consumer and other 3,678 1,029 4,707 478,214 482,921 1,029 Total $ 72,678 $ 44,187 $ 116,865 $ 15,715,805 $ 15,832,670 $ 23,725
Troubled Debt Restructurings	Troubled debt restructurings during the nine months ended September 30, 2021 and 2020 are set forth in the following table. Nine Months Ended Nine Months Ended Balance at Balance at Balance at Balance at Commercial and industrial $ 1,312 $ 1,237 $ 3,660 $ 1,415 Energy 3,817 3,566 — — Commercial real estate: Buildings, land and other 1,888 1,875 6,606 6,585 Construction — — 1,192 1,181 Consumer and other — — 1,104 104 $ 7,017 $ 6,678 $ 12,562 $ 9,285
Restructured Loans Past Due in Excess of 90 Days on Financing Receivables	Additional information related to restructured loans as of or for the nine months ended September 30, 2021 and 2020 is set forth in the following table. September 30, 2021 September 30, 2020 Restructured loans past due in excess of 90 days at period-end: Number of loans 1 2 Dollar amount of loans $ 306 $ 3,682 Restructured loans on non-accrual status at period end 5,072 5,353 Charge-offs of restructured loans: Recognized in connection with restructuring — — Recognized on previously restructured loans 1,433 2,188
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 September 30, 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 $ 1,009,752 $ 730,976 $ 405,216 $ 201,578 $ 160,431 $ 149,611 $ 1,875,496 $ 65,603 $ 4,598,663 6.04 Risk grade 9 9,569 35,274 30,953 26,216 12,258 18,329 70,412 5,272 208,283 9.00 Risk grade 10 28,117 6,546 1,871 4,653 1,358 112 12,954 1,652 57,263 10.00 Risk grade 11 1,326 8,603 9,644 4,160 3,098 279 7,189 11,799 46,098 11.00 Risk grade 12 135 7,922 3,263 676 411 370 962 1,974 15,713 12.00 Risk grade 13 180 — 948 — 471 — 1,105 932 3,636 13.00 $ 1,049,079 $ 789,321 $ 451,895 $ 237,283 $ 178,027 $ 168,701 $ 1,968,118 $ 87,232 $ 4,929,656 6.29 W/A risk grade 5.97 6.35 6.91 7.04 6.20 5.94 6.17 7.60 6.29 Energy Risk grades 1-8 $ 350,757 $ 10,562 $ 10,339 $ 7,060 $ 3,958 $ 4,518 $ 392,527 $ 68,437 $ 848,158 5.94 Risk grade 9 32,434 712 1,831 270 — 737 10,136 2,574 48,694 9.00 Risk grade 10 — 115 703 668 — — 860 478 2,824 10.00 Risk grade 11 11,517 1,168 17,711 1,607 — 573 19,133 3,479 55,188 11.00 Risk grade 12 — 34 4,777 90 — 323 4,641 99 9,964 12.00 Risk grade 13 — 443 1,690 1,423 — — 7,529 2,298 13,383 13.00 $ 394,708 $ 13,034 $ 37,051 $ 11,118 $ 3,958 $ 6,151 $ 434,826 $ 77,365 $ 978,211 6.55 W/A risk grade 6.37 8.13 9.98 8.77 7.19 7.84 6.26 6.76 6.55 Commercial real estate: Buildings, land, other Risk grades 1-8 $ 1,153,181 $ 1,146,764 $ 862,629 $ 578,768 $ 517,822 $ 842,409 $ 44,525 $ 75,360 $ 5,221,458 6.90 Risk grade 9 13,873 143,504 70,218 48,372 29,691 34,746 4,669 6,858 351,931 9.00 Risk grade 10 14,376 16,956 69,882 51,903 65,363 55,732 — — 274,212 10.00 Risk grade 11 3,995 19,766 11,351 7,615 32,698 40,245 3,000 5,872 124,542 11.00 Risk grade 12 1,896 1,723 2,511 824 2,051 2,492 — — 11,497 12.00 Risk grade 13 200 — 300 — — 250 — — 750 13.00 $ 1,187,521 $ 1,328,713 $ 1,016,891 $ 687,482 $ 647,625 $ 975,874 $ 52,194 $ 88,090 $ 5,984,390 7.27 W/A risk grade 7.21 7.27 7.40 7.37 7.38 7.04 7.30 7.15 7.27 Construction Risk grades 1-8 $ 429,990 $ 284,149 $ 299,927 $ 20,457 $ — $ 2,593 $ 144,950 $ — $ 1,182,066 7.03 Risk grade 9 37,113 32,002 — — 513 — 13,307 — 82,935 9.00 Risk grade 10 — — — — — — — — — 10.00 Risk grade 11 — — — — — — — — — 11.00 Risk grade 12 — 1,432 — — — — — — 1,432 12.00 Risk grade 13 — 200 — — — — — — 200 13.00 $ 467,103 $ 317,783 $ 299,927 $ 20,457 $ 513 $ 2,593 $ 158,257 $ — $ 1,266,633 7.16 W/A risk grade 7.13 6.87 7.46 7.45 9.00 6.79 7.24 — 7.16 Total commercial real estate $ 1,654,624 $ 1,646,496 $ 1,316,818 $ 707,939 $ 648,138 $ 978,467 $ 210,451 $ 88,090 $ 7,251,023 7.25 W/A risk grade 7.19 7.19 7.41 7.38 7.38 7.04 7.26 7.15 7.25
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 September 30, 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 $ 396 $ 34 $ 277 $ 370 $ 326 $ 1,187 $ 174 $ 1,384 $ 4,148 Past due 90 or more days — — 110 11 226 645 967 142 2,101 Total past due 396 34 387 381 552 1,832 1,141 1,526 6,249 Current loans 235,725 283,853 111,449 62,416 54,827 132,108 464,440 11,972 1,356,790 Total $ 236,121 $ 283,887 $ 111,836 $ 62,797 $ 55,379 $ 133,940 $ 465,581 $ 13,498 $ 1,363,039 Consumer and other: Past due 30-89 days $ 1,527 $ 283 $ 182 $ 89 $ 63 $ 18 $ 481 $ 1,035 $ 3,678 Past due 90 or more days 576 10 — 90 — — — 353 1,029 Total past due 2,103 293 182 179 63 18 481 1,388 4,707 Current loans 39,156 20,924 10,442 3,268 1,844 1,391 374,993 26,196 478,214 Total $ 41,259 $ 21,217 $ 10,624 $ 3,447 $ 1,907 $ 1,409 $ 375,474 $ 27,584 $ 482,921
Revolving Loans Converted to Term [Table Text Block]	Revolving loans that converted to term during the three and nine months ended September 30, 2021 and 2020 were as follows: Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Commercial and industrial $ 12,009 $ 10,224 $ 39,551 $ 25,340 Energy 52,055 7,144 57,871 38,642 Commercial real estate: Buildings, land and other 6,012 637 37,093 8,094 Construction — — — — Consumer real estate 330 421 1,172 2,132 Consumer and other 1,523 5,494 7,017 15,338 Total $ 71,929 $ 23,920 $ 142,704 $ 89,546
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 September 30, 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. September 30, 2021 Commercial Energy Commercial Consumer Consumer Total Modeled expected credit losses $ 45,395 $ 6,962 $ 17,033 $ 5,934 $ 6,412 $ 81,736 Q-Factor and other qualitative adjustments 14,081 104 134,071 59 1,440 149,755 Specific allocations 4,290 13,383 950 36 — 18,659 Total $ 63,766 $ 20,449 $ 152,054 $ 6,029 $ 7,852 $ 250,150 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 and nine months ended September 30, 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: September 30, 2021 Beginning balance $ 65,271 $ 28,010 $ 147,993 $ 6,154 $ 7,860 $ 255,288 Credit loss expense (benefit) (1,020) (7,720) 4,007 (982) 2,692 (3,023) Charge-offs (1,611) — — — (5,073) (6,684) Recoveries 1,126 159 54 857 2,373 4,569 Net charge-offs (485) 159 54 857 (2,700) (2,115) Ending balance $ 63,766 $ 20,449 $ 152,054 $ 6,029 $ 7,852 $ 250,150 September 30, 2020 Beginning balance $ 98,536 $ 40,817 $ 93,425 $ 8,998 $ 8,285 $ 250,061 Credit loss expense (benefit) (18,547) 13,814 25,368 1,794 1,161 23,590 Charge-offs (8,605) — (242) (1,088) (4,219) (14,154) Recoveries 1,121 — 42 573 2,242 3,978 Net charge-offs (7,484) — (200) (515) (1,977) (10,176) Ending balance $ 72,505 $ 54,631 $ 118,593 $ 10,277 $ 7,469 $ 263,475 Commercial Energy Commercial Consumer Consumer Total Nine months ended: September 30, 2021 Beginning balance $ 73,843 $ 39,553 $ 134,892 $ 7,926 $ 6,963 $ 263,177 Credit loss expense (benefit) (8,886) (19,048) 16,583 (3,093) 7,042 (7,402) Charge-offs (4,485) (1,433) (137) (672) (13,015) (19,742) Recoveries 3,294 1,377 716 1,868 6,862 14,117 Net (charge-offs) recoveries (1,191) (56) 579 1,196 (6,153) (5,625) Ending balance $ 63,766 $ 20,449 $ 152,054 $ 6,029 $ 7,852 $ 250,150 September 30, 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 (benefit) 10,737 96,478 104,716 3,716 8,096 223,743 Charge-offs (14,815) (68,842) (3,826) (1,508) (13,402) (102,393) Recoveries 3,727 66 185 1,564 6,981 12,523 Net (charge-offs) recoveries (11,088) (68,776) (3,641) 56 (6,421) (89,870) Ending balance $ 72,505 $ 54,631 $ 118,593 $ 10,277 $ 7,469 $ 263,475
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 September 30, 2021 and December 31, 2020. September 30, 2021 December 31, 2020 Loan Specific Allocations Loan Specific Allocations Commercial and industrial $ 22,717 $ 4,290 $ 21,287 $ 5,321 Energy 25,671 13,383 22,888 9,427 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 28,619 750 34,057 513 Construction 1,632 200 1,684 — Consumer real estate 302 36 561 — Consumer and other — — 18 18 Total $ 78,941 $ 18,659 $ 80,495 $ 15,279

Goodwill and Other Intangible_2

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets and Goodwill	Goodwill and other intangible assets are presented in the table below. September 30, December 31, Goodwill $ 654,952 $ 654,952 Other intangible assets: Core deposits $ 845 $ 1,296 Customer relationships 174 267 $ 1,019 $ 1,563
Estimated Aggregate Future Amortization Expense for Intangible Assets	The estimated aggregate future amortization expense for intangible assets remaining as of September 30, 2021 is as follows: Remainder of 2021 $ 153 2022 481 2023 282 2024 87 2025 11 Thereafter 5 $ 1,019

Deposits (Tables)

Deposits (Tables)	9 Months Ended
	Sep. 30, 2021
Disclosure Text Block [Abstract]
Deposit Liabilities, Type	Deposits were as follows: September 30, Percentage December 31, Percentage Non-interest-bearing demand deposits: Commercial and individual $ 15,997,472 40.4 % $ 13,914,754 39.7 % Correspondent banks 223,466 0.6 242,225 0.7 Public funds 930,645 2.3 960,072 2.8 Total non-interest-bearing demand deposits 17,151,583 43.3 15,117,051 43.2 Interest-bearing deposits: Private accounts: Savings and interest checking 10,326,315 26.1 9,132,789 26.1 Money market accounts 10,381,703 26.2 8,977,585 25.6 Time accounts 1,086,894 2.7 1,135,575 3.3 Total private accounts 21,794,912 55.0 19,245,949 55.0 Public funds: Savings and interest checking 580,174 1.5 597,503 1.7 Money market accounts 61,028 0.1 50,070 0.1 Time accounts 25,070 0.1 5,188 — Total public funds 666,272 1.7 652,761 1.8 Total interest-bearing deposits 22,461,184 56.7 19,898,710 56.8 Total deposits $ 39,612,767 100.0 % $ 35,015,761 100.0 %
Additional Information About Corporation's Deposits	The following table presents additional information about our deposits: September 30, December 31, Deposits from the Certificate of Deposit Account Registry Service (CDARS) deposits $ — $ 372 Deposits from foreign sources (primarily Mexico) 1,007,416 884,169 Total deposits not covered by deposit insurance 21,419,285 18,694,320 Time deposits not covered by deposit insurance 239,598 237,298

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Financial Instruments with Off-Balance-Sheet Risk	Financial instruments with off-balance-sheet risk were as follows: September 30, December 31, Commitments to extend credit $ 10,262,077 $ 9,814,475 Standby letters of credit 225,311 241,345 Deferred standby letter of credit fees 1,935 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 Nine Months Ended 2021 2020 2021 2020 Beginning balance $ 48,596 $ 46,939 $ 44,152 $ 500 Impact of adopting ASC 326 — — — 39,377 Credit loss expense 3,023 (3,276) 7,467 3,786 Ending balance $ 51,619 $ 43,663 $ 51,619 $ 43,663
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 Nine Months Ended 2021 2020 2021 2020 Amortization of lease right-of-use assets $ 8,172 $ 8,521 $ 24,584 $ 24,545 Short-term lease expense 384 172 1,118 1,343 Non-lease components (including taxes, insurance, common maintenance, etc.) 2,864 2,586 8,205 8,410 Total $ 11,420 $ 11,279 $ 33,907 $ 34,298

Capital and Regulatory Matters

Capital and Regulatory Matters (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 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 September 30, 2021 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 3,300,979 13.42 % $ 1,722,459 7.00 % $ 1,599,426 6.50 % Frost Bank 3,215,116 13.09 1,719,451 7.00 1,596,633 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 3,446,431 14.01 2,091,558 8.50 1,968,525 8.00 Frost Bank 3,215,116 13.09 2,087,905 8.50 1,965,087 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,911,272 15.90 2,583,689 10.50 2,460,656 10.00 Frost Bank 3,446,957 14.03 2,579,177 10.50 2,456,359 10.00 Leverage Ratio Cullen/Frost 3,446,431 7.52 1,833,156 4.00 2,291,445 5.00 Frost Bank 3,215,116 7.02 1,832,498 4.00 2,290,623 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)	9 Months Ended
	Sep. 30, 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 methods 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 September 30, 2021 and December 31, 2020. September 30, 2021 December 31, 2020 Notional Estimated Notional Estimated Derivatives designated as hedges of fair value: Financial institution counterparties: Loan/lease interest rate swaps – liabilities 2,758 $ (59) 3,724 $ (134) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets 127,420 641 — — Loan/lease interest rate swaps – liabilities 997,411 (22,874) 1,173,173 (33,812) Loan/lease interest rate caps – assets 405,163 2,915 356,601 1,241 Customer counterparties: Loan/lease interest rate swaps – assets 997,411 49,630 1,173,173 84,424 Loan/lease interest rate swaps – liabilities 127,420 (1,761) — — Loan/lease interest rate caps – liabilities 405,163 (2,915) 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 September 30, 2021 were as follows: Weighted-Average Interest Interest Interest rate swaps: Fair value hedge loan/lease interest rate swaps 2.75 % 0.09 % Non-hedging interest rate swaps – financial institution counterparties 3.78 1.87 Non-hedging interest rate swaps – customer counterparties 1.87 3.78
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 methods with observable market data inputs to value our commodity derivative positions. September 30, 2021 December 31, 2020 Notional Notional Estimated Notional Estimated Financial institution counterparties: Oil – assets Barrels 5,632 $ 21,271 3,056 $ 8,341 Oil – liabilities Barrels 12,228 (147,576) 6,391 (32,112) Natural gas – assets MMBTUs 18,908 3,312 9,281 1,529 Natural gas – liabilities MMBTUs 39,901 (53,923) 15,079 (3,265) Customer counterparties: Oil – assets Barrels 12,276 148,742 6,391 32,670 Oil – liabilities Barrels 5,584 (21,245) 3,056 (8,264) Natural gas – assets MMBTUs 39,901 54,202 17,636 3,451 Natural gas – liabilities MMBTUs 18,908 (3,310) 6,724 (1,458)
Schedule of Foreign Exchange Contracts, Statement of Financial Position	The notional amounts and fair values of open foreign currency forward contracts were as follows: September 30, 2021 December 31, 2020 Notional Notional Estimated Notional Estimated Financial institution counterparties: Forward contracts – assets CAD 658 $ 2 — — Customer counterparties: Forward contracts – assets CAD 658 2 — —
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 Nine Months Ended 2021 2020 2021 2020 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ (21) $ (36) $ (74) $ (79) Amount of (gain) loss included in other non-interest expense 3 2 8 6
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 Nine Months Ended 2021 2020 2021 2020 Non-hedging interest rate derivatives: Other non-interest income $ 946 $ 280 $ 2,674 $ 2,615 Other non-interest expense — 1 (1) 1 Non-hedging commodity derivatives: Other non-interest income 1,143 379 3,266 1,057 Non-hedging foreign currency derivatives: Other non-interest income 4 10 34 28 Non-hedging put options: Other non-interest income — — — 5,980

Balance Sheet Offsetting (Table

Balance Sheet Offsetting (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2021 is presented in the following tables. Gross Amount Gross Amount Net Amount September 30, 2021 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 3,556 $ — $ 3,556 Commodity swaps and options 24,583 — 24,583 Foreign currency forward contracts 2 — 2 Total derivatives 28,141 — 28,141 Resell agreements 7,903 — 7,903 Total $ 36,044 $ — $ 36,044 Financial liabilities: Derivatives: Loan/lease interest rate swaps and caps $ 22,933 $ — $ 22,933 Commodity swaps and options 201,499 — 201,499 Total derivatives 224,432 — 224,432 Repurchase agreements 2,200,029 — 2,200,029 Total $ 2,424,461 $ — $ 2,424,461 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 and caps $ 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 September 30, 2021 Financial assets: Derivatives: Counterparty A $ 4 $ (4) $ — $ — Counterparty B 12,537 (12,537) — — Other counterparties 15,600 (15,600) — — Total derivatives 28,141 (28,141) — — Resell agreements 7,903 — (7,903) — Total $ 36,044 $ (28,141) $ (7,903) $ — Financial liabilities: Derivatives: Counterparty A $ 4,577 $ (4) $ (4,573) $ — Counterparty B 77,110 (12,537) (60,860) 3,713 Counterparty C 19 — (19) — Other counterparties 142,726 (15,600) (122,984) 4,142 Total derivatives 224,432 (28,141) (188,436) 7,855 Repurchase agreements 2,200,029 — (2,200,029) — Total $ 2,424,461 $ (28,141) $ (2,388,465) $ 7,855 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 September 30, 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 September 30, 2021 Repurchase agreements: U.S. Treasury $ 1,122,968 $ — $ — $ — $ 1,122,968 Residential mortgage-backed securities 1,077,061 — — — 1,077,061 Total borrowings $ 2,200,029 $ — $ — $ — $ 2,200,029 Gross amount of recognized liabilities for repurchase agreements $ 2,200,029 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)	9 Months Ended
	Sep. 30, 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 September 30, 2021, there were 917,440 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 5,940 117.90 — — — — — — Exercised/vested (2,499) 92.03 (1,213) 98.90 (35,131) 92.27 (611,201) 61.86 Forfeited/expired — — (26,439) 87.08 (9,752) 75.70 — — Balance, September 30, 2021 56,301 $ 79.21 442,707 $ 86.16 156,374 $ 73.88 1,128,358 $ 68.41
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 Nine Months Ended 2021 2020 2021 2020 New shares issued from available authorized shares — — — — Shares issued from available treasury stock 21,968 53,425 650,044 196,948 Total 21,968 53,425 650,044 196,948 Proceeds from stock option exercises $ 1,128 $ 2,698 $ 37,810 $ 7,544
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 Nine Months Ended 2021 2020 2021 2020 Non-vested stock awards/stock units $ 1,559 $ 1,999 $ 5,210 $ 6,325 Director deferred stock units — — 700 770 Performance stock units 426 444 1,846 1,132 Total $ 1,985 $ 2,443 $ 7,756 $ 8,227 Income tax benefit $ 281 $ 450 $ 1,162 $ 1,539
Unrecognized Stock-Based Compensation Expense	Unrecognized stock-based compensation expense at September 30, 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 $ 10,918 Performance stock units 4,450 Total $ 15,368

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	9 Months Ended
	Sep. 30, 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 Nine Months Ended 2021 2020 2021 2020 Net income $ 107,975 $ 95,056 $ 342,058 $ 242,881 Less: Preferred stock dividends 1,668 — 5,488 2,016 Redemption of preferred stock — — — 5,514 Net income available to common shareholders 106,307 95,056 336,570 235,351 Less: Earnings allocated to participating securities 932 877 3,228 2,236 Net earnings allocated to common stock $ 105,375 $ 94,179 $ 333,342 $ 233,115 Distributed earnings allocated to common stock $ 47,740 $ 44,546 $ 139,224 $ 133,511 Undistributed earnings allocated to common stock 57,635 49,633 194,118 99,604 Net earnings allocated to common stock $ 105,375 $ 94,179 $ 333,342 $ 233,115 Weighted-average shares outstanding for basic earnings per common share 63,652,020 62,726,542 63,522,884 62,655,393 Dilutive effect of stock compensation 444,636 193,116 489,219 263,028 Weighted-average shares outstanding for diluted earnings per common share 64,096,656 62,919,658 64,012,103 62,918,421

Defined Benefit Plans (Tables)

Defined Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 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 Nine Months Ended 2021 2020 2021 2020 Expected return on plan assets, net of expenses $ (3,210) $ (3,073) $ (9,630) $ (9,217) Interest cost on projected benefit obligation 835 1,252 2,506 3,757 Net amortization and deferral 1,529 1,330 4,587 3,989 Net periodic expense (benefit) $ (846) $ (491) $ (2,537) $ (1,471)

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Tax Expense	Income tax expense was as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Current income tax expense (benefit) $ 13,849 $ (33,614) $ 31,639 $ 20,328 Deferred income tax expense (benefit) (516) 43,130 4,672 (8,803) Income tax expense, as reported $ 13,333 $ 9,516 $ 36,311 $ 11,525 Effective tax rate 11.0 % 9.1 % 9.6 % 4.5 %

Other Comprehensive Income (L_2

Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 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 $ (80,668) $ (16,941) $ (63,727) $ 20,332 $ 4,269 $ 16,063 Change in net unrealized gain on securities transferred to held to maturity (237) (49) (188) (294) (62) (232) Reclassification adjustment for net (gains) losses included in net income — — — — — — Total securities available for sale and transferred securities (80,905) (16,990) (63,915) 20,038 4,207 15,831 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 1,330 280 1,050 Total defined-benefit post-retirement benefit plans 1,529 321 1,208 1,330 280 1,050 Total other comprehensive income (loss) $ (79,376) $ (16,669) $ (62,707) $ 21,368 $ 4,487 $ 16,881 Nine Months Ended Nine 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 $ (209,779) $ (44,054) $ (165,725) $ 406,959 $ 85,462 $ 321,497 Change in net unrealized gain on securities transferred to held to maturity (741) (155) (586) (979) (206) (773) Reclassification adjustment for net (gains) losses included in net income — — — (108,989) (22,888) (86,101) Total securities available for sale and transferred securities (210,520) (44,209) (166,311) 296,991 62,368 234,623 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) 4,587 963 3,624 3,989 837 3,152 Total defined-benefit post-retirement benefit plans 4,587 963 3,624 3,989 837 3,152 Total other comprehensive income (loss) $ (205,933) $ (43,246) $ (162,687) $ 300,980 $ 63,205 $ 237,775
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 (166,311) — (166,311) Reclassification of amounts included in net income — 3,624 3,624 Net other comprehensive income (loss) during period (166,311) 3,624 (162,687) Balance at September 30, 2021 $ 397,490 $ (47,207) $ 350,283 Balance January 1, 2020 $ 313,304 $ (45,934) $ 267,370 Other comprehensive income (loss) before reclassifications 320,724 — 320,724 Reclassification of amounts included in net income (86,101) 3,152 (82,949) Net other comprehensive income (loss) during period 234,623 3,152 237,775 Balance at September 30, 2020 $ 547,927 $ (42,782) $ 505,145

Operating Segments (Tables)

Operating Segments (Tables)	9 Months Ended
	Sep. 30, 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: September 30, 2021 Net interest income (expense) $ 247,381 $ 536 $ (1,795) $ 246,122 Credit loss expense (6) 6 — — Non-interest income 51,279 42,369 (459) 93,189 Non-interest expense 185,891 31,086 1,026 218,003 Income (loss) before income taxes 112,775 11,813 (3,280) 121,308 Income tax expense (benefit) 12,029 2,481 (1,177) 13,333 Net income (loss) 100,746 9,332 (2,103) 107,975 Preferred stock dividends — — 1,668 1,668 Net income (loss) available to common shareholders $ 100,746 $ 9,332 $ (3,771) $ 106,307 Revenues from (expenses to) external customers $ 298,660 $ 42,905 $ (2,254) $ 339,311 September 30, 2020 Net interest income (expense) $ 244,685 $ 602 $ (1,864) $ 243,423 Credit loss expense (benefit) 20,300 2 — 20,302 Non-interest income 49,170 34,680 (249) 83,601 Non-interest expense 170,290 30,052 1,808 202,150 Income (loss) before income taxes 103,265 5,228 (3,921) 104,572 Income tax expense (benefit) 9,758 1,098 (1,340) 9,516 Net income (loss) available to common shareholders $ 93,507 $ 4,130 $ (2,581) $ 95,056 Revenues from (expenses to) external customers $ 293,855 $ 35,282 $ (2,113) $ 327,024 Nine months ended: September 30, 2021 Net interest income (expense) $ 748,036 $ 1,530 $ (5,407) $ 744,159 Credit loss expense 57 6 — 63 Non-interest income 154,003 124,512 (843) 277,672 Non-interest expense 548,151 91,037 4,211 643,399 Income (loss) before income taxes 353,831 34,999 (10,461) 378,369 Income tax expense (benefit) 32,811 7,350 (3,850) 36,311 Net income (loss) 321,020 27,649 (6,611) 342,058 Preferred stock dividends — — 5,488 5,488 Net income (loss) available to common shareholders $ 321,020 $ 27,649 $ (12,099) $ 336,570 Revenues from (expenses to) external customers $ 902,039 $ 126,042 $ (6,250) $ 1,021,831 September 30, 2020 Net interest income (expense) $ 737,977 $ 2,163 $ (6,385) $ 733,755 Credit loss expense 227,474 — — 227,474 Non-interest income 265,492 109,330 (705) 374,117 Non-interest expense 525,882 94,225 5,885 625,992 Income (loss) before income taxes 250,113 17,268 (12,975) 254,406 Income tax expense (benefit) 12,090 3,626 (4,191) 11,525 Net income (loss) 238,023 13,642 (8,784) 242,881 Preferred stock dividends — — 2,016 2,016 Redemption of preferred stock — — 5,514 5,514 Net income (loss) available to common shareholders $ 238,023 $ 13,642 $ (16,314) $ 235,351 Revenues from (expenses to) external customers $ 1,003,469 $ 111,493 $ (7,090) $ 1,107,872

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 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 September 30, 2021 Securities available for sale: U.S. Treasury $ 1,685,221 $ — $ — $ 1,685,221 Residential mortgage-backed securities — 2,491,154 — 2,491,154 States and political subdivisions — 7,512,489 — 7,512,489 Other — 42,358 — 42,358 Trading account securities: U.S. Treasury 24,087 — — 24,087 States and political subdivisions — 1,205 — 1,205 Derivative assets: Interest rate swaps, caps and floors — 53,186 — 53,186 Commodity swaps and options — 227,527 — 227,527 Foreign currency forward contracts 4 — — 4 Derivative liabilities: Interest rate swaps, caps and floors — 27,609 — 27,609 Commodity swaps and options — 226,054 — 226,054 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 Derivative liabilities: Interest rate swaps, caps and floors — 35,187 — 35,187 Commodity swaps and options — 45,099 — 45,099
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. Nine Months Ended Nine Months Ended Level 2 Level 3 Level 2 Level 3 Carrying value before allocations $ 3,329 $ 15,274 $ 6,388 $ 8,660 Specific (allocations) reversals of prior allocations (336) (4,791) (930) 14,928 Fair value $ 2,993 $ 10,483 $ 5,458 $ 23,588
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: Nine 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: September 30, 2021 December 31, 2020 Carrying Estimated Carrying Estimated Financial assets: Level 2 inputs: Cash and cash equivalents $ 16,253,971 $ 16,253,971 $ 10,288,853 $ 10,288,853 Securities held to maturity 1,759,270 1,825,568 1,945,673 2,052,896 Cash surrender value of life insurance policies 189,941 189,941 189,984 189,984 Accrued interest receivable 119,232 119,232 181,432 181,432 Level 3 inputs: Loans, net 15,582,520 15,656,269 17,218,132 17,390,683 Financial liabilities: Level 2 inputs: Deposits 39,612,767 39,613,343 35,015,761 35,018,185 Federal funds purchased 27,200 27,200 48,850 48,850 Repurchase agreements 2,200,029 2,200,029 2,068,147 2,068,147 Junior subordinated deferrable interest debentures 136,400 137,115 136,357 137,115 Subordinated notes 99,139 113,223 99,021 115,717 Accrued interest payable 2,126 2,126 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	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accounting Policies [Abstract]
Cash paid for interest		$ 23,722	$ 41,634
Cash paid for income taxes		28,043	44,140
Unsettled securities transactions		600,874	6,698
Loans foreclosed and transferred to other real estate owned and foreclosed assets		3,464	140
Right-of-use lease assets obtained in exchange for lessee operating lease liabilities		4,045	18,284
Treasury stock issued to 401(k) stock purchase plan	$ 4,131	$ 1,749	$ 6,961

Securities - Additional Informa

Securities - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 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	$ 2,800	$ 3,500
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	$ 2,200	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	76.90%
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	553,100	659,200
Accrued interest receivable	10,800	21,700
Available-for-sale Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	4,900,000	4,400,000
Accrued interest receivable	$ 69,000	$ 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	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 1,759,428	$ 1,945,833
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	66,172	107,063
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	32	0
Debt Securities, Held-to-maturity, Fair Value	1,825,568	2,052,896
Securities, held-to-maturity, allowance for credit loss	(158)	(160)
Debt Securities, Held-to-Maturity Net Carrying Amount	1,759,270	1,945,673
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	527,649	528,784
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	24,332	41,742
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	551,981	570,526
Securities, held-to-maturity, allowance for credit loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	527,649	528,784
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,230,279	1,415,549
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	41,840	65,321
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	32	0
Debt Securities, Held-to-maturity, Fair Value	1,272,087	1,480,870
Securities, held-to-maturity, allowance for credit loss	(158)	(160)
Debt Securities, Held-to-Maturity Net Carrying Amount	1,230,121	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	Sep. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	$ 1,759,428	$ 1,945,833
US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,230,279	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	1,124,168	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	106,111	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	0	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	201,931	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	95,820	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	106,111	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	0	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	466,028	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	466,028	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	562,320	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	562,320	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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Securities, held-to-maturity, allowance for credit loss	$ 158		$ 158			$ 160
Impact of adopting ASC 326	0	$ 0	0	$ 39,377
Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities, held-to-maturity, allowance for credit loss	158	160	158	160	$ 158	$ 160	$ 172	$ 0
Impact of adopting ASC 326	0	0	0	215
Credit loss expense (benefit)	$ 0	$ (12)	$ (2)	$ (55)

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	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 11,230,837	$ 9,727,401
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	530,402	710,168
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	30,017	4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	11,731,222	10,437,565
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,670,692	1,084,542
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	20,609	35,091
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6,080	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	1,685,221	1,119,633
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	2,467,915	1,916,581
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	37,910	71,102
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	14,671	4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	2,491,154	1,987,679
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	7,049,872	6,683,927
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	471,883	603,975
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	9,266	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	7,512,489	7,287,902
Other Investments [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	42,358	42,351
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	$ 42,358	$ 42,351

Securities - Securities Availab

Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 2,308,547
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	30,017
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Debt Securities, Available-for-sale, Unrealized Loss Position	2,308,547
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	30,017	$ 4
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	687,535
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6,080
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Debt Securities, Available-for-sale, Unrealized Loss Position	687,535
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6,080	0
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	984,236
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	14,671
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Debt Securities, Available-for-sale, Unrealized Loss Position	984,236
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	14,671	4
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	636,776
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9,266
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Debt Securities, Available-for-sale, Unrealized Loss Position	636,776
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 9,266	$ 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	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 458,343
Held to Maturity, Due after one year through five years, Amortized Cost	181,302
Held to Maturity, Due after five years through ten years, Amortized Cost	604,218
Held to Maturity, Due after ten years, Amortized Cost	515,565
Held to Maturity, Amortized Cost	1,759,428	$ 1,945,833
Held to Maturity, Due in one year or less, Estimated Fair Value	465,632
Held to Maturity, Due after one year through five years, Estimated Fair Value	187,495
Held to Maturity, Due after five years through ten years, Estimated Fair Value	629,437
Held to Maturity, Due after ten years, Estimated Fair Value	543,004
Held to Maturity, Estimated Fair Value	1,825,568	2,052,896
Available-for-sale Securities, Due in one year or less, Amortized Cost	582,966
Available for Sale, Due after one year through five years, Amortized Cost	1,943,087
Available for Sale, Due after five years through ten years, Amortized Cost	1,331,530
Available for Sale, Due after ten years, Amortized Cost	7,330,896
Debt Securities, Available-for-sale, Amortized Cost	11,230,837	9,727,401
Available for Sale, Due in one year or less, Estimated Fair Value	586,193
Available for Sale, Due after one year through five years, Estimated Fair Value	2,057,801
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,382,879
Available for Sale, Due after ten years, Estimated Fair Value	7,661,991
Securities available for sale, at estimated fair value	11,731,222	10,437,565
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	499,857
Available for Sale, Due after one year through five years, Amortized Cost	533,532
Available for Sale, Due after five years through ten years, Amortized Cost	445,697
Available for Sale, Due after ten years, Amortized Cost	191,606
Debt Securities, Available-for-sale, Amortized Cost	1,670,692	1,084,542
Available for Sale, Due in one year or less, Estimated Fair Value	501,489
Available for Sale, Due after one year through five years, Estimated Fair Value	548,216
Available for Sale, Due after five years through ten years, Estimated Fair Value	440,328
Available for Sale, Due after ten years, Estimated Fair Value	195,188
Securities available for sale, at estimated fair value	1,685,221	1,119,633
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	65
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	515,356
Held to Maturity, Due after ten years, Amortized Cost	12,228
Held to Maturity, Amortized Cost	527,649	528,784
Held to Maturity, Due in one year or less, Estimated Fair Value	65
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	539,142
Held to Maturity, Due after ten years, Estimated Fair Value	12,774
Held to Maturity, Estimated Fair Value	551,981	570,526
Available-for-sale Securities, Due in one year or less, Amortized Cost	743
Available for Sale, Due after one year through five years, Amortized Cost	16,678
Available for Sale, Due after five years through ten years, Amortized Cost	22,146
Available for Sale, Due after ten years, Amortized Cost	2,428,348
Debt Securities, Available-for-sale, Amortized Cost	2,467,915	1,916,581
Available for Sale, Due in one year or less, Estimated Fair Value	772
Available for Sale, Due after one year through five years, Estimated Fair Value	17,559
Available for Sale, Due after five years through ten years, Estimated Fair Value	22,743
Available for Sale, Due after ten years, Estimated Fair Value	2,450,080
Securities available for sale, at estimated fair value	2,491,154	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	456,778
Held to Maturity, Due after one year through five years, Amortized Cost	181,302
Held to Maturity, Due after five years through ten years, Amortized Cost	88,862
Held to Maturity, Due after ten years, Amortized Cost	503,337
Held to Maturity, Amortized Cost	1,230,279	1,415,549
Held to Maturity, Due in one year or less, Estimated Fair Value	464,067
Held to Maturity, Due after one year through five years, Estimated Fair Value	187,495
Held to Maturity, Due after five years through ten years, Estimated Fair Value	90,295
Held to Maturity, Due after ten years, Estimated Fair Value	530,230
Held to Maturity, Estimated Fair Value	1,272,087	1,480,870
Available-for-sale Securities, Due in one year or less, Amortized Cost	82,366
Available for Sale, Due after one year through five years, Amortized Cost	1,392,877
Available for Sale, Due after five years through ten years, Amortized Cost	863,687
Available for Sale, Due after ten years, Amortized Cost	4,710,942
Debt Securities, Available-for-sale, Amortized Cost	7,049,872	6,683,927
Available for Sale, Due in one year or less, Estimated Fair Value	83,932
Available for Sale, Due after one year through five years, Estimated Fair Value	1,492,026
Available for Sale, Due after five years through ten years, Estimated Fair Value	919,808
Available for Sale, Due after ten years, Estimated Fair Value	5,016,723
Securities available for sale, at estimated fair value	7,512,489	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
Debt Securities, Available-for-sale, Amortized Cost	42,358	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, at estimated fair value	$ 42,358	$ 42,351

Securities - Sales of Securitie

Securities - Sales of Securities Available for Sale (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Sales	$ 0	$ 0	$ 0	$ 1,162,352
Debt Securities, Available-for-sale, Realized Gain	0	0	0	108,989
Debt Securities, Available-for-sale, Realized Loss	0	0	0	0
Tax (expense)benefit of securities gains/losses	$ 0	$ 0	$ 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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Premium amortization	$ (30,240)	$ (31,540)	$ (92,016)	$ (94,873)
Discount accretion	657	637	1,999	1,793
Net (premium amortization) discount accretion	$ (29,583)	$ (30,903)	$ (90,017)	$ (93,080)

Securities - Trading Account Se

Securities - Trading Account Securities, at Estimated Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	$ 25,292	$ 24,456
U.S. Treasury [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	24,087	23,996
States and political subdivisions [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	$ 1,205	$ 460

Securities - Net Gains and Loss

Securities - Net Gains and Losses on Trading Account Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Net gain on sales transactions	$ 314	$ 159	$ 716	$ 933
Net mark-to-market gains (losses)	(2)	1	(71)	94
Net gain (loss) on trading account securities	$ 312	$ 160	$ 645	$ 1,027

Loans - Additional Information

Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Aug. 31, 2021	Jun. 30, 2021USD ($)	Dec. 31, 2020USD ($)	Jun. 30, 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%		0.00%
Maximum percentage loan related to single industry	10.00%		10.00%
Loans and Leases Receivable, Related Parties	$ 318,800		$ 318,800				$ 353,100
Texas Leading Index economic condition indicator					130.4		118.1
Financing Receivable, Allowance for Credit Loss	250,150		250,150				$ 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	$ 474	$ 670	$ 1,400	$ 2,300
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%		6.20%				7.10%
Percentage of total energy loans to total loans excluding PPP Loans	6.50%		6.50%
Financing Receivable, Allowance for Credit Loss	$ 20,449	$ 54,631	$ 20,449	$ 54,631		$ 28,010	$ 39,553	$ 40,817	$ 37,382
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss	0		0				0
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,000,000		1,000,000
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	63,700		63,700
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest Receivable	$ 39,000		$ 39,000				$ 48,700

Loans - Loans (Detail)

Loans - Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 15,832,670	$ 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	$ 4,929,656	$ 4,955,341
Total commercial and industrial percentage of total loans	31.10%	28.40%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 774,753	$ 976,473
Percentage of energy production loans to total loans	4.90%	5.60%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 107,585	$ 116,825
Percentage of energy service loans to total loans	0.70%	0.70%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 95,873	$ 141,900
Percentage of other energy loans to total loans	0.60%	0.80%
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 978,211	$ 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	$ 827,820	$ 2,433,849
Percentage Of Payment Protection Program Loans To Loans	5.20%	13.90%
Commercial mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 5,648,541	$ 5,478,806
Commercial mortgages percentage of total loans	35.70%	31.30%
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,266,633	$ 1,223,814
Construction percentage of total loans	8.00%	7.00%
Commercial land loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 335,849	$ 317,847
Land percentage of total loans	2.10%	1.80%
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 7,251,023	$ 7,020,467
Total commercial real estate percentage of total loans	45.80%	40.10%
Home Equity Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 320,597	$ 329,390
Home equity loans percentage of total loans	2.00%	1.90%
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 478,978	$ 452,854
Home equity lines of credit percentage of total loans	3.00%	2.60%
Other consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 563,464	$ 548,530
Other percentage of total loans	3.60%	3.10%
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,363,039	$ 1,330,774
Total consumer real estate percentage of total loans	8.60%	7.60%
Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 8,614,062	$ 8,351,241
Total real estate percentage of total loans	54.40%	47.70%
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 482,921	$ 505,680
Total consumer and other percentage of total loans	3.10%	2.90%

Loans - Non-Accrual Loans, Segr

Loans - Non-Accrual Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 57,055	$ 61,449
Non-accrual with no credit loss allowance	28,447	21,911
Commercial Portfolio Segment [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	19,349	19,849
Non-accrual with no credit loss allowance	11,611	4,479
Commercial and Industrial Total Energy Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	23,347	23,168
Non-accrual with no credit loss allowance	7,224	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	12,247	15,737
Non-accrual with no credit loss allowance	8,770	14,116
Construction Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	1,632	1,684
Non-accrual with no credit loss allowance	664	1,684
Total consumer real estate loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	480	993
Non-accrual with no credit loss allowance	178	993
Consumer Loan [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	0	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	Sep. 30, 2021	Dec. 31, 2020
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	$ 57,055	$ 61,449
Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	2,411
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	11,754
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	13,489
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	3,013
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,933
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	3,878
Financing Receivable, Revolving	14,237
Financing Receivable, Revolving, Converted to Term Loan	5,340
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	1,049,079
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	789,321
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	451,895
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	237,283
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	178,027
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	168,701
Financing Receivable, Revolving	1,968,118
Non-accrual loans	19,349	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	315
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	7,922
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	4,211
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	676
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	882
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	370
Financing Receivable, Revolving	2,067
Financing Receivable, Revolving, Converted to Term Loan	2,906
Commercial and Industrial Total Energy Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	394,708
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	13,034
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	37,051
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	11,118
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	3,958
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	6,151
Financing Receivable, Revolving	434,826
Non-accrual loans	23,347	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	477
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	6,467
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,513
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	323
Financing Receivable, Revolving	12,170
Financing Receivable, Revolving, Converted to Term Loan	2,397
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	1,187,521
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,328,713
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,016,891
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	687,482
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	647,625
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	975,874
Financing Receivable, Revolving	52,194
Non-accrual loans	12,247	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	2,096
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,723
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	2,811
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	824
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,051
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,742
Financing Receivable, Revolving	0
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	467,103
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	317,783
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	299,927
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	20,457
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	513
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,593
Financing Receivable, Revolving	158,257
Non-accrual loans	1,632	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	1,632
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	480	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	443
Financing Receivable, Revolving	0
Financing Receivable, Revolving, Converted to Term Loan	37
Consumer Loan [Member]
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	0	$ 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	0
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	87,232
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	77,365
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	88,090
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	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	$ 116,865
Current Loans	15,715,805
Total loans	15,832,670	$ 17,481,309
Accruing Loans 90 or More Days Past Due	23,725
Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	51,339
Current Loans	4,878,317
Total loans	4,929,656	4,955,341
Accruing Loans 90 or More Days Past Due	7,522
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	14,470
Current Loans	963,741
Total loans	978,211	1,235,198
Accruing Loans 90 or More Days Past Due	965
Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	827,820
Total loans	827,820	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	40,100
Current Loans	5,944,290
Total loans	5,984,390	5,796,653
Accruing Loans 90 or More Days Past Due	12,224
Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	1,266,633
Total loans	1,266,633	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	6,249
Current Loans	1,356,790
Total loans	1,363,039	1,330,774
Accruing Loans 90 or More Days Past Due	1,985
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	4,707
Current Loans	478,214
Total loans	482,921	$ 505,680
Accruing Loans 90 or More Days Past Due	1,029
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	72,678
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	42,705
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	1,237
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	20,910
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
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	4,148
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,678
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	44,187
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	8,634
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	13,233
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	19,190
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	2,101
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	$ 1,029

Loans - Troubled Debt Restructu

Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	$ 7,017	$ 12,562
Balance at period end	6,678	9,285
Commercial Portfolio Segment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	1,312	3,660
Balance at period end	1,237	1,415
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	3,817	0
Balance at period end	3,566	0
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	1,888	6,606
Balance at period end	1,875	6,585
Construction Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	0	1,192
Balance at period end	0	1,181
Consumer Loan [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	0	1,104
Balance at period end	$ 0	$ 104

Loans Loans - Restructured Loan

Loans Loans - Restructured Loans Past Due in Excess of 90 Days on Financing Receivables (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)Item	Sep. 30, 2020USD ($)Item
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number Of Restructured Loans Past Due In Excess of 90 Days At Period-End | Item	1	2
Dollar amount of previously restructured loans past due	$ 306	$ 3,682
Restructured loans on non-accrual status at period end	5,072	5,353
Charge-offs of restructured loans recognized in connection with restructuring	0	0
Charge-offs of restructured loans recognized on previously restructured loans	$ 1,433	$ 2,188

Loans Loans - Weighted Average

Loans Loans - Weighted Average Risk Grade for All Commercial Loans by Class and Year of Origination (Details) $ in Thousands	Sep. 30, 2021USD ($)Grade	Dec. 31, 2020USD ($)Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 15,832,670	$ 17,481,309
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	1,049,079
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	789,321
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	451,895
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	237,283
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	178,027
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	168,701
Financing Receivable, Revolving	1,968,118
Loans and Leases Receivable, Net of Deferred Income	$ 4,929,656	$ 4,955,341
Weighted Average Risk Grade For Loans | Grade	6.29	6.45
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 394,708
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	13,034
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	37,051
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	11,118
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	3,958
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	6,151
Financing Receivable, Revolving	434,826
Loans and Leases Receivable, Net of Deferred Income	$ 978,211	$ 1,235,198
Weighted Average Risk Grade For Loans | Grade	6.55	6.85
Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,187,521
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,328,713
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,016,891
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	687,482
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	647,625
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	975,874
Financing Receivable, Revolving	52,194
Loans and Leases Receivable, Net of Deferred Income	$ 5,984,390	$ 5,796,653
Weighted Average Risk Grade For Loans | Grade	7.27	7.34
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 467,103
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	317,783
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	299,927
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	20,457
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	513
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,593
Financing Receivable, Revolving	158,257
Loans and Leases Receivable, Net of Deferred Income	$ 1,266,633	$ 1,223,814
Weighted Average Risk Grade For Loans | Grade	7.16	7.22
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,654,624
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,646,496
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,316,818
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	707,939
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	648,138
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	978,467
Financing Receivable, Revolving	210,451
Loans and Leases Receivable, Net of Deferred Income	$ 7,251,023	$ 7,020,467
Weighted Average Risk Grade For Loans | Grade	7.25	7.32
Risk Grade One To Eight [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,009,752
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	730,976
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	405,216
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	201,578
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	160,431
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	149,611
Financing Receivable, Revolving	1,875,496
Loans and Leases Receivable, Net of Deferred Income	$ 4,598,663	$ 4,506,121
Weighted Average Risk Grade For Loans | Grade	6.04	6.13
Risk Grade One To Eight [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 350,757
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	10,562
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	10,339
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	7,060
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	3,958
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	4,518
Financing Receivable, Revolving	392,527
Loans and Leases Receivable, Net of Deferred Income	$ 848,158	$ 968,144
Weighted Average Risk Grade For Loans | Grade	5.94	5.99
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, Year One, Originated, Current Fiscal Year	$ 1,153,181
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,146,764
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	862,629
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	578,768
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	517,822
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	842,409
Financing Receivable, Revolving	44,525
Loans and Leases Receivable, Net of Deferred Income	$ 5,221,458	$ 5,047,873
Weighted Average Risk Grade For Loans | Grade	6.90	6.97
Risk Grade One To Eight [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 429,990
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	284,149
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	299,927
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	20,457
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,593
Financing Receivable, Revolving	144,950
Loans and Leases Receivable, Net of Deferred Income	$ 1,182,066	$ 1,110,025
Weighted Average Risk Grade For Loans | Grade	7.03	6.99
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]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 9,569
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	35,274
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	30,953
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	26,216
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	12,258
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	18,329
Financing Receivable, Revolving	70,412
Loans and Leases Receivable, Net of Deferred Income	$ 208,283	$ 256,198
Weighted Average Risk Grade For Loans | Grade	9	9
Risk Grade Nine [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 32,434
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	712
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,831
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	270
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	737
Financing Receivable, Revolving	10,136
Loans and Leases Receivable, Net of Deferred Income	$ 48,694	$ 133,547
Weighted Average Risk Grade For Loans | Grade	9	9
Risk Grade Nine [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 13,873
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	143,504
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	70,218
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	48,372
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	29,691
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	34,746
Financing Receivable, Revolving	4,669
Loans and Leases Receivable, Net of Deferred Income	$ 351,931	$ 325,227
Weighted Average Risk Grade For Loans | Grade	9	9
Risk Grade Nine [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 37,113
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	32,002
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	513
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	13,307
Loans and Leases Receivable, Net of Deferred Income	$ 82,935	$ 72,287
Weighted Average Risk Grade For Loans | Grade	9	9
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]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 28,117
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	6,546
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,871
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	4,653
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	1,358
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	112
Financing Receivable, Revolving	12,954
Loans and Leases Receivable, Net of Deferred Income	$ 57,263	$ 125,977
Weighted Average Risk Grade For Loans | Grade	10	10
Risk Grade Ten [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	115
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	703
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	668
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	860
Loans and Leases Receivable, Net of Deferred Income	$ 2,824	$ 46,427
Weighted Average Risk Grade For Loans | Grade	10	10
Risk Grade Ten [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 14,376
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	16,956
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	69,882
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	51,903
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	65,363
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	55,732
Financing Receivable, Revolving	0
Loans and Leases Receivable, Net of Deferred Income	$ 274,212	$ 258,454
Weighted Average Risk Grade For Loans | Grade	10	10
Risk Grade Ten [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [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
Loans and Leases Receivable, Net of Deferred Income	$ 0	$ 38,962
Weighted Average Risk Grade For Loans | Grade	10	10
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]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,326
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	8,603
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	9,644
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	4,160
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	3,098
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	279
Financing Receivable, Revolving	7,189
Loans and Leases Receivable, Net of Deferred Income	$ 46,098	$ 47,196
Weighted Average Risk Grade For Loans | Grade	11	11
Risk Grade Eleven [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 11,517
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,168
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	17,711
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,607
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	573
Financing Receivable, Revolving	19,133
Loans and Leases Receivable, Net of Deferred Income	$ 55,188	$ 63,912
Weighted Average Risk Grade For Loans | Grade	11	11
Risk Grade Eleven [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 3,995
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	19,766
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	11,351
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	7,615
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	32,698
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	40,245
Financing Receivable, Revolving	3,000
Loans and Leases Receivable, Net of Deferred Income	$ 124,542	$ 149,362
Weighted Average Risk Grade For Loans | Grade	11	11
Risk Grade Eleven [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [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
Loans and Leases Receivable, Net of Deferred Income	$ 0	$ 856
Weighted Average Risk Grade For Loans | Grade	11	11
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]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 135
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	7,922
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	3,263
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	676
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	411
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	370
Financing Receivable, Revolving	962
Loans and Leases Receivable, Net of Deferred Income	$ 15,713	$ 14,528
Weighted Average Risk Grade For Loans | Grade	12	12
Risk Grade Twelve [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	34
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	4,777
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	90
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	323
Financing Receivable, Revolving	4,641
Loans and Leases Receivable, Net of Deferred Income	$ 9,964	$ 13,741
Weighted Average Risk Grade For Loans | Grade	12	12
Risk Grade Twelve [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,896
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,723
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	2,511
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	824
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,051
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,492
Financing Receivable, Revolving	0
Loans and Leases Receivable, Net of Deferred Income	$ 11,497	$ 15,224
Weighted Average Risk Grade For Loans | Grade	12	12
Risk Grade Twelve [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,432
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
Loans and Leases Receivable, Net of Deferred Income	$ 1,432	$ 1,684
Weighted Average Risk Grade For Loans | Grade	12	12
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]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 180
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	948
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	471
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	1,105
Loans and Leases Receivable, Net of Deferred Income	$ 3,636	$ 5,321
Weighted Average Risk Grade For Loans | Grade	13	13
Risk Grade Thirteen [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	443
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	7,529
Loans and Leases Receivable, Net of Deferred Income	$ 13,383	$ 9,427
Weighted Average Risk Grade For Loans | Grade	13	13
Risk Grade Thirteen [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 200
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	300
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
Loans and Leases Receivable, Net of Deferred Income	$ 750	$ 513
Weighted Average Risk Grade For Loans | Grade	13	13
Risk Grade Thirteen [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	200
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
Loans and Leases Receivable, Net of Deferred Income	$ 200	$ 0
Weighted Average Risk Grade For Loans | Grade	13	13
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	6.17
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	6.26
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	7.30
Revolving Credit Facility [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.24
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	7.26
Revolving Loans Converted to Term [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	$ 87,232
Weighted Average Risk Grade For Loans | Grade	7.60
Revolving Loans Converted to Term [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	$ 77,365
Weighted Average Risk Grade For Loans | Grade	6.76
Revolving Loans Converted to Term [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	$ 88,090
Weighted Average Risk Grade For Loans | Grade	7.15
Revolving Loans Converted to Term [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	$ 0
Weighted Average Risk Grade For Loans | Grade	0
Revolving Loans Converted to Term [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	$ 88,090
Weighted Average Risk Grade For Loans | Grade	7.15
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	$ 65,603
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	68,437
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	75,360
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	5,272
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	2,574
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	6,858
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	1,652
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	478
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	0
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	11,799
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	3,479
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	5,872
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	1,974
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	99
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	932
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	2,298
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	5.97
2021 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.37
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	7.13
2021 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.19
2020 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.35
2020 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	8.13
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	7.27
2020 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.87
2020 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.19
2019 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.91
2019 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	9.98
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	7.40
2019 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.46
2019 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.41
2018 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.04
2018 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	8.77
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	7.37
2018 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.45
2018 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.38
2017 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.20
2017 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.19
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	7.38
2017 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	9
2017 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.38
Prior Years [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	5.94
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	7.84
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	7.04
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	7.04

Loans - Weighted Average Risk G

Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) $ in Thousands	Sep. 30, 2021USD ($)Grade	Dec. 31, 2020USD ($)Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 15,832,670	$ 17,481,309
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.29	6.45
Loans and Leases Receivable, Net of Deferred Income	$ 4,929,656	$ 4,955,341
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.55	6.85
Loans and Leases Receivable, Net of Deferred Income	$ 978,211	$ 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	7.27	7.34
Loans and Leases Receivable, Net of Deferred Income	$ 5,984,390	$ 5,796,653
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.16	7.22
Loans and Leases Receivable, Net of Deferred Income	$ 1,266,633	$ 1,223,814
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.25	7.32
Loans and Leases Receivable, Net of Deferred Income	$ 7,251,023	$ 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	6.04	6.13
Loans and Leases Receivable, Net of Deferred Income	$ 4,598,663	$ 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	5.94	5.99
Loans and Leases Receivable, Net of Deferred Income	$ 848,158	$ 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	6.90	6.97
Loans and Leases Receivable, Net of Deferred Income	$ 5,221,458	$ 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	7.03	6.99
Loans and Leases Receivable, Net of Deferred Income	$ 1,182,066	$ 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	$ 208,283	$ 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	$ 48,694	$ 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	$ 351,931	$ 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	$ 82,935	$ 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	$ 57,263	$ 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	$ 2,824	$ 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	$ 274,212	$ 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	$ 0	$ 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	$ 46,098	$ 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	$ 55,188	$ 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	$ 124,542	$ 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	$ 0	$ 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	$ 15,713	$ 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	$ 9,964	$ 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	$ 11,497	$ 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	$ 1,432	$ 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	$ 3,636	$ 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	$ 13,383	$ 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	$ 750	$ 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	$ 200	$ 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	Sep. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	$ 116,865
Current Loans	15,715,805
Loans and Leases Receivable, Net of Deferred Income	15,832,670	$ 17,481,309
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	6,249
Current Loans	1,356,790
Loans and Leases Receivable, Net of Deferred Income	1,363,039	1,330,774
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	4,707
Current Loans	478,214
Loans and Leases Receivable, Net of Deferred Income	482,921	$ 505,680
Financing Receivables, 30 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	72,678
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	4,148
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	3,678
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	44,187
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	2,101
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	1,029
Revolving Credit Facility [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,141
Current Loans	464,440
Loans and Leases Receivable, Net of Deferred Income	465,581
Revolving Credit Facility [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	481
Current Loans	374,993
Loans and Leases Receivable, Net of Deferred Income	375,474
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	174
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	481
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	967
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	0
Revolving Loans Converted to Term [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,526
Current Loans	11,972
Loans and Leases Receivable, Net of Deferred Income	13,498
Revolving Loans Converted to Term [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,388
Current Loans	26,196
Loans and Leases Receivable, Net of Deferred Income	27,584
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	1,384
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	1,035
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	142
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	353
2021 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	396
Current Loans	235,725
Loans and Leases Receivable, Net of Deferred Income	236,121
2021 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	2,103
Current Loans	39,156
Loans and Leases Receivable, Net of Deferred Income	41,259
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	396
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	1,527
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	576
2020 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	34
Current Loans	283,853
Loans and Leases Receivable, Net of Deferred Income	283,887
2020 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	293
Current Loans	20,924
Loans and Leases Receivable, Net of Deferred Income	21,217
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	34
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	283
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	10
2019 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	387
Current Loans	111,449
Loans and Leases Receivable, Net of Deferred Income	111,836
2019 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	182
Current Loans	10,442
Loans and Leases Receivable, Net of Deferred Income	10,624
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	277
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	182
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	110
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	381
Current Loans	62,416
Loans and Leases Receivable, Net of Deferred Income	62,797
2018 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	179
Current Loans	3,268
Loans and Leases Receivable, Net of Deferred Income	3,447
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	370
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	89
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	11
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	90
2017 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	552
Current Loans	54,827
Loans and Leases Receivable, Net of Deferred Income	55,379
2017 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	63
Current Loans	1,844
Loans and Leases Receivable, Net of Deferred Income	1,907
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	326
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	63
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	226
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	1,832
Current Loans	132,108
Loans and Leases Receivable, Net of Deferred Income	133,940
Prior Years [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	18
Current Loans	1,391
Loans and Leases Receivable, Net of Deferred Income	1,409
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	1,187
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	18
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	645
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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	$ 71,929	$ 23,920	$ 142,704	$ 89,546
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	12,009	10,224	39,551	25,340
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	52,055	7,144	57,871	38,642
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	6,012	637	37,093	8,094
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	0	0	0	0
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	330	421	1,172	2,132
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	$ 1,523	$ 5,494	$ 7,017	$ 15,338

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	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	$ 250,150		$ 263,177
Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	63,766	$ 65,271	73,843	$ 72,505	$ 98,536	$ 51,593
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	20,449	28,010	39,553	54,631	40,817	37,382
Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	152,054	147,993	134,892	118,593	93,425	31,037
Total consumer real estate loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	6,029	6,154	7,926	10,277	8,998	4,113
Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	7,852	7,860	6,963	7,469	8,285	8,042
Loans Receivable [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	250,150	$ 255,288	263,177	$ 263,475	$ 250,061	$ 132,167
Modeled Expected Credit Losses [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	81,736		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	45,395		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	6,962		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	17,033		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	5,934		7,926
Modeled Expected Credit Losses [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	6,412		6,945
Q-factor adjustments [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	149,755		33,346
Q-factor adjustments [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	14,081		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	104		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	134,071		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	59		0
Q-factor adjustments [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	1,440		0
Specific Valuation Allowances [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	18,659		15,279
Specific Valuation Allowances [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	4,290		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	13,383		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	950		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	36		0
Specific Valuation Allowances [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss	$ 0		$ 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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance			$ 263,177
Impact of adopting ASC 326	$ 0	$ 0	0	$ 39,377
Ending balance	250,150		250,150
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	65,271	98,536	73,843	51,593
Impact of adopting ASC 326				21,263
Credit loss, expense (reversal)	(1,020)	(18,547)	(8,886)	10,737
Charge-offs	(1,611)	(8,605)	(4,485)	(14,815)
Recoveries	1,126	1,121	3,294	3,727
Net charge-offs	(485)	(7,484)	(1,191)	(11,088)
Ending balance	63,766	72,505	63,766	72,505
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	28,010	40,817	39,553	37,382
Impact of adopting ASC 326				(10,453)
Credit loss, expense (reversal)	(7,720)	13,814	(19,048)	96,478
Charge-offs	0	0	(1,433)	(68,842)
Recoveries	159	0	1,377	66
Net charge-offs	159	0	(56)	(68,776)
Ending balance	20,449	54,631	20,449	54,631
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	147,993	93,425	134,892	31,037
Impact of adopting ASC 326				(13,519)
Credit loss, expense (reversal)	4,007	25,368	16,583	104,716
Charge-offs	0	(242)	(137)	(3,826)
Recoveries	54	42	716	185
Net charge-offs	54	(200)	579	(3,641)
Ending balance	152,054	118,593	152,054	118,593
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	6,154	8,998	7,926	4,113
Impact of adopting ASC 326				2,392
Credit loss, expense (reversal)	(982)	1,794	(3,093)	3,716
Charge-offs	0	(1,088)	(672)	(1,508)
Recoveries	857	573	1,868	1,564
Net charge-offs	857	(515)	1,196	56
Ending balance	6,029	10,277	6,029	10,277
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	7,860	8,285	6,963	8,042
Impact of adopting ASC 326				(2,248)
Credit loss, expense (reversal)	2,692	1,161	7,042	8,096
Charge-offs	(5,073)	(4,219)	(13,015)	(13,402)
Recoveries	2,373	2,242	6,862	6,981
Net charge-offs	(2,700)	(1,977)	(6,153)	(6,421)
Ending balance	7,852	7,469	7,852	7,469
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	255,288	250,061	263,177	132,167
Impact of adopting ASC 326				(2,565)
Credit loss, expense (reversal)	(3,023)	23,590	(7,402)	223,743
Charge-offs	(6,684)	(14,154)	(19,742)	(102,393)
Recoveries	4,569	3,978	14,117	12,523
Net charge-offs	(2,115)	(10,176)	(5,625)	(89,870)
Ending balance	$ 250,150	$ 263,475	$ 250,150	$ 263,475

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	Sep. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 15,832,670	$ 17,481,309
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	4,929,656	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	978,211	1,235,198
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	827,820	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	5,984,390	5,796,653
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,266,633	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	1,363,039	1,330,774
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	482,921	505,680
Specific Valuation Allowances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	18,659	15,279
Specific Valuation Allowances [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	4,290	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	13,383	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	750	513
Specific Valuation Allowances [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	200	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	36	0
Specific Valuation Allowances [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	0	18
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	78,941	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	22,717	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	25,671	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	28,619	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	1,632	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	$ 0	$ 18

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Schedule of Goodwill (Detail) - USD ($) $ in Thousands	Sep. 30, 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	Sep. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, net	$ 1,019	$ 1,563
Core deposits [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, net	845	1,296
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, net	$ 174	$ 267

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Estimated Aggregate Future Amortization Expense for Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Remainder of 2021	$ 153
2022	481
2023	282
2024	87
2025	11
Thereafter	5
Future Amortization Expense, Total	$ 1,019	$ 1,563

Deposits - Schedule of Deposits

Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 17,151,583	$ 15,117,051
Interest-bearing Deposit Liabilities, Total	22,461,184	19,898,710
Total deposits	$ 39,612,767	$ 35,015,761
Total non-interest-bearing demand deposits, Percentage of Total	43.30%	43.20%
Percentage of Interest-bearing Domestic Deposits to Deposits	56.70%	56.80%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 15,997,472	$ 13,914,754
Total non-interest-bearing demand deposits, Percentage of Total	40.40%	39.70%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 223,466	$ 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	$ 10,326,315	$ 9,132,789
Money market accounts	10,381,703	8,977,585
Time Deposits	1,086,894	1,135,575
Interest-bearing Deposit Liabilities, Total	$ 21,794,912	$ 19,245,949
Savings and interest checking, Percentage of Total	26.10%	26.10%
Money market accounts, Percentage of Total	26.20%	25.60%
Percentage Of Time Accounts To Deposits	2.70%	3.30%
Percentage of Interest-bearing Domestic Deposits to Deposits	55.00%	55.00%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 930,645	$ 960,072
Savings and interest checking	580,174	597,503
Money market accounts	61,028	50,070
Time Deposits	25,070	5,188
Interest-bearing Deposit Liabilities, Total	$ 666,272	$ 652,761
Total non-interest-bearing demand deposits, Percentage of Total	2.30%	2.80%
Savings and interest checking, Percentage of Total	1.50%	1.70%
Money market accounts, Percentage of Total	0.10%	0.10%
Percentage Of Time Accounts To Deposits	0.10%	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	Sep. 30, 2021	Dec. 31, 2020
Disclosure Text Block [Abstract]
Deposits from the Certificate of Deposit Account Registry Service (CDARS) deposits	$ 0	$ 372
Deposits from foreign sources (primarily Mexico)	1,007,416	884,169
Total deposits not covered by deposit insurance	21,419,285	18,694,320
Time deposits not covered by deposit insurance	$ 239,598	$ 237,298

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Operating Lease, Right-of-Use Asset	$ 278,600		$ 278,600		$ 292,100
Operating Lease, Liability	310,300		310,300		$ 323,000
Lessee, Operating Lease, Liability, Payments, Due	$ 7,900	$ 8,000	24,000	$ 23,800
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	Sep. 30, 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	$ 225,311	$ 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	1,935	1,723
Commitments to Extend Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 10,262,077	$ 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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Off-Balance Sheet, Credit Loss, Liability	$ 48,596	$ 46,939	$ 44,152	$ 500
Impact of adopting ASC 326	0	0	0	39,377
Credit loss expense	3,023	(3,276)	7,467	3,786
Off-Balance Sheet, Credit Loss, Liability	$ 51,619	$ 43,663	$ 51,619	$ 43,663

Commitments and Contingencies_3

Commitments and Contingencies Commitments and Contingencies - Components of Lease Commitments Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Amortization of lease right-of-use assets	$ 8,172	$ 8,521	$ 24,584	$ 24,545
Short-term lease expense	384	172	1,118	1,343
Non-lease components (including taxes, insurance, common maintenance, etc.)	2,864	2,586	8,205	8,410
Total operating lease expense, excluding rent	$ 11,420	$ 11,279	$ 33,907	$ 34,298

Capital and Regulatory Matter_2

Capital and Regulatory Matters - Additional Information (Detail)	Jan. 27, 2021USD ($)	Nov. 19, 2020USD ($)$ / sharesshares	Mar. 16, 2020USD ($)shares	Feb. 15, 2013USD ($)	Dec. 31, 2021USD ($)	Sep. 30, 2021USD ($)$ / sharesshares	Sep. 30, 2020USD ($)shares	Mar. 31, 2020USD ($)shares	Sep. 30, 2021USD ($)quarter$ / sharesshares	Sep. 30, 2020USD ($)shares	Dec. 31, 2020USD ($)$ / sharesshares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
CECL Transition Amount						$ 62,300,000			$ 62,300,000		$ 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 (478 shares QTD and 12,103 shares during 2021 and 400 shares QTD and 182,225 shares during 2020) | shares						478	400		12,103	182,225
Treasury Stock, Value, Acquired, Cost Method						$ (54,000)	$ (27,000)		$ (1,342,000)	$ (14,000,000)
Maximum dividends available without prior regulatory approval						445,800,000			$ 445,800,000
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		$ 1,000
Preferred stock, shares issued | shares		150,000				150,000			150,000		150,000
Preferred stock, par value | $ / shares		$ 0.01				$ 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		$ 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
Cullen/Frost [Member] | Trust Preferred Securities Tier Two Capital Allowable Portion [Member] | WNB Capital Trust I | Subsequent Event
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Payments for Repurchase of Trust Preferred Securities					$ 13,000,000
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1						0			$ 0		0
stock repurchase 2019 plan [Member] | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Purchase of treasury stock (478 shares QTD and 12,103 shares during 2021 and 400 shares QTD and 182,225 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 (478 shares QTD and 12,103 shares during 2021 and 400 shares QTD and 182,225 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		$ 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	Sep. 30, 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	$ 3,300,979	$ 3,058,447
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	13.42%	12.86%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,722,459	$ 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	$ 1,599,426	$ 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	$ 3,446,431	$ 3,203,899
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.01%	13.47%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,091,558	$ 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	$ 1,968,525	$ 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	$ 3,911,272	$ 3,672,912
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.90%	15.44%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,583,689	$ 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	$ 2,460,656	$ 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	$ 3,446,431	$ 3,203,899
Leverage Ratio, Actual, Ratio	7.52%	8.07%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,833,156	$ 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	$ 2,291,445	$ 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	$ 3,215,116	$ 3,030,093
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	13.09%	12.77%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,719,451	$ 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	$ 1,596,633	$ 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	$ 3,215,116	$ 3,030,093
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.09%	12.77%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,087,905	$ 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	$ 1,965,087	$ 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	$ 3,446,957	$ 3,266,106
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.03%	13.76%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,579,177	$ 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	$ 2,456,359	$ 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	$ 3,215,116	$ 3,030,093
Leverage Ratio, Actual, Ratio	7.02%	7.63%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,832,498	$ 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	$ 2,290,623	$ 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			9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Derivative [Line Items]
Weighted-average strike rate for outstanding interest rate caps				3.12%
Approximate credit exposure related to swaps with bank customers	$ 227,400			$ 227,400
Cash collateral for borrowed securities deposited with other financial institutions	200,900			200,900
Interest rate swaps with upstream financial institution counterparties [Member]
Derivative [Line Items]
Approximate credit exposure related to swaps with bank customers	12,500			12,500
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		$ 0
Other Income [Member] | Not Designated as Hedging Instrument [Member] | Put Option [Member]
Derivative [Line Items]
Amount of gain (loss) included in income	$ 0	$ 0	$ 6,000	$ 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	Sep. 30, 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	$ 2,758	$ 3,724
Estimated Fair Value	(59)	(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	997,411	1,173,173
Estimated Fair Value	(22,874)	(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	127,420	0
Estimated Fair Value	641	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	405,163	356,601
Estimated Fair Value	2,915	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	997,411	1,173,173
Estimated Fair Value	49,630	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	127,420	0
Estimated Fair Value	(1,761)	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	405,163	356,601
Estimated Fair Value	$ (2,915)	$ (1,241)

Derivative Financial Instrume_5

Derivative Financial Instruments - Schedule of Weighted-Average Rates Paid and Received for Interest Rate Swaps Outstanding (Detail)	Sep. 30, 2021
Weighted Average Interest Rate Paid [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Fair value hedge loan/lease interest rate swaps	2.75%
Weighted-Average Interest Rate Received [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Fair value hedge loan/lease interest rate swaps	0.09%
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	3.78%
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	1.87%
Customer Counterparties [Member] | Weighted Average Interest Rate Paid [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Non-hedging interest rate swaps – customer counterparties	1.87%
Customer Counterparties [Member] | Weighted-Average Interest Rate Received [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Non-hedging interest rate swaps – customer counterparties	3.78%

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	Sep. 30, 2021USD ($)MMBTUbbl	Dec. 31, 2020USD ($)bblMMBTU
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	5,632	3,056
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	12,228	6,391
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 21,271	$ 8,341
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (147,576)	$ (32,112)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	18,908	9,281
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	39,901	15,079
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 3,312	$ 1,529
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (53,923)	$ (3,265)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	12,276	6,391
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	5,584	3,056
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 148,742	$ 32,670
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (21,245)	$ (8,264)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	39,901	17,636
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	18,908	6,724
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 54,202	$ 3,451
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (3,310)	$ (1,458)

Derivative Financial Instrume_7

Derivative Financial Instruments - Schedule of Notional Amounts and Fair Values of Open Foreign Currency Forward Contracts (Details) $ in Thousands, $ in Thousands	Sep. 30, 2021USD ($)	Sep. 30, 2021CAD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2020CAD ($)
Financial Institution Counterparties [Member] | Foreign Exchange Forward [Member]
Derivative Instruments [Line Items]
Forward Contracts – Assets Estimated Fair Value	$ 2		$ 0
Financial Institution Counterparties [Member] | Other Assets [Member] | Non Designated as Hedging Instrument [Member]
Derivative Instruments [Line Items]
Forward Contracts – Assets Notional Amount		$ 658		$ 0
Customer Counterparties [Member] | Foreign Exchange Forward [Member]
Derivative Instruments [Line Items]
Forward Contracts – Assets Estimated Fair Value	$ 2		$ 0
Customer Counterparties [Member] | Other Assets [Member] | Non Designated as Hedging Instrument [Member]
Derivative Instruments [Line Items]
Forward Contracts – Assets Notional Amount		$ 658		$ 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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Interest Income on Loans [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ (21)	$ (36)	$ (74)	$ (79)
Other Non-Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ 3	$ 2	$ 8	$ 6

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			9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 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)	$ 946	$ 280		$ 2,674	$ 2,615
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)	0	1		(1)	1
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)	1,143	379		3,266	1,057
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)	4	10		34	28
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	$ 0	$ 6,000	$ 0	$ 5,980

Balance Sheet Offsetting - Fina

Balance Sheet Offsetting - Financial Instruments Eligible for Offset Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	$ 28,141	$ 11,111
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	28,141	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	36,044	11,111
Gross Amount Offset, Financial assets	0	0
Net Amount Recognized, Financial assets	36,044	11,111
Gross Amount Recognized, Derivatives, Financial Liabilities	224,432	69,323
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	224,432	69,323
Gross Amount Recognized, Repurchase agreements, Financial liabilities	2,200,029	2,068,147
Gross Amount Offset, Repurchase agreements, Financial liabilities	0	0
Net Amount Recognized, Repurchase agreements, Financial liabilities	2,200,029	2,068,147
Gross Amount Recognized, Financial liabilities	2,424,461	2,137,470
Gross Amount Offset, Financial liabilities	0	0
Net Amount Recognized, Financial Liabilities	2,424,461	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	3,556	1,241
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	3,556	1,241
Commodity swaps and options [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	24,583	9,870
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	24,583	9,870
Gross Amount Recognized, Derivatives, Financial Liabilities	201,499	35,377
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	201,499	35,377
Loan/lease interest rate swaps [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Liabilities	22,933	33,946
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	22,933	$ 33,946
Foreign currency forward contracts [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	2
Gross Amount Offset, Derivatives, Financial assets	0
Net Amount Recognized, Derivatives, Financial assets	$ 2

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	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 28,141	$ 11,111
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(28,141)	(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	36,044	11,111
Gross Amounts Not Offset Collateral, Financial Assets	(7,903)	0
Net Amount, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	224,432	69,323
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(28,141)	(11,111)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(188,436)	(57,031)
Net Amount, Derivatives, Financial Liabilities	7,855	1,181
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	2,200,029	2,068,147
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(2,200,029)	(2,068,147)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	2,424,461	2,137,470
Gross Amounts Not Offset Collateral, Financial Liabilities	(2,388,465)	(2,125,178)
Net Amount, Financial Liabilities	7,855	1,181
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	4	2
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(4)	(2)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	4,577	6,430
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(4)	(2)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(4,573)	(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	12,537	5,838
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(12,537)	(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	77,110	20,722
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(12,537)	(5,838)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(60,860)	(14,700)
Net Amount, Derivatives, Financial Liabilities	3,713	184
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Liabilities	19	71
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(19)	(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	15,600	5,271
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(15,600)	(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	142,726	42,100
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(15,600)	(5,271)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(122,984)	(35,832)
Net Amount, Derivatives, Financial Liabilities	$ 4,142	$ 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	Sep. 30, 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	$ 2,200,029	$ 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	2,200,029	2,068,147
Securities Sold under Agreements to Repurchase, Gross	2,200,029	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	1,122,968	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	1,122,968	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	1,077,061	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	$ 1,077,061	$ 1,375,287

Stock-Based Compensation Stock-

Stock-Based Compensation Stock-Based Compensation - Narrative (Details)	Sep. 30, 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	917,440

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Activity in Corporation's Active Stock Plans (Detail) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options exercised, Number of Shares	(21,968)	(53,425)	(650,044)	(196,948)
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
Granted, Number of Units			5,940
Stock Awards exercised/vested, Number of Shares			(2,499)
Ending Balance, Number of Shares/Units	56,301		56,301
Beginning Balance, Weighted-Average Grant-Date Fair Value			$ 75.47
Granted Weighted-Average Grant-Date Fair Value			117.90
Stock Award Exercised/Vested, Weighted Average Grant Date Fair Value			92.03
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 79.21		$ 79.21
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
Stock Awards exercised/vested, Number of Shares			(1,213)
Forfeited/expired, Number of Shares/Units			(26,439)
Ending Balance, Number of Shares/Units	442,707		442,707
Beginning Balance, Weighted-Average Grant-Date Fair Value			$ 86.24
Stock Award Exercised/Vested, Weighted Average Grant Date Fair Value			98.90
Forfeited/Expired, Weighted-Average Price			87.08
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 86.16		$ 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			(611,201)
Ending Balance, Number of Shares	1,128,358		1,128,358
Beginning Balance, Weighted-Average Exercise Price			$ 66.11
Stock options exercised, Weighted-Average Exercise Price			61.86
Ending Balance, Weighted-Average Exercise Price	$ 68.41		$ 68.41
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			(9,752)
Ending Balance, Number of Shares/Units	156,374		156,374
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			75.70
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 73.88		$ 73.88

Stock-Based Compensation - Shar

Stock-Based Compensation - Shares Issued in Connection with Stock Compensation Awards (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
New shares issued from available authorized shares	0	0	0	0
Shares issued from available treasury stock	21,968	53,425	650,044	196,948
Total	21,968	53,425	650,044	196,948
Proceeds from stock option exercises	$ 1,128	$ 2,698	$ 37,810	$ 7,544

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	$ 1,985	$ 2,443	$ 7,756	$ 8,227
Income tax benefit	281	450	1,162	1,539
Non-Vested Stock Awards/Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	1,559	1,999	5,210	6,325
Director deferred stock units outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	0	0	700	770
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	$ 426	$ 444	$ 1,846	$ 1,132

Stock-Based Compensation - Unre

Stock-Based Compensation - Unrecognized Stock-Based Compensation Expense (Detail) $ in Thousands	Sep. 30, 2021USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 15,368
Non-Vested Stock Awards/Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	10,918
Maximum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 4,450

Earnings Per Common Share - Bas

Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income	$ 107,975	$ 95,056	$ 342,058	$ 242,881
Less: Preferred stock dividends	1,668	0	5,488	2,016
Redemption of preferred stock	0	0	0	5,514
Net income available to common shareholders	106,307	95,056	336,570	235,351
Less: Earnings allocated to participating securities	932	877	3,228	2,236
Distributed earnings allocated to common stock	47,740	44,546	139,224	133,511
Undistributed earnings allocated to common stock	57,635	49,633	194,118	99,604
Net earnings allocated to common stock	$ 105,375	$ 94,179	$ 333,342	$ 233,115
Weighted-average shares outstanding for basic earnings per common share	63,652,020	62,726,542	63,522,884	62,655,393
Dilutive effect of stock compensation	444,636	193,116	489,219	263,028
Weighted-average shares outstanding for diluted earnings per common share	64,096,656	62,919,658	64,012,103	62,918,421

Defined Benefit Plans Additiona

Defined Benefit Plans Additional Information (Details) $ in Thousands	9 Months Ended
	Sep. 30, 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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Retirement Benefits [Abstract]
Expected return on plan assets, net of expenses	$ (3,210)	$ (3,073)	$ (9,630)	$ (9,217)
Interest cost on projected benefit obligation	835	1,252	2,506	3,757
Net amortization and deferral	1,529	1,330	4,587	3,989
Net periodic expense (benefit)	$ (846)	$ (491)	$ (2,537)	$ (1,471)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Deferred Tax Liabilities, Net	$ 78.9		$ 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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Current income tax expense (benefit)	$ 13,849	$ (33,614)	$ 31,639	$ 20,328
Deferred income tax expense (benefit)	(516)	43,130	4,672	(8,803)
Income tax expense, as reported	$ 13,333	$ 9,516	$ 36,311	$ 11,525
Effective tax rate	11.00%	9.10%	9.60%	4.50%

Other Comprehensive Income (L_3

Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (80,668)	$ 20,332	$ (209,779)	$ 406,959
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(237)	(294)	(741)	(979)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	0	0	(108,989)
Total securities available for sale and transferred securities, before tax amount	(80,905)	20,038	(210,520)	296,991
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	4,587	3,989
Total defined-benefit post-retirement benefit plans	1,529	1,330	4,587	3,989
Total other comprehensive income (loss), before tax	(79,376)	21,368	(205,933)	300,980
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(16,941)	4,269	(44,054)	85,462
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(49)	(62)	(155)	(206)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	0	0	(22,888)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(16,990)	4,207	(44,209)	62,368
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	280	963	837
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	321	280	963	837
Total other comprehensive income (loss), Tax Expense (Benefit)	(16,669)	4,487	(43,246)	63,205
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(63,727)	16,063	(165,725)	321,497
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(188)	(232)	(586)	(773)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	0	0	(86,101)
Total securities available for sale and transferred securities, Net of Tax	(63,915)	15,831	(166,311)	234,623
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	1,050	3,624	3,152
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	1,208	1,050	3,624	3,152
Other comprehensive income (loss), net of tax	$ (62,707)	$ 16,881	$ (162,687)	$ 237,775

Other Comprehensive Income - Sc

Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 512,970
Other comprehensive income (loss), net of tax	$ (62,707)	$ 16,881	(162,687)	$ 237,775
Ending Balance	350,283		350,283
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			563,801	313,304
Other comprehensive income (loss) before reclassifications			(166,311)	320,724
Reclassification of amounts included in net income			0	(86,101)
Other comprehensive income (loss), net of tax			(166,311)	234,623
Ending Balance	397,490	547,927	397,490	547,927
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			3,624	3,152
Other comprehensive income (loss), net of tax			3,624	3,152
Ending Balance	(47,207)	(42,782)	(47,207)	(42,782)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			512,970	267,370
Other comprehensive income (loss) before reclassifications			(166,311)	320,724
Reclassification of amounts included in net income			3,624	(82,949)
Other comprehensive income (loss), net of tax			(162,687)	237,775
Ending Balance	$ 350,283	$ 505,145	$ 350,283	$ 505,145

Operating Segments - Additional

Operating Segments - Additional Information (Detail)	9 Months Ended
	Sep. 30, 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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 246,122	$ 243,423	$ 744,159	$ 733,755
Credit loss expense	0	20,302	63	227,474
Total non-interest income	93,189	83,601	277,672	374,117
Total non-interest expense	218,003	202,150	643,399	625,992
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	121,308	104,572	378,369	254,406
Income taxes	13,333	9,516	36,311	11,525
Net income	107,975	95,056	342,058	242,881
Less: Preferred stock dividends	1,668	0	5,488	2,016
Redemption of preferred stock	0	0	0	5,514
Net Income (Loss) Available to Common Stockholders, Basic	106,307	95,056	336,570	235,351
Revenues from (expenses to) external customers	339,311	327,024	1,021,831	1,107,872
Bank
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	247,381	244,685	748,036	737,977
Credit loss expense	(6)	20,300	57	227,474
Total non-interest income	51,279	49,170	154,003	265,492
Total non-interest expense	185,891	170,290	548,151	525,882
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	112,775	103,265	353,831	250,113
Income taxes	12,029	9,758	32,811	12,090
Net income	100,746		321,020	238,023
Less: Preferred stock dividends	0		0	0
Redemption of preferred stock				0
Net Income (Loss) Available to Common Stockholders, Basic	100,746	93,507	321,020	238,023
Revenues from (expenses to) external customers	298,660	293,855	902,039	1,003,469
Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	536	602	1,530	2,163
Credit loss expense	6	2	6	0
Total non-interest income	42,369	34,680	124,512	109,330
Total non-interest expense	31,086	30,052	91,037	94,225
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	11,813	5,228	34,999	17,268
Income taxes	2,481	1,098	7,350	3,626
Net income	9,332		27,649	13,642
Less: Preferred stock dividends	0		0	0
Redemption of preferred stock				0
Net Income (Loss) Available to Common Stockholders, Basic	9,332	4,130	27,649	13,642
Revenues from (expenses to) external customers	42,905	35,282	126,042	111,493
Non-Banks [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	(1,795)	(1,864)	(5,407)	(6,385)
Credit loss expense	0	0	0	0
Total non-interest income	(459)	(249)	(843)	(705)
Total non-interest expense	1,026	1,808	4,211	5,885
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(3,280)	(3,921)	(10,461)	(12,975)
Income taxes	(1,177)	(1,340)	(3,850)	(4,191)
Net income	(2,103)		(6,611)	(8,784)
Less: Preferred stock dividends	1,668		5,488	2,016
Redemption of preferred stock				5,514
Net Income (Loss) Available to Common Stockholders, Basic	(3,771)	(2,581)	(12,099)	(16,314)
Revenues from (expenses to) external customers	$ (2,254)	$ (2,113)	$ (6,250)	$ (7,090)

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 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	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	$ 25,292	$ 24,456
Derivative assets	28,141	11,111
Derivative liabilities	224,432	69,323
Securities available for sale, at estimated fair value	11,731,222	10,437,565
Interest rate swaps, caps and floors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	53,186	85,665
Derivative liabilities	27,609	35,187
Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	227,527	45,991
Derivative liabilities	226,054	45,099
Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	4
U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	24,087	23,996
Securities available for sale, at estimated fair value	1,685,221	1,119,633
Residential Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	2,491,154	1,987,679
States and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	1,205	460
Securities available for sale, at estimated fair value	7,512,489	7,287,902
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	42,358	42,351
Level 1 [Member] | Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	4
Level 1 [Member] | U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	24,087	23,996
Securities available for sale, at estimated fair value	1,685,221	1,119,633
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	53,186	85,665
Derivative liabilities	27,609	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	227,527	45,535
Derivative liabilities	226,054	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, at estimated fair value	2,491,154	1,987,679
Level 2 [Member] | States and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	1,205	460
Securities available for sale, at estimated fair value	7,512,489	7,287,902
Level 2 [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	42,358	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	Sep. 30, 2021	Sep. 30, 2020
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	$ 3,329	$ 6,388
Impaired Financing Receivable, Reversal of Specific Valuation Allowance Allocations	(336)	(930)
Loans receivable, fair value disclosure	2,993	5,458
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	15,274	8,660
Impaired Financing Receivable, Reversal of Specific Valuation Allowance Allocations	(4,791)
Impaired Financing Receivable, Specific Valuation Allowance Allocation		14,928
Loans receivable, fair value disclosure	$ 10,483	$ 23,588

Fair Value Measurements - Forec

Fair Value Measurements - Foreclosed Assets Remeasured and Reported at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 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	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 16,253,971	$ 10,288,853	$ 7,175,680	$ 3,788,181
Securities held to maturity	1,759,428	1,945,833
Loans, net	15,582,520	17,218,132
Deposits	39,612,767	35,015,761
Federal funds purchased	27,200	48,850
Repurchase agreements	2,200,029	2,068,147
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,400	136,357
Subordinated notes	99,139	99,021
Carrying Amount [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	16,253,971	10,288,853
Securities held to maturity	1,759,270	1,945,673
Cash surrender value of life insurance policies	189,941	189,984
Accrued interest receivable	119,232	181,432
Deposits	39,612,767	35,015,761
Federal funds purchased	27,200	48,850
Repurchase agreements	2,200,029	2,068,147
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,400	136,357
Subordinated notes	99,139	99,021
Accrued interest payable	2,126	8,127
Carrying Amount [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	15,582,520	17,218,132
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	16,253,971	10,288,853
Securities held to maturity	1,825,568	2,052,896
Cash surrender value of life insurance policies	189,941	189,984
Accrued interest receivable	119,232	181,432
Deposits	39,613,343	35,018,185
Federal funds purchased	27,200	48,850
Repurchase agreements	2,200,029	2,068,147
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	137,115	137,115
Subordinated notes	113,223	115,717
Accrued interest payable	2,126	8,127
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 15,656,269	$ 17,390,683