Cullen/Frost Bankers (CFR) 10-Q 30 Oct 20 2020 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - $ / shares	9 Months Ended
	Sep. 30, 2020	Oct. 22, 2020	Dec. 31, 2019
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
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
Title of 12(b) Security	Common Stock, $.01 Par Value
Trading Symbol	CFR
Security Exchange Name	NYSE
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		62,854,581
Common stock, par value	$ 0.01		$ 0.01
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Entity Central Index Key	0000039263
Current Fiscal Year End Date	--12-31

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Cash and due from banks	$ 499,396	$ 581,857
Interest-bearing deposits	6,655,784	2,849,950
Federal funds sold and resell agreements	20,500	356,374
Total cash and cash equivalents	7,175,680	3,788,181
Securities held to maturity, net of allowance for credit losses of $160 at September 30, 2020	1,951,384	2,030,005
Securities available-for-sale, at estimated fair value	10,604,480	11,269,591
Trading account securities	25,808	24,298
Loans, net of unearned discounts	18,223,877	14,750,332
Less: Allowance for credit losses on loans	(263,475)	(132,167)
Net loans	17,960,402	14,618,165
Premises and equipment, net	1,055,064	1,011,947
Goodwill	654,952	654,952
Other intangible assets, net	1,771	2,481
Cash surrender value of life insurance policies	189,148	187,156
Accrued interest receivable and other assets	482,551	440,652
Total assets	40,101,240	34,027,428
Liabilities:
Non-interest-bearing demand deposits	14,845,770	10,873,629
Interest-bearing deposits	18,653,733	16,765,935
Total deposits	33,499,503	27,639,564
Federal funds purchased and repurchase agreements	1,608,667	1,695,342
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,342	136,299
Subordinated notes, net of unamortized issuance costs	98,982	98,865
Accrued interest payable and other liabilities	672,720	545,690
Total liabilities	36,016,214	30,115,760
Shareholders’ Equity:
Preferred stock, par value $0.01 per share; 10,000,000 shares authorized; 6,000,000 Series A shares ($25 liquidation preference) issued at December 31, 2019	0	144,486
Common stock, par value $0.01 per share; 210,000,000 shares authorized; 64,236,306 shares issued at September 30, 2020 and December 31, 2019	642	642
Additional paid-in capital	991,477	983,250
Retained earnings	2,724,681	2,667,534
Accumulated other comprehensive income (loss), net of tax	505,145	267,370
Treasury stock, at cost; 1,454,736 shares at September 30, 2020 and 1,567,302 shares at December 31, 2019	(136,919)	(151,614)
Total shareholders’ equity	4,085,026	3,911,668
Total liabilities and shareholders’ equity	$ 40,101,240	$ 34,027,428

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Securities, held-to-maturity, allowance for credit loss	$ 160,000	$ 0
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	1,454,736	1,567,302
Series A Preferred Stock [Member]
Preferred stock, par value		$ 0.01
Preferred stock, shares authorized		10,000,000
Preferred stock, shares issued		6,000,000
Preferred stock liquidation preference value		$ 25

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Interest income:
Loans, including fees	$ 169,002	$ 186,870	$ 512,444	$ 561,990
Securities:
Taxable	22,560	32,122	71,950	87,125
Tax-exempt	58,073	57,328	175,968	175,067
Interest-bearing deposits	1,512	8,759	10,894	27,226
Federal funds sold and resell agreements	19	1,194	884	4,323
Total interest income	251,166	286,273	772,140	855,731
Interest expense:
Deposits	5,397	25,637	27,677	79,655
Federal funds purchased and repurchase agreements	482	5,040	4,005	15,276
Junior subordinated deferrable interest debentures	700	1,425	2,893	4,401
Subordinated notes	1,164	1,164	3,492	3,492
Federal Home Loan Bank advances	0	0	318	0
Total interest expense	7,743	33,266	38,385	102,824
Net interest income	243,423	253,007	733,755	752,907
Credit loss expense	20,302	8,001	227,474	25,404
Net interest income after credit loss expense	223,121	245,006	506,281	727,503
Non-interest income:
Trust and investment management fees	31,469	31,649	97,002	93,794
Service charges on deposit accounts	19,812	22,941	60,043	65,529
Insurance commissions and fees	11,456	11,683	38,609	40,207
Interchange and debit card transaction fees	3,503	4,117	9,724	11,265
Other charges, commissions and fees	8,370	10,108	25,398	28,103
Net gain (loss) on securities transactions	0	96	108,989	265
Other	8,991	8,630	34,352	29,484
Total non-interest income	83,601	89,224	374,117	268,647
Non-interest expense:
Salaries and wages	93,323	93,812	282,485	277,078
Employee benefits	16,074	21,002	59,824	64,579
Net occupancy	25,466	24,202	76,116	64,602
Technology, furniture and equipment	26,482	22,415	77,768	66,236
Deposit insurance	2,372	2,491	7,796	7,752
Intangible amortization	212	274	710	904
Other	38,221	44,668	121,293	132,722
Total non-interest expense	202,150	208,864	625,992	613,873
Income before income taxes	104,572	125,366	254,406	382,277
Income taxes	9,516	13,530	11,525	42,359
Net income	95,056	111,836	242,881	339,918
Preferred stock dividends	0	2,016	2,016	6,047
Redemption of preferred stock	0	0	5,514	0
Net income available to common shareholders	$ 95,056	$ 109,820	$ 235,351	$ 333,871
Earnings per common share:
Basic	$ 1.50	$ 1.74	$ 3.72	$ 5.28
Diluted	$ 1.50	$ 1.73	$ 3.71	$ 5.24

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net income	$ 95,056	$ 111,836	$ 242,881	$ 339,918
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss on securities available for sale during the period	20,332	107,786	406,959	464,072
Change in net unrealized gain on securities transferred to held to maturity	(294)	(305)	(979)	(957)
Reclassification adjustment for net (gains) losses included in net income	0	(97)	(108,989)	(266)
Total securities available for sale and transferred securities, before tax amount	20,038	107,384	296,991	462,849
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)	1,330	1,406	3,989	4,218
Total defined-benefit post-retirement benefit plans	1,330	1,406	3,989	4,218
Other comprehensive income (loss), before tax	21,368	108,790	300,980	467,067
Deferred tax expense (benefit)	4,487	22,845	63,205	98,084
Other comprehensive income (loss), net of tax	16,881	85,945	237,775	368,983
Comprehensive income (loss)	$ 111,937	$ 197,781	$ 480,656	$ 708,901

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	$ (14,672)				$ (14,672)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	3,354,245	$ 144,486	$ 642	$ 967,304	2,425,330	$ (63,600)	$ (119,917)
Total shareholders’ equity at beginning of period at Dec. 31, 2018	3,368,917	144,486	642	967,304	2,440,002	(63,600)	(119,917)
Net income	339,918				339,918
Other comprehensive income (loss)	368,983					368,983
Stock option exercises/stock unit conversions (53,425 shares QTD and 196,948 shares YTD in 2020 and 101,284 shares QTD and 255,184 shares YTD in 2019)	13,506				(9,569)		23,075
Stock compensation expense recognized in earnings	10,693			10,693
Purchase of treasury stock (400 shares QTD and 182,225 shares YTD in 2020 and 202,724 shares QTD and 704,382 shares YTD in 2019)	(67,709)						(67,709)
Treasury stock issued to the 401(k) stock purchase plan (58,900 shares QTD and 97,843 YTD in 2020)	0
Cash dividends – preferred stock (approximately $0.34 per share YTD in 2020 and $0.34 QTD and $1.01 YTD in 2019)	(6,047)				(6,047)
Cash dividends – common stock ($0.71 per share QTD and $2.13 per share YTD in 2020 and $0.71 per share and $2.09 per share YTD in 2019)	(132,132)				(132,132)
Total shareholders’ equity at end of period at Sep. 30, 2019	3,881,457	144,486	642	977,997	2,617,500	305,383	(164,551)
Total shareholders’ equity at beginning of period at Jun. 30, 2019	3,739,246	144,486	642	975,322	2,556,235	219,438	(156,877)
Net income	111,836				111,836
Other comprehensive income (loss)	85,945					85,945
Stock option exercises/stock unit conversions (53,425 shares QTD and 196,948 shares YTD in 2020 and 101,284 shares QTD and 255,184 shares YTD in 2019)	5,665				(3,851)		9,516
Stock compensation expense recognized in earnings	2,675			2,675
Purchase of treasury stock (400 shares QTD and 182,225 shares YTD in 2020 and 202,724 shares QTD and 704,382 shares YTD in 2019)	(17,190)						(17,190)
Cash dividends – preferred stock (approximately $0.34 per share YTD in 2020 and $0.34 QTD and $1.01 YTD in 2019)	(2,016)				(2,016)
Cash dividends – common stock ($0.71 per share QTD and $2.13 per share YTD in 2020 and $0.71 per share and $2.09 per share YTD in 2019)	(44,704)				(44,704)
Total shareholders’ equity at end of period at Sep. 30, 2019	3,881,457	144,486	642	977,997	2,617,500	305,383	(164,551)
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 (53,425 shares QTD and 196,948 shares YTD in 2020 and 101,284 shares QTD and 255,184 shares YTD in 2019)	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 (400 shares QTD and 182,225 shares YTD in 2020 and 202,724 shares QTD and 704,382 shares YTD in 2019)	(14,000)						(14,000)
Treasury stock issued to the 401(k) stock purchase plan (58,900 shares QTD and 97,843 YTD in 2020)	6,961				(2,563)		9,524
Cash dividends – preferred stock (approximately $0.34 per share YTD in 2020 and $0.34 QTD and $1.01 YTD in 2019)	(2,016)				(2,016)
Cash dividends – common stock ($0.71 per share QTD and $2.13 per share YTD in 2020 and $0.71 per share and $2.09 per share YTD in 2019)	(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)
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 (53,425 shares QTD and 196,948 shares YTD in 2020 and 101,284 shares QTD and 255,184 shares YTD in 2019)	2,698				(2,502)		5,200
Stock compensation expense recognized in earnings	2,443			2,443
Purchase of treasury stock (400 shares QTD and 182,225 shares YTD in 2020 and 202,724 shares QTD and 704,382 shares YTD in 2019)	(27)						(27)
Treasury stock issued to the 401(k) stock purchase plan (58,900 shares QTD and 97,843 YTD in 2020)	4,131				(1,602)		5,733
Cash dividends – common stock ($0.71 per share QTD and $2.13 per share YTD in 2020 and $0.71 per share and $2.09 per share YTD in 2019)	(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)

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Shareholders' Equity (Parenthetical) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Stockholders' Equity [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	53,425	101,284	196,948	255,184
Stock Redeemed or Called During Period, Shares			6,000,000
Treasury Stock, Shares, Acquired	400	202,724	182,225	704,382
Stock Issued During Period, Shares, Treasury Stock Reissued	58,900		97,843
Approximate cash dividends declared on preferred stock, per share		$ 0.34	$ 0.34	$ 1.01
Cash dividends declared on common stock, per share	$ 0.71	$ 0.71	$ 2.13	$ 2.09

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Operating Activities:
Net income	$ 242,881	$ 339,918
Adjustments to reconcile net income to net cash from operating activities:
Credit loss expense	227,474	25,404
Deferred tax expense (benefit)	(8,803)	(1,625)
Accretion of loan discounts	(11,905)	(11,364)
Securities premium amortization (discount accretion), net	93,080	84,963
Net (gain) loss on securities transactions	(108,989)	(265)
Depreciation and amortization	47,752	39,182
Net (gain) loss on sale/write-down of assets/foreclosed assets	(6)	(6,016)
Stock-based compensation	8,227	10,693
Net tax benefit from stock-based compensation	694	1,602
Earnings on life insurance policies	(2,895)	(2,744)
Net change in:
Trading account securities	(197)	(3,132)
Lease right-of-use assets	17,419	14,618
Accrued interest receivable and other assets	(52,842)	37,608
Accrued interest payable and other liabilities	23,208	56,354
Net cash from operating activities	475,098	585,196
Investing Activities:
Securities held-to-maturity - Purchases	(1,500)	0
Securities held to maturity: Maturities, calls and principal repayments	63,333	69,273
Securities available for sale:
Purchases	(8,324,364)	(9,815,936)
Sales	1,162,352	8,236,066
Maturities, calls and principal repayments	8,162,036	1,007,783
Proceeds from sale of loans	0	24,036
Net change in loans	(3,551,650)	(569,223)
Benefits received on life insurance policies	903	0
Proceeds from sales of premises and equipment	3,054	8,019
Purchases of premises and equipment	(81,866)	(168,326)
Proceeds from sales of repossessed properties	73	131
Net cash from investing activities	(2,567,629)	(1,208,177)
Financing Activities:
Net change in deposits	5,859,939	(65,622)
Net change in short-term borrowings	(86,675)	(166,893)
Proceeds from Federal Home Loan Bank advances	1,250,000	0
Principal payments on Federal Home Loan Bank advances	(1,250,000)	0
Redemption of preferred stock	(150,000)	0
Proceeds from stock option exercises	7,544	13,506
Purchase of treasury stock	(14,000)	(67,709)
Cash dividends paid on preferred stock	(2,016)	(6,047)
Cash dividends paid on common stock	(134,762)	(132,132)
Net cash from financing activities	5,480,030	(424,897)
Net change in cash and cash equivalents	3,387,499	(1,047,878)
Cash and cash equivalents at beginning of period	3,788,181	3,955,779
Cash and cash equivalents at end of period	$ 7,175,680	$ 2,907,901

Significant Accounting Policies

Significant Accounting Policies	9 Months Ended
	Sep. 30, 2020
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, 2019, included in our Annual Report on Form 10-K filed with the SEC on February 4, 2020 (the “2019 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 2020 2019 Cash paid for interest $ 41,634 $ 100,041 Cash paid for income taxes 44,140 42,352 Significant non-cash transactions: Unsettled securities transactions 6,698 82,064 Loans foreclosed and transferred to other real estate owned and foreclosed assets 140 1,302 Loans to facilitate the sale of other real estate owned — 847 Right-of-use lease assets obtained in exchange for lessee operating lease liabilities 18,284 315,542 Treasury stock issued to 401(k) stock purchase plan 6,961 — Accounting Changes, Reclassifications and Restatements. Certain items in prior financial statements have been reclassified to conform to the current presentation. In addition, 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. Accounting Standards Codification (“ASC”) Topic 326 (“ASC 326”) replaces the previous “incurred loss” model for measuring credit losses, which encompassed allowances for current known and inherent losses within the portfolio, with an “expected loss” model, which encompasses allowances for losses expected to be incurred over the life of the portfolio. The new current expected credit loss (“CECL”) model requires the measurement of all expected credit losses for financial assets measured at amortized cost and certain off-balance-sheet credit exposures based on historical experience, current conditions, and reasonable and supportable forecasts. ASC 326 also requires enhanced disclosures related to the significant estimates and judgments used in estimating credit losses, as well as the credit quality and underwriting standards of an organization’s portfolio. In addition, ASC 326 includes certain changes to the accounting for available-for-sale securities including the requirement to present credit losses as an allowance rather than as a direct write-down for available-for-sale securities management does not intend to sell or believes that it is more likely than not they will be required to sell. We adopted ASC 326 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 the table below. Operating results for periods after January 1, 2020 are presented in accordance with ASC 326 while prior period amounts continue to be reported in accordance with previously applicable standards and the accounting policies described in our 2019 Form 10-K. The following table details the impact of the adoption of ASC 326 on the allowance for credit losses as of January 1, 2020. January 1, 2020 Pre-Adoption Allowance Impact of Adoption Post-Adoption Allowance Cumulative Effect on Retained Earnings Securities held to maturity: U.S. Treasury $ — $ — $ — $ — Residential mortgage-backed securities — — — — States and political subdivisions — 215 215 (170) Other — — — — Total $ — $ 215 $ 215 $ (170) Loans: Commercial and industrial $ 51,593 $ 21,263 $ 72,856 $ (16,798) Energy 37,382 (10,453) 26,929 8,258 Commercial real estate 31,037 (13,519) 17,518 10,680 Consumer real estate 4,113 2,392 6,505 (1,890) Consumer and other 8,042 (2,248) 5,794 1,776 Total $ 132,167 $ (2,565) $ 129,602 $ 2,026 Off-balance-sheet credit exposures $ 500 $ 39,377 $ 39,877 $ (31,108) In connection with the adoption of ASC 326, we revised certain accounting policies and implemented certain accounting policy elections. The revised accounting policies are described below. Securities : Securities are classified as held to maturity and carried at amortized cost when management has the positive intent and ability to hold them until maturity. Securities to be held for indefinite periods of time are classified as available for sale and carried at fair value, with the unrealized holding gains and losses (those for which no allowance for credit losses are recorded) reported as a component of other comprehensive income, net of tax. Securities held for resale in anticipation of short-term market movements are classified as trading and are carried at fair value, with changes in unrealized holding gains and losses included in income. Management determines the appropriate classification of securities at the time of purchase. Securities with limited marketability, such as stock in the Federal Reserve Bank and the Federal Home Loan Bank, are carried at cost. Interest income on securities includes amortization of purchase premiums and discounts. Premiums and discounts on securities are generally amortized using the interest method with a constant effective yield without anticipating prepayments, except for mortgage-backed securities where prepayments are anticipated. Premiums on callable securities are amortized to their earliest call date. A security is placed on non-accrual status if (i) principal or interest has been in default for a period of 90 days or more or (ii) full payment of principal and interest is not expected. Interest accrued but not received for a security placed on non-accrual status is reversed against interest income. Gains and losses on sales are recorded on the trade date and are derived from the amortized cost of the security sold. Allowance For Credit Losses - Held-to-Maturity Securities : The allowance for credit losses on held-to-maturity securities is a contra-asset valuation account, calculated in accordance with ASC 326, that is deducted from the amortized cost basis of held-to-maturity securities to present management's best estimate of the net amount expected to be collected. Held-to-maturity securities are charged-off against the allowance when deemed uncollectible by management. Adjustments to the allowance are reported in our income statement as a component of credit loss expense. Management measures expected credit losses on held-to-maturity securities on a collective basis by major security type with each type sharing similar risk characteristics, and considers historical credit loss information that is adjusted for current conditions and reasonable and supportable forecasts. Management has made the accounting policy election to exclude accrued interest receivable on held-to-maturity securities from the estimate of credit losses. Further information regarding our policies and methodology used to estimate the allowance for credit losses on held-to-maturity securities is presented in Note 2 - Securities. Allowance For Credit Losses - Available-for-Sale Securities : For available-for-sale securities in an unrealized loss position, we first assess whether (i) we intend to sell, or (ii) it is more likely than not that we will be required to sell the security before recovery of its amortized cost basis. If either case is affirmative, any previously recognized allowances are charged-off and the security's amortized cost is written down to fair value through income. If neither case is affirmative, the security is evaluated to determine whether the decline in fair value has resulted from credit losses or other factors. In making this assessment, management considers the extent to which fair value is less than amortized cost, any changes to the rating of the security by a rating agency and any adverse conditions specifically related to the security, among other factors. If this assessment indicates that a credit loss exists, the present value of cash flows expected to be collected from the security are compared to the amortized cost basis of the security. If the present value of cash flows expected to be collected is less than the amortized cost basis, a credit loss exists and an allowance for credit losses is recorded for the credit loss, limited by the amount that the fair value is less than the amortized cost basis. Any impairment that has not been recorded through an allowance for credit losses is recognized in other comprehensive income. Adjustments to the allowance are reported in our income statement as a component of credit loss expense. Management has made the accounting policy election to exclude accrued interest receivable on available-for-sale securities from the estimate of credit losses. Available-for-sale securities are charged-off against the allowance or, in the absence of any allowance, written down through income when deemed uncollectible by management or when either of the aforementioned criteria regarding intent or requirement to sell is met. Loans : Loans are reported at the principal balance outstanding net of unearned discounts. Interest income on loans is reported on the level-yield method and includes amortization of deferred loan fees and costs over the terms of the individual loans to which they relate, or, in certain cases, over the average expected term for loans where deferred fees and costs are accounted for on a pooled basis. Net loan commitment fees or costs for commitment periods greater than one year are deferred and amortized into fee income or other expense on a straight-line basis over the commitment period. Income on direct financing leases is recognized on a basis that achieves a constant periodic rate of return on the outstanding investment. Further information regarding our accounting policies related to past due loans, non-accrual loans, collateral dependent loans and troubled-debt restructurings is presented in Note 3 - Loans. 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 management's best estimate of the net amount expected to be collected. Loans are charged-off against the allowance when deemed uncollectible by management. Expected recoveries do not exceed the aggregate of amounts previously charged-off and expected to be charged-off. Adjustments to the allowance are reported in our income statement as a component of credit loss expense. Management has made the accounting policy election to exclude accrued interest receivable on loans from the estimate of credit losses. Further information regarding our policies and methodology used to estimate the allowance for credit losses on loans is presented in Note 3 - Loans. 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. The allowance is reported as a component of accrued interest payable and other liabilities in our consolidated balance sheets. Adjustments to the allowance are reported in our income statement as a component of credit loss expense. Further information regarding our policies and methodology used to estimate the allowance for credit losses on off-balance-sheet credit exposures is presented in Note 6 - Off-Balance-Sheet Arrangements, Commitments, Guarantees and Contingencies. As more fully discussed in our 2019 Form 10-K, we adopted certain accounting standard updates related to accounting for leases as of January 1, 2019 using a modified-retrospective transition approach and recognized right-of-use lease assets and related lease liabilities totaling $170.5 million and $174.4 million, respectively, as of that date. We also adopted an accounting standard update which shortened the amortization period for certain callable debt securities held at a premium as of January 1, 2019 using a modified retrospective transition adoption approach and recognized a cumulative effect reduction to retained earnings totaling $14.7 million.

Securities

Securities	9 Months Ended
	Sep. 30, 2020
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, 2020 and December 31, 2019 is presented below. Amortized Gross Gross Estimated Allowance Net September 30, 2020 Residential mortgage-backed securities $ 529,290 $ 45,504 $ — $ 574,794 $ — $ 529,290 States and political subdivisions 1,420,754 61,808 — 1,482,562 (160) 1,420,594 Other 1,500 — 1 1,499 — 1,500 Total $ 1,951,544 $ 107,312 $ 1 $ 2,058,855 $ (160) $ 1,951,384 December 31, 2019 Residential mortgage-backed securities $ 530,861 $ 22 $ 9,365 $ 521,518 $ — $ 530,861 States and political subdivisions 1,497,644 28,909 896 1,525,657 — 1,497,644 Other 1,500 — — 1,500 — 1,500 Total $ 2,030,005 $ 28,931 $ 10,261 $ 2,048,675 $ — $ 2,030,005 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 $663.2 million and $561.4 million at September 30, 2020 and December 31, 2019, respectively. Accrued interest receivable on held-to-maturity securities totaled $11.9 million and $21.1 million at September 30, 2020 and December 31, 2019, 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. During the fourth quarter of 2019, we reclassified securities with an aggregate fair value of $377.8 million and an aggregate net unrealized gain of $3.3 million ($2.6 million, net of tax) on the date of the transfer. The net unamortized, unrealized gain remaining on transferred securities, including those transferred in 2019 and in years prior, included in accumulated other comprehensive income in the accompanying balance sheet totaled $3.8 million ($3.0 million, net of tax) at September 30, 2020 and $4.8 million ($3.8 million, net of tax) at December 31, 2019. 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 allowance for credit losses on held-to-maturity securities is a contra-asset valuation account that is deducted from the amortized cost basis of held-to-maturity securities to present the net amount expected to be collected. Management measures expected credit losses on held-to-maturity securities on a collective basis by major security type with each type sharing similar risk characteristics, and considers historical credit loss information that is adjusted for current conditions and reasonable and supportable forecasts. With regard to U.S. Treasury and residential mortgage-backed securities issued by the U.S. government, or agencies thereof, it is expected that the securities will not be settled at prices less than the amortized cost bases of the securities as such securities are backed by the full faith and credit of and/or guaranteed by the U.S. government. Accordingly, no allowance for credit losses has been recorded for these securities. With regard to securities issued by States and political subdivisions and other held-to-maturity securities, management considers (i) issuer bond ratings, (ii) historical loss rates for given bond ratings, (iii) whether issuers continue to make timely principal and interest payments under the contractual terms of the securities, (iv) internal forecasts and (v) whether or not such securities are guaranteed by the Texas Permanent School Fund (“PSF”) or pre-refunded by the issuers. 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, 2020: States and Political Subdivisions Not Guaranteed or Pre-Refunded Guaranteed by the Texas PSF Pre-Refunded Total Other Aaa/AAA $ 115,385 $ 797,659 $ 332,240 $ 1,245,284 $ — Aa/AA 112,614 — — 112,614 — A 62,856 — — 62,856 — Not rated — — — — 1,500 Total $ 290,855 $ 797,659 $ 332,240 $ 1,420,754 $ 1,500 Historical loss rates associated with securities having similar grades as those in our portfolio have generally not been significant. Furthermore, as of September 30, 2020, there were no past due principal or interest payments associated with these securities. The PSF is a sovereign wealth fund which serves to provide revenues for funding of public primary and secondary education in the State of Texas. Based upon (i) the PSF's AAA insurer financial strength rating, (ii) the PSF's substantial capitalization and excess guarantee capacity and (iii) a zero historical loss rate, no allowance for credit losses has been recorded for securities guaranteed by the PSF as there is no current expectation of credit losses related to these securities. Pre-refunded securities have been defeased by the issuer and are fully secured by cash and/or U.S. Treasury securities held in escrow for payment to holders when the underlying call dates of the securities are reached. Accordingly, no allowance for credit losses has been recorded for securities that have been defeased as there is no current expectation of credit losses related to these securities. 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, 2020. Three Months Ended Nine Months Ended September 30, 2020 Beginning balance $ 172 $ — Impact of adopting ASC 326 — 215 Credit loss expense (benefit) (12) (55) Ending balance $ 160 $ 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, 2020 and December 31, 2019 is presented below. Amortized Gross Gross Allowance Estimated September 30, 2020 U.S. Treasury $ 1,084,210 $ 40,755 $ — $ — $ 1,124,965 Residential mortgage-backed securities 2,092,184 72,021 43 — 2,164,162 States and political subdivisions 6,695,946 577,059 — — 7,273,005 Other 42,348 — — — 42,348 Total $ 9,914,688 $ 689,835 $ 43 $ — $ 10,604,480 December 31, 2019 U.S. Treasury $ 1,941,283 $ 18,934 $ 12,084 $ — $ 1,948,133 Residential mortgage-backed securities 2,176,275 32,608 1,289 — 2,207,594 States and political subdivisions 6,717,344 353,857 204 — 7,070,997 Other 42,867 — — — 42,867 Total $ 10,877,769 $ 405,399 $ 13,577 $ — $ 11,269,591 All mortgage-backed securities included in the above table were issued by U.S. government agencies and corporations. At September 30, 2020, 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 75.8% 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 $3.8 billion and $3.4 billion at September 30, 2020 and December 31, 2019, respectively. Accrued interest receivable on available-for-sale securities totaled $66.6 million and $115.9 million at September 30, 2020 and December 31, 2019, 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, 2020, 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 Residential mortgage-backed securities $ 7,527 $ 42 $ 124 $ 1 $ 7,651 $ 43 Total $ 7,527 $ 42 $ 124 $ 1 $ 7,651 $ 43 As of September 30, 2020, 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 (such as a PSF guarantee) 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, 2020. 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 $ — $ 231 $ 516,368 $ 12,691 $ 529,290 States and political subdivisions 55,180 355,409 399,377 610,788 1,420,754 Other — 1,500 — — 1,500 Total $ 55,180 $ 357,140 $ 915,745 $ 623,479 $ 1,951,544 Estimated Fair Value Residential mortgage-backed securities $ — $ 233 $ 560,456 $ 14,105 $ 574,794 States and political subdivisions 56,043 369,536 411,517 645,466 1,482,562 Other — 1,499 — — 1,499 Total $ 56,043 $ 371,268 $ 971,973 $ 659,571 $ 2,058,855 Available For Sale Amortized Cost U. S. Treasury $ 299,355 $ 784,855 $ — $ — $ 1,084,210 Residential mortgage-backed securities 175 45,832 22,949 2,023,228 2,092,184 States and political subdivisions 201,529 1,029,750 696,654 4,768,013 6,695,946 Other — — — — 42,348 Total $ 501,059 $ 1,860,437 $ 719,603 $ 6,791,241 $ 9,914,688 Estimated Fair Value U. S. Treasury $ 305,121 $ 819,844 $ — $ — $ 1,124,965 Residential mortgage-backed securities 182 47,995 23,439 2,092,546 2,164,162 States and political subdivisions 204,107 1,127,267 757,126 5,184,505 7,273,005 Other — — — — 42,348 Total $ 509,410 $ 1,995,106 $ 780,565 $ 7,277,051 $ 10,604,480 Sales of Securities. Sales of securities available for sale were as follows: Three Months Ended Nine Months Ended 2020 2019 2020 2019 Proceeds from sales $ — $ 4,944,537 $ 1,162,352 $ 8,236,066 Gross realized gains — 96 108,989 899 Gross realized losses — — — (634) Tax (expense) benefit of securities gains/losses — (21) (22,888) (56) Premiums and Discounts . Premium amortization and discount accretion included in interest income on securities was as follows: Three Months Ended Nine Months Ended 2020 2019 2020 2019 Premium amortization $ (31,540) $ (29,511) $ (94,873) $ (88,798) Discount accretion 637 1,274 1,793 3,835 Net (premium amortization) discount accretion $ (30,903) $ (28,237) $ (93,080) $ (84,963) Trading Account Securities. Trading account securities, at estimated fair value, were as follows: September 30, December 31, U.S. Treasury $ 24,495 $ 24,298 States and political subdivisions 1,313 — Total $ 25,808 $ 24,298 Net gains and losses on trading account securities were as follows: Three Months Ended Nine Months Ended 2020 2019 2020 2019 Net gain on sales transactions $ 159 $ 731 $ 933 $ 1,789 Net mark-to-market gains (losses) 1 (201) 94 (170) Net gain (loss) on trading account securities $ 160 $ 530 $ 1,027 $ 1,619

Loans

Loans	9 Months Ended
	Sep. 30, 2020
Receivables [Abstract]
Loans	Loans Loans were as follows: September 30, Percentage December 31, Percentage Commercial and industrial $ 4,820,018 26.4 % $ 5,187,466 35.2 % Energy: Production 1,108,753 6.1 1,348,900 9.2 Service 163,197 0.9 192,996 1.3 Other 91,418 0.4 110,986 0.8 Total energy 1,363,368 7.4 1,652,882 11.2 Paycheck Protection Program 3,226,980 17.7 — — Commercial real estate: Commercial mortgages 5,412,731 29.7 4,594,113 31.1 Construction 1,274,080 7.0 1,312,659 8.9 Land 320,407 1.8 289,467 2.0 Total commercial real estate 7,007,218 38.5 6,196,239 42.0 Consumer real estate: Home equity loans 340,226 1.9 375,596 2.6 Home equity lines of credit 434,171 2.4 354,671 2.4 Other 533,356 2.9 464,146 3.1 Total consumer real estate 1,307,753 7.2 1,194,413 8.1 Total real estate 8,314,971 45.7 7,390,652 50.1 Consumer and other 498,540 2.8 519,332 3.5 Total loans $ 18,223,877 100.0 % $ 14,750,332 100.0 % 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, 2020, 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 7.4% of total loans, or 9.1% excluding PPP loans. Unfunded commitments to extend credit and standby letters of credit issued to customers in the energy industry totaled $910.3 million and $62.6 million, respectively, as of September 30, 2020. 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, 2020 or December 31, 2019. 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 $316.0 million at September 30, 2020 and $298.5 million at December 31, 2019. Accrued Interest Receivable. Accrued interest receivable on loans totaled $43.9 million and $45.5 million at September 30, 2020 and December 31, 2019, respectively and is included in accrued interest receivable and other assets in the accompany consolidated balance sheets. COVID-19 Loan Deferments. Certain borrowers are currently unable to meet their contractual payment obligations because of the adverse effects of COVID-19. To help mitigate these effects, loan customers may apply for a deferral of payments, or portions thereof, for up to 90 days. After 90 days, customers may apply for an additional deferral, and a small proportion of our customers have requested such an additional deferral. In the absence of other intervening factors, such short-term modifications made on a good faith basis are not categorized as troubled debt restructurings, nor are loans granted payment deferrals related to COVID-19 reported as past due or placed on non-accrual status (provided the loans were not past due or on non-accrual status prior to the deferral). At September 30, 2020, there were approximately 300 loans in COVID-19 related deferment with an aggregate outstanding balance of approximately $157.2 million. Non-Accrual and Past Due Loans. Loans are considered past due if the required principal and interest payments have not been received as of the date such payments were due. Loans are placed on non-accrual status when, in management’s opinion, the borrower may be unable to meet payment obligations as they become due, as well as when required by regulatory provisions. In determining whether or not a borrower may be unable to meet payment obligations for each class of loans, we consider the borrower’s debt service capacity through the analysis of current financial information, if available, and/or current information with regards to our collateral position. Regulatory provisions would typically require the placement of a loan on non-accrual status if (i) principal or interest has been in default for a period of 90 days or more unless the loan is both well secured and in the process of collection or (ii) full payment of principal and interest is not expected. Loans may be placed on non-accrual status regardless of whether or not such loans are considered past due. When interest accrual is discontinued, all unpaid accrued interest is reversed against income. Interest income on non-accrual loans is recognized only to the extent that cash payments are received in excess of principal due. A loan may be returned to accrual status when all the principal and interest amounts contractually due are brought current and future principal and interest amounts contractually due are reasonably assured, which is typically evidenced by a sustained period (at least six months) of repayment performance by the borrower. Non-accrual loans, segregated by class of loans, were as follows: September 30, 2020 December 31, 2019 Total Non-Accrual Non-Accrual with No Credit Loss Allowance Total Non-Accrual Non-Accrual with No Credit Loss Allowance Commercial and industrial $ 21,566 $ 6,583 $ 26,038 $ 13,266 Energy 54,041 33,324 65,761 3,281 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 13,525 6,754 8,912 6,558 Construction 680 680 665 665 Consumer real estate 1,748 1,748 922 922 Consumer and other 18 — 5 — Total $ 91,578 $ 49,089 $ 102,303 $ 24,692 The following table presents non-accrual loans as of September 30, 2020 by class and year of origination. 2020 2019 2018 2017 2016 Prior Revolving Loans Revolving Loans Converted to Term Total Commercial and industrial $ 9,317 $ 3,743 $ 1,886 $ 1,263 $ 110 $ 51 $ 1,879 $ 3,317 $ 21,566 Energy 24,149 7,021 2,204 — — — 15,054 5,613 54,041 Paycheck Protection Program — — — — — — — — — Commercial real estate: Buildings, land and other 1,426 6,014 169 1,372 840 3,590 114 — 13,525 Construction 680 — — — — — — — 680 Consumer real estate — — 421 211 350 382 261 123 1,748 Consumer and other — — — — — — 18 — 18 Total $ 35,572 $ 16,778 $ 4,680 $ 2,846 $ 1,300 $ 4,023 $ 17,326 $ 9,053 $ 91,578 In the table above, loans reported as 2020 originations were, for the most part, first originated in various years prior to 2020 but were renewed in the current year. Had non-accrual loans performed in accordance with their original contract terms, we would have recognized additional interest income, net of tax, of approximately $670 thousand and $2.3 million for the three and nine months ended September 30, 2020, and approximately $937 thousand and $3.0 million for the three and nine months ended September 30, 2019. An age analysis of past due loans (including both accruing and non-accruing loans), segregated by class of loans, as of September 30, 2020 was as follows: Loans Loans Total Current Total Accruing Commercial and industrial $ 36,067 $ 25,115 $ 61,182 $ 4,758,836 $ 4,820,018 $ 19,392 Energy 31,071 9,588 40,659 1,322,709 1,363,368 559 Paycheck Protection Program — — — 3,226,980 3,226,980 — Commercial real estate: Buildings, land and other 25,113 19,571 44,684 5,688,454 5,733,138 11,676 Construction — — — 1,274,080 1,274,080 — Consumer real estate 11,054 4,761 15,815 1,291,938 1,307,753 3,406 Consumer and other 6,629 673 7,302 491,238 498,540 673 Total $ 109,934 $ 59,708 $ 169,642 $ 18,054,235 $ 18,223,877 $ 35,706 Troubled Debt Restructurings . Troubled debt restructurings during the nine months ended September 30, 2020 and September 30, 2019 are set forth in the following table. Nine Months Ended Nine Months Ended Balance at Balance at Balance at Balance at Commercial and industrial $ 3,660 $ 1,415 $ 3,845 $ 2,188 Commercial real estate: Buildings, land and other 6,606 6,585 9,456 9,494 Construction 1,192 1,181 — — Consumer real estate — — 124 123 Consumer and other 1,104 104 — — $ 12,562 $ 9,285 $ 13,425 $ 11,805 Loan modifications are typically related to extending amortization periods, converting loans to interest only for a limited period of time, deferral of interest payments, waiver of certain covenants, consolidating notes and/or reducing collateral or interest rates. The modifications during the reported periods did not significantly impact our determination of the allowance for credit losses on loans. Additional information related to restructured loans as of or for the three months ended September 30, 2020 and September 30, 2019 is set forth in the following table. September 30, 2020 September 30, 2019 Restructured loans past due in excess of 90 days at period-end: Number of loans 2 4 Dollar amount of loans $ 3,682 $ 3,244 Restructured loans on non-accrual status at period end 5,353 5,645 Charge-offs of restructured loans: Recognized in connection with restructuring — 1,500 Recognized on previously restructured loans 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 2019 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, 2020. Paycheck Protection Program (“PPP”) loans are excluded as such loans are fully guaranteed by the Small Business Administration (“SBA”). 2020 2019 2018 2017 2016 Prior Revolving Loans Revolving Loans Converted to Term Total W/A Risk Grade Commercial and industrial Risk grades 1-8 $ 1,101,917 $ 625,692 $ 340,799 $ 251,180 $ 120,777 $ 129,757 $ 1,788,761 $ 43,468 $ 4,402,351 6.12 Risk grade 9 69,846 27,843 36,271 21,352 9,910 1,087 77,802 4,462 248,573 9.00 Risk grade 10 4,394 7,722 13,867 6,875 812 824 60,028 4,347 98,869 10.00 Risk grade 11 3,675 4,507 6,720 4,422 1,896 167 17,909 9,363 48,659 11.00 Risk grade 12 6,917 2,791 1,544 708 110 51 1,350 3,022 16,493 12.00 Risk grade 13 2,400 952 342 555 — — 529 295 5,073 13.00 $ 1,189,149 $ 669,507 $ 399,543 $ 285,092 $ 133,505 $ 131,886 $ 1,946,379 $ 64,957 $ 4,820,018 6.42 W/A risk grade 6.12 6.81 7.20 6.32 6.37 5.87 6.32 7.92 6.42 Energy Risk grades 1-8 $ 454,901 $ 25,589 $ 10,956 $ 6,732 $ 1,481 $ 4,709 $ 407,478 $ 24,936 $ 936,782 6.18 Risk grade 9 108,859 5,542 3,190 — — — 90,292 14,974 222,857 9.00 Risk grade 10 486 4,113 1,461 — 907 — 7,837 2,105 16,909 10.00 Risk grade 11 78,832 15,724 3,280 1,210 — 1,066 32,398 269 132,779 11.00 Risk grade 12 24,149 4,829 714 — — — 10,629 5,613 45,934 12.00 Risk grade 13 — 2,192 1,490 — — — 4,425 — 8,107 13.00 $ 667,227 $ 57,989 $ 21,091 $ 7,942 $ 2,388 $ 5,775 $ 553,059 $ 47,897 $ 1,363,368 7.39 W/A risk grade 7.43 9.19 8.78 7.83 7.93 8.06 6.98 8.76 7.39 2020 2019 2018 2017 2016 Prior Revolving Loans Revolving Loans Converted to Term Total W/A Risk Grade Commercial real estate: Buildings, land, other Risk grades 1-8 $ 1,167,872 $ 1,010,418 $ 829,640 $ 660,047 $ 430,971 $ 752,461 $ 62,672 $ 53,480 $ 4,967,561 6.98 Risk grade 9 26,056 93,113 67,546 66,365 45,477 92,060 4,413 1,042 396,072 9.00 Risk grade 10 1,362 26,080 29,357 39,853 65,702 42,528 4,147 2,876 211,905 10.00 Risk grade 11 7,416 7,108 13,425 42,660 10,487 59,730 3,038 211 144,075 11.00 Risk grade 12 1,226 5,214 169 1,372 840 3,340 51 — 12,212 12.00 Risk grade 13 200 800 — — — 250 63 — 1,313 13.00 $ 1,204,132 $ 1,142,733 $ 940,137 $ 810,297 $ 553,477 $ 950,369 $ 74,384 $ 57,609 $ 5,733,138 7.34 W/A risk grade 7.09 7.34 7.46 7.48 7.73 7.26 7.41 6.84 7.34 Construction Risk grades 1-8 $ 272,025 $ 480,166 $ 224,389 $ 753 $ 1,164 $ 1,786 $ 160,654 $ — $ 1,140,937 7.01 Risk grade 9 31,402 8,370 — 39,688 — — 14,608 — 94,068 9.00 Risk grade 10 6,133 — 27,479 — — — 3,838 — 37,450 10.00 Risk grade 11 — — — — — 945 — — 945 11.00 Risk grade 12 680 — — — — — — — 680 12.00 Risk grade 13 — — — — — — — — — 13.00 $ 310,240 $ 488,536 $ 251,868 $ 40,441 $ 1,164 $ 2,731 $ 179,100 $ — $ 1,274,080 7.25 W/A risk grade 6.87 7.18 7.74 8.98 7.29 8.02 7.00 — 7.25 Total commercial real estate $ 1,514,372 $ 1,631,269 $ 1,192,005 $ 850,738 $ 554,641 $ 953,100 $ 253,484 $ 57,609 $ 7,007,218 7.33 W/A risk grade 7.04 7.29 7.52 7.55 7.73 7.26 7.12 6.84 7.33 In the tables above, certain loans are reported as 2020 originations and have risk grades of 11 or higher. These loans were, for the most part, first originated in various years prior to 2020 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, 2019. 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 6.17 $ 4,788,857 5.90 $ 1,488,301 6.78 $ 4,523,271 7.25 $ 1,274,098 6.88 $ 5,797,369 Risk grade 9 9.00 247,212 9.00 32,163 9.00 163,714 9.00 21,509 9.00 185,223 Risk grade 10 10.00 71,472 10.00 51,898 10.00 103,626 10.00 15,243 10.00 118,869 Risk grade 11 11.00 53,887 11.00 14,760 11.00 84,057 11.00 1,144 11.00 85,201 Risk grade 12 12.00 18,189 12.00 45,514 12.00 8,529 12.00 665 12.00 9,194 Risk grade 13 13.00 7,849 13.00 20,246 13.00 383 13.00 — 13.00 383 Total 6.44 $ 5,187,466 6.39 $ 1,652,882 7.01 $ 4,883,580 7.31 $ 1,312,659 7.07 $ 6,196,239 Information about the payment status of consumer loans, segregated by portfolio segment and year of origination, as of September 30, 2020 was as follows: 2020 2019 2018 2017 2016 Prior Revolving Loans Revolving Loans Converted to Term Total Consumer real estate: Past due 30-89 days $ 787 $ 850 $ 1,434 $ 858 $ 540 $ 4,558 $ 1,535 $ 492 $ 11,054 Past due 90 or more days — 139 894 542 351 2,028 600 207 4,761 Total past due 787 989 2,328 1,400 891 6,586 2,135 699 15,815 Current loans 253,392 188,770 106,947 93,143 75,216 142,983 414,331 17,156 1,291,938 Total $ 254,179 $ 189,759 $ 109,275 $ 94,543 $ 76,107 $ 149,569 $ 416,466 $ 17,855 $ 1,307,753 Consumer and other: Past due 30-89 days $ 1,328 $ 326 $ 288 $ 75 $ 2 $ 1 $ 4,313 $ 296 $ 6,629 Past due 90 or more days 69 189 3 — — 1 372 39 673 Total past due 1,397 515 291 75 2 2 4,685 335 7,302 Current loans 38,114 32,214 7,985 3,131 2,070 950 376,762 30,012 491,238 Total $ 39,511 $ 32,729 $ 8,276 $ 3,206 $ 2,072 $ 952 $ 381,447 $ 30,347 $ 498,540 Revolving loans that converted to term during the three and nine months ended September 30, 2020 were as follows: Three Months Ended Nine Months Ended Commercial and industrial $ 10,224 $ 25,340 Energy 7,144 38,642 Commercial real estate: Buildings, land and other 637 8,094 Construction — — Consumer real estate 421 2,132 Consumer and other 5,494 15,338 Total $ 23,920 $ 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 2019 Form 10-K, totaled 113.1 at September 30, 2020 and 127.9 at December 31, 2019. A lower TLI value implies less 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. The contractual term excludes expected extensions, renewals and modifications unless (i) management has a reasonable expectation that a trouble debt restructuring will be executed with an individual borrower or (ii) such extension or renewal options are not unconditionally cancellable by us and, in such cases, the borrower is likely to meet applicable conditions and likely to request extension or renewal. Relevant available information includes historical credit loss experience, current conditions and reasonable and supportable forecasts. While historical credit loss experience provides the basis for the estimation of expected credit losses, adjustments to historical loss information may be made for differences in current portfolio-specific risk characteristics, environmental conditions or other relevant factors. The allowance for credit losses is measured on a collective basis for portfolios of loans when similar risk characteristics exist. Loans that do not share risk characteristics are evaluated for expected credit losses on an individual basis and excluded from the collective evaluation. Expected credit losses for collateral dependent loans, including loans where the borrower is experiencing financial difficulty but foreclosure is not probable, are based on the fair value of the collateral at the reporting date, adjusted for selling costs as 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. In calculating the allowance for credit losses, most loans are segmented into pools based upon similar characteristics and risk profiles. Common characteristics and risk profiles include the type/purpose of loan, underlying collateral, geographical similarity and historical/expected credit loss patterns. In developing these loan pools for the purposes of modeling expected credit losses, we also analyzed the degree of correlation in how loans within each portfolio respond when subjected to varying economic conditions and scenarios as well as other portfolio stress factors. For modeling purposes, our loan pools include (i) commercial and industrial and energy - non-revolving, (ii) commercial and industrial and energy - revolving, (iii) commercial real estate - owner occupied, (iv) commercial real estate - non-owner occupied, (v) commercial real estate - construction/land development, (vi) consumer real estate and (vii) consumer and other. We periodically reassess each pool to ensure the loans within the pool continue to share similar characteristics and risk profiles and to determine whether further segmentation is necessary. For each loan pool, we measure expected credit losses over the life of each loan utilizing a combination of models which measure (i) probability of default (“PD”), which is the likelihood that loan will stop performing/default, (ii) probability of attrition (“PA”), which is the likelihood that a loan will pay-off prior to maturity, (iii) loss given default (“LGD”), which is the expected loss rate for loans in default and (iv) exposure at default (“EAD”), which is the estimated outstanding principal balance of the loans upon default, including the expected funding of unfunded commitments outstanding as of the measurement date. For certain commercial loan portfolios, the PD is calculated using a transition matrix to determine the likelihood of a customer’s risk grade migrating from one specified range of risk grades to a different specified range. Expected credit losses are calculated as the product of PD (adjusted for attrition), LGD and EAD. This methodology builds on default probabilities already incorporated into our risk grading process by utilizing pool-specific historical loss rates to calculate expected credit losses. These pool-specific historical loss rates may be adjusted for current macroeconomic assumptions, as further discussed below, and other factors such as differences in underwriting standards, portfolio mix, or when historical asset terms do not reflect the contractual terms of the financial assets being evaluated as of the measurement date. Each time we measure expected credit losses, we assess the relevancy of historical loss information and consider any necessary adjustments to address any differences in asset-specific characteristics. Due to their short-term nature, expected credit losses for overdrafts included in consumer and other loans are based solely upon a weighting of recent historical charge-offs over a period of three years. The measurement of expected credit losses is impacted by loan/borrower attributes and certain macroeconomic variables. Significant loan/borrower attributes utilized in our modeling processes include, among other things, (i) origination date, (ii) maturity date, (iii) payment type, (iv) collateral type and amount, (v) current risk grade, (vi) current unpaid balance and commitment utilization rate, (vii) payment status/delinquency history and (viii) expected recoveries of previously charged-off amounts. Significant macroeconomic variables utilized in our modeling processes include, among other things, (i) Gross State Product for Texas and U.S. Gross Domestic Product, (ii) selected market interest rates including U.S. Treasury rates, bank prime rate, 30-year fixed mortgage rate, BBB corporate bond rate, among others, (iii) unemployment rates, (iv) commercial and residential property prices in Texas and the U.S. as a whole, (v) West Texas Intermediate crude oil price and (vi) total stock market index. PD and PA were estimated by analyzing internally-sourced data related to historical performance of each loan pool over a complete economic cycle. PD and PA are adjusted to reflect the current impact of certain macroeconomic variables as well as their expected changes over a reasonable and supportable forecast period. We have determined that we are reasonably able to forecast the macroeconomic variables used in our modeling processes with an acceptable degree of confidence for a total of two years with the last twelve months of the forecast period encompassing a reversion process whereby the forecasted macroeconomic variables are reverted to their historical mean utilizing a rational, systematic basis. The macroeconomic variables utilized as inputs in our modeling processes were subjected to a variety of analysis procedures and were selected primarily based on statistical relevancy and correlation to our historical credit losses. By reverting these modeling inputs to their historical mean and considering loan/borrower specific attributes, our models will yield a measurement of expected credit losses that reflects our average historical loss rates for periods subsequent to the twelve-month reversion period. The LGD is based on historical recovery averages for each loan pool, adjusted to reflect the current impact of certain macroeconomic variables as well as their expected changes over a two-year forecast period, with the final twelve months of the forecast period encompassing a reversion process, which management considers to be both reasonable and supportable. This same forecast/reversion period is used for all macroeconomic variables used in all of our models. EAD is estimated using a linear regression model that estimates the average percentage of the loan balance that remains at the time of a default event. Management qualitatively adjusts model results for risk factors that are not considered within our modeling processes but are nonetheless relevant in assessing the expected credit losses within our loan pools. These qualitative factor (“Q-Factor”) adjustments may increase or decrease management's estimate of expected credit losses by a calculated percentage or amount based upon the estimated level of risk. The various risks that may be considered in making Q-Factor adjustments include, among other things, the impact of (i) changes in lending policies and procedures, including changes in underwriting standards and practices for collections, write-offs, and recoveries, (ii) actual and expected changes in international, national, regional, and local economic and business conditions and developments that affect the collectibility of the loan pools, (iii) changes in the nature and volume of the loan pools and in the terms of the underlying loans, (iv) changes in the experience, ability, and depth of our lending management and staff, (v) changes in volume and severity of past due financial assets, the volume of non-accrual assets, and the volume and severity of adversely classified or graded assets, (vi) changes in the quality of our credit review function, (vii) changes in the value of the underlying collateral for loans that are non-collateral dependent, (viii) the existence, growth, and effect of any concentrations of credit and (ix) other factors such as the regulatory, legal and technological environments; competition; and events such as natural disasters or health pandemics. In some cases, management may determine that an individual loan exhibits unique risk characteristics which differentiate the loan from other loans within our loan pools. In such cases, the loans are evaluated for expected credit losses on an individual basis and excluded from the collective evaluation. Specific allocations of the allowance for credit losses are determined by analyzing the borrower’s ability to repay amounts owed, collateral deficiencies, the relative risk grade of the loan and economic conditions affecting the borrower’s industry, among other things. A loan is considered to be collateral dependent when, based upon management's assessment, the borrower is experiencing financial difficulty and repayment is expected to be provided substantially through the operation or sale of the collateral. In such cases, expected credit losses are based on the fair value of the collateral at the measurement date, adjusted for estimated selling costs if satisfaction of the loan depends on the sale of the collateral. We reevaluate the fair value of collateral supporting collateral dependent loans on a quarterly basis. The fair value of real estate collateral supporting collateral dependent loans is evaluated by our internal appraisal services using a methodology that is consistent with the Uniform Standards of Professional Appraisal Practice. The fair value of collateral supporting collateral dependent construction loans is based on an “as is” valuation. The following table presents details of the allowance for credit losses on loans segregated by loan portfolio segment as of September 30, 2020, calculated in accordance with the CECL methodology described above. 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 Modeled expected credit losses $ 68,896 $ 21,644 $ 125,965 $ 10,226 $ 7,420 $ 234,151 Q-Factor and other qualitative adjustments (1,647) 24,630 (8,685) 51 31 14,380 Specific allocations 5,256 8,357 1,313 — 18 14,944 Total $ 72,505 $ 54,631 $ 118,593 $ 10,277 $ 7,469 $ 263,475 The following table presents details of the allowance for credit losses on loans segregated by loan portfolio segment as of December 31, 2019, calculated in accordance with our prior incurred loss methodology described in our 2019 Form 10-K. Commercial Energy Commercial Consumer Consumer Total Historical valuation allowances $ 29,015 $ 7,873 $ 21,947 $ 2,690 $ 7,562 $ 69,087 Specific valuation allowances 7,849 20,246 383 — 5 28,483 General valuation allowances 9,840 5,196 4,201 904 (409) 19,732 Macroeconomic valuation allowances 4,889 4,067 4,506 519 884 14,865 Total $ 51,593 $ 37,382 $ 31,037 $ 4,113 $ 8,042 $ 132,167 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, 2020 and 2019. 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, 2020 Beginning balance $ 98,536 $ 40,817 $ 93,425 $ 8,998 $ 8,285 $ 250,061 Credit loss expense (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 September 30, 2019 Beginning balance $ 57,714 $ 25,818 $ 35,914 $ 5,637 $ 9,846 $ 134,929 Credit loss expense (3,527) 8,788 (607) (650) 3,997 8,001 Charge-offs (2,705) (2,000) — (557) (6,357) (11,619) Recoveries 1,185 740 46 454 2,823 5,248 Net charge-offs (1,520) (1,260) 46 (103) (3,534) (6,371) Ending balance $ 52,667 $ 33,346 $ 35,353 $ 4,884 $ 10,309 $ 136,559 Nine months ended: 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 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 (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 September 30, 2019 Beginning balance $ 48,580 $ 29,052 $ 38,777 $ 6,103 $ 9,620 $ 132,132 Credit loss expense 9,999 7,478 (2,972) 859 10,040 25,404 Charge-offs (8,782) (4,000) (617) (2,936) (17,157) (33,492) Recoveries 2,870 816 165 858 7,806 12,515 Net charge-offs (5,912) (3,184) (452) (2,078) (9,351) (20,977) Ending balance $ 52,667 $ 33,346 $ 35,353 $ 4,884 $ 10,309 $ 136,559 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, 2020 and December 31, 2019. September 30, 2020 December 31, 2019 Loan Specific Allocations Loan Specific Allocations Commercial and industrial $ 22,930 $ 5,256 $ 24,360 $ 7,849 Energy 56,420 8,357 65,244 20,246 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 25,611 1,313 8,609 383 Construction 680 — 665 — Consumer real estate 1,335 — 570 — Consumer and other 18 18 5 5 Total $ 106,994 $ 14,944 $ 99,453 $ 28,483

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	9 Months Ended
	Sep. 30, 2020
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. As of September 30, 2020, we evaluated recent potential triggering events that might be indicators that our goodwill was impaired. The events include the economic disruption and uncertainty surrounding the COVID-19 pandemic and the circumstances surrounding recent volatility in the market price of crude oil. Based on our evaluation, we concluded that our goodwill was not more than likely impaired as of that date. September 30, December 31, Goodwill $ 654,952 $ 654,952 Other intangible assets: Core deposits $ 1,465 $ 2,043 Customer relationships 306 438 $ 1,771 $ 2,481 The estimated aggregate future amortization expense for intangible assets remaining as of September 30, 2020 is as follows: Remainder of 2020 $ 208 2021 697 2022 481 2023 282 2024 87 Thereafter 16 $ 1,771

Deposits

Deposits	9 Months Ended
	Sep. 30, 2020
Disclosure Text Block [Abstract]
Deposits	Deposits Deposits were as follows: September 30, Percentage December 31, Percentage Non-interest-bearing demand deposits: Commercial and individual $ 13,852,404 41.4 % $ 10,212,265 36.9 % Correspondent banks 208,836 0.6 246,181 0.9 Public funds 784,530 2.3 415,183 1.5 Total non-interest-bearing demand deposits 14,845,770 44.3 10,873,629 39.3 Interest-bearing deposits: Private accounts: Savings and interest checking 8,176,608 24.4 7,147,327 25.9 Money market accounts 8,684,470 25.9 7,888,433 28.5 Time accounts of $100,000 or more 791,004 2.4 736,481 2.7 Time accounts under $100,000 339,174 1.0 347,418 1.2 Total private accounts 17,991,256 53.7 16,119,659 58.3 Public funds: Savings and interest checking 583,214 1.8 548,399 2.0 Money market accounts 76,088 0.2 73,180 0.3 Time accounts of $100,000 or more 3,092 — 24,672 0.1 Time accounts under $100,000 83 — 25 — Total public funds 662,477 2.0 646,276 2.4 Total interest-bearing deposits 18,653,733 55.7 16,765,935 60.7 Total deposits $ 33,499,503 100.0 % $ 27,639,564 100.0 % The following table presents additional information about our deposits: September 30, December 31, Deposits from the Certificate of Deposit Account Registry Service (CDARS) deposits $ 369 $ 361 Deposits from foreign sources (primarily Mexico) 845,950 805,828 Deposits not covered by deposit insurance 17,265,358 13,115,796

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2020
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 2019 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 $ 9,586,212 $ 9,306,043 Standby letters of credit 260,683 260,587 Deferred standby letter of credit fees 1,716 1,276 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. For the period of exposure, the estimate of expected credit losses considers both the likelihood that funding will occur and the amount expected to be funded over the estimated remaining life of the commitment or other off-balance-sheet exposure. The likelihood and expected amount of funding are based on historical utilization rates. 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. Estimating credit losses on amounts expected to be funded uses the same methodology as described for loans in Note 3 - Loans as if such commitments were funded. The following table details activity in the allowance for credit losses on off-balance-sheet credit exposures. Three Months Ended Nine Months Ended 2020 2019 2020 2019 Beginning balance $ 46,939 $ 500 $ 500 $ 500 Impact of adopting ASC 326 — — 39,377 — Credit loss expense (benefit) (3,276) — 3,786 — Ending balance $ 43,663 $ 500 $ 43,663 $ 500 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 2020 2019 2020 2019 Amortization of lease right-of-use assets $ 8,521 $ 7,415 $ 24,545 $ 19,698 Short-term lease expense 172 1,199 1,343 3,316 Non-lease components (including taxes, insurance, common maintenance, etc.) 2,586 3,114 8,410 7,022 Total $ 11,279 $ 11,728 $ 34,298 $ 30,036 Right-of-use lease assets totaled $298.6 million at September 30, 2020 and $297.7 million at December 31, 2019 and are reported as a component of premises and equipment on our accompanying consolidated balance sheets. The related lease liabilities totaled $328.9 million at September 30, 2020 and $323.7 million at December 31, 2019 and are reported as a component of accrued interest payable and other liabilities in the accompanying consolidated balance sheets. Lease payments under operating leases that were applied to our operating lease liability totaled $8.0 million and $23.8 million during the three and nine months ended September 30, 2020 and $7.6 million and $19.7 million during the three and nine months ended September 30, 2019. There has been no significant change in our expected future minimum lease payments since December 31, 2019. See the 2019 Form 10-K for information regarding these commitments. Litigation. We are subject to various claims and legal actions that have arisen in the course of conducting business. Management does not expect the ultimate disposition of these matters to have a material adverse impact on our financial statements. In May of 2020, a purported class action lawsuit was filed against Frost Bank in a Texas Federal court alleging that Frost Bank had refused to pay agent fees to purported agents of borrowers under the PPP in violation of SBA regulations. The Plaintiff's motion to dismiss the Federal lawsuit was effected and as a result the Federal lawsuit is resolved. In July of 2020, another purported class action lawsuit was filed against Frost Bank in a California Federal court alleging that Frost Bank had refused to pay agent fees to purported agents of borrowers under the PPP in violation of SBA regulations. Frost Bank believes the claims to be without merit. In October of 2020, a lawsuit was filed against Frost Bank in Texas State court alleging, among other claims, that Frost Bank refused to provide a PPP loan to the purported applicant. Frost Bank believes the claims to be without merit.

Capital and Regulatory Matters

Capital and Regulatory Matters	9 Months Ended
	Sep. 30, 2020
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 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 Common Equity Tier 1. We also elected to exclude, for a five-year transitional period, the effects of credit loss accounting under CECL from Common Equity Tier 1, as further discussed below. Common Equity Tier 1 for both Cullen/Frost and Frost Bank is reduced by goodwill and other intangible assets, net of associated deferred tax liabilities. Frost Bank's Common Equity Tier 1 is also reduced by its equity investment in its financial subsidiary, Frost Insurance Agency (“FIA”). Tier 1 capital includes Common Equity Tier 1 capital and additional Tier 1 capital. Cullen/Frost and Frost Bank did not have any additional Tier 1 capital beyond Common Equity Tier 1 at September 30, 2020. For Cullen/Frost, additional Tier 1 capital included $144.5 million of preferred stock at December 31, 2019, the details of which are further discussed below. Frost Bank did not have any additional Tier 1 capital beyond Common Equity Tier 1 at December 31, 2019. 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, 2020 and December 31, 2019. As discussed in Note 1 - Significant Accounting Policies, in connection with the adoption of ASC 326, we recognized an after-tax cumulative effect reduction to retained earnings totaling $29.3 million. In February 2019, the federal bank regulatory agencies issued a final rule (the “2019 CECL Rule”) that revised certain capital regulations to account for changes to credit loss accounting under U.S. GAAP. The 2019 CECL Rule included a transition option that allows banking organizations to phase in, over a three-year period, the day-one adverse effects of CECL on their regulatory capital ratios (three-year transition option). In March 2020, the federal bank regulatory agencies issued an interim final rule that maintains the three-year transition option of the 2019 CECL Rule and also provides banking organizations that were required under U.S. GAAP (as of January 2020) to implement CECL before the end of 2020 the option to delay for two years an estimate of the effect of CECL on regulatory capital, relative to the incurred loss methodology’s effect on regulatory capital, followed by a three-year transition period (five-year transition option). We elected to adopt the five-year transition option. Accordingly, a CECL transitional amount totaling $63.7 million has been added back to CET1 as of September 30, 2020. The CECL transitional amount includes $29.3 million related to cumulative effect of adopting CECL and $34.4 million related to the estimated incremental effect of CECL since adoption. In April 2020, we began originating loans to qualified small businesses under the PPP administered by the SBA. Federal bank regulatory agencies have issued an interim final rule that permits banks to neutralize the regulatory capital effects of participating in the Paycheck Protection Program Lending Facility (the “PPP Facility”) and clarify that PPP loans have a zero percent risk weight under applicable risk-based capital rules. Specifically, a bank may exclude all PPP loans pledged as collateral to the PPP Facility from its average total consolidated assets for the purposes of calculating its leverage ratio, while PPP loans that are not pledged as collateral to the PPP Facility will be included. Our PPP loans are included in the calculation of our leverage ratio as of September 30, 2020 as we did not utilize the PPP Facility for funding purposes. The following table presents actual and required capital ratios as of September 30, 2020 and December 31, 2019 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 2019 Form 10-K for a more detailed discussion of the Basel III Capital Rules. After a review of risk-weight classifications during the first quarter of 2019, risk-weightings for certain loans were reclassified. Amounts reported as of December 31, 2019 have been revised to reflect these reclassifications. Actual Minimum Capital Required - Basel III Required to be Capital Ratio Capital Ratio Capital Ratio September 30, 2020 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 3,003,029 12.71 % $ 1,654,073 7.00 % $ 1,535,925 6.50 % Frost Bank 3,010,290 12.76 1,651,191 7.00 1,533,249 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 3,003,029 12.71 2,008,517 8.50 1,890,369 8.00 Frost Bank 3,010,290 12.76 2,005,018 8.50 1,887,076 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,471,898 14.69 2,481,109 10.50 2,362,961 10.00 Frost Bank 3,246,159 13.76 2,476,787 10.50 2,358,845 10.00 Leverage Ratio Cullen/Frost 3,003,029 7.85 1,530,584 4.00 1,913,230 5.00 Frost Bank 3,010,290 7.87 1,530,076 4.00 1,912,596 5.00 December 31, 2019 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,857,250 12.36 % $ 1,617,886 7.00 % $ 1,502,323 6.50 % Frost Bank 2,958,326 12.82 1,615,206 7.00 1,499,834 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 3,001,736 12.99 1,964,576 8.50 1,849,013 8.00 Frost Bank 2,958,326 12.82 1,961,322 8.50 1,845,950 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,367,403 14.57 2,426,829 10.50 2,311,266 10.00 Frost Bank 3,090,993 13.40 2,422,809 10.50 2,307,438 10.00 Leverage Ratio Cullen/Frost 3,001,736 9.28 1,293,188 4.00 1,616,485 5.00 Frost Bank 2,958,326 9.15 1,292,743 4.00 1,615,929 5.00 As of September 30, 2020, 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, 2020 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, 2020, 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. 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 July 24, 2019, 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, 2020, Frost Bank could pay aggregate dividends of up to $482.1 million to Cullen/Frost without prior regulatory approval. Under the terms of the junior subordinated deferrable interest debentures that Cullen/Frost has issued to Cullen/Frost Capital Trust II and WNB Capital Trust I, Cullen/Frost has the right at any time during the term of the debentures to defer the payment of interest at any time or from time to time for an extension period not exceeding 20 consecutive quarterly periods with respect to each extension period. In the event that we have elected to defer interest on the debentures, we may not, with certain exceptions, declare or pay any dividends or distributions on our capital stock or purchase or acquire any of our capital stock.

Derivative Financial Instrument

Derivative Financial Instruments	9 Months Ended
	Sep. 30, 2020
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 2019 Form 10-K. The notional amounts and estimated fair values of interest rate derivative contracts are presented in the following table. The fair values of interest rate derivative contracts are estimated utilizing internal valuation models with observable market data inputs, or as determined by the Chicago Mercantile Exchange (“CME”) for centrally cleared derivative contracts. CME rules legally characterize variation margin payments for centrally cleared derivatives as settlements of the derivatives' exposure rather than collateral. As a result, the variation margin payment and the related derivative instruments are considered a single unit of account for accounting and financial reporting purposes. Variation margin, as determined by the CME, is settled daily. As a result, derivative contracts that clear through the CME have an estimated fair value of zero as of September 30, 2020 and December 31, 2019. September 30, 2020 December 31, 2019 Notional Estimated Notional Estimated Derivatives designated as hedges of fair value: Financial institution counterparties: Loan/lease interest rate swaps – assets $ — $ — $ 2,545 $ 6 Loan/lease interest rate swaps – liabilities 4,036 (167) 6,000 (138) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets — — 122,788 67 Loan/lease interest rate swaps – liabilities 1,192,762 (36,975) 1,002,860 (19,483) Loan/lease interest rate caps – assets 306,171 1,197 107,835 266 Customer counterparties: Loan/lease interest rate swaps – assets 1,192,762 95,462 1,002,860 43,857 Loan/lease interest rate swaps – liabilities — — 122,788 (310) Loan/lease interest rate caps – liabilities 306,171 (1,197) 107,835 (266) The weighted-average rates paid and received for interest rate swaps outstanding at September 30, 2020 were as follows: Weighted-Average Interest Interest Interest rate swaps: Fair value hedge loan/lease interest rate swaps 3.39 % 0.15 % Non-hedging interest rate swaps – financial institution counterparties 4.00 1.93 Non-hedging interest rate swaps – customer counterparties 1.93 4.00 The weighted-average strike rate for outstanding interest rate caps was 3.67% at September 30, 2020. Commodity Derivatives. We enter into commodity swaps and option contracts that are not designated as hedging instruments primarily to accommodate the business needs of our customers. Upon the origination of a commodity swap or option contract with a customer, we simultaneously enter into an offsetting contract with a third party financial institution to mitigate the exposure to fluctuations in commodity prices. The notional amounts and estimated fair values of non-hedging commodity swap and option derivative positions outstanding are presented in the following table. We obtain dealer quotations and use internal valuation models with observable market data inputs to value our commodity derivative positions. September 30, 2020 December 31, 2019 Notional Notional Estimated Notional Estimated Financial institution counterparties: Oil – assets Barrels 2,296 $ 18,718 1,214 $ 2,796 Oil – liabilities Barrels 3,089 (9,132) 2,148 (6,916) Natural gas – assets MMBTUs 5,623 932 8,295 2,131 Natural gas – liabilities MMBTUs 18,181 (5,879) 2,689 (70) Customer counterparties: Oil – assets Barrels 3,106 9,461 2,172 7,208 Oil – liabilities Barrels 2,279 (18,506) 1,190 (2,652) Natural gas – assets MMBTUs 19,940 6,161 2,689 83 Natural gas – liabilities MMBTUs 3,864 (915) 8,295 (2,039) 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, 2020 December 31, 2019 Notional Notional Estimated Notional Estimated Financial institution counterparties: Forward contracts – liabilities CAD — — 4,593 $ (33) Customer counterparties: Forward contracts – assets CAD — — 4,583 45 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 2020 2019 2020 2019 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ (36) $ 24 $ (79) $ 78 Amount of (gain) loss included in other non-interest expense 2 (1) 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 quarter 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 2020 2019 2020 2019 Non-hedging interest rate derivatives: Other non-interest income $ 280 $ 215 $ 2,615 $ 1,188 Other non-interest expense 1 — 1 — Non-hedging commodity derivatives: Other non-interest income 379 109 1,057 322 Non-hedging foreign currency derivatives: Other non-interest income 10 12 28 41 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 $103.0 million at September 30, 2020. 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 $19.2 million at September 30, 2020. 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, 2020, we had $48.6 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, 2020
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, 2020 is presented in the following tables. Gross Amount Gross Amount Net Amount September 30, 2020 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 1,197 $ — $ 1,197 Commodity swaps and options 19,650 — 19,650 Foreign currency forward contracts — — — Total derivatives 20,847 — 20,847 Resell agreements 20,000 — 20,000 Total $ 40,847 $ — $ 40,847 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 37,142 $ — $ 37,142 Commodity swaps and options 15,011 — 15,011 Foreign currency forward contracts — — — Total derivatives 52,153 — 52,153 Repurchase agreements 1,574,367 — 1,574,367 Total $ 1,626,520 $ — $ 1,626,520 Gross Amounts Not Offset Net Amount Financial Collateral Net September 30, 2020 Financial assets: Derivatives: Counterparty A $ 8 $ (8) $ — $ — Counterparty B 5,996 (5,996) — — Other counterparties 14,843 (14,510) (327) 6 Total derivatives 20,847 (20,514) (327) 6 Resell agreements 20,000 — (20,000) — Total $ 40,847 $ (20,514) $ (20,327) $ 6 Financial liabilities: Derivatives: Counterparty A $ 7,178 $ (8) $ (7,170) $ — Counterparty B 14,386 (5,996) (8,210) 180 Counterparty C 94 — (94) — Other counterparties 30,495 (14,510) (13,971) 2,014 Total derivatives 52,153 (20,514) (29,445) 2,194 Repurchase agreements 1,574,367 — (1,574,367) — Total $ 1,626,520 $ (20,514) $ (1,603,812) $ 2,194 Information about financial instruments that are eligible for offset in the consolidated balance sheet as of December 31, 2019 is presented in the following tables. Gross Amount Gross Amount Net Amount December 31, 2019 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 339 $ — $ 339 Commodity swaps and options 4,927 — 4,927 Total derivatives 5,266 — 5,266 Resell agreements 31,299 — 31,299 Total $ 36,565 $ — $ 36,565 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 19,621 $ — $ 19,621 Commodity swaps and options 6,986 — 6,986 Foreign currency forward contracts 33 — 33 Total derivatives 26,640 — 26,640 Repurchase agreements 1,668,142 — 1,668,142 Total $ 1,694,782 $ — $ 1,694,782 Gross Amounts Not Offset Net Amount Financial Collateral Net December 31, 2019 Financial assets: Derivatives: Counterparty A $ 39 $ (39) $ — $ — Counterparty B 1,650 (1,650) — — Counterparty C 1 (1) — — Other counterparties 3,576 (3,546) — 30 Total derivatives 5,266 (5,236) — 30 Resell agreements 31,299 — (31,299) — Total $ 36,565 $ (5,236) $ (31,299) $ 30 Financial liabilities: Derivatives: Counterparty A $ 5,192 $ (39) $ (5,153) $ — Counterparty B 7,424 (1,650) (5,774) — Counterparty C 135 (1) (134) — Other counterparties 13,889 (3,546) (10,343) — Total derivatives 26,640 (5,236) (21,404) — Repurchase agreements 1,668,142 — (1,668,142) — Total $ 1,694,782 $ (5,236) $ (1,689,546) $ — 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, 2020 and December 31, 2019 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, 2020 Repurchase agreements: U.S. Treasury $ 99,365 $ — $ — $ — $ 99,365 Residential mortgage-backed securities 1,475,002 — — — 1,475,002 Total borrowings $ 1,574,367 $ — $ — $ — $ 1,574,367 Gross amount of recognized liabilities for repurchase agreements $ 1,574,367 Amounts related to agreements not included in offsetting disclosures above $ — December 31, 2019 Repurchase agreements: U.S. Treasury $ 435,904 $ — $ — $ — $ 435,904 Residential mortgage-backed securities 1,232,238 — — — 1,232,238 Total borrowings $ 1,668,142 $ — $ — $ — $ 1,668,142 Gross amount of recognized liabilities for repurchase agreements $ 1,668,142 Amounts related to agreements not included in offsetting disclosures above $ —

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2020
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, 2020, there were 1,109,864 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, 2020 55,370 $ 74.76 440,647 $ 90.22 177,288 $ 83.48 1,980,866 $ 64.60 Authorized — — — — — — — — Granted 10,428 73.84 458 65.43 — — — — Exercised/vested (12,938) 71.09 (1,640) 76.07 (41,755) 69.70 (140,615) 53.65 Forfeited/expired — — (2,813) 90.20 (6,894) 81.33 (5,427) 75.74 Balance, September 30, 2020 52,860 $ 75.47 436,652 $ 90.24 128,639 $ 88.07 1,834,824 $ 65.41 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 2020 2019 2020 2019 New shares issued from available authorized shares — — — — Issued from available treasury stock 53,425 71,284 196,948 225,184 Total 53,425 71,284 196,948 225,184 Proceeds from stock option exercises $ 2,698 $ 5,665 $ 7,544 $ 13,506 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 2020 2019 2020 2019 Stock options $ — $ 330 $ — $ 1,091 Non-vested stock awards/stock units 1,999 1,536 6,325 5,727 Director deferred stock units — — 770 780 Performance stock units 444 809 1,132 3,095 Total $ 2,443 $ 2,675 $ 8,227 $ 10,693 Income tax benefit $ 450 $ 452 $ 1,539 $ 1,768 Unrecognized stock-based compensation expense at September 30, 2020 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 $ 12,337 Performance stock units 4,651 Total $ 16,988

Earnings Per Common Share

Earnings Per Common Share	9 Months Ended
	Sep. 30, 2020
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 2019 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 2020 2019 2020 2019 Net income $ 95,056 $ 111,836 $ 242,881 $ 339,918 Less: Preferred stock dividends — 2,016 2,016 6,047 Redemption of preferred stock — — 5,514 — Net income available to common shareholders 95,056 109,820 235,351 333,871 Less: Earnings allocated to participating securities 877 876 2,236 2,761 Net earnings allocated to common stock $ 94,179 $ 108,944 $ 233,115 $ 331,110 Distributed earnings allocated to common stock $ 44,546 $ 44,370 $ 133,511 $ 131,069 Undistributed earnings allocated to common stock 49,633 64,574 99,604 200,041 Net earnings allocated to common stock $ 94,179 $ 108,944 $ 233,115 $ 331,110 Weighted-average shares outstanding for basic earnings per common share 62,726,542 62,566,128 62,655,393 62,786,501 Dilutive effect of stock compensation 193,116 592,866 263,028 725,911 Weighted-average shares outstanding for diluted earnings per common share 62,919,658 63,158,994 62,918,421 63,512,412

Defined Benefit Plans

Defined Benefit Plans	9 Months Ended
	Sep. 30, 2020
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 2020 2019 2020 2019 Expected return on plan assets, net of expenses $ (3,073) $ (2,693) $ (9,217) $ (8,079) Interest cost on projected benefit obligation 1,252 1,618 3,757 4,854 Net amortization and deferral 1,330 1,406 3,989 4,218 Net periodic expense (benefit) $ (491) $ 331 $ (1,471) $ 993 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, 2020. We do not expect to make any contributions to the qualified defined benefit plan during the remainder of 2020.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Income tax expense was as follows: Three Months Ended Nine Months Ended 2020 2019 2020 2019 Current income tax expense (benefit) $ (33,614) $ 14,480 $ 20,328 $ 43,984 Deferred income tax expense (benefit) 43,130 (950) (8,803) (1,625) Income tax expense, as reported $ 9,516 $ 13,530 $ 11,525 $ 42,359 Effective tax rate 9.1 % 10.8 % 4.5 % 11.1 % We had a net deferred tax liability totaling $122.4 million at September 30, 2020 and $75.8 million at December 31, 2019. No valuation allowance for deferred tax assets was recorded at September 30, 2020 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 2016.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2020
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 $ 20,332 $ 4,269 $ 16,063 $ 107,786 $ 22,635 $ 85,151 Change in net unrealized gain on securities transferred to held to maturity (294) (62) (232) (305) (64) (241) Reclassification adjustment for net (gains) losses included in net income — — — (97) (21) (76) Total securities available for sale and transferred securities 20,038 4,207 15,831 107,384 22,550 84,834 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) 1,330 280 1,050 1,406 295 1,111 Total defined-benefit post-retirement benefit plans 1,330 280 1,050 1,406 295 1,111 Total other comprehensive income (loss) $ 21,368 $ 4,487 $ 16,881 $ 108,790 $ 22,845 $ 85,945 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 $ 406,959 $ 85,462 $ 321,497 $ 464,072 $ 97,455 $ 366,617 Change in net unrealized gain on securities transferred to held to maturity (979) (206) (773) (957) (201) (756) Reclassification adjustment for net (gains) losses included in net income (108,989) (22,888) (86,101) (266) (56) (210) Total securities available for sale and transferred securities 296,991 62,368 234,623 462,849 97,198 365,651 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) 3,989 837 3,152 4,218 886 3,332 Total defined-benefit post-retirement benefit plans 3,989 837 3,152 4,218 886 3,332 Total other comprehensive income (loss) $ 300,980 $ 63,205 $ 237,775 $ 467,067 $ 98,084 $ 368,983 Activity in accumulated other comprehensive income (loss), net of tax, was as follows: Securities Defined Accumulated 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 Balance January 1, 2019 $ (16,103) $ (47,497) $ (63,600) Other comprehensive income (loss) before reclassifications 365,861 — 365,861 Reclassification of amounts included in net income (210) 3,332 3,122 Net other comprehensive income (loss) during period 365,651 3,332 368,983 Balance at September 30, 2019 $ 349,548 $ (44,165) $ 305,383

Operating Segments

Operating Segments	9 Months Ended
	Sep. 30, 2020
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 2019 Form 10-K for additional information regarding our operating segments. Summarized operating results by segment were as follows: Banking Frost Wealth Non-Banks Consolidated Revenues from (expenses to) external customers: Three months ended: September 30, 2020 $ 293,855 $ 35,282 $ (2,113) $ 327,024 September 30, 2019 307,312 37,549 (2,630) 342,231 Nine months ended: September 30, 2020 $ 1,003,469 $ 111,493 $ (7,090) $ 1,107,872 September 30, 2019 918,672 111,064 (8,182) 1,021,554 Net income (loss): Three months ended: September 30, 2020 $ 93,507 $ 4,130 $ (2,581) $ 95,056 September 30, 2019 110,583 4,106 (2,853) 111,836 Nine months ended: September 30, 2020 $ 238,023 $ 13,642 $ (8,784) $ 242,881 September 30, 2019 334,393 15,403 (9,878) 339,918

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value MeasurementsThe 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 2019 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, 2020 and December 31, 2019, 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, 2020 Securities available for sale: U.S. Treasury $ 1,124,965 $ — $ — $ 1,124,965 Residential mortgage-backed securities — 2,164,162 — 2,164,162 States and political subdivisions — 7,273,005 — 7,273,005 Other — 42,348 — 42,348 Trading account securities: U.S. Treasury 24,495 — — 24,495 States and political subdivisions — 1,313 — 1,313 Derivative assets: Interest rate swaps, caps and floors — 96,659 — 96,659 Commodity swaps and options — 35,272 — 35,272 Foreign currency forward contracts — — — — Derivative liabilities: Interest rate swaps, caps and floors — 38,339 — 38,339 Commodity swaps and options — 34,432 — 34,432 Foreign currency forward contracts — — — — December 31, 2019 Securities available for sale: U.S. Treasury $ 1,948,133 $ — $ — $ 1,948,133 Residential mortgage-backed securities — 2,207,594 — 2,207,594 States and political subdivisions — 7,070,997 — 7,070,997 Other — 42,867 — 42,867 Trading account securities: U.S. Treasury 24,298 — — 24,298 Derivative assets: Interest rate swaps, caps and floors — 44,196 — 44,196 Commodity swaps and options — 12,218 — 12,218 Foreign currency forward contracts 45 — — 45 Derivative liabilities: Interest rate swaps, caps and floors — 20,197 — 20,197 Commodity swaps and options — 11,677 — 11,677 Foreign currency forward contracts 33 — — 33 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 $ 6,388 $ 8,660 $ 2,416 $ 60,010 Specific (allocations) reversals of prior allocations (930) 14,928 1,116 (12,437) Fair value $ 5,458 $ 23,588 $ 3,532 $ 47,573 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 2020 2019 Foreclosed assets remeasured at initial recognition: Carrying value of foreclosed assets prior to remeasurement $ — $ 1,302 Charge-offs recognized in the allowance for credit losses on loan — (76) Fair value $ — $ 1,226 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, 2020 December 31, 2019 Carrying Estimated Carrying Estimated Financial assets: Level 2 inputs: Cash and cash equivalents $ 7,175,680 $ 7,175,680 $ 3,788,181 $ 3,788,181 Securities held to maturity 1,951,384 2,058,855 2,030,005 2,048,675 Cash surrender value of life insurance policies 189,148 189,148 187,156 187,156 Accrued interest receivable 122,618 122,618 183,850 183,850 Level 3 inputs: Loans, net 17,960,402 18,160,904 14,618,165 14,654,615 Financial liabilities: Level 2 inputs: Deposits 33,499,503 33,503,668 27,639,564 27,641,255 Federal funds purchased and repurchase agreements 1,608,667 1,608,667 1,695,342 1,695,342 Junior subordinated deferrable interest debentures 136,342 137,115 136,299 137,115 Subordinated notes 98,982 112,912 98,865 89,077 Accrued interest payable 9,144 9,144 12,393 12,393 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, 2020
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 2019 Form 10-K for additional information related to previously issued accounting standards updates. ASU 2016-13, “Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” ASU 2016-13 requires the measurement of all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts and requires enhanced disclosures related to the significant estimates and judgments used in estimating credit losses, as well as the credit quality and underwriting standards of an organization’s portfolio. We adopted ASU 2016-13, as subsequently updated for certain clarifications, targeted relief and codification improvements, as of January 1, 2020. See Note 1 - Significant Accounting Policies for additional information. ASU 2020-4, “Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting.” ASU 2020-4 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 2020-4 was effective upon issuance and generally can be applied through December 31, 2022. The adoption of ASU 2020-4 did not significantly impact our financial statements. ASU 2020-8, “Codification Improvements to Subtopic 310-20, Receivables - Nonrefundable Fees and Other Costs.” ASU 2020-8 clarifies the accounting for the amortization of purchase premiums for callable debt securities with multiple call dates. ASU 2020-8 will be effective for us on January 1, 2021 and is not expected to have a significant impact on our financial statements.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2020
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, 2019, included in our Annual Report on Form 10-K filed with the SEC on February 4, 2020 (the “2019 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]	In addition, 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. Accounting Standards Codification (“ASC”) Topic 326 (“ASC 326”) replaces the previous “incurred loss” model for measuring credit losses, which encompassed allowances for current known and inherent losses within the portfolio, with an “expected loss” model, which encompasses allowances for losses expected to be incurred over the life of the portfolio. The new current expected credit loss (“CECL”) model requires the measurement of all expected credit losses for financial assets measured at amortized cost and certain off-balance-sheet credit exposures based on historical experience, current conditions, and reasonable and supportable forecasts. ASC 326 also requires enhanced disclosures related to the significant estimates and judgments used in estimating credit losses, as well as the credit quality and underwriting standards of an organization’s portfolio. In addition, ASC 326 includes certain changes to the accounting for available-for-sale securities including the requirement to present credit losses as an allowance rather than as a direct write-down for available-for-sale securities management does not intend to sell or believes that it is more likely than not they will be required to sell. We adopted ASC 326 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 the table below. Operating results for periods after January 1, 2020 are presented in accordance with ASC 326 while prior period amounts continue to be reported in accordance with previously applicable standards and the accounting policies described in our 2019 Form 10-K.
Marketable Securities, Policy [Policy Text Block]	Securities : Securities are classified as held to maturity and carried at amortized cost when management has the positive intent and ability to hold them until maturity. Securities to be held for indefinite periods of time are classified as available for sale and carried at fair value, with the unrealized holding gains and losses (those for which no allowance for credit losses are recorded) reported as a component of other comprehensive income, net of tax. Securities held for resale in anticipation of short-term market movements are classified as trading and are carried at fair value, with changes in unrealized holding gains and losses included in income. Management determines the appropriate classification of securities at the time of purchase. Securities with limited marketability, such as stock in the Federal Reserve Bank and the Federal Home Loan Bank, are carried at cost. Interest income on securities includes amortization of purchase premiums and discounts. Premiums and discounts on securities are generally amortized using the interest method with a constant effective yield without anticipating prepayments, except for mortgage-backed securities where prepayments are anticipated. Premiums on callable securities are amortized to their earliest call date. A security is placed on non-accrual status if (i) principal or interest has been in default for a period of 90 days or more or (ii) full payment of principal and interest is not expected. Interest accrued but not received for a security placed on non-accrual status is reversed against interest income. Gains and losses on sales are recorded on the trade date and are derived from the amortized cost of the security sold.
Credit Loss, Financial Instrument [Policy Text Block]	Allowance For Credit Losses - Held-to-Maturity Securities : The allowance for credit losses on held-to-maturity securities is a contra-asset valuation account, calculated in accordance with ASC 326, that is deducted from the amortized cost basis of held-to-maturity securities to present management's best estimate of the net amount expected to be collected. Held-to-maturity securities are charged-off against the allowance when deemed uncollectible by management. Adjustments to the allowance are reported in our income statement as a component of credit loss expense. Management measures expected credit losses on held-to-maturity securities on a collective basis by major security type with each type sharing similar risk characteristics, and considers historical credit loss information that is adjusted for current conditions and reasonable and supportable forecasts. Management has made the accounting policy election to exclude accrued interest receivable on held-to-maturity securities from the estimate of credit losses. Further information regarding our policies and methodology used to estimate the allowance for credit losses on held-to-maturity securities is presented in Note 2 - Securities. Allowance For Credit Losses - Available-for-Sale Securities : For available-for-sale securities in an unrealized loss position, we first assess whether (i) we intend to sell, or (ii) it is more likely than not that we will be required to sell the security before recovery of its amortized cost basis. If either case is affirmative, any previously recognized allowances are charged-off and the security's amortized cost is written down to fair value through income. If neither case is affirmative, the security is evaluated to determine whether the decline in fair value has resulted from credit losses or other factors. In making this assessment, management considers the extent to which fair value is less than amortized cost, any changes to the rating of the security by a rating agency and any adverse conditions specifically related to the security, among other factors. If this assessment indicates that a credit loss exists, the present value of cash flows expected to be collected from the security are compared to the amortized cost basis of the security. If the present value of cash flows expected to be collected is less than the amortized cost basis, a credit loss exists and an allowance for credit losses is recorded for the credit loss, limited by the amount that the fair value is less than the amortized cost basis. Any impairment that has not been recorded through an allowance for credit losses is recognized in other comprehensive income. Adjustments to the allowance are reported in our income statement as a component of credit loss expense. Management has made the accounting policy election to exclude accrued interest receivable on available-for-sale securities from the estimate of credit losses. Available-for-sale securities are charged-off against the allowance or, in the absence of any allowance, written down through income when deemed uncollectible by management or when either of the aforementioned criteria regarding intent or requirement to sell is met.
Financing Receivable [Policy Text Block]	Loans : Loans are reported at the principal balance outstanding net of unearned discounts. Interest income on loans is reported on the level-yield method and includes amortization of deferred loan fees and costs over the terms of the individual loans to which they relate, or, in certain cases, over the average expected term for loans where deferred fees and costs are accounted for on a pooled basis. Net loan commitment fees or costs for commitment periods greater than one year are deferred and amortized into fee income or other expense on a straight-line basis over the commitment period. Income on direct financing leases is recognized on a basis that achieves a constant periodic rate of return on the outstanding investment. Further information regarding our accounting policies related to past due loans, non-accrual loans, collateral dependent loans and troubled-debt restructurings is presented in Note 3 - Loans.
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 management's best estimate of the net amount expected to be collected. Loans are charged-off against the allowance when deemed uncollectible by management. Expected recoveries do not exceed the aggregate of amounts previously charged-off and expected to be charged-off. Adjustments to the allowance are reported in our income statement as a component of credit loss expense. Management has made the accounting policy election to exclude accrued interest receivable on loans from the estimate of credit losses. Further information regarding our policies and methodology used to estimate the allowance for credit losses on loans is presented in Note 3 - Loans. 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. The contractual term excludes expected extensions, renewals and modifications unless (i) management has a reasonable expectation that a trouble debt restructuring will be executed with an individual borrower or (ii) such extension or renewal options are not unconditionally cancellable by us and, in such cases, the borrower is likely to meet applicable conditions and likely to request extension or renewal. Relevant available information includes historical credit loss experience, current conditions and reasonable and supportable forecasts. While historical credit loss experience provides the basis for the estimation of expected credit losses, adjustments to historical loss information may be made for differences in current portfolio-specific risk characteristics, environmental conditions or other relevant factors. The allowance for credit losses is measured on a collective basis for portfolios of loans when similar risk characteristics exist. Loans that do not share risk characteristics are evaluated for expected credit losses on an individual basis and excluded from the collective evaluation. Expected credit losses for collateral dependent loans, including loans where the borrower is experiencing financial difficulty but foreclosure is not probable, are based on the fair value of the collateral at the reporting date, adjusted for selling costs as 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. In calculating the allowance for credit losses, most loans are segmented into pools based upon similar characteristics and risk profiles. Common characteristics and risk profiles include the type/purpose of loan, underlying collateral, geographical similarity and historical/expected credit loss patterns. In developing these loan pools for the purposes of modeling expected credit losses, we also analyzed the degree of correlation in how loans within each portfolio respond when subjected to varying economic conditions and scenarios as well as other portfolio stress factors. For modeling purposes, our loan pools include (i) commercial and industrial and energy - non-revolving, (ii) commercial and industrial and energy - revolving, (iii) commercial real estate - owner occupied, (iv) commercial real estate - non-owner occupied, (v) commercial real estate - construction/land development, (vi) consumer real estate and (vii) consumer and other. We periodically reassess each pool to ensure the loans within the pool continue to share similar characteristics and risk profiles and to determine whether further segmentation is necessary. For each loan pool, we measure expected credit losses over the life of each loan utilizing a combination of models which measure (i) probability of default (“PD”), which is the likelihood that loan will stop performing/default, (ii) probability of attrition (“PA”), which is the likelihood that a loan will pay-off prior to maturity, (iii) loss given default (“LGD”), which is the expected loss rate for loans in default and (iv) exposure at default (“EAD”), which is the estimated outstanding principal balance of the loans upon default, including the expected funding of unfunded commitments outstanding as of the measurement date. For certain commercial loan portfolios, the PD is calculated using a transition matrix to determine the likelihood of a customer’s risk grade migrating from one specified range of risk grades to a different specified range. Expected credit losses are calculated as the product of PD (adjusted for attrition), LGD and EAD. This methodology builds on default probabilities already incorporated into our risk grading process by utilizing pool-specific historical loss rates to calculate expected credit losses. These pool-specific historical loss rates may be adjusted for current macroeconomic assumptions, as further discussed below, and other factors such as differences in underwriting standards, portfolio mix, or when historical asset terms do not reflect the contractual terms of the financial assets being evaluated as of the measurement date. Each time we measure expected credit losses, we assess the relevancy of historical loss information and consider any necessary adjustments to address any differences in asset-specific characteristics. Due to their short-term nature, expected credit losses for overdrafts included in consumer and other loans are based solely upon a weighting of recent historical charge-offs over a period of three years. The measurement of expected credit losses is impacted by loan/borrower attributes and certain macroeconomic variables. Significant loan/borrower attributes utilized in our modeling processes include, among other things, (i) origination date, (ii) maturity date, (iii) payment type, (iv) collateral type and amount, (v) current risk grade, (vi) current unpaid balance and commitment utilization rate, (vii) payment status/delinquency history and (viii) expected recoveries of previously charged-off amounts. Significant macroeconomic variables utilized in our modeling processes include, among other things, (i) Gross State Product for Texas and U.S. Gross Domestic Product, (ii) selected market interest rates including U.S. Treasury rates, bank prime rate, 30-year fixed mortgage rate, BBB corporate bond rate, among others, (iii) unemployment rates, (iv) commercial and residential property prices in Texas and the U.S. as a whole, (v) West Texas Intermediate crude oil price and (vi) total stock market index. PD and PA were estimated by analyzing internally-sourced data related to historical performance of each loan pool over a complete economic cycle. PD and PA are adjusted to reflect the current impact of certain macroeconomic variables as well as their expected changes over a reasonable and supportable forecast period. We have determined that we are reasonably able to forecast the macroeconomic variables used in our modeling processes with an acceptable degree of confidence for a total of two years with the last twelve months of the forecast period encompassing a reversion process whereby the forecasted macroeconomic variables are reverted to their historical mean utilizing a rational, systematic basis. The macroeconomic variables utilized as inputs in our modeling processes were subjected to a variety of analysis procedures and were selected primarily based on statistical relevancy and correlation to our historical credit losses. By reverting these modeling inputs to their historical mean and considering loan/borrower specific attributes, our models will yield a measurement of expected credit losses that reflects our average historical loss rates for periods subsequent to the twelve-month reversion period. The LGD is based on historical recovery averages for each loan pool, adjusted to reflect the current impact of certain macroeconomic variables as well as their expected changes over a two-year forecast period, with the final twelve months of the forecast period encompassing a reversion process, which management considers to be both reasonable and supportable. This same forecast/reversion period is used for all macroeconomic variables used in all of our models. EAD is estimated using a linear regression model that estimates the average percentage of the loan balance that remains at the time of a default event. Management qualitatively adjusts model results for risk factors that are not considered within our modeling processes but are nonetheless relevant in assessing the expected credit losses within our loan pools. These qualitative factor (“Q-Factor”) adjustments may increase or decrease management's estimate of expected credit losses by a calculated percentage or amount based upon the estimated level of risk. The various risks that may be considered in making Q-Factor adjustments include, among other things, the impact of (i) changes in lending policies and procedures, including changes in underwriting standards and practices for collections, write-offs, and recoveries, (ii) actual and expected changes in international, national, regional, and local economic and business conditions and developments that affect the collectibility of the loan pools, (iii) changes in the nature and volume of the loan pools and in the terms of the underlying loans, (iv) changes in the experience, ability, and depth of our lending management and staff, (v) changes in volume and severity of past due financial assets, the volume of non-accrual assets, and the volume and severity of adversely classified or graded assets, (vi) changes in the quality of our credit review function, (vii) changes in the value of the underlying collateral for loans that are non-collateral dependent, (viii) the existence, growth, and effect of any concentrations of credit and (ix) other factors such as the regulatory, legal and technological environments; competition; and events such as natural disasters or health pandemics. In some cases, management may determine that an individual loan exhibits unique risk characteristics which differentiate the loan from other loans within our loan pools. In such cases, the loans are evaluated for expected credit losses on an individual basis and excluded from the collective evaluation. Specific allocations of the allowance for credit losses are determined by analyzing the borrower’s ability to repay amounts owed, collateral deficiencies, the relative risk grade of the loan and economic conditions affecting the borrower’s industry, among other things. A loan is considered to be collateral dependent when, based upon management's assessment, the borrower is experiencing financial difficulty and repayment is expected to be provided substantially through the operation or sale of the collateral. In such cases, expected credit losses are based on the fair value of the collateral at the measurement date, adjusted for estimated selling costs if satisfaction of the loan depends on the sale of the collateral. We reevaluate the fair value of collateral supporting collateral dependent loans on a quarterly basis. The fair value of real estate collateral supporting collateral dependent loans is evaluated by our internal appraisal services using a methodology that is consistent with the Uniform Standards of Professional Appraisal Practice. The fair value of collateral supporting collateral dependent construction loans is based on an “as is” valuation.
Off-Balance-Sheet Credit Exposure, Policy [Policy Text Block]	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. The allowance is reported as a component of accrued interest payable and other liabilities in our consolidated balance sheets. Adjustments to the allowance are reported in our income statement as a component of credit loss expense. Further information regarding our policies and methodology used to estimate the allowance for credit losses on off-balance-sheet credit exposures is presented in Note 6 - Off-Balance-Sheet Arrangements, Commitments, Guarantees and Contingencies.

Loans Loans - Allowance For Cre

Loans Loans - Allowance For Credit Losses (Policies)	9 Months Ended
	Sep. 30, 2020
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 management's best estimate of the net amount expected to be collected. Loans are charged-off against the allowance when deemed uncollectible by management. Expected recoveries do not exceed the aggregate of amounts previously charged-off and expected to be charged-off. Adjustments to the allowance are reported in our income statement as a component of credit loss expense. Management has made the accounting policy election to exclude accrued interest receivable on loans from the estimate of credit losses. Further information regarding our policies and methodology used to estimate the allowance for credit losses on loans is presented in Note 3 - Loans. 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. The contractual term excludes expected extensions, renewals and modifications unless (i) management has a reasonable expectation that a trouble debt restructuring will be executed with an individual borrower or (ii) such extension or renewal options are not unconditionally cancellable by us and, in such cases, the borrower is likely to meet applicable conditions and likely to request extension or renewal. Relevant available information includes historical credit loss experience, current conditions and reasonable and supportable forecasts. While historical credit loss experience provides the basis for the estimation of expected credit losses, adjustments to historical loss information may be made for differences in current portfolio-specific risk characteristics, environmental conditions or other relevant factors. The allowance for credit losses is measured on a collective basis for portfolios of loans when similar risk characteristics exist. Loans that do not share risk characteristics are evaluated for expected credit losses on an individual basis and excluded from the collective evaluation. Expected credit losses for collateral dependent loans, including loans where the borrower is experiencing financial difficulty but foreclosure is not probable, are based on the fair value of the collateral at the reporting date, adjusted for selling costs as 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. In calculating the allowance for credit losses, most loans are segmented into pools based upon similar characteristics and risk profiles. Common characteristics and risk profiles include the type/purpose of loan, underlying collateral, geographical similarity and historical/expected credit loss patterns. In developing these loan pools for the purposes of modeling expected credit losses, we also analyzed the degree of correlation in how loans within each portfolio respond when subjected to varying economic conditions and scenarios as well as other portfolio stress factors. For modeling purposes, our loan pools include (i) commercial and industrial and energy - non-revolving, (ii) commercial and industrial and energy - revolving, (iii) commercial real estate - owner occupied, (iv) commercial real estate - non-owner occupied, (v) commercial real estate - construction/land development, (vi) consumer real estate and (vii) consumer and other. We periodically reassess each pool to ensure the loans within the pool continue to share similar characteristics and risk profiles and to determine whether further segmentation is necessary. For each loan pool, we measure expected credit losses over the life of each loan utilizing a combination of models which measure (i) probability of default (“PD”), which is the likelihood that loan will stop performing/default, (ii) probability of attrition (“PA”), which is the likelihood that a loan will pay-off prior to maturity, (iii) loss given default (“LGD”), which is the expected loss rate for loans in default and (iv) exposure at default (“EAD”), which is the estimated outstanding principal balance of the loans upon default, including the expected funding of unfunded commitments outstanding as of the measurement date. For certain commercial loan portfolios, the PD is calculated using a transition matrix to determine the likelihood of a customer’s risk grade migrating from one specified range of risk grades to a different specified range. Expected credit losses are calculated as the product of PD (adjusted for attrition), LGD and EAD. This methodology builds on default probabilities already incorporated into our risk grading process by utilizing pool-specific historical loss rates to calculate expected credit losses. These pool-specific historical loss rates may be adjusted for current macroeconomic assumptions, as further discussed below, and other factors such as differences in underwriting standards, portfolio mix, or when historical asset terms do not reflect the contractual terms of the financial assets being evaluated as of the measurement date. Each time we measure expected credit losses, we assess the relevancy of historical loss information and consider any necessary adjustments to address any differences in asset-specific characteristics. Due to their short-term nature, expected credit losses for overdrafts included in consumer and other loans are based solely upon a weighting of recent historical charge-offs over a period of three years. The measurement of expected credit losses is impacted by loan/borrower attributes and certain macroeconomic variables. Significant loan/borrower attributes utilized in our modeling processes include, among other things, (i) origination date, (ii) maturity date, (iii) payment type, (iv) collateral type and amount, (v) current risk grade, (vi) current unpaid balance and commitment utilization rate, (vii) payment status/delinquency history and (viii) expected recoveries of previously charged-off amounts. Significant macroeconomic variables utilized in our modeling processes include, among other things, (i) Gross State Product for Texas and U.S. Gross Domestic Product, (ii) selected market interest rates including U.S. Treasury rates, bank prime rate, 30-year fixed mortgage rate, BBB corporate bond rate, among others, (iii) unemployment rates, (iv) commercial and residential property prices in Texas and the U.S. as a whole, (v) West Texas Intermediate crude oil price and (vi) total stock market index. PD and PA were estimated by analyzing internally-sourced data related to historical performance of each loan pool over a complete economic cycle. PD and PA are adjusted to reflect the current impact of certain macroeconomic variables as well as their expected changes over a reasonable and supportable forecast period. We have determined that we are reasonably able to forecast the macroeconomic variables used in our modeling processes with an acceptable degree of confidence for a total of two years with the last twelve months of the forecast period encompassing a reversion process whereby the forecasted macroeconomic variables are reverted to their historical mean utilizing a rational, systematic basis. The macroeconomic variables utilized as inputs in our modeling processes were subjected to a variety of analysis procedures and were selected primarily based on statistical relevancy and correlation to our historical credit losses. By reverting these modeling inputs to their historical mean and considering loan/borrower specific attributes, our models will yield a measurement of expected credit losses that reflects our average historical loss rates for periods subsequent to the twelve-month reversion period. The LGD is based on historical recovery averages for each loan pool, adjusted to reflect the current impact of certain macroeconomic variables as well as their expected changes over a two-year forecast period, with the final twelve months of the forecast period encompassing a reversion process, which management considers to be both reasonable and supportable. This same forecast/reversion period is used for all macroeconomic variables used in all of our models. EAD is estimated using a linear regression model that estimates the average percentage of the loan balance that remains at the time of a default event. Management qualitatively adjusts model results for risk factors that are not considered within our modeling processes but are nonetheless relevant in assessing the expected credit losses within our loan pools. These qualitative factor (“Q-Factor”) adjustments may increase or decrease management's estimate of expected credit losses by a calculated percentage or amount based upon the estimated level of risk. The various risks that may be considered in making Q-Factor adjustments include, among other things, the impact of (i) changes in lending policies and procedures, including changes in underwriting standards and practices for collections, write-offs, and recoveries, (ii) actual and expected changes in international, national, regional, and local economic and business conditions and developments that affect the collectibility of the loan pools, (iii) changes in the nature and volume of the loan pools and in the terms of the underlying loans, (iv) changes in the experience, ability, and depth of our lending management and staff, (v) changes in volume and severity of past due financial assets, the volume of non-accrual assets, and the volume and severity of adversely classified or graded assets, (vi) changes in the quality of our credit review function, (vii) changes in the value of the underlying collateral for loans that are non-collateral dependent, (viii) the existence, growth, and effect of any concentrations of credit and (ix) other factors such as the regulatory, legal and technological environments; competition; and events such as natural disasters or health pandemics. In some cases, management may determine that an individual loan exhibits unique risk characteristics which differentiate the loan from other loans within our loan pools. In such cases, the loans are evaluated for expected credit losses on an individual basis and excluded from the collective evaluation. Specific allocations of the allowance for credit losses are determined by analyzing the borrower’s ability to repay amounts owed, collateral deficiencies, the relative risk grade of the loan and economic conditions affecting the borrower’s industry, among other things. A loan is considered to be collateral dependent when, based upon management's assessment, the borrower is experiencing financial difficulty and repayment is expected to be provided substantially through the operation or sale of the collateral. In such cases, expected credit losses are based on the fair value of the collateral at the measurement date, adjusted for estimated selling costs if satisfaction of the loan depends on the sale of the collateral. We reevaluate the fair value of collateral supporting collateral dependent loans on a quarterly basis. The fair value of real estate collateral supporting collateral dependent loans is evaluated by our internal appraisal services using a methodology that is consistent with the Uniform Standards of Professional Appraisal Practice. The fair value of collateral supporting collateral dependent construction loans is based on an “as is” valuation.

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Additional Cash Flow Information	Additional cash flow information was as follows: Nine Months Ended 2020 2019 Cash paid for interest $ 41,634 $ 100,041 Cash paid for income taxes 44,140 42,352 Significant non-cash transactions: Unsettled securities transactions 6,698 82,064 Loans foreclosed and transferred to other real estate owned and foreclosed assets 140 1,302 Loans to facilitate the sale of other real estate owned — 847 Right-of-use lease assets obtained in exchange for lessee operating lease liabilities 18,284 315,542 Treasury stock issued to 401(k) stock purchase plan 6,961 —
Schedule of New Accounting Pronouncements and Changes in Accounting Principles [Table Text Block]	The following table details the impact of the adoption of ASC 326 on the allowance for credit losses as of January 1, 2020. January 1, 2020 Pre-Adoption Allowance Impact of Adoption Post-Adoption Allowance Cumulative Effect on Retained Earnings Securities held to maturity: U.S. Treasury $ — $ — $ — $ — Residential mortgage-backed securities — — — — States and political subdivisions — 215 215 (170) Other — — — — Total $ — $ 215 $ 215 $ (170) Loans: Commercial and industrial $ 51,593 $ 21,263 $ 72,856 $ (16,798) Energy 37,382 (10,453) 26,929 8,258 Commercial real estate 31,037 (13,519) 17,518 10,680 Consumer real estate 4,113 2,392 6,505 (1,890) Consumer and other 8,042 (2,248) 5,794 1,776 Total $ 132,167 $ (2,565) $ 129,602 $ 2,026 Off-balance-sheet credit exposures $ 500 $ 39,377 $ 39,877 $ (31,108)

Securities (Tables)

Securities (Tables)	9 Months Ended
	Sep. 30, 2020
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, 2020 and December 31, 2019 is presented below. Amortized Gross Gross Estimated Allowance Net September 30, 2020 Residential mortgage-backed securities $ 529,290 $ 45,504 $ — $ 574,794 $ — $ 529,290 States and political subdivisions 1,420,754 61,808 — 1,482,562 (160) 1,420,594 Other 1,500 — 1 1,499 — 1,500 Total $ 1,951,544 $ 107,312 $ 1 $ 2,058,855 $ (160) $ 1,951,384 December 31, 2019 Residential mortgage-backed securities $ 530,861 $ 22 $ 9,365 $ 521,518 $ — $ 530,861 States and political subdivisions 1,497,644 28,909 896 1,525,657 — 1,497,644 Other 1,500 — — 1,500 — 1,500 Total $ 2,030,005 $ 28,931 $ 10,261 $ 2,048,675 $ — $ 2,030,005
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, 2020: States and Political Subdivisions Not Guaranteed or Pre-Refunded Guaranteed by the Texas PSF Pre-Refunded Total Other Aaa/AAA $ 115,385 $ 797,659 $ 332,240 $ 1,245,284 $ — Aa/AA 112,614 — — 112,614 — A 62,856 — — 62,856 — Not rated — — — — 1,500 Total $ 290,855 $ 797,659 $ 332,240 $ 1,420,754 $ 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, 2020. Three Months Ended Nine Months Ended September 30, 2020 Beginning balance $ 172 $ — Impact of adopting ASC 326 — 215 Credit loss expense (benefit) (12) (55) Ending balance $ 160 $ 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, 2020 and December 31, 2019 is presented below. Amortized Gross Gross Allowance Estimated September 30, 2020 U.S. Treasury $ 1,084,210 $ 40,755 $ — $ — $ 1,124,965 Residential mortgage-backed securities 2,092,184 72,021 43 — 2,164,162 States and political subdivisions 6,695,946 577,059 — — 7,273,005 Other 42,348 — — — 42,348 Total $ 9,914,688 $ 689,835 $ 43 $ — $ 10,604,480 December 31, 2019 U.S. Treasury $ 1,941,283 $ 18,934 $ 12,084 $ — $ 1,948,133 Residential mortgage-backed securities 2,176,275 32,608 1,289 — 2,207,594 States and political subdivisions 6,717,344 353,857 204 — 7,070,997 Other 42,867 — — — 42,867 Total $ 10,877,769 $ 405,399 $ 13,577 $ — $ 11,269,591
Securities, with Unrealized Losses Segregated by Length of Impairment	The table below summarizes, as of September 30, 2020, 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 Residential mortgage-backed securities $ 7,527 $ 42 $ 124 $ 1 $ 7,651 $ 43 Total $ 7,527 $ 42 $ 124 $ 1 $ 7,651 $ 43
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, 2020. 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 $ — $ 231 $ 516,368 $ 12,691 $ 529,290 States and political subdivisions 55,180 355,409 399,377 610,788 1,420,754 Other — 1,500 — — 1,500 Total $ 55,180 $ 357,140 $ 915,745 $ 623,479 $ 1,951,544 Estimated Fair Value Residential mortgage-backed securities $ — $ 233 $ 560,456 $ 14,105 $ 574,794 States and political subdivisions 56,043 369,536 411,517 645,466 1,482,562 Other — 1,499 — — 1,499 Total $ 56,043 $ 371,268 $ 971,973 $ 659,571 $ 2,058,855 Available For Sale Amortized Cost U. S. Treasury $ 299,355 $ 784,855 $ — $ — $ 1,084,210 Residential mortgage-backed securities 175 45,832 22,949 2,023,228 2,092,184 States and political subdivisions 201,529 1,029,750 696,654 4,768,013 6,695,946 Other — — — — 42,348 Total $ 501,059 $ 1,860,437 $ 719,603 $ 6,791,241 $ 9,914,688 Estimated Fair Value U. S. Treasury $ 305,121 $ 819,844 $ — $ — $ 1,124,965 Residential mortgage-backed securities 182 47,995 23,439 2,092,546 2,164,162 States and political subdivisions 204,107 1,127,267 757,126 5,184,505 7,273,005 Other — — — — 42,348 Total $ 509,410 $ 1,995,106 $ 780,565 $ 7,277,051 $ 10,604,480
Sales of Securities Available for Sale	Sales of securities available for sale were as follows: Three Months Ended Nine Months Ended 2020 2019 2020 2019 Proceeds from sales $ — $ 4,944,537 $ 1,162,352 $ 8,236,066 Gross realized gains — 96 108,989 899 Gross realized losses — — — (634) Tax (expense) benefit of securities gains/losses — (21) (22,888) (56)
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 2020 2019 2020 2019 Premium amortization $ (31,540) $ (29,511) $ (94,873) $ (88,798) Discount accretion 637 1,274 1,793 3,835 Net (premium amortization) discount accretion $ (30,903) $ (28,237) $ (93,080) $ (84,963)
Trading Account Securities, at Estimated Fair Value	Trading account securities, at estimated fair value, were as follows: September 30, December 31, U.S. Treasury $ 24,495 $ 24,298 States and political subdivisions 1,313 — Total $ 25,808 $ 24,298
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 2020 2019 2020 2019 Net gain on sales transactions $ 159 $ 731 $ 933 $ 1,789 Net mark-to-market gains (losses) 1 (201) 94 (170) Net gain (loss) on trading account securities $ 160 $ 530 $ 1,027 $ 1,619

Loans (Tables)

Loans (Tables)	9 Months Ended
	Sep. 30, 2020
Receivables [Abstract]
Loans	Loans were as follows: September 30, Percentage December 31, Percentage Commercial and industrial $ 4,820,018 26.4 % $ 5,187,466 35.2 % Energy: Production 1,108,753 6.1 1,348,900 9.2 Service 163,197 0.9 192,996 1.3 Other 91,418 0.4 110,986 0.8 Total energy 1,363,368 7.4 1,652,882 11.2 Paycheck Protection Program 3,226,980 17.7 — — Commercial real estate: Commercial mortgages 5,412,731 29.7 4,594,113 31.1 Construction 1,274,080 7.0 1,312,659 8.9 Land 320,407 1.8 289,467 2.0 Total commercial real estate 7,007,218 38.5 6,196,239 42.0 Consumer real estate: Home equity loans 340,226 1.9 375,596 2.6 Home equity lines of credit 434,171 2.4 354,671 2.4 Other 533,356 2.9 464,146 3.1 Total consumer real estate 1,307,753 7.2 1,194,413 8.1 Total real estate 8,314,971 45.7 7,390,652 50.1 Consumer and other 498,540 2.8 519,332 3.5 Total loans $ 18,223,877 100.0 % $ 14,750,332 100.0 %
Non-Accrual Loans, Segregated by Class of Loans	Non-accrual loans, segregated by class of loans, were as follows: September 30, 2020 December 31, 2019 Total Non-Accrual Non-Accrual with No Credit Loss Allowance Total Non-Accrual Non-Accrual with No Credit Loss Allowance Commercial and industrial $ 21,566 $ 6,583 $ 26,038 $ 13,266 Energy 54,041 33,324 65,761 3,281 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 13,525 6,754 8,912 6,558 Construction 680 680 665 665 Consumer real estate 1,748 1,748 922 922 Consumer and other 18 — 5 — Total $ 91,578 $ 49,089 $ 102,303 $ 24,692
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, 2020 by class and year of origination. 2020 2019 2018 2017 2016 Prior Revolving Loans Revolving Loans Converted to Term Total Commercial and industrial $ 9,317 $ 3,743 $ 1,886 $ 1,263 $ 110 $ 51 $ 1,879 $ 3,317 $ 21,566 Energy 24,149 7,021 2,204 — — — 15,054 5,613 54,041 Paycheck Protection Program — — — — — — — — — Commercial real estate: Buildings, land and other 1,426 6,014 169 1,372 840 3,590 114 — 13,525 Construction 680 — — — — — — — 680 Consumer real estate — — 421 211 350 382 261 123 1,748 Consumer and other — — — — — — 18 — 18 Total $ 35,572 $ 16,778 $ 4,680 $ 2,846 $ 1,300 $ 4,023 $ 17,326 $ 9,053 $ 91,578
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, 2020 was as follows: Loans Loans Total Current Total Accruing Commercial and industrial $ 36,067 $ 25,115 $ 61,182 $ 4,758,836 $ 4,820,018 $ 19,392 Energy 31,071 9,588 40,659 1,322,709 1,363,368 559 Paycheck Protection Program — — — 3,226,980 3,226,980 — Commercial real estate: Buildings, land and other 25,113 19,571 44,684 5,688,454 5,733,138 11,676 Construction — — — 1,274,080 1,274,080 — Consumer real estate 11,054 4,761 15,815 1,291,938 1,307,753 3,406 Consumer and other 6,629 673 7,302 491,238 498,540 673 Total $ 109,934 $ 59,708 $ 169,642 $ 18,054,235 $ 18,223,877 $ 35,706
Troubled Debt Restructurings	Troubled debt restructurings during the nine months ended September 30, 2020 and September 30, 2019 are set forth in the following table. Nine Months Ended Nine Months Ended Balance at Balance at Balance at Balance at Commercial and industrial $ 3,660 $ 1,415 $ 3,845 $ 2,188 Commercial real estate: Buildings, land and other 6,606 6,585 9,456 9,494 Construction 1,192 1,181 — — Consumer real estate — — 124 123 Consumer and other 1,104 104 — — $ 12,562 $ 9,285 $ 13,425 $ 11,805
Restructured Loans Past Due in Excess of 90 Days on Financing Receivables	Additional information related to restructured loans as of or for the three months ended September 30, 2020 and September 30, 2019 is set forth in the following table. September 30, 2020 September 30, 2019 Restructured loans past due in excess of 90 days at period-end: Number of loans 2 4 Dollar amount of loans $ 3,682 $ 3,244 Restructured loans on non-accrual status at period end 5,353 5,645 Charge-offs of restructured loans: Recognized in connection with restructuring — 1,500 Recognized on previously restructured loans 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, 2020. Paycheck Protection Program (“PPP”) loans are excluded as such loans are fully guaranteed by the Small Business Administration (“SBA”). 2020 2019 2018 2017 2016 Prior Revolving Loans Revolving Loans Converted to Term Total W/A Risk Grade Commercial and industrial Risk grades 1-8 $ 1,101,917 $ 625,692 $ 340,799 $ 251,180 $ 120,777 $ 129,757 $ 1,788,761 $ 43,468 $ 4,402,351 6.12 Risk grade 9 69,846 27,843 36,271 21,352 9,910 1,087 77,802 4,462 248,573 9.00 Risk grade 10 4,394 7,722 13,867 6,875 812 824 60,028 4,347 98,869 10.00 Risk grade 11 3,675 4,507 6,720 4,422 1,896 167 17,909 9,363 48,659 11.00 Risk grade 12 6,917 2,791 1,544 708 110 51 1,350 3,022 16,493 12.00 Risk grade 13 2,400 952 342 555 — — 529 295 5,073 13.00 $ 1,189,149 $ 669,507 $ 399,543 $ 285,092 $ 133,505 $ 131,886 $ 1,946,379 $ 64,957 $ 4,820,018 6.42 W/A risk grade 6.12 6.81 7.20 6.32 6.37 5.87 6.32 7.92 6.42 Energy Risk grades 1-8 $ 454,901 $ 25,589 $ 10,956 $ 6,732 $ 1,481 $ 4,709 $ 407,478 $ 24,936 $ 936,782 6.18 Risk grade 9 108,859 5,542 3,190 — — — 90,292 14,974 222,857 9.00 Risk grade 10 486 4,113 1,461 — 907 — 7,837 2,105 16,909 10.00 Risk grade 11 78,832 15,724 3,280 1,210 — 1,066 32,398 269 132,779 11.00 Risk grade 12 24,149 4,829 714 — — — 10,629 5,613 45,934 12.00 Risk grade 13 — 2,192 1,490 — — — 4,425 — 8,107 13.00 $ 667,227 $ 57,989 $ 21,091 $ 7,942 $ 2,388 $ 5,775 $ 553,059 $ 47,897 $ 1,363,368 7.39 W/A risk grade 7.43 9.19 8.78 7.83 7.93 8.06 6.98 8.76 7.39 2020 2019 2018 2017 2016 Prior Revolving Loans Revolving Loans Converted to Term Total W/A Risk Grade Commercial real estate: Buildings, land, other Risk grades 1-8 $ 1,167,872 $ 1,010,418 $ 829,640 $ 660,047 $ 430,971 $ 752,461 $ 62,672 $ 53,480 $ 4,967,561 6.98 Risk grade 9 26,056 93,113 67,546 66,365 45,477 92,060 4,413 1,042 396,072 9.00 Risk grade 10 1,362 26,080 29,357 39,853 65,702 42,528 4,147 2,876 211,905 10.00 Risk grade 11 7,416 7,108 13,425 42,660 10,487 59,730 3,038 211 144,075 11.00 Risk grade 12 1,226 5,214 169 1,372 840 3,340 51 — 12,212 12.00 Risk grade 13 200 800 — — — 250 63 — 1,313 13.00 $ 1,204,132 $ 1,142,733 $ 940,137 $ 810,297 $ 553,477 $ 950,369 $ 74,384 $ 57,609 $ 5,733,138 7.34 W/A risk grade 7.09 7.34 7.46 7.48 7.73 7.26 7.41 6.84 7.34 Construction Risk grades 1-8 $ 272,025 $ 480,166 $ 224,389 $ 753 $ 1,164 $ 1,786 $ 160,654 $ — $ 1,140,937 7.01 Risk grade 9 31,402 8,370 — 39,688 — — 14,608 — 94,068 9.00 Risk grade 10 6,133 — 27,479 — — — 3,838 — 37,450 10.00 Risk grade 11 — — — — — 945 — — 945 11.00 Risk grade 12 680 — — — — — — — 680 12.00 Risk grade 13 — — — — — — — — — 13.00 $ 310,240 $ 488,536 $ 251,868 $ 40,441 $ 1,164 $ 2,731 $ 179,100 $ — $ 1,274,080 7.25 W/A risk grade 6.87 7.18 7.74 8.98 7.29 8.02 7.00 — 7.25 Total commercial real estate $ 1,514,372 $ 1,631,269 $ 1,192,005 $ 850,738 $ 554,641 $ 953,100 $ 253,484 $ 57,609 $ 7,007,218 7.33 W/A risk grade 7.04 7.29 7.52 7.55 7.73 7.26 7.12 6.84 7.33
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, 2019. 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 6.17 $ 4,788,857 5.90 $ 1,488,301 6.78 $ 4,523,271 7.25 $ 1,274,098 6.88 $ 5,797,369 Risk grade 9 9.00 247,212 9.00 32,163 9.00 163,714 9.00 21,509 9.00 185,223 Risk grade 10 10.00 71,472 10.00 51,898 10.00 103,626 10.00 15,243 10.00 118,869 Risk grade 11 11.00 53,887 11.00 14,760 11.00 84,057 11.00 1,144 11.00 85,201 Risk grade 12 12.00 18,189 12.00 45,514 12.00 8,529 12.00 665 12.00 9,194 Risk grade 13 13.00 7,849 13.00 20,246 13.00 383 13.00 — 13.00 383 Total 6.44 $ 5,187,466 6.39 $ 1,652,882 7.01 $ 4,883,580 7.31 $ 1,312,659 7.07 $ 6,196,239
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, 2020 was as follows: 2020 2019 2018 2017 2016 Prior Revolving Loans Revolving Loans Converted to Term Total Consumer real estate: Past due 30-89 days $ 787 $ 850 $ 1,434 $ 858 $ 540 $ 4,558 $ 1,535 $ 492 $ 11,054 Past due 90 or more days — 139 894 542 351 2,028 600 207 4,761 Total past due 787 989 2,328 1,400 891 6,586 2,135 699 15,815 Current loans 253,392 188,770 106,947 93,143 75,216 142,983 414,331 17,156 1,291,938 Total $ 254,179 $ 189,759 $ 109,275 $ 94,543 $ 76,107 $ 149,569 $ 416,466 $ 17,855 $ 1,307,753 Consumer and other: Past due 30-89 days $ 1,328 $ 326 $ 288 $ 75 $ 2 $ 1 $ 4,313 $ 296 $ 6,629 Past due 90 or more days 69 189 3 — — 1 372 39 673 Total past due 1,397 515 291 75 2 2 4,685 335 7,302 Current loans 38,114 32,214 7,985 3,131 2,070 950 376,762 30,012 491,238 Total $ 39,511 $ 32,729 $ 8,276 $ 3,206 $ 2,072 $ 952 $ 381,447 $ 30,347 $ 498,540
Revolving Loans Converted to Term [Table Text Block]	Revolving loans that converted to term during the three and nine months ended September 30, 2020 were as follows: Three Months Ended Nine Months Ended Commercial and industrial $ 10,224 $ 25,340 Energy 7,144 38,642 Commercial real estate: Buildings, land and other 637 8,094 Construction — — Consumer real estate 421 2,132 Consumer and other 5,494 15,338 Total $ 23,920 $ 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, 2020, calculated in accordance with the CECL methodology described above. 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 Modeled expected credit losses $ 68,896 $ 21,644 $ 125,965 $ 10,226 $ 7,420 $ 234,151 Q-Factor and other qualitative adjustments (1,647) 24,630 (8,685) 51 31 14,380 Specific allocations 5,256 8,357 1,313 — 18 14,944 Total $ 72,505 $ 54,631 $ 118,593 $ 10,277 $ 7,469 $ 263,475
Allowance for Credit Losses By Loan Class in Accordance With Methodology Used Prior to CECL	The following table presents details of the allowance for credit losses on loans segregated by loan portfolio segment as of December 31, 2019, calculated in accordance with our prior incurred loss methodology described in our 2019 Form 10-K. Commercial Energy Commercial Consumer Consumer Total Historical valuation allowances $ 29,015 $ 7,873 $ 21,947 $ 2,690 $ 7,562 $ 69,087 Specific valuation allowances 7,849 20,246 383 — 5 28,483 General valuation allowances 9,840 5,196 4,201 904 (409) 19,732 Macroeconomic valuation allowances 4,889 4,067 4,506 519 884 14,865 Total $ 51,593 $ 37,382 $ 31,037 $ 4,113 $ 8,042 $ 132,167
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, 2020 and 2019. 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, 2020 Beginning balance $ 98,536 $ 40,817 $ 93,425 $ 8,998 $ 8,285 $ 250,061 Credit loss expense (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 September 30, 2019 Beginning balance $ 57,714 $ 25,818 $ 35,914 $ 5,637 $ 9,846 $ 134,929 Credit loss expense (3,527) 8,788 (607) (650) 3,997 8,001 Charge-offs (2,705) (2,000) — (557) (6,357) (11,619) Recoveries 1,185 740 46 454 2,823 5,248 Net charge-offs (1,520) (1,260) 46 (103) (3,534) (6,371) Ending balance $ 52,667 $ 33,346 $ 35,353 $ 4,884 $ 10,309 $ 136,559 Nine months ended: 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 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 (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 September 30, 2019 Beginning balance $ 48,580 $ 29,052 $ 38,777 $ 6,103 $ 9,620 $ 132,132 Credit loss expense 9,999 7,478 (2,972) 859 10,040 25,404 Charge-offs (8,782) (4,000) (617) (2,936) (17,157) (33,492) Recoveries 2,870 816 165 858 7,806 12,515 Net charge-offs (5,912) (3,184) (452) (2,078) (9,351) (20,977) Ending balance $ 52,667 $ 33,346 $ 35,353 $ 4,884 $ 10,309 $ 136,559
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, 2020 and December 31, 2019. September 30, 2020 December 31, 2019 Loan Specific Allocations Loan Specific Allocations Commercial and industrial $ 22,930 $ 5,256 $ 24,360 $ 7,849 Energy 56,420 8,357 65,244 20,246 Paycheck Protection Program — — — — Commercial real estate: Buildings, land and other 25,611 1,313 8,609 383 Construction 680 — 665 — Consumer real estate 1,335 — 570 — Consumer and other 18 18 5 5 Total $ 106,994 $ 14,944 $ 99,453 $ 28,483

Goodwill and Other Intangible_2

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets and Goodwill	Goodwill and other intangible assets are presented in the table below. As of September 30, 2020, we evaluated recent potential triggering events that might be indicators that our goodwill was impaired. The events include the economic disruption and uncertainty surrounding the COVID-19 pandemic and the circumstances surrounding recent volatility in the market price of crude oil. Based on our evaluation, we concluded that our goodwill was not more than likely impaired as of that date. September 30, December 31, Goodwill $ 654,952 $ 654,952 Other intangible assets: Core deposits $ 1,465 $ 2,043 Customer relationships 306 438 $ 1,771 $ 2,481
Estimated Aggregate Future Amortization Expense for Intangible Assets	The estimated aggregate future amortization expense for intangible assets remaining as of September 30, 2020 is as follows: Remainder of 2020 $ 208 2021 697 2022 481 2023 282 2024 87 Thereafter 16 $ 1,771

Deposits (Tables)

Deposits (Tables)	9 Months Ended
	Sep. 30, 2020
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 $ 13,852,404 41.4 % $ 10,212,265 36.9 % Correspondent banks 208,836 0.6 246,181 0.9 Public funds 784,530 2.3 415,183 1.5 Total non-interest-bearing demand deposits 14,845,770 44.3 10,873,629 39.3 Interest-bearing deposits: Private accounts: Savings and interest checking 8,176,608 24.4 7,147,327 25.9 Money market accounts 8,684,470 25.9 7,888,433 28.5 Time accounts of $100,000 or more 791,004 2.4 736,481 2.7 Time accounts under $100,000 339,174 1.0 347,418 1.2 Total private accounts 17,991,256 53.7 16,119,659 58.3 Public funds: Savings and interest checking 583,214 1.8 548,399 2.0 Money market accounts 76,088 0.2 73,180 0.3 Time accounts of $100,000 or more 3,092 — 24,672 0.1 Time accounts under $100,000 83 — 25 — Total public funds 662,477 2.0 646,276 2.4 Total interest-bearing deposits 18,653,733 55.7 16,765,935 60.7 Total deposits $ 33,499,503 100.0 % $ 27,639,564 100.0 %
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 $ 369 $ 361 Deposits from foreign sources (primarily Mexico) 845,950 805,828 Deposits not covered by deposit insurance 17,265,358 13,115,796

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2020
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 $ 9,586,212 $ 9,306,043 Standby letters of credit 260,683 260,587 Deferred standby letter of credit fees 1,716 1,276
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 2020 2019 2020 2019 Beginning balance $ 46,939 $ 500 $ 500 $ 500 Impact of adopting ASC 326 — — 39,377 — Credit loss expense (benefit) (3,276) — 3,786 — Ending balance $ 43,663 $ 500 $ 43,663 $ 500
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 2020 2019 2020 2019 Amortization of lease right-of-use assets $ 8,521 $ 7,415 $ 24,545 $ 19,698 Short-term lease expense 172 1,199 1,343 3,316 Non-lease components (including taxes, insurance, common maintenance, etc.) 2,586 3,114 8,410 7,022 Total $ 11,279 $ 11,728 $ 34,298 $ 30,036

Capital and Regulatory Matters

Capital and Regulatory Matters (Tables)	9 Months Ended
	Sep. 30, 2020
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, 2020 and December 31, 2019 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 2019 Form 10-K for a more detailed discussion of the Basel III Capital Rules. After a review of risk-weight classifications during the first quarter of 2019, risk-weightings for certain loans were reclassified. Amounts reported as of December 31, 2019 have been revised to reflect these reclassifications. Actual Minimum Capital Required - Basel III Required to be Capital Ratio Capital Ratio Capital Ratio September 30, 2020 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 3,003,029 12.71 % $ 1,654,073 7.00 % $ 1,535,925 6.50 % Frost Bank 3,010,290 12.76 1,651,191 7.00 1,533,249 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 3,003,029 12.71 2,008,517 8.50 1,890,369 8.00 Frost Bank 3,010,290 12.76 2,005,018 8.50 1,887,076 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,471,898 14.69 2,481,109 10.50 2,362,961 10.00 Frost Bank 3,246,159 13.76 2,476,787 10.50 2,358,845 10.00 Leverage Ratio Cullen/Frost 3,003,029 7.85 1,530,584 4.00 1,913,230 5.00 Frost Bank 3,010,290 7.87 1,530,076 4.00 1,912,596 5.00 December 31, 2019 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,857,250 12.36 % $ 1,617,886 7.00 % $ 1,502,323 6.50 % Frost Bank 2,958,326 12.82 1,615,206 7.00 1,499,834 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 3,001,736 12.99 1,964,576 8.50 1,849,013 8.00 Frost Bank 2,958,326 12.82 1,961,322 8.50 1,845,950 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,367,403 14.57 2,426,829 10.50 2,311,266 10.00 Frost Bank 3,090,993 13.40 2,422,809 10.50 2,307,438 10.00 Leverage Ratio Cullen/Frost 3,001,736 9.28 1,293,188 4.00 1,616,485 5.00 Frost Bank 2,958,326 9.15 1,292,743 4.00 1,615,929 5.00

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts and Estimated Fair Values of Interest Rate Derivative Contracts Outstanding	The notional amounts and estimated fair values of interest rate derivative contracts are presented in the following table. The fair values of interest rate derivative contracts are estimated utilizing internal valuation models with observable market data inputs, or as determined by the Chicago Mercantile Exchange (“CME”) for centrally cleared derivative contracts. CME rules legally characterize variation margin payments for centrally cleared derivatives as settlements of the derivatives' exposure rather than collateral. As a result, the variation margin payment and the related derivative instruments are considered a single unit of account for accounting and financial reporting purposes. Variation margin, as determined by the CME, is settled daily. As a result, derivative contracts that clear through the CME have an estimated fair value of zero as of September 30, 2020 and December 31, 2019. September 30, 2020 December 31, 2019 Notional Estimated Notional Estimated Derivatives designated as hedges of fair value: Financial institution counterparties: Loan/lease interest rate swaps – assets $ — $ — $ 2,545 $ 6 Loan/lease interest rate swaps – liabilities 4,036 (167) 6,000 (138) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets — — 122,788 67 Loan/lease interest rate swaps – liabilities 1,192,762 (36,975) 1,002,860 (19,483) Loan/lease interest rate caps – assets 306,171 1,197 107,835 266 Customer counterparties: Loan/lease interest rate swaps – assets 1,192,762 95,462 1,002,860 43,857 Loan/lease interest rate swaps – liabilities — — 122,788 (310) Loan/lease interest rate caps – liabilities 306,171 (1,197) 107,835 (266)
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, 2020 were as follows: Weighted-Average Interest Interest Interest rate swaps: Fair value hedge loan/lease interest rate swaps 3.39 % 0.15 % Non-hedging interest rate swaps – financial institution counterparties 4.00 1.93 Non-hedging interest rate swaps – customer counterparties 1.93 4.00
Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions	The notional amounts and estimated fair values of non-hedging commodity swap and option derivative positions outstanding are presented in the following table. We obtain dealer quotations and use internal valuation models with observable market data inputs to value our commodity derivative positions. September 30, 2020 December 31, 2019 Notional Notional Estimated Notional Estimated Financial institution counterparties: Oil – assets Barrels 2,296 $ 18,718 1,214 $ 2,796 Oil – liabilities Barrels 3,089 (9,132) 2,148 (6,916) Natural gas – assets MMBTUs 5,623 932 8,295 2,131 Natural gas – liabilities MMBTUs 18,181 (5,879) 2,689 (70) Customer counterparties: Oil – assets Barrels 3,106 9,461 2,172 7,208 Oil – liabilities Barrels 2,279 (18,506) 1,190 (2,652) Natural gas – assets MMBTUs 19,940 6,161 2,689 83 Natural gas – liabilities MMBTUs 3,864 (915) 8,295 (2,039)
Notional Amounts and Fair Values of Open Foreign Currency Forward Contracts	The notional amounts and fair values of open foreign currency forward contracts were as follows: September 30, 2020 December 31, 2019 Notional Notional Estimated Notional Estimated Financial institution counterparties: Forward contracts – liabilities CAD — — 4,593 $ (33) Customer counterparties: Forward contracts – assets CAD — — 4,583 45
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 2020 2019 2020 2019 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ (36) $ 24 $ (79) $ 78 Amount of (gain) loss included in other non-interest expense 2 (1) 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 2020 2019 2020 2019 Non-hedging interest rate derivatives: Other non-interest income $ 280 $ 215 $ 2,615 $ 1,188 Other non-interest expense 1 — 1 — Non-hedging commodity derivatives: Other non-interest income 379 109 1,057 322 Non-hedging foreign currency derivatives: Other non-interest income 10 12 28 41 Non-hedging put options: Other non-interest income — — 5,980 —

Balance Sheet Offsetting (Table

Balance Sheet Offsetting (Tables)	9 Months Ended
	Sep. 30, 2020
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, 2020 is presented in the following tables. Gross Amount Gross Amount Net Amount September 30, 2020 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 1,197 $ — $ 1,197 Commodity swaps and options 19,650 — 19,650 Foreign currency forward contracts — — — Total derivatives 20,847 — 20,847 Resell agreements 20,000 — 20,000 Total $ 40,847 $ — $ 40,847 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 37,142 $ — $ 37,142 Commodity swaps and options 15,011 — 15,011 Foreign currency forward contracts — — — Total derivatives 52,153 — 52,153 Repurchase agreements 1,574,367 — 1,574,367 Total $ 1,626,520 $ — $ 1,626,520 Information about financial instruments that are eligible for offset in the consolidated balance sheet as of December 31, 2019 is presented in the following tables. Gross Amount Gross Amount Net Amount December 31, 2019 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 339 $ — $ 339 Commodity swaps and options 4,927 — 4,927 Total derivatives 5,266 — 5,266 Resell agreements 31,299 — 31,299 Total $ 36,565 $ — $ 36,565 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 19,621 $ — $ 19,621 Commodity swaps and options 6,986 — 6,986 Foreign currency forward contracts 33 — 33 Total derivatives 26,640 — 26,640 Repurchase agreements 1,668,142 — 1,668,142 Total $ 1,694,782 $ — $ 1,694,782
Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset	Gross Amounts Not Offset Net Amount Financial Collateral Net September 30, 2020 Financial assets: Derivatives: Counterparty A $ 8 $ (8) $ — $ — Counterparty B 5,996 (5,996) — — Other counterparties 14,843 (14,510) (327) 6 Total derivatives 20,847 (20,514) (327) 6 Resell agreements 20,000 — (20,000) — Total $ 40,847 $ (20,514) $ (20,327) $ 6 Financial liabilities: Derivatives: Counterparty A $ 7,178 $ (8) $ (7,170) $ — Counterparty B 14,386 (5,996) (8,210) 180 Counterparty C 94 — (94) — Other counterparties 30,495 (14,510) (13,971) 2,014 Total derivatives 52,153 (20,514) (29,445) 2,194 Repurchase agreements 1,574,367 — (1,574,367) — Total $ 1,626,520 $ (20,514) $ (1,603,812) $ 2,194 Gross Amounts Not Offset Net Amount Financial Collateral Net December 31, 2019 Financial assets: Derivatives: Counterparty A $ 39 $ (39) $ — $ — Counterparty B 1,650 (1,650) — — Counterparty C 1 (1) — — Other counterparties 3,576 (3,546) — 30 Total derivatives 5,266 (5,236) — 30 Resell agreements 31,299 — (31,299) — Total $ 36,565 $ (5,236) $ (31,299) $ 30 Financial liabilities: Derivatives: Counterparty A $ 5,192 $ (39) $ (5,153) $ — Counterparty B 7,424 (1,650) (5,774) — Counterparty C 135 (1) (134) — Other counterparties 13,889 (3,546) (10,343) — Total derivatives 26,640 (5,236) (21,404) — Repurchase agreements 1,668,142 — (1,668,142) — Total $ 1,694,782 $ (5,236) $ (1,689,546) $ —
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, 2020 and December 31, 2019 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, 2020 Repurchase agreements: U.S. Treasury $ 99,365 $ — $ — $ — $ 99,365 Residential mortgage-backed securities 1,475,002 — — — 1,475,002 Total borrowings $ 1,574,367 $ — $ — $ — $ 1,574,367 Gross amount of recognized liabilities for repurchase agreements $ 1,574,367 Amounts related to agreements not included in offsetting disclosures above $ — December 31, 2019 Repurchase agreements: U.S. Treasury $ 435,904 $ — $ — $ — $ 435,904 Residential mortgage-backed securities 1,232,238 — — — 1,232,238 Total borrowings $ 1,668,142 $ — $ — $ — $ 1,668,142 Gross amount of recognized liabilities for repurchase agreements $ 1,668,142 Amounts related to agreements not included in offsetting disclosures above $ —

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2020
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, 2020, there were 1,109,864 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, 2020 55,370 $ 74.76 440,647 $ 90.22 177,288 $ 83.48 1,980,866 $ 64.60 Authorized — — — — — — — — Granted 10,428 73.84 458 65.43 — — — — Exercised/vested (12,938) 71.09 (1,640) 76.07 (41,755) 69.70 (140,615) 53.65 Forfeited/expired — — (2,813) 90.20 (6,894) 81.33 (5,427) 75.74 Balance, September 30, 2020 52,860 $ 75.47 436,652 $ 90.24 128,639 $ 88.07 1,834,824 $ 65.41
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 2020 2019 2020 2019 New shares issued from available authorized shares — — — — Issued from available treasury stock 53,425 71,284 196,948 225,184 Total 53,425 71,284 196,948 225,184 Proceeds from stock option exercises $ 2,698 $ 5,665 $ 7,544 $ 13,506
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 2020 2019 2020 2019 Stock options $ — $ 330 $ — $ 1,091 Non-vested stock awards/stock units 1,999 1,536 6,325 5,727 Director deferred stock units — — 770 780 Performance stock units 444 809 1,132 3,095 Total $ 2,443 $ 2,675 $ 8,227 $ 10,693 Income tax benefit $ 450 $ 452 $ 1,539 $ 1,768
Unrecognized Stock-Based Compensation Expense	Unrecognized stock-based compensation expense at September 30, 2020 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 $ 12,337 Performance stock units 4,651 Total $ 16,988

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	9 Months Ended
	Sep. 30, 2020
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 2020 2019 2020 2019 Net income $ 95,056 $ 111,836 $ 242,881 $ 339,918 Less: Preferred stock dividends — 2,016 2,016 6,047 Redemption of preferred stock — — 5,514 — Net income available to common shareholders 95,056 109,820 235,351 333,871 Less: Earnings allocated to participating securities 877 876 2,236 2,761 Net earnings allocated to common stock $ 94,179 $ 108,944 $ 233,115 $ 331,110 Distributed earnings allocated to common stock $ 44,546 $ 44,370 $ 133,511 $ 131,069 Undistributed earnings allocated to common stock 49,633 64,574 99,604 200,041 Net earnings allocated to common stock $ 94,179 $ 108,944 $ 233,115 $ 331,110 Weighted-average shares outstanding for basic earnings per common share 62,726,542 62,566,128 62,655,393 62,786,501 Dilutive effect of stock compensation 193,116 592,866 263,028 725,911 Weighted-average shares outstanding for diluted earnings per common share 62,919,658 63,158,994 62,918,421 63,512,412

Defined Benefit Plans (Tables)

Defined Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2020
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 2020 2019 2020 2019 Expected return on plan assets, net of expenses $ (3,073) $ (2,693) $ (9,217) $ (8,079) Interest cost on projected benefit obligation 1,252 1,618 3,757 4,854 Net amortization and deferral 1,330 1,406 3,989 4,218 Net periodic expense (benefit) $ (491) $ 331 $ (1,471) $ 993

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income Tax Expense	Income tax expense was as follows: Three Months Ended Nine Months Ended 2020 2019 2020 2019 Current income tax expense (benefit) $ (33,614) $ 14,480 $ 20,328 $ 43,984 Deferred income tax expense (benefit) 43,130 (950) (8,803) (1,625) Income tax expense, as reported $ 9,516 $ 13,530 $ 11,525 $ 42,359 Effective tax rate 9.1 % 10.8 % 4.5 % 11.1 %

Other Comprehensive Income (L_2

Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2020
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 $ 20,332 $ 4,269 $ 16,063 $ 107,786 $ 22,635 $ 85,151 Change in net unrealized gain on securities transferred to held to maturity (294) (62) (232) (305) (64) (241) Reclassification adjustment for net (gains) losses included in net income — — — (97) (21) (76) Total securities available for sale and transferred securities 20,038 4,207 15,831 107,384 22,550 84,834 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) 1,330 280 1,050 1,406 295 1,111 Total defined-benefit post-retirement benefit plans 1,330 280 1,050 1,406 295 1,111 Total other comprehensive income (loss) $ 21,368 $ 4,487 $ 16,881 $ 108,790 $ 22,845 $ 85,945 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 $ 406,959 $ 85,462 $ 321,497 $ 464,072 $ 97,455 $ 366,617 Change in net unrealized gain on securities transferred to held to maturity (979) (206) (773) (957) (201) (756) Reclassification adjustment for net (gains) losses included in net income (108,989) (22,888) (86,101) (266) (56) (210) Total securities available for sale and transferred securities 296,991 62,368 234,623 462,849 97,198 365,651 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) 3,989 837 3,152 4,218 886 3,332 Total defined-benefit post-retirement benefit plans 3,989 837 3,152 4,218 886 3,332 Total other comprehensive income (loss) $ 300,980 $ 63,205 $ 237,775 $ 467,067 $ 98,084 $ 368,983
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, 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 Balance January 1, 2019 $ (16,103) $ (47,497) $ (63,600) Other comprehensive income (loss) before reclassifications 365,861 — 365,861 Reclassification of amounts included in net income (210) 3,332 3,122 Net other comprehensive income (loss) during period 365,651 3,332 368,983 Balance at September 30, 2019 $ 349,548 $ (44,165) $ 305,383

Operating Segments (Tables)

Operating Segments (Tables)	9 Months Ended
	Sep. 30, 2020
Segment Reporting [Abstract]
Summary of Operating Results by Segment	Summarized operating results by segment were as follows: Banking Frost Wealth Non-Banks Consolidated Revenues from (expenses to) external customers: Three months ended: September 30, 2020 $ 293,855 $ 35,282 $ (2,113) $ 327,024 September 30, 2019 307,312 37,549 (2,630) 342,231 Nine months ended: September 30, 2020 $ 1,003,469 $ 111,493 $ (7,090) $ 1,107,872 September 30, 2019 918,672 111,064 (8,182) 1,021,554 Net income (loss): Three months ended: September 30, 2020 $ 93,507 $ 4,130 $ (2,581) $ 95,056 September 30, 2019 110,583 4,106 (2,853) 111,836 Nine months ended: September 30, 2020 $ 238,023 $ 13,642 $ (8,784) $ 242,881 September 30, 2019 334,393 15,403 (9,878) 339,918

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2020
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, 2020 and December 31, 2019, 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, 2020 Securities available for sale: U.S. Treasury $ 1,124,965 $ — $ — $ 1,124,965 Residential mortgage-backed securities — 2,164,162 — 2,164,162 States and political subdivisions — 7,273,005 — 7,273,005 Other — 42,348 — 42,348 Trading account securities: U.S. Treasury 24,495 — — 24,495 States and political subdivisions — 1,313 — 1,313 Derivative assets: Interest rate swaps, caps and floors — 96,659 — 96,659 Commodity swaps and options — 35,272 — 35,272 Foreign currency forward contracts — — — — Derivative liabilities: Interest rate swaps, caps and floors — 38,339 — 38,339 Commodity swaps and options — 34,432 — 34,432 Foreign currency forward contracts — — — — December 31, 2019 Securities available for sale: U.S. Treasury $ 1,948,133 $ — $ — $ 1,948,133 Residential mortgage-backed securities — 2,207,594 — 2,207,594 States and political subdivisions — 7,070,997 — 7,070,997 Other — 42,867 — 42,867 Trading account securities: U.S. Treasury 24,298 — — 24,298 Derivative assets: Interest rate swaps, caps and floors — 44,196 — 44,196 Commodity swaps and options — 12,218 — 12,218 Foreign currency forward contracts 45 — — 45 Derivative liabilities: Interest rate swaps, caps and floors — 20,197 — 20,197 Commodity swaps and options — 11,677 — 11,677 Foreign currency forward contracts 33 — — 33
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 $ 6,388 $ 8,660 $ 2,416 $ 60,010 Specific (allocations) reversals of prior allocations (930) 14,928 1,116 (12,437) Fair value $ 5,458 $ 23,588 $ 3,532 $ 47,573
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 2020 2019 Foreclosed assets remeasured at initial recognition: Carrying value of foreclosed assets prior to remeasurement $ — $ 1,302 Charge-offs recognized in the allowance for credit losses on loan — (76) Fair value $ — $ 1,226 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, 2020 December 31, 2019 Carrying Estimated Carrying Estimated Financial assets: Level 2 inputs: Cash and cash equivalents $ 7,175,680 $ 7,175,680 $ 3,788,181 $ 3,788,181 Securities held to maturity 1,951,384 2,058,855 2,030,005 2,048,675 Cash surrender value of life insurance policies 189,148 189,148 187,156 187,156 Accrued interest receivable 122,618 122,618 183,850 183,850 Level 3 inputs: Loans, net 17,960,402 18,160,904 14,618,165 14,654,615 Financial liabilities: Level 2 inputs: Deposits 33,499,503 33,503,668 27,639,564 27,641,255 Federal funds purchased and repurchase agreements 1,608,667 1,608,667 1,695,342 1,695,342 Junior subordinated deferrable interest debentures 136,342 137,115 136,299 137,115 Subordinated notes 98,982 112,912 98,865 89,077 Accrued interest payable 9,144 9,144 12,393 12,393

Significant Accounting Polici_4

Significant Accounting Policies Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			$ (29,252)		$ (14,672)
Debt Securities, Threshold Period Past Due	90 days
Operating Lease, Right-of-Use Asset	$ 298,600		297,700	$ 170,500
Operating Lease, Liability	$ 328,900		323,700	174,400
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change		$ (29,300)	$ (29,252)	$ (14,700)	$ (14,672)

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, 2020	Sep. 30, 2019
Accounting Policies [Abstract]
Cash paid for interest		$ 41,634	$ 100,041
Cash paid for income taxes		44,140	42,352
Unsettled securities transactions		6,698	82,064
Loans foreclosed and transferred to other real estate owned and foreclosed assets		140	1,302
Loans to facilitate the sale of other real estate owned		0	847
Right-of-use lease assets obtained in exchange for lessee operating lease liabilities		18,284	315,542
Treasury stock issued to 401(k) stock purchase plan	$ 4,131	$ 6,961	$ 0

Significant Accounting Polici_6

Significant Accounting Policies Significant Accounting Policies - Impact of Adoption of New Accounting Standard Related to CECL (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss	$ 160			$ 0
Financing Receivable, Allowance for Credit Loss	263,475			132,167
Off-Balance Sheet, Credit Loss, Liability	43,663	$ 46,939		500	$ 500	$ 500		$ 500
Cumulative effect of accounting change				(29,252)				(14,672)
Commercial Portfolio Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss	72,505	98,536		51,593	52,667	57,714		48,580
Commercial and Industrial Total Energy Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss	54,631	40,817		37,382	33,346	25,818		29,052
Commercial Real Estate Portfolio Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss	118,593	93,425		31,037	35,353	35,914		38,777
Total consumer real estate loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss	10,277	8,998		4,113	4,884	5,637		6,103
Consumer Loan [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss	7,469	8,285		8,042	10,309	9,846		9,620
Loans Receivable [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss	263,475	250,061		132,167	$ 136,559	$ 134,929		132,132
Residential Mortgage Backed Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss	0			0
US States and Political Subdivisions Debt Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss	160			0
Other Investments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss	0			0
Debt Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss	$ 160	$ 172		0
Pre-Adoption Allowance [Member] | Commercial Portfolio Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			$ 51,593
Pre-Adoption Allowance [Member] | Commercial and Industrial Total Energy Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			37,382
Pre-Adoption Allowance [Member] | Commercial Real Estate Portfolio Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			31,037
Pre-Adoption Allowance [Member] | Total consumer real estate loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			4,113
Pre-Adoption Allowance [Member] | Consumer Loan [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			8,042
Pre-Adoption Allowance [Member] | Loans Receivable [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			132,167
Pre-Adoption Allowance [Member] | US Treasury Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			0
Pre-Adoption Allowance [Member] | Residential Mortgage Backed Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			0
Pre-Adoption Allowance [Member] | US States and Political Subdivisions Debt Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			0
Pre-Adoption Allowance [Member] | Other Investments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			0
Pre-Adoption Allowance [Member] | Debt Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			0
Pre-Adoption Allowance [Member] | Commitments to Extend Credit [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Off-Balance Sheet, Credit Loss, Liability			500
Impact of Adoption [Member] | Commercial Portfolio Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			21,263
Impact of Adoption [Member] | Commercial and Industrial Total Energy Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			(10,453)
Impact of Adoption [Member] | Commercial Real Estate Portfolio Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			(13,519)
Impact of Adoption [Member] | Total consumer real estate loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			2,392
Impact of Adoption [Member] | Consumer Loan [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			(2,248)
Impact of Adoption [Member] | Loans Receivable [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			(2,565)
Impact of Adoption [Member] | US Treasury Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			0
Impact of Adoption [Member] | Residential Mortgage Backed Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			0
Impact of Adoption [Member] | US States and Political Subdivisions Debt Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			215
Impact of Adoption [Member] | Other Investments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			0
Impact of Adoption [Member] | Debt Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			215
Impact of Adoption [Member] | Commitments to Extend Credit [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Off-Balance Sheet, Credit Loss, Liability			39,377
Post-Adoption Allowance [Member] | Commercial Portfolio Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			72,856
Post-Adoption Allowance [Member] | Commercial and Industrial Total Energy Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			26,929
Post-Adoption Allowance [Member] | Commercial Real Estate Portfolio Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			17,518
Post-Adoption Allowance [Member] | Total consumer real estate loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			6,505
Post-Adoption Allowance [Member] | Consumer Loan [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			5,794
Post-Adoption Allowance [Member] | Loans Receivable [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss			129,602
Post-Adoption Allowance [Member] | US Treasury Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			0
Post-Adoption Allowance [Member] | Residential Mortgage Backed Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			0
Post-Adoption Allowance [Member] | US States and Political Subdivisions Debt Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			215
Post-Adoption Allowance [Member] | Other Investments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			0
Post-Adoption Allowance [Member] | Debt Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities, held-to-maturity, allowance for credit loss			215
Post-Adoption Allowance [Member] | Commitments to Extend Credit [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Off-Balance Sheet, Credit Loss, Liability			39,877
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			(29,300)	$ (29,252)			$ (14,700)	$ (14,672)
Retained Earnings [Member] | Commercial Portfolio Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			(16,798)
Retained Earnings [Member] | Commercial and Industrial Total Energy Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			8,258
Retained Earnings [Member] | Commercial Real Estate Portfolio Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			10,680
Retained Earnings [Member] | Total consumer real estate loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			(1,890)
Retained Earnings [Member] | Consumer Loan [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			1,776
Retained Earnings [Member] | Loans Receivable [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			2,026
Retained Earnings [Member] | US Treasury Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			0
Retained Earnings [Member] | Residential Mortgage Backed Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			0
Retained Earnings [Member] | US States and Political Subdivisions Debt Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			(170)
Retained Earnings [Member] | Other Investments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			0
Retained Earnings [Member] | Debt Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			(170)
Retained Earnings [Member] | Commitments to Extend Credit [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			$ (31,108)

Securities - Additional Informa

Securities - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule Of Marketable Securities [Line Items]
Securities available for sale, estimated fair value	$ 10,604,480	$ 11,269,591
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income Pre Tax	3,800	4,800
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	$ 3,000	3,800
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	75.80%
Held-to-maturity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	$ 663,200	561,400
Accrued interest receivable	11,900	21,100
Reclassified To Held To Maturity [Member]
Schedule Of Marketable Securities [Line Items]
Securities available for sale, estimated fair value		377,800
Available for sale securities reclassified to held to maturity, unrealized gain		3,300
Available for sale securities reclassified to held to maturity, unrealized gain net of tax		2,600
Available-for-sale Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	3,800,000	3,400,000
Accrued interest receivable	$ 66,600	$ 115,900

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, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 1,951,544	$ 2,030,005
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	107,312	28,931
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	1	10,261
Debt Securities, Held-to-maturity, Fair Value	2,058,855	2,048,675
Securities, held-to-maturity, allowance for credit loss	(160)	0
Debt Securities, Held-to-Maturity Net Carrying Amount	1,951,384	2,030,005
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	529,290	530,861
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	45,504	22
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	9,365
Debt Securities, Held-to-maturity, Fair Value	574,794	521,518
Securities, held-to-maturity, allowance for credit loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	529,290	530,861
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,420,754	1,497,644
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	61,808	28,909
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	896
Debt Securities, Held-to-maturity, Fair Value	1,482,562	1,525,657
Securities, held-to-maturity, allowance for credit loss	(160)	0
Debt Securities, Held-to-Maturity Net Carrying Amount	1,420,594	1,497,644
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	1	0
Debt Securities, Held-to-maturity, Fair Value	1,499	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, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	$ 1,951,544	$ 2,030,005
US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,420,754	1,497,644
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	1,245,284
Aaa/AAA [Member] | Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	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	112,614
Aa/AA [Member] | Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0
A [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	62,856
A [Member] | Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	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
Not rated [Member] | Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	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	290,855
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	115,385
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	112,614
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	62,856
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
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	797,659
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	797,659
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
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
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
Pre-Refunded [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	332,240
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	332,240
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
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
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

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Securities, held-to-maturity, allowance for credit loss	$ 160		$ 160				$ 0
Impact of adopting ASC 326	0	$ 0	39,377	$ 0
Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities, held-to-maturity, allowance for credit loss	160		160		$ 172		0
Impact of adopting ASC 326	0		215
Credit loss expense (benefit)	(12)		(55)
Debt Securities [Member] | Impact of Adoption [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities, held-to-maturity, allowance for credit loss						$ 215
States and political subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities, held-to-maturity, allowance for credit loss	$ 160		$ 160				$ 0
States and political subdivisions [Member] | Impact of Adoption [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities, held-to-maturity, allowance for credit loss						$ 215

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, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 9,914,688	$ 10,877,769
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	689,835	405,399
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	43	13,577
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	10,604,480	11,269,591
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,084,210	1,941,283
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	40,755	18,934
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	12,084
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	1,124,965	1,948,133
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,092,184	2,176,275
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	72,021	32,608
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	43	1,289
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	2,164,162	2,207,594
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	6,695,946	6,717,344
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	577,059	353,857
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	204
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	7,273,005	7,070,997
Other Investments [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	42,348	42,867
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	$ 42,348	$ 42,867

Securities - Securities Availab

Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	$ 7,527
Available for Sale, Less than 12 Months, Unrealized Losses	42
Available for Sale, More than 12 Months, Estimated Fair Value	124
Available for Sale, More than 12 Months, Unrealized Losses	1
Available for Sale, Total Estimated Fair Value	7,651
Available-for-sale Securities, Gross Unrealized Loss, before Tax	43	$ 13,577
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	7,527
Available for Sale, Less than 12 Months, Unrealized Losses	42
Available for Sale, More than 12 Months, Estimated Fair Value	124
Available for Sale, More than 12 Months, Unrealized Losses	1
Available for Sale, Total Estimated Fair Value	7,651
Available-for-sale Securities, Gross Unrealized Loss, before Tax	$ 43	$ 1,289

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, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 55,180
Held to Maturity, Due after one year through five years, Amortized Cost	357,140
Held to Maturity, Due after five years through ten years, Amortized Cost	915,745
Held to Maturity, Due after ten years, Amortized Cost	623,479
Held to Maturity, Amortized Cost	1,951,544	$ 2,030,005
Held to Maturity, Due in one year or less, Estimated Fair Value	56,043
Held to Maturity, Due after one year through five years, Estimated Fair Value	371,268
Held to Maturity, Due after five years through ten years, Estimated Fair Value	971,973
Held to Maturity, Due after ten years, Estimated Fair Value	659,571
Held to Maturity, Estimated Fair Value	2,058,855	2,048,675
Available-for-sale Securities, Due in one year or less, Amortized Cost	501,059
Available for Sale, Due after one year through five years, Amortized Cost	1,860,437
Available for Sale, Due after five years through ten years, Amortized Cost	719,603
Available for Sale, Due after ten years, Amortized Cost	6,791,241
Available for Sale, Amortized Cost	9,914,688	10,877,769
Available for Sale, Due in one year or less, Estimated Fair Value	509,410
Available for Sale, Due after one year through five years, Estimated Fair Value	1,995,106
Available for Sale, Due after five years through ten years, Estimated Fair Value	780,565
Available for Sale, Due after ten years, Estimated Fair Value	7,277,051
Securities available for sale, estimated fair value	10,604,480	11,269,591
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	299,355
Available for Sale, Due after one year through five years, Amortized Cost	784,855
Available for Sale, Due after five years through ten years, Amortized Cost	0
Available for Sale, Due after ten years, Amortized Cost	0
Available for Sale, Amortized Cost	1,084,210	1,941,283
Available for Sale, Due in one year or less, Estimated Fair Value	305,121
Available for Sale, Due after one year through five years, Estimated Fair Value	819,844
Available for Sale, Due after five years through ten years, Estimated Fair Value	0
Available for Sale, Due after ten years, Estimated Fair Value	0
Securities available for sale, estimated fair value	1,124,965	1,948,133
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	0
Held to Maturity, Due after one year through five years, Amortized Cost	231
Held to Maturity, Due after five years through ten years, Amortized Cost	516,368
Held to Maturity, Due after ten years, Amortized Cost	12,691
Held to Maturity, Amortized Cost	529,290	530,861
Held to Maturity, Due in one year or less, Estimated Fair Value	0
Held to Maturity, Due after one year through five years, Estimated Fair Value	233
Held to Maturity, Due after five years through ten years, Estimated Fair Value	560,456
Held to Maturity, Due after ten years, Estimated Fair Value	14,105
Held to Maturity, Estimated Fair Value	574,794	521,518
Available-for-sale Securities, Due in one year or less, Amortized Cost	175
Available for Sale, Due after one year through five years, Amortized Cost	45,832
Available for Sale, Due after five years through ten years, Amortized Cost	22,949
Available for Sale, Due after ten years, Amortized Cost	2,023,228
Available for Sale, Amortized Cost	2,092,184	2,176,275
Available for Sale, Due in one year or less, Estimated Fair Value	182
Available for Sale, Due after one year through five years, Estimated Fair Value	47,995
Available for Sale, Due after five years through ten years, Estimated Fair Value	23,439
Available for Sale, Due after ten years, Estimated Fair Value	2,092,546
Securities available for sale, estimated fair value	2,164,162	2,207,594
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	55,180
Held to Maturity, Due after one year through five years, Amortized Cost	355,409
Held to Maturity, Due after five years through ten years, Amortized Cost	399,377
Held to Maturity, Due after ten years, Amortized Cost	610,788
Held to Maturity, Amortized Cost	1,420,754	1,497,644
Held to Maturity, Due in one year or less, Estimated Fair Value	56,043
Held to Maturity, Due after one year through five years, Estimated Fair Value	369,536
Held to Maturity, Due after five years through ten years, Estimated Fair Value	411,517
Held to Maturity, Due after ten years, Estimated Fair Value	645,466
Held to Maturity, Estimated Fair Value	1,482,562	1,525,657
Available-for-sale Securities, Due in one year or less, Amortized Cost	201,529
Available for Sale, Due after one year through five years, Amortized Cost	1,029,750
Available for Sale, Due after five years through ten years, Amortized Cost	696,654
Available for Sale, Due after ten years, Amortized Cost	4,768,013
Available for Sale, Amortized Cost	6,695,946	6,717,344
Available for Sale, Due in one year or less, Estimated Fair Value	204,107
Available for Sale, Due after one year through five years, Estimated Fair Value	1,127,267
Available for Sale, Due after five years through ten years, Estimated Fair Value	757,126
Available for Sale, Due after ten years, Estimated Fair Value	5,184,505
Securities available for sale, estimated fair value	7,273,005	7,070,997
Other Investments [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	0
Held to Maturity, Due after one year through five years, Amortized Cost	1,500
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	0
Held to Maturity, Due after one year through five years, Estimated Fair Value	1,499
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	1,499	1,500
Available-for-sale Securities, Due in one year or less, Amortized Cost	0
Available for Sale, Due after one year through five years, Amortized Cost	0
Available for Sale, Due after five years through ten years, Amortized Cost	0
Available for Sale, Due after ten years, Amortized Cost	0
Available for Sale, Amortized Cost	42,348	42,867
Available for Sale, Due in one year or less, Estimated Fair Value	0
Available for Sale, Due after one year through five years, Estimated Fair Value	0
Available for Sale, Due after five years through ten years, Estimated Fair Value	0
Available for Sale, Due after ten years, Estimated Fair Value	0
Securities available for sale, estimated fair value	$ 42,348	$ 42,867

Securities - Sales of Securitie

Securities - Sales of Securities Available for Sale (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 0	$ 4,944,537	$ 1,162,352	$ 8,236,066
Gross realized gains	0	96	108,989	899
Gross realized losses	0	0	0	(634)
Tax (expense)benefit of securities gains/losses	$ 0	$ (21)	$ (22,888)	$ (56)

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Premium amortization	$ (31,540)	$ (29,511)	$ (94,873)	$ (88,798)
Discount accretion	637	1,274	1,793	3,835
Net (premium amortization) discount accretion	$ (30,903)	$ (28,237)	$ (93,080)	$ (84,963)

Securities - Trading Account Se

Securities - Trading Account Securities, at Estimated Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	$ 25,808	$ 24,298
U.S. Treasury [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	24,495	24,298
States and political subdivisions [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	$ 1,313	$ 0

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Net gain on sales transactions	$ 159	$ 731	$ 933	$ 1,789
Net mark-to-market gains (losses)	1	(201)	94	(170)
Net gain (loss) on trading account securities	$ 160	$ 530	$ 1,027	$ 1,619

Loans - Additional Information

Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020USD ($)Item	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)Item	Sep. 30, 2019USD ($)	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	$ 316,000		$ 316,000		$ 298,500
Number Of COVID-19 Loan Deferments At Period-End | Item	300		300
COVID 19 Loan Deferment	$ 157,200		$ 157,200
Financing Receivable, Threshold Period Past Due	90 days		90 days
Texas Leading Index economic condition indicator	113.1		113.1		127.9
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	$ 670	$ 937	$ 2,300	$ 3,000
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total energy loans to total loans	7.40%		7.40%		11.20%
Percentage of total energy loans to total loans excluding PPP Loans	9.10%		9.10%
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 910,300		$ 910,300
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	62,600		62,600
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest Receivable	$ 43,900		$ 43,900		$ 45,500

Loans - Loans (Detail)

Loans - Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 18,223,877	$ 14,750,332
Total loans percentage	100.00%	100.00%
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 4,820,018	$ 5,187,466
Total commercial and industrial percentage of total loans	26.40%	35.20%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,108,753	$ 1,348,900
Percentage of energy production loans to total loans	6.10%	9.20%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 163,197	$ 192,996
Percentage of energy service loans to total loans	0.90%	1.30%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 91,418	$ 110,986
Percentage of other energy loans to total loans	0.40%	0.80%
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,363,368	$ 1,652,882
Percentage of total energy loans to total loans	7.40%	11.20%
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 3,226,980	$ 0
Percentage Of Payment Protection Program Loans To Loans	17.70%	0.00%
Commercial mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 5,412,731	$ 4,594,113
Commercial mortgages percentage of total loans	29.70%	31.10%
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,274,080	$ 1,312,659
Construction percentage of total loans	7.00%	8.90%
Commercial land loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 320,407	$ 289,467
Land percentage of total loans	1.80%	2.00%
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 7,007,218	$ 6,196,239
Total commercial real estate percentage of total loans	38.50%	42.00%
Home Equity Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 340,226	$ 375,596
Home equity loans percentage of total loans	1.90%	2.60%
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 434,171	$ 354,671
Home equity lines of credit percentage of total loans	2.40%	2.40%
Other consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 533,356	$ 464,146
Other percentage of total loans	2.90%	3.10%
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,307,753	$ 1,194,413
Total consumer real estate percentage of total loans	7.20%	8.10%
Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 8,314,971	$ 7,390,652
Total real estate percentage of total loans	45.70%	50.10%
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 498,540	$ 519,332
Total consumer and other percentage of total loans	2.80%	3.50%

Loans - Non-Accrual Loans, Segr

Loans - Non-Accrual Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 91,578	$ 102,303
Non-accrual with no credit loss allowance	49,089	24,692
Commercial Portfolio Segment [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	21,566	26,038
Non-accrual with no credit loss allowance	6,583	13,266
Commercial and Industrial Total Energy Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	54,041	65,761
Non-accrual with no credit loss allowance	33,324	3,281
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	13,525	8,912
Non-accrual with no credit loss allowance	6,754	6,558
Construction Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	680	665
Non-accrual with no credit loss allowance	680	665
Total consumer real estate loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	1,748	922
Non-accrual with no credit loss allowance	1,748	922
Consumer Loan [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	18	5
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, 2020	Dec. 31, 2019
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	$ 91,578	$ 102,303
Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	35,572
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	16,778
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	4,680
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	2,846
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	1,300
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	4,023
Financing Receivable, Revolving	17,326
Financing Receivable, Revolving, Converted to Term Loan	9,053
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	1,189,149
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	669,507
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	399,543
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	285,092
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	133,505
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	131,886
Financing Receivable, Revolving	1,946,379
Non-accrual loans	21,566	26,038
Commercial Portfolio Segment [Member] | Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	9,317
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	3,743
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,886
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,263
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	110
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	51
Financing Receivable, Revolving	1,879
Financing Receivable, Revolving, Converted to Term Loan	3,317
Commercial and Industrial Total Energy Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	667,227
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	57,989
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	21,091
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	7,942
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,388
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	5,775
Financing Receivable, Revolving	553,059
Non-accrual loans	54,041	65,761
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	24,149
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	7,021
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	2,204
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	15,054
Financing Receivable, Revolving, Converted to Term Loan	5,613
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	1,204,132
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,142,733
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	940,137
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	810,297
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	553,477
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	950,369
Financing Receivable, Revolving	74,384
Non-accrual loans	13,525	8,912
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	1,426
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	6,014
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	169
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,372
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	840
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	3,590
Financing Receivable, Revolving	114
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	310,240
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	488,536
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	251,868
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	40,441
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	1,164
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,731
Financing Receivable, Revolving	179,100
Non-accrual loans	680	665
Construction Loans [Member] | Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	680
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
Total consumer real estate loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	1,748	922
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	421
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	211
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	350
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	382
Financing Receivable, Revolving	261
Financing Receivable, Revolving, Converted to Term Loan	123
Consumer Loan [Member]
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	18	$ 5
Consumer Loan [Member] | Nonperforming Financial Instruments [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	18
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	64,957
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	47,897
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	57,609
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, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	$ 169,642
Current Loans	18,054,235
Total loans	18,223,877	$ 14,750,332
Accruing Loans 90 or More Days Past Due	35,706
Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	61,182
Current Loans	4,758,836
Total loans	4,820,018	5,187,466
Accruing Loans 90 or More Days Past Due	19,392
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	40,659
Current Loans	1,322,709
Total loans	1,363,368	1,652,882
Accruing Loans 90 or More Days Past Due	559
Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	3,226,980
Total loans	3,226,980	0
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	44,684
Current Loans	5,688,454
Total loans	5,733,138	4,883,580
Accruing Loans 90 or More Days Past Due	11,676
Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	1,274,080
Total loans	1,274,080	1,312,659
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	15,815
Current Loans	1,291,938
Total loans	1,307,753	1,194,413
Accruing Loans 90 or More Days Past Due	3,406
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	7,302
Current Loans	491,238
Total loans	498,540	$ 519,332
Accruing Loans 90 or More Days Past Due	673
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	109,934
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	36,067
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	31,071
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	25,113
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	11,054
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	6,629
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	59,708
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	25,115
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	9,588
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	19,571
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	4,761
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	$ 673

Loans - Troubled Debt Restructu

Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	$ 12,562	$ 13,425
Balance at period end	9,285	11,805
Commercial Portfolio Segment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	3,660	3,845
Balance at period end	1,415	2,188
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	6,606	9,456
Balance at period end	6,585	9,494
Construction Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	1,192	0
Balance at period end	1,181	0
Total consumer real estate loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	0	124
Balance at period end	0	123
Consumer Loan [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Balance at time of restructuring	1,104	0
Balance at period end	$ 104	$ 0

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, 2020USD ($)Item	Sep. 30, 2019USD ($)Item
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number Of Restructured Loans Past Due In Excess of 90 Days At Period-End | Item	2	4
Dollar amount of previously restructured loans past due	$ 3,682	$ 3,244
Restructured loans on non-accrual status at period end	5,353	5,645
Charge-offs of restructured loans recognized in connection with restructuring	0	1,500
Charge-offs of restructured loans recognized on previously restructured loans	$ 2,188	$ 0

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, 2020USD ($)Grade	Dec. 31, 2019USD ($)Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 18,223,877	$ 14,750,332
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 4,820,018	$ 5,187,466
Weighted Average Risk Grade For Loans | Grade	6.42	6.44
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,189,149
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	669,507
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	399,543
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	285,092
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	133,505
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	131,886
Financing Receivable, Revolving	1,946,379
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 1,363,368	$ 1,652,882
Weighted Average Risk Grade For Loans | Grade	7.39	6.39
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 667,227
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	57,989
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	21,091
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	7,942
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,388
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	5,775
Financing Receivable, Revolving	553,059
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	$ 5,733,138	$ 4,883,580
Weighted Average Risk Grade For Loans | Grade	7.34	7.01
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,204,132
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,142,733
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	940,137
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	810,297
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	553,477
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	950,369
Financing Receivable, Revolving	74,384
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 1,274,080	$ 1,312,659
Weighted Average Risk Grade For Loans | Grade	7.25	7.31
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 310,240
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	488,536
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	251,868
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	40,441
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	1,164
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,731
Financing Receivable, Revolving	179,100
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 7,007,218	$ 6,196,239
Weighted Average Risk Grade For Loans | Grade	7.33	7.07
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,514,372
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,631,269
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,192,005
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	850,738
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	554,641
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	953,100
Financing Receivable, Revolving	253,484
Risk Grade One To Eight [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 4,402,351	$ 4,788,857
Weighted Average Risk Grade For Loans | Grade	6.12	6.17
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,101,917
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	625,692
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	340,799
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	251,180
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	120,777
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	129,757
Financing Receivable, Revolving	1,788,761
Risk Grade One To Eight [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 936,782	$ 1,488,301
Weighted Average Risk Grade For Loans | Grade	6.18	5.90
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 454,901
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	25,589
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	10,956
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	6,732
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	1,481
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	4,709
Financing Receivable, Revolving	407,478
Risk Grade One To Eight [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 4,967,561	$ 4,523,271
Weighted Average Risk Grade For Loans | Grade	6.98	6.78
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,167,872
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,010,418
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	829,640
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	660,047
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	430,971
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	752,461
Financing Receivable, Revolving	62,672
Risk Grade One To Eight [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 1,140,937	$ 1,274,098
Weighted Average Risk Grade For Loans | Grade	7.01	7.25
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 272,025
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	480,166
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	224,389
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	753
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	1,164
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,786
Financing Receivable, Revolving	160,654
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		$ 5,797,369
Weighted Average Risk Grade For Loans | Grade		6.88
Risk Grade Nine [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 248,573	$ 247,212
Weighted Average Risk Grade For Loans | Grade	9	9
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 69,846
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	27,843
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	36,271
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	21,352
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	9,910
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,087
Financing Receivable, Revolving	77,802
Risk Grade Nine [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 222,857	$ 32,163
Weighted Average Risk Grade For Loans | Grade	9	9
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 108,859
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	5,542
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	3,190
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	90,292
Risk Grade Nine [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 396,072	$ 163,714
Weighted Average Risk Grade For Loans | Grade	9	9
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 26,056
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	93,113
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	67,546
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	66,365
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	45,477
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	92,060
Financing Receivable, Revolving	4,413
Risk Grade Nine [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 94,068	$ 21,509
Weighted Average Risk Grade For Loans | Grade	9	9
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 31,402
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	8,370
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	39,688
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	14,608
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		$ 185,223
Weighted Average Risk Grade For Loans | Grade		9
Risk Grade Ten [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 98,869	$ 71,472
Weighted Average Risk Grade For Loans | Grade	10	10
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 4,394
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	7,722
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	13,867
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	6,875
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	812
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	824
Financing Receivable, Revolving	60,028
Risk Grade Ten [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 16,909	$ 51,898
Weighted Average Risk Grade For Loans | Grade	10	10
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 486
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	4,113
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,461
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	907
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	7,837
Risk Grade Ten [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 211,905	$ 103,626
Weighted Average Risk Grade For Loans | Grade	10	10
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,362
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	26,080
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	29,357
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	39,853
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	65,702
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	42,528
Financing Receivable, Revolving	4,147
Risk Grade Ten [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 37,450	$ 15,243
Weighted Average Risk Grade For Loans | Grade	10	10
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 6,133
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	27,479
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	3,838
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		$ 118,869
Weighted Average Risk Grade For Loans | Grade		10
Risk Grade Eleven [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 48,659	$ 53,887
Weighted Average Risk Grade For Loans | Grade	11	11
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 3,675
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	4,507
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	6,720
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	4,422
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	1,896
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	167
Financing Receivable, Revolving	17,909
Risk Grade Eleven [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 132,779	$ 14,760
Weighted Average Risk Grade For Loans | Grade	11	11
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 78,832
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	15,724
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	3,280
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,210
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,066
Financing Receivable, Revolving	32,398
Risk Grade Eleven [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 144,075	$ 84,057
Weighted Average Risk Grade For Loans | Grade	11	11
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 7,416
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	7,108
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	13,425
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	42,660
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	10,487
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	59,730
Financing Receivable, Revolving	3,038
Risk Grade Eleven [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 945	$ 1,144
Weighted Average Risk Grade For Loans | Grade	11	11
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	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	945
Financing Receivable, Revolving	0
Risk Grade Eleven [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income		$ 85,201
Weighted Average Risk Grade For Loans | Grade		11
Risk Grade Twelve [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 16,493	$ 18,189
Weighted Average Risk Grade For Loans | Grade	12	12
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 6,917
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	2,791
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,544
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	708
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	110
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	51
Financing Receivable, Revolving	1,350
Risk Grade Twelve [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 45,934	$ 45,514
Weighted Average Risk Grade For Loans | Grade	12	12
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 24,149
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	4,829
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	714
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	10,629
Risk Grade Twelve [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 12,212	$ 8,529
Weighted Average Risk Grade For Loans | Grade	12	12
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,226
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	5,214
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	169
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,372
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	840
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	3,340
Financing Receivable, Revolving	51
Risk Grade Twelve [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 680	$ 665
Weighted Average Risk Grade For Loans | Grade	12	12
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 680
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	0
Risk Grade Twelve [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income		$ 9,194
Weighted Average Risk Grade For Loans | Grade		12
Risk Grade Thirteen [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 5,073	$ 7,849
Weighted Average Risk Grade For Loans | Grade	13	13
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 2,400
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	952
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	342
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	555
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	529
Risk Grade Thirteen [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 8,107	$ 20,246
Weighted Average Risk Grade For Loans | Grade	13	13
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	2,192
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,490
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	4,425
Risk Grade Thirteen [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 1,313	$ 383
Weighted Average Risk Grade For Loans | Grade	13	13
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 200
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	800
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	250
Financing Receivable, Revolving	63
Risk Grade Thirteen [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 0	$ 0
Weighted Average Risk Grade For Loans | Grade	13	13
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0
Financing Receivable, Revolving	$ 0
Risk Grade Thirteen [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income		$ 383
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	6.32
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	6.98
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	7.41
Revolving Credit Facility [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7
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	7.12
Revolving Loans Converted to Term [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.92
Financing Receivable, Revolving, Converted to Term Loan	$ 64,957
Revolving Loans Converted to Term [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	8.76
Financing Receivable, Revolving, Converted to Term Loan	$ 47,897
Revolving Loans Converted to Term [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.84
Financing Receivable, Revolving, Converted to Term Loan	$ 57,609
Revolving Loans Converted to Term [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	0
Financing Receivable, Revolving, Converted to Term Loan	$ 0
Revolving Loans Converted to Term [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.84
Financing Receivable, Revolving, Converted to Term Loan	$ 57,609
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	43,468
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	24,936
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	53,480
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	4,462
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	14,974
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	1,042
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	4,347
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	2,105
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	2,876
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	9,363
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	269
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	211
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	3,022
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	5,613
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	295
Revolving Loans Converted to Term [Member] | Risk Grade Thirteen [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Risk Grade Thirteen [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	0
Revolving Loans Converted to Term [Member] | Risk Grade Thirteen [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Revolving, Converted to Term Loan	$ 0
2020 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.12
2020 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.43
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	7.09
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	7.04
2019 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.81
2019 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	9.19
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	7.34
2019 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.18
2019 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.29
2018 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.20
2018 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	8.78
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	7.46
2018 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.74
2018 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.52
2017 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.32
2017 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.83
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	7.48
2017 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	8.98
2017 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.55
2016 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	6.37
2016 [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.93
2016 [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	7.73
2016 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.29
2016 [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.73
Prior Years [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	5.87
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	8.06
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	7.26
Prior Years [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	8.02
Prior Years [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade For Loans | Grade	7.26

Loans - Weighted Average Risk G

Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) $ in Thousands	Sep. 30, 2020USD ($)Grade	Dec. 31, 2019USD ($)Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 18,223,877	$ 14,750,332
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.42	6.44
Loans and Leases Receivable, Net of Deferred Income	$ 4,820,018	$ 5,187,466
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.39	6.39
Loans and Leases Receivable, Net of Deferred Income	$ 1,363,368	$ 1,652,882
Commercial Real Estate Buildings Land And Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.34	7.01
Loans and Leases Receivable, Net of Deferred Income	$ 5,733,138	$ 4,883,580
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.25	7.31
Loans and Leases Receivable, Net of Deferred Income	$ 1,274,080	$ 1,312,659
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.33	7.07
Loans and Leases Receivable, Net of Deferred Income	$ 7,007,218	$ 6,196,239
Risk Grade One To Eight [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.12	6.17
Loans and Leases Receivable, Net of Deferred Income	$ 4,402,351	$ 4,788,857
Risk Grade One To Eight [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.18	5.90
Loans and Leases Receivable, Net of Deferred Income	$ 936,782	$ 1,488,301
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	6.98	6.78
Loans and Leases Receivable, Net of Deferred Income	$ 4,967,561	$ 4,523,271
Risk Grade One To Eight [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.01	7.25
Loans and Leases Receivable, Net of Deferred Income	$ 1,140,937	$ 1,274,098
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		6.88
Loans and Leases Receivable, Net of Deferred Income		$ 5,797,369
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	$ 248,573	$ 247,212
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	$ 222,857	$ 32,163
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	$ 396,072	$ 163,714
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	$ 94,068	$ 21,509
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		$ 185,223
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	$ 98,869	$ 71,472
Risk Grade Ten [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10	10
Loans and Leases Receivable, Net of Deferred Income	$ 16,909	$ 51,898
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	$ 211,905	$ 103,626
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	$ 37,450	$ 15,243
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		$ 118,869
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	$ 48,659	$ 53,887
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	$ 132,779	$ 14,760
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	$ 144,075	$ 84,057
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	$ 945	$ 1,144
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		$ 85,201
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	$ 16,493	$ 18,189
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	$ 45,934	$ 45,514
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	$ 12,212	$ 8,529
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	$ 680	$ 665
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		$ 9,194
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	$ 5,073	$ 7,849
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	$ 8,107	$ 20,246
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	$ 1,313	$ 383
Risk Grade Thirteen [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13	13
Loans and Leases Receivable, Net of Deferred Income	$ 0	$ 0
Risk Grade Thirteen [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		13
Loans and Leases Receivable, Net of Deferred Income		$ 383

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, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	$ 169,642
Current Loans	18,054,235
Loans and Leases Receivable, Net of Deferred Income	18,223,877	$ 14,750,332
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	15,815
Current Loans	1,291,938
Loans and Leases Receivable, Net of Deferred Income	1,307,753	1,194,413
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	7,302
Current Loans	491,238
Loans and Leases Receivable, Net of Deferred Income	498,540	$ 519,332
Financing Receivables, 30 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	109,934
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	11,054
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	6,629
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	59,708
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	4,761
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	673
Revolving Credit Facility [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	2,135
Current Loans	414,331
Loans and Leases Receivable, Net of Deferred Income	416,466
Revolving Credit Facility [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	4,685
Current Loans	376,762
Loans and Leases Receivable, Net of Deferred Income	381,447
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	1,535
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	4,313
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	600
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	372
Revolving Loans Converted to Term [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	699
Current Loans	17,156
Loans and Leases Receivable, Net of Deferred Income	17,855
Revolving Loans Converted to Term [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	335
Current Loans	30,012
Loans and Leases Receivable, Net of Deferred Income	30,347
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	492
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	296
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	207
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	39
2020 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	787
Current Loans	253,392
Loans and Leases Receivable, Net of Deferred Income	254,179
2020 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,397
Current Loans	38,114
Loans and Leases Receivable, Net of Deferred Income	39,511
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	787
2020 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,328
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	69
2019 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	989
Current Loans	188,770
Loans and Leases Receivable, Net of Deferred Income	189,759
2019 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	515
Current Loans	32,214
Loans and Leases Receivable, Net of Deferred Income	32,729
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	850
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	326
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	139
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	189
2018 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	2,328
Current Loans	106,947
Loans and Leases Receivable, Net of Deferred Income	109,275
2018 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	291
Current Loans	7,985
Loans and Leases Receivable, Net of Deferred Income	8,276
2018 [Member] | Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,434
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	288
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	894
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	3
2017 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	1,400
Current Loans	93,143
Loans and Leases Receivable, Net of Deferred Income	94,543
2017 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	75
Current Loans	3,131
Loans and Leases Receivable, Net of Deferred Income	3,206
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	858
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	75
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	542
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
2016 [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	891
Current Loans	75,216
Loans and Leases Receivable, Net of Deferred Income	76,107
2016 [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	2
Current Loans	2,070
Loans and Leases Receivable, Net of Deferred Income	2,072
2016 [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	540
2016 [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	2
2016 [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	351
2016 [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	6,586
Current Loans	142,983
Loans and Leases Receivable, Net of Deferred Income	149,569
Prior Years [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due Loans	2
Current Loans	950
Loans and Leases Receivable, Net of Deferred Income	952
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	4,558
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	1
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	2,028
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	$ 1

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, 2020	Sep. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	$ 23,920	$ 89,546
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	10,224	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	7,144	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	637	8,094
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	0	0
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	421	2,132
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Remaining Revolving Loans Converted to Term During Period	$ 5,494	$ 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, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance		$ 132,167
Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance		51,593
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance		37,382
Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance		31,037
Total consumer real estate loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance		4,113
Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance		8,042
Modeled Expected Credit Losses [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	$ 234,151
Modeled Expected Credit Losses [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	68,896
Modeled Expected Credit Losses [Member] | Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	21,644
Modeled Expected Credit Losses [Member] | Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	125,965
Modeled Expected Credit Losses [Member] | Total consumer real estate loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	10,226
Modeled Expected Credit Losses [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	7,420
Q-factor adjustments [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	14,380
Q-factor adjustments [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	(1,647)
Q-factor adjustments [Member] | Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	24,630
Q-factor adjustments [Member] | Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	(8,685)
Q-factor adjustments [Member] | Total consumer real estate loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	51
Q-factor adjustments [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	31
Specific Valuation Allowances [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	14,944	28,483
Specific Valuation Allowances [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	5,256	7,849
Specific Valuation Allowances [Member] | Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	8,357	20,246
Specific Valuation Allowances [Member] | Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	1,313	383
Specific Valuation Allowances [Member] | Total consumer real estate loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	0	0
Specific Valuation Allowances [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans and Leases Receivable, Allowance	$ 18	$ 5

Loans - Allowance for Credit Lo

Loans - Allowance for Credit Losses By Loan Class Accordance With Methodology Used Prior to CECL (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		$ 132,167
Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		69,087
Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance	$ 14,944	28,483
General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		19,732
Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		14,865
Commercial Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		51,593
Commercial Portfolio Segment [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		29,015
Commercial Portfolio Segment [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance	5,256	7,849
Commercial Portfolio Segment [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		9,840
Commercial Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		4,889
Commercial and Industrial Total Energy Loans [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		37,382
Commercial and Industrial Total Energy Loans [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		7,873
Commercial and Industrial Total Energy Loans [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance	8,357	20,246
Commercial and Industrial Total Energy Loans [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		5,196
Commercial and Industrial Total Energy Loans [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		4,067
Commercial Real Estate Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		31,037
Commercial Real Estate Portfolio Segment [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		21,947
Commercial Real Estate Portfolio Segment [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance	1,313	383
Commercial Real Estate Portfolio Segment [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		4,201
Commercial Real Estate Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		4,506
Total consumer real estate loans [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		4,113
Total consumer real estate loans [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		2,690
Total consumer real estate loans [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance	0	0
Total consumer real estate loans [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		904
Total consumer real estate loans [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		519
Consumer Loan [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		8,042
Consumer Loan [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		7,562
Consumer Loan [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance	$ 18	5
Consumer Loan [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		(409)
Consumer Loan [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Loans and Leases Receivable, Allowance		$ 884

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance			$ 132,167
Impact of adopting ASC 326	$ 0	$ 0	39,377	$ 0
Ending balance	263,475		263,475
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	98,536	57,714	51,593	48,580
Impact of adopting ASC 326			21,263
Credit loss expense	(18,547)	(3,527)	10,737	9,999
Charge-offs	(8,605)	(2,705)	(14,815)	(8,782)
Recoveries	1,121	1,185	3,727	2,870
Net charge-offs	(7,484)	(1,520)	(11,088)	(5,912)
Ending balance	72,505	52,667	72,505	52,667
Commercial and Industrial Total Energy Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	40,817	25,818	37,382	29,052
Impact of adopting ASC 326			(10,453)
Credit loss expense	13,814	8,788	96,478	7,478
Charge-offs	0	(2,000)	(68,842)	(4,000)
Recoveries	0	740	66	816
Net charge-offs	0	(1,260)	(68,776)	(3,184)
Ending balance	54,631	33,346	54,631	33,346
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	93,425	35,914	31,037	38,777
Impact of adopting ASC 326			(13,519)
Credit loss expense	25,368	(607)	104,716	(2,972)
Charge-offs	(242)	0	(3,826)	(617)
Recoveries	42	46	185	165
Net charge-offs	(200)	46	(3,641)	(452)
Ending balance	118,593	35,353	118,593	35,353
Total consumer real estate loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,998	5,637	4,113	6,103
Impact of adopting ASC 326			2,392
Credit loss expense	1,794	(650)	3,716	859
Charge-offs	(1,088)	(557)	(1,508)	(2,936)
Recoveries	573	454	1,564	858
Net charge-offs	(515)	(103)	56	(2,078)
Ending balance	10,277	4,884	10,277	4,884
Consumer Loan [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,285	9,846	8,042	9,620
Impact of adopting ASC 326			(2,248)
Credit loss expense	1,161	3,997	8,096	10,040
Charge-offs	(4,219)	(6,357)	(13,402)	(17,157)
Recoveries	2,242	2,823	6,981	7,806
Net charge-offs	(1,977)	(3,534)	(6,421)	(9,351)
Ending balance	7,469	10,309	7,469	10,309
Loans Receivable [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	250,061	134,929	132,167	132,132
Impact of adopting ASC 326			(2,565)
Credit loss expense	23,590	8,001	223,743	25,404
Charge-offs	(14,154)	(11,619)	(102,393)	(33,492)
Recoveries	3,978	5,248	12,523	12,515
Net charge-offs	(10,176)	(6,371)	(89,870)	(20,977)
Ending balance	$ 263,475	$ 136,559	$ 263,475	$ 136,559

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, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 18,223,877	$ 14,750,332
Loans and Leases Receivable, Specific Allocations		132,167
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	4,820,018	5,187,466
Loans and Leases Receivable, Specific Allocations		51,593
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,363,368	1,652,882
Loans and Leases Receivable, Specific Allocations		37,382
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,226,980	0
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	5,733,138	4,883,580
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,274,080	1,312,659
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,307,753	1,194,413
Loans and Leases Receivable, Specific Allocations		4,113
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	498,540	519,332
Loans and Leases Receivable, Specific Allocations		8,042
Specific Valuation Allowances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	14,944	28,483
Specific Valuation Allowances [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	5,256	7,849
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	8,357	20,246
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	1,313	383
Specific Valuation Allowances [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	0	0
Specific Valuation Allowances [Member] | Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	0	0
Specific Valuation Allowances [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Specific Allocations	18	5
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	106,994	99,453
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	22,930	24,360
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	56,420	65,244
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	25,611	8,609
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	680	665
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	1,335	570
Loans Individually Evaluated for Impairment [Member] | Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 18	$ 5

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Schedule of Goodwill (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
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, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	$ 1,771	$ 2,481
Core deposits [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Net	1,465	2,043
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Net	$ 306	$ 438

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Estimated Aggregate Future Amortization Expense for Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Remainder of 2020	$ 208
2021	697
2022	481
2023	282
2024	87
Thereafter	16
Future Amortization Expense, Total	$ 1,771	$ 2,481

Deposits - Schedule of Deposits

Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 14,845,770	$ 10,873,629
Interest-bearing Deposit Liabilities, Total	18,653,733	16,765,935
Total deposits	$ 33,499,503	$ 27,639,564
Total non-interest-bearing demand deposits, Percentage of Total	44.30%	39.30%
Percentage of Interest-bearing Domestic Deposits to Deposits	55.70%	60.70%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 13,852,404	$ 10,212,265
Total non-interest-bearing demand deposits, Percentage of Total	41.40%	36.90%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 208,836	$ 246,181
Total non-interest-bearing demand deposits, Percentage of Total	0.60%	0.90%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 8,176,608	$ 7,147,327
Money market accounts	8,684,470	7,888,433
Time accounts of $100,000 or More	791,004	736,481
Time accounts under $100,000	339,174	347,418
Interest-bearing Deposit Liabilities, Total	$ 17,991,256	$ 16,119,659
Savings and interest checking, Percentage of Total	24.40%	25.90%
Money market accounts, Percentage of Total	25.90%	28.50%
Time accounts of $100,000 or more, Percentage of Total	2.40%	2.70%
Time accounts under $100,000, Percentage of Total	1.00%	1.20%
Percentage of Interest-bearing Domestic Deposits to Deposits	53.70%	58.30%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 784,530	$ 415,183
Savings and interest checking	583,214	548,399
Money market accounts	76,088	73,180
Time accounts of $100,000 or More	3,092	24,672
Time accounts under $100,000	83	25
Interest-bearing Deposit Liabilities, Total	$ 662,477	$ 646,276
Total non-interest-bearing demand deposits, Percentage of Total	2.30%	1.50%
Savings and interest checking, Percentage of Total	1.80%	2.00%
Money market accounts, Percentage of Total	0.20%	0.30%
Time accounts of $100,000 or more, Percentage of Total	0.00%	0.10%
Time accounts under $100,000, Percentage of Total	0.00%	0.00%
Percentage of Interest-bearing Domestic Deposits to Deposits	2.00%	2.40%

Deposits - Additional Informati

Deposits - Additional Information About Corporation's Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Disclosure Text Block [Abstract]
Deposits from the Certificate of Deposit Account Registry Service (CDARS) deposits	$ 369	$ 361
Deposits from foreign sources (primarily Mexico)	845,950	805,828
Deposits not covered by deposit insurance	$ 17,265,358	$ 13,115,796

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jan. 01, 2019
Operating Lease, Right-of-Use Asset	$ 298,600		$ 298,600		$ 297,700	$ 170,500
Operating Lease, Liability	328,900		328,900		$ 323,700	$ 174,400
Lessee, Operating Lease, Liability, Payments, Due	$ 8,000	$ 7,600	23,800	$ 19,700
Significant change in expected future minimum lease payments since year end			$ 0

Commitments and Contingencies_2

Commitments and Contingencies - Financial Instruments with Off-Balance-Sheet Risk (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Standby Letters of Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 260,683	$ 260,587
Deferred Standby Letter Of Credit Fees [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	1,716	1,276
Commitments to Extend Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 9,586,212	$ 9,306,043

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Off-Balance Sheet, Credit Loss, Liability	$ 46,939	$ 500	$ 500	$ 500
Impact of adopting ASC 326	0	0	39,377	0
Credit loss expense (benefit)	(3,276)	0	3,786	0
Off-Balance Sheet, Credit Loss, Liability	$ 43,663	$ 500	$ 43,663	$ 500

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Amortization of lease right-of-use assets	$ 8,521	$ 7,415	$ 24,545	$ 19,698
Short-term lease expense	172	1,199	1,343	3,316
Non-lease components (including taxes, insurance, common maintenance, etc.)	2,586	3,114	8,410	7,022
Total operating lease expense, excluding rent	$ 11,279	$ 11,728	$ 34,298	$ 30,036

Capital and Regulatory Matter_2

Capital and Regulatory Matters - Additional Information (Detail)	Mar. 16, 2020USD ($)shares	Jul. 24, 2019USD ($)	Feb. 15, 2013USD ($)	Sep. 30, 2020USD ($)shares	Mar. 31, 2020USD ($)shares	Sep. 30, 2019USD ($)shares	Sep. 30, 2020USD ($)quartershares	Sep. 30, 2019USD ($)shares	Dec. 31, 2019USD ($)	Jan. 01, 2020USD ($)	Jan. 01, 2019USD ($)	Dec. 31, 2018USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Cumulative effect of accounting change									$ (29,252,000)			$ (14,672,000)
CECL Transition Amount				$ (63,700,000)			$ (63,700,000)
Estimated Incremental Effect of CECL							$ (34,400,000)
Proceeds from Issuance of Preferred Stock and Preference Stock			$ 5,500,000
Stock Redeemed or Called During Period, 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
Stock Repurchase Program, Authorized Amount		$ 100,000,000
Stock Repurchase Program, Period in Force		1 year
Treasury Stock, Shares, Acquired | shares				400		202,724	182,225	704,382
Treasury Stock, Value, Acquired, Cost Method				$ (27,000)		$ (17,190,000)	$ (14,000,000)	$ (67,709,000)
Maximum dividends available without prior regulatory approval				482,100,000			$ 482,100,000
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								25
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1				0			$ 0		144,500,000
Cullen/Frost [Member] | Trust Preferred Securities Tier Two Capital Allowable Portion [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities							133,000,000		133,000,000
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1				0			0		0
stock repurchase 2019 plan [Member] | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Shares, Acquired | shares					177,834	202,724
Treasury Stock, Value, Acquired, Cost Method					$ (13,700,000)	$ (17,200,000)
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]
Proceeds from Issuance of Preferred Stock and Preference Stock			$ 144,500,000
Preferred stock, redemption amount	144,500,000						144,486,000
Retained Earnings [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Cumulative effect of accounting change									$ (29,252,000)	$ (29,300,000)	$ (14,700,000)	$ (14,672,000)
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, 2020	Dec. 31, 2019
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 3,003,029	$ 2,857,250
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.71%	12.36%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,654,073	$ 1,617,886
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	$ 1,535,925	$ 1,502,323
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	$ 3,003,029	$ 3,001,736
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.71%	12.99%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,008,517	$ 1,964,576
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	$ 1,890,369	$ 1,849,013
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	$ 3,471,898	$ 3,367,403
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.69%	14.57%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,481,109	$ 2,426,829
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	$ 2,362,961	$ 2,311,266
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 3,003,029	$ 3,001,736
Leverage Ratio, Actual, Ratio	7.85%	9.28%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,530,584	$ 1,293,188
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	$ 1,913,230	$ 1,616,485
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	$ 3,010,290	$ 2,958,326
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.76%	12.82%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,651,191	$ 1,615,206
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	$ 1,533,249	$ 1,499,834
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	$ 3,010,290	$ 2,958,326
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.76%	12.82%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,005,018	$ 1,961,322
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	$ 1,887,076	$ 1,845,950
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	$ 3,246,159	$ 3,090,993
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.76%	13.40%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,476,787	$ 2,422,809
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	$ 2,358,845	$ 2,307,438
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 3,010,290	$ 2,958,326
Leverage Ratio, Actual, Ratio	7.87%	9.15%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,530,076	$ 1,292,743
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	$ 1,912,596	$ 1,615,929
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, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Derivative [Line Items]
Weighted-average strike rate for outstanding interest rate caps				3.67%
Approximate credit exposure related to swaps with bank customers	$ 103,000			$ 103,000
Cash collateral for borrowed securities deposited with other financial institutions	48,600			48,600
Interest rate swaps with upstream financial institution counterparties [Member]
Derivative [Line Items]
Approximate credit exposure related to swaps with bank customers	19,200			19,200
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
Notional Amount	1,192,762			1,192,762		$ 1,002,860
Other Income [Member] | Not Designated as Hedging Instrument [Member] | Put Option [Member]
Derivative [Line Items]
Amount of gain (loss) included in income	$ 0	$ 6,000	$ 0	$ 5,980	$ 0
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, 2020	Dec. 31, 2019
Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	$ 0	$ 2,545
Estimated Fair Value	0	6
Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	4,036	6,000
Estimated Fair Value	(167)	(138)
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	0	122,788
Estimated Fair Value	0	67
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	1,192,762	1,002,860
Estimated Fair Value	(36,975)	(19,483)
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest-rate caps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	306,171	107,835
Estimated Fair Value	1,197	266
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	1,192,762	1,002,860
Estimated Fair Value	95,462	43,857
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	0	122,788
Estimated Fair Value	0	(310)
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate caps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	306,171	107,835
Estimated Fair Value	$ (1,197)	$ (266)

Derivative Financial Instrume_5

Derivative Financial Instruments - Schedule of Weighted-Average Rates Paid and Received for Interest Rate Swaps Outstanding (Detail)	Sep. 30, 2020
Weighted Average Interest Rate Paid [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Fair value hedge loan/lease interest rate swaps	3.39%
Weighted-Average Interest Rate Received [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Fair value hedge loan/lease interest rate swaps	0.15%
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	4.00%
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	1.93%
Customer Counterparties [Member] | Weighted Average Interest Rate Paid [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Non-hedging interest rate swaps – customer counterparties	1.93%
Customer Counterparties [Member] | Weighted-Average Interest Rate Received [Member]
Short-duration insurance contracts, discounted liabilities [Line Items]
Non-hedging interest rate swaps – customer counterparties	4.00%

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, 2020USD ($)bblMMBTU	Dec. 31, 2019USD ($)bblMMBTU
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	2,296	1,214
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	3,089	2,148
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 18,718	$ 2,796
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (9,132)	$ (6,916)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	5,623	8,295
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	18,181	2,689
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 932	$ 2,131
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (5,879)	$ (70)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	3,106	2,172
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	2,279	1,190
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 9,461	$ 7,208
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (18,506)	$ (2,652)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	19,940	2,689
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	3,864	8,295
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 6,161	$ 83
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (915)	$ (2,039)

Derivative Financial Instrume_7

Derivative Financial Instruments - Notional Amount and Fair Value of Foreign Currency Forward Contract (Detail) $ in Thousands, $ in Thousands	Sep. 30, 2020CAD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2019CAD ($)	Dec. 31, 2019USD ($)
Financial Institution Counterparties [Member] | Other Liabilities [Member] | Non Designated as Hedging Instrument [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Notional Amount	$ 0		$ 4,593
Customer Counterparties [Member] | Other Assets [Member] | Non Designated as Hedging Instrument [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Notional Amount	$ 0		$ 4,583
Foreign Exchange Forward [Member] | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Estimated Fair Value		$ 0		$ (33)
Foreign Exchange Forward [Member] | Customer Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value		$ 0		$ 45

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Interest Income on Loans [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ (36)	$ 24	$ (79)	$ 78
Other Non-Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ 2	$ (1)	$ 6	$ 0

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, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
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)	$ 280		$ 215	$ 2,615	$ 1,188
Interest Rate Contract [Member] | Other Non-Interest Expense [Member]
Other Non Interest Income Non Hedging Interest Rate Derivatives [Line Items]
Amount of gain (loss) included in income (expense)	1		0	1	0
Commodity Contract [Member] | Other Non-Interest Income [Member]
Other Non Interest Income Non Hedging Interest Rate Derivatives [Line Items]
Amount of gain (loss) included in income (expense)	379		109	1,057	322
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)	10		12	28	41
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	$ 6,000	$ 0	$ 5,980	$ 0

Balance Sheet Offsetting - Fina

Balance Sheet Offsetting - Financial Instruments Eligible for Offset Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	$ 20,847	$ 5,266
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	20,847	5,266
Gross Amount Recognized, Resell agreements, Financial assets	20,000	31,299
Gross Amount Offset, Resell agreements, Financial assets	0	0
Net Amount Recognized, Resell agreements, Financial assets	20,000	31,299
Gross Amount Recognized, Financial assets	40,847	36,565
Gross Amount Offset, Financial assets	0	0
Net Amount Recognized, Financial assets	40,847	36,565
Gross Amount Recognized, Derivatives, Financial Liabilities	52,153	26,640
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	52,153	26,640
Gross Amount Recognized, Repurchase agreements, Financial liabilities	1,574,367	1,668,142
Gross Amount Offset, Repurchase agreements, Financial liabilities	0	0
Net Amount Recognized, Repurchase agreements, Financial liabilities	1,574,367	1,668,142
Gross Amount Recognized, Financial liabilities	1,626,520	1,694,782
Gross Amount Offset, Financial liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,626,520	1,694,782
Loan/lease interest rate swaps and caps [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	1,197	339
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	1,197	339
Commodity swaps and options [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	19,650	4,927
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	19,650	4,927
Gross Amount Recognized, Derivatives, Financial Liabilities	15,011	6,986
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	15,011	6,986
Foreign currency forward contracts [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	0
Gross Amount Offset, Derivatives, Financial assets	0
Net Amount Recognized, Derivatives, Financial assets	0
Gross Amount Recognized, Derivatives, Financial Liabilities	0	33
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	0	33
Loan/lease interest rate swaps [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Liabilities	37,142	19,621
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	$ 37,142	$ 19,621

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, 2020	Dec. 31, 2019
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 20,847	$ 5,266
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(20,514)	(5,236)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(327)	0
Net Amount, Derivatives, Financial Assets	6	30
Net Amount Recognized, Resell agreements, Financial Assets	20,000	31,299
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(20,000)	(31,299)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	40,847	36,565
Gross Amounts Not Offset Collateral, Financial Assets	(20,327)	(31,299)
Net Amount, Financial Assets	6	30
Net Amount Recognized, Derivatives, Financial Liabilities	52,153	26,640
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(20,514)	(5,236)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(29,445)	(21,404)
Net Amount, Derivatives, Financial Liabilities	2,194	0
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	1,574,367	1,668,142
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(1,574,367)	(1,668,142)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,626,520	1,694,782
Gross Amounts Not Offset Collateral, Financial Liabilities	(1,603,812)	(1,689,546)
Net Amount, Financial Liabilities	2,194	0
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	8	39
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(8)	(39)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	7,178	5,192
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(8)	(39)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(7,170)	(5,153)
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	5,996	1,650
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(5,996)	(1,650)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	14,386	7,424
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(5,996)	(1,650)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(8,210)	(5,774)
Net Amount, Derivatives, Financial Liabilities	180	0
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets		1
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets		(1)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets		0
Net Amount, Derivatives, Financial Assets		0
Net Amount Recognized, Derivatives, Financial Liabilities	94	135
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	0	(1)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(94)	(134)
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	14,843	3,576
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(14,510)	(3,546)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(327)	0
Net Amount, Derivatives, Financial Assets	6	30
Net Amount Recognized, Derivatives, Financial Liabilities	30,495	13,889
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(14,510)	(3,546)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(13,971)	(10,343)
Net Amount, Derivatives, Financial Liabilities	$ 2,014	$ 0

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	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase [Line Items]
Remaining contractual maturity of the agreements overnight and continuous	$ 1,574,367	$ 1,668,142
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	1,574,367	1,668,142
Securities Sold under Agreements to Repurchase, Gross	1,574,367	1,668,142
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	99,365	435,904
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	99,365	435,904
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	1,475,002	1,232,238
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	$ 1,475,002	$ 1,232,238

Stock-Based Compensation Stock-

Stock-Based Compensation Stock-Based Compensation - Narrative (Details)	Sep. 30, 2020shares
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	1,109,864

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options exercised, Number of Shares	(53,425)	(101,284)	(196,948)	(255,184)
Director deferred stock units outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares/Units			55,370
Granted, Number of Units			10,428
Stock Awards exercised/vested, Number of Shares			(12,938)
Ending Balance, Number of Shares/Units	52,860		52,860
Beginning Balance, Weighted-Average Grant-Date Fair Value			$ 74.76
Granted Weighted-Average Grant-Date Fair Value			73.84
Stock Award Exercised/Vested, Weighted Average Grant Date Fair Value			71.09
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 75.47		$ 75.47
Non-Vested Stock Awards/Stock Units Outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares/Units			440,647
Granted, Number of Units			458
Stock Awards exercised/vested, Number of Shares			(1,640)
Forfeited/expired, Number of Shares/Units			(2,813)
Ending Balance, Number of Shares/Units	436,652		436,652
Beginning Balance, Weighted-Average Grant-Date Fair Value			$ 90.22
Granted Weighted-Average Grant-Date Fair Value			65.43
Stock Award Exercised/Vested, Weighted Average Grant Date Fair Value			76.07
Forfeited/Expired, Weighted-Average Price			90.20
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 90.24		$ 90.24
Stock Options Outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares			1,980,866
Stock options exercised, Number of Shares			(140,615)
Forfeited/expired, Number of Shares			(5,427)
Ending Balance, Number of Shares	1,834,824		1,834,824
Beginning Balance, Weighted-Average Exercise Price			$ 64.60
Stock options exercised, Weighted-Average Exercise Price			53.65
Forfeited/expired, Weighted-Average Exercise Price			75.74
Ending Balance, Weighted-Average Exercise Price	$ 65.41		$ 65.41
Maximum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares/Units			177,288
Stock Awards exercised/vested, Number of Shares			(41,755)
Forfeited/expired, Number of Shares/Units			(6,894)
Ending Balance, Number of Shares/Units	128,639		128,639
Beginning Balance, Weighted-Average Grant-Date Fair Value			$ 83.48
Stock Award Exercised/Vested, Weighted Average Grant Date Fair Value			69.70
Forfeited/Expired, Weighted-Average Price			81.33
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 88.07		$ 88.07

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Payment Arrangement [Abstract]
New shares issued from available authorized shares	0	0	0	0
Issued from available treasury stock	53,425	71,284	196,948	225,184
Total	53,425	71,284	196,948	225,184
Proceeds from stock option exercises	$ 2,698	$ 5,665	$ 7,544	$ 13,506

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	$ 2,443	$ 2,675	$ 8,227	$ 10,693
Income tax benefit	450	452	1,539	1,768
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	0	330	0	1,091
Non-Vested Stock Awards/Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	1,999	1,536	6,325	5,727
Director deferred stock units outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	0	0	770	780
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expense	$ 444	$ 809	$ 1,132	$ 3,095

Stock-Based Compensation - Unre

Stock-Based Compensation - Unrecognized Stock-Based Compensation Expense (Detail) $ in Thousands	Sep. 30, 2020USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 16,988
Non-Vested Stock Awards/Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested stock awards/stock units and Performance stock units	12,337
Maximum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested stock awards/stock units and Performance stock units	$ 4,651

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net income	$ 95,056	$ 111,836	$ 242,881	$ 339,918
Less: Preferred stock dividends	0	2,016	2,016	6,047
Redemption of preferred stock	0	0	5,514	0
Net income available to common shareholders	95,056	109,820	235,351	333,871
Less: Earnings allocated to participating securities	877	876	2,236	2,761
Distributed earnings allocated to common stock	44,546	44,370	133,511	131,069
Undistributed earnings allocated to common stock	49,633	64,574	99,604	200,041
Net earnings allocated to common stock	$ 94,179	$ 108,944	$ 233,115	$ 331,110
Weighted-average shares outstanding for basic earnings per common share	62,726,542	62,566,128	62,655,393	62,786,501
Dilutive effect of stock compensation	193,116	592,866	263,028	725,911
Weighted-average shares outstanding for diluted earnings per common share	62,919,658	63,158,994	62,918,421	63,512,412

Defined Benefit Plans Additiona

Defined Benefit Plans Additional Information (Details)	Sep. 30, 2020USD ($)
Retirement Benefits [Abstract]
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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Retirement Benefits [Abstract]
Expected return on plan assets, net of expenses	$ (3,073)	$ (2,693)	$ (9,217)	$ (8,079)
Interest cost on projected benefit obligation	1,252	1,618	3,757	4,854
Net amortization and deferral	1,330	1,406	3,989	4,218
Net periodic expense (benefit)	$ (491)	$ 331	$ (1,471)	$ 993

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Deferred Tax Liabilities, Net	$ 122.4		$ 122.4		$ 75.8
Valuation allowance	$ 0		$ 0
U.S. federal statutory income tax rate, percent	21.00%	21.00%	21.00%	21.00%
Unrecognized tax benefits	$ 0	$ 0	$ 0	$ 0

Income Taxes - Income Tax Expen

Income Taxes - Income Tax Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Current income tax expense (benefit)	$ (33,614)	$ 14,480	$ 20,328	$ 43,984
Deferred income tax expense (benefit)	43,130	(950)	(8,803)	(1,625)
Income tax expense, as reported	$ 9,516	$ 13,530	$ 11,525	$ 42,359
Effective tax rate	9.10%	10.80%	4.50%	11.10%

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 20,332	$ 107,786	$ 406,959	$ 464,072
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(294)	(305)	(979)	(957)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	(97)	(108,989)	(266)
Total securities available for sale and transferred securities, before tax amount	20,038	107,384	296,991	462,849
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	1,330	1,406	3,989	4,218
Total defined-benefit post-retirement benefit plans	1,330	1,406	3,989	4,218
Total other comprehensive income (loss), before tax	21,368	108,790	300,980	467,067
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	4,269	22,635	85,462	97,455
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(62)	(64)	(206)	(201)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	(21)	(22,888)	(56)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	4,207	22,550	62,368	97,198
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	280	295	837	886
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	280	295	837	886
Total other comprehensive income (loss), Tax Expense (Benefit)	4,487	22,845	63,205	98,084
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	16,063	85,151	321,497	366,617
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(232)	(241)	(773)	(756)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	(76)	(86,101)	(210)
Total securities available for sale and transferred securities, Net of Tax	15,831	84,834	234,623	365,651
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	1,050	1,111	3,152	3,332
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	1,050	1,111	3,152	3,332
Other comprehensive income (loss), net of tax	$ 16,881	$ 85,945	$ 237,775	$ 368,983

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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 267,370
Other comprehensive income (loss), net of tax	$ 16,881	$ 85,945	237,775	$ 368,983
Ending Balance	505,145		505,145
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			313,304	(16,103)
Other comprehensive income (loss) before reclassifications			320,724	365,861
Reclassification of amounts included in net income			(86,101)	(210)
Other comprehensive income (loss), net of tax			234,623	365,651
Ending Balance	547,927	349,548	547,927	349,548
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(45,934)	(47,497)
Other comprehensive income (loss) before reclassifications			0	0
Reclassification of amounts included in net income			3,152	3,332
Other comprehensive income (loss), net of tax			3,152	3,332
Ending Balance	(42,782)	(44,165)	(42,782)	(44,165)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			267,370	(63,600)
Other comprehensive income (loss) before reclassifications			320,724	365,861
Reclassification of amounts included in net income			(82,949)	3,122
Other comprehensive income (loss), net of tax			237,775	368,983
Ending Balance	$ 505,145	$ 305,383	$ 505,145	$ 305,383

Operating Segments - Additional

Operating Segments - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2020Segment
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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	$ 327,024	$ 342,231	$ 1,107,872	$ 1,021,554
Net income	95,056	111,836	242,881	339,918
Operating Segments [Member] | Banking [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	293,855	307,312	1,003,469	918,672
Net income	93,507	110,583	238,023	334,393
Operating Segments [Member] | Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	35,282	37,549	111,493	111,064
Net income	4,130	4,106	13,642	15,403
Operating Segments [Member] | Non-Banks [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	(2,113)	(2,630)	(7,090)	(8,182)
Net income	$ (2,581)	$ (2,853)	$ (8,784)	$ (9,878)

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
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, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	$ 10,604,480	$ 11,269,591
Trading account securities	25,808	24,298
Derivative assets	20,847	5,266
Derivative liabilities	52,153	26,640
Interest rate swaps, caps and floors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	96,659	44,196
Derivative liabilities	38,339	20,197
Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	35,272	12,218
Derivative liabilities	34,432	11,677
Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		45
Derivative liabilities		33
U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	1,124,965	1,948,133
Trading account securities	24,495	24,298
Residential Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	2,164,162	2,207,594
States and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	7,273,005	7,070,997
Trading account securities	1,313
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	42,348	42,867
Level 1 [Member] | Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		45
Derivative liabilities		33
Level 1 [Member] | U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	1,124,965	1,948,133
Trading account securities	24,495	24,298
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	96,659	44,196
Derivative liabilities	38,339	20,197
Level 2 [Member] | Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	35,272	12,218
Derivative liabilities	34,432	11,677
Level 2 [Member] | Residential Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	2,164,162	2,207,594
Level 2 [Member] | States and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	7,273,005	7,070,997
Trading account securities	1,313
Level 2 [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	$ 42,348	$ 42,867

Fair Value Measurements - Impai

Fair Value Measurements - Impaired Loans Remeasured and Reported at Fair Value of Underlying Collateral (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	$ 6,388	$ 2,416
Impaired Financing Receivable Related Allowance Reversal		1,116
Specific (allocations) reversals of prior allocations	(930)
Loans receivable, fair value disclosure	5,458	3,532
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	8,660	60,010
Impaired Financing Receivable Related Allowance Reversal	14,928
Specific (allocations) reversals of prior allocations		(12,437)
Loans receivable, fair value disclosure	$ 23,588	$ 47,573

Fair Value Measurements - Forec

Fair Value Measurements - Foreclosed Assets Remeasured and Reported at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed Assets Remeasured at Initial Recognition Carrying Value Of Foreclosed Assets Prior To Remeasurement	$ 0	$ 1,302
Foreclosed Assets Remeasured at Initial Recognition Charge-offs Recognized in the Allowance for credit losses	0	(76)
Fair Value of Foreclosed Assets Remeasured at Initial Recognition	0	1,226
Foreclosed Assets Remeasured Subsequent to initial Recognition Carrying value of foreclosed assets prior to remeasurement	328	0
Foreclosed Assets Remeasured Subsequent to Initial Recognition Write-downs included in other non-interest expense	(231)	0
Fair Value of Foreclosed Assets remeasured subsequent to initial recognition	$ 97	$ 0

Fair Value Measurements - Estim

Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 7,175,680	$ 3,788,181	$ 2,907,901	$ 3,955,779
Securities held to maturity	1,951,544	2,030,005
Loans, net	17,960,402	14,618,165
Deposits	33,499,503	27,639,564
Federal funds purchased and repurchase agreements	1,608,667	1,695,342
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,342	136,299
Subordinated notes	98,982	98,865
Carrying Amount [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7,175,680	3,788,181
Securities held to maturity	1,951,384	2,030,005
Cash surrender value of life insurance policies	189,148	187,156
Accrued interest receivable	122,618	183,850
Deposits	33,499,503	27,639,564
Federal funds purchased and repurchase agreements	1,608,667	1,695,342
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,342	136,299
Subordinated notes	98,982	98,865
Accrued interest payable	9,144	12,393
Carrying Amount [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	17,960,402	14,618,165
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7,175,680	3,788,181
Securities held to maturity	2,058,855	2,048,675
Cash surrender value of life insurance policies	189,148	187,156
Accrued interest receivable	122,618	183,850
Deposits	33,503,668	27,641,255
Federal funds purchased and repurchase agreements	1,608,667	1,695,342
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	137,115	137,115
Subordinated notes	112,912	89,077
Accrued interest payable	9,144	12,393
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 18,160,904	$ 14,654,615