Cullen/Frost Bankers (CFR) 10-Q 31 Oct 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - $ / shares	9 Months Ended
	Sep. 30, 2019	Oct. 24, 2019	Dec. 31, 2018
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2019
Document Transition Report	false
Entity File Number	001-13221
Entity Registrant Name	Cullen/Frost Bankers, Inc.
Entity Incorporation, State or Country Code	TX
Entity Tax Identification Number	74-1751768
Entity Address, Address Line One	111 W. Houston Street,
Entity Address, City or Town	San Antonio,
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	78205
City Area Code	(210)
Local Phone Number	220-4011
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		62,565,469
Common stock, par value	$ 0.01		$ 0.01
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Entity Central Index Key	0000039263
Current Fiscal Year End Date	--12-31
Common Stock [Member] | NEW YORK STOCK EXCHANGE, INC. [Member]
Entity Information [Line Items]
Title of 12(b) Security	Common Stock, $.01 Par Value
Trading Symbol	CFR
Security Exchange Name	NYSE
Preferred Class A [Member] | NEW YORK STOCK EXCHANGE, INC. [Member]
Entity Information [Line Items]
Title of 12(b) Security	5.375% Non-Cumulative Perpetual Preferred Stock, Series A
Trading Symbol	CFR.PRA
Security Exchange Name	NYSE

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Cash and due from banks	$ 653,969	$ 678,791
Interest-bearing deposits	1,865,883	2,641,971
Federal funds sold and resell agreements	388,049	635,017
Total cash and cash equivalents	2,907,901	3,955,779
Securities held to maturity, at amortized cost	1,020,838	1,106,057
Securities available for sale, at estimated fair value	12,420,446	11,387,321
Trading account securities	27,669	24,086
Loans, net of unearned discounts	14,634,928	14,099,733
Less: Allowance for loan losses	(136,559)	(132,132)
Net loans	14,498,369	13,967,601
Premises and equipment, net	987,503	552,330
Goodwill	654,952	654,952
Other intangible assets, net	2,745	3,649
Cash surrender value of life insurance policies	186,217	183,473
Accrued interest receivable and other assets	391,536	457,718
Total assets	33,098,176	32,292,966
Liabilities:
Non-interest-bearing demand deposits	10,750,244	10,997,494
Interest-bearing deposits	16,333,338	16,151,710
Total deposits	27,083,582	27,149,204
Federal funds purchased and repurchase agreements	1,200,655	1,367,548
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,285	136,242
Subordinated notes, net of unamortized issuance costs	98,826	98,708
Accrued interest payable and other liabilities	697,371	172,347
Total liabilities	29,216,719	28,924,049
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 September 30, 2019 and December 31, 2018	144,486	144,486
Common stock, par value $0.01 per share; 210,000,000 shares authorized; 64,236,306 shares issued at September 30, 2019 and December 31, 2018	642	642
Additional paid-in capital	977,997	967,304
Retained earnings	2,617,500	2,440,002
Accumulated other comprehensive income (loss), net of tax	305,383	(63,600)
Treasury stock, at cost; 1,699,662 shares at September 30, 2019 and 1,250,464 shares at December 31, 2018	(164,551)	(119,917)
Total shareholders’ equity	3,881,457	3,368,917
Total liabilities and shareholders’ equity	$ 33,098,176	$ 32,292,966

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	6,000,000	6,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	210,000,000	210,000,000
Common stock, shares issued	64,236,306	64,236,306
Treasury stock, shares	1,699,662	1,250,464
Series A Preferred Stock [Member]
Preferred stock liquidation preference value	$ 25	$ 25

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Interest income:
Loans, including fees	$ 186,870	$ 172,333	$ 561,990	$ 487,668
Securities:
Taxable	32,122	21,339	87,125	63,085
Tax-exempt	57,328	59,200	175,067	173,209
Interest-bearing deposits	8,759	14,445	27,226	42,456
Federal funds sold and resell agreements	1,194	1,399	4,323	3,575
Total interest income	286,273	268,716	855,731	769,993
Interest expense:
Deposits	25,637	22,201	79,655	50,414
Federal funds purchased and repurchase agreements	5,040	2,292	15,276	3,557
Junior subordinated deferrable interest debentures	1,425	1,394	4,401	3,847
Other long-term borrowings	1,164	1,164	3,492	3,492
Total interest expense	33,266	27,051	102,824	61,310
Net interest income	253,007	241,665	752,907	708,683
Provision for loan losses	8,001	2,650	25,404	17,846
Net interest income after provision for loan losses	245,006	239,015	727,503	690,837
Non-interest income:
Trust and investment management fees	31,649	30,801	93,794	89,509
Service charges on deposit accounts	22,941	21,569	65,529	63,554
Insurance commissions and fees	11,683	11,037	40,207	37,573
Interchange and debit card transaction fees	4,117	3,499	11,265	10,103
Other charges, commissions and fees	10,108	9,580	28,103	27,860
Net gain (loss) on securities transactions	96	(34)	265	(113)
Other	8,630	11,205	29,484	35,682
Total non-interest income	89,224	87,657	268,647	264,168
Non-interest expense:
Salaries and wages	93,812	87,547	277,078	259,434
Employee benefits	21,002	18,355	64,579	58,257
Net occupancy	24,202	19,894	64,602	59,089
Technology, furniture and equipment	22,415	21,004	66,236	61,142
Deposit insurance	2,491	4,694	7,752	14,178
Intangible amortization	274	336	904	1,093
Other	44,668	41,838	132,722	125,994
Total non-interest expense	208,864	193,668	613,873	579,187
Income before income taxes	125,366	133,004	382,277	375,818
Income taxes	13,530	15,160	42,359	40,153
Net income	111,836	117,844	339,918	335,665
Preferred stock dividends	2,016	2,016	6,047	6,047
Net income available to common shareholders	$ 109,820	$ 115,828	$ 333,871	$ 329,618
Earnings per common share:
Basic	$ 1.74	$ 1.80	$ 5.28	$ 5.13
Diluted	$ 1.73	$ 1.78	$ 5.24	$ 5.08

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net income	$ 111,836	$ 117,844	$ 339,918	$ 335,665
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss on securities available for sale during the period	107,786	(96,147)	464,072	(286,935)
Change in net unrealized gain on securities transferred to held to maturity	(305)	(3,764)	(957)	(8,424)
Reclassification adjustment for net (gains) losses included in net income	(97)	34	(266)	113
Total securities available for sale and transferred securities, before tax amount	107,384	(99,877)	462,849	(295,246)
Defined-benefit post-retirement benefit plans:
Change in the net actuarial gain/loss	0	0	0	0
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit)	1,406	1,250	4,218	3,751
Total defined-benefit post-retirement benefit plans	1,406	1,250	4,218	3,751
Other comprehensive income (loss), before tax	108,790	(98,627)	467,067	(291,495)
Deferred tax expense (benefit) related to other comprehensive income	22,845	(20,711)	98,084	(61,213)
Other comprehensive income (loss), net of tax	85,945	(77,916)	368,983	(230,282)
Comprehensive income (loss)	$ 197,781	$ 39,928	$ 708,901	$ 105,383

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 new accounting principle in period of adoption	$ (2,285)				$ (2,285)
Total shareholders’ equity at beginning of period at Dec. 31, 2017	3,297,863	$ 144,486	$ 642	$ 953,361	2,187,069	$ 79,512	$ (67,207)
Total shareholders' equity at beginning of period, as adjusted at Dec. 31, 2017	3,295,578	144,486	642	953,361	2,184,784	79,512	(67,207)
Net income	335,665				335,665
Other comprehensive income (loss)	(230,282)					(230,282)
Reclassification of certain income tax effects related to U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act					(9,535)	9,535
Stock option exercises/stock unit conversions (101,284 shares QTD and 255,184 shares YTD in 2019 and 19,232 shares QTD and 447,831 shares YTD in 2018)	26,567				(12,887)		39,454
Stock compensation expense recognized in earnings	9,556			9,556
Purchase of treasury stock (202,724 shares QTD and 704,382 shares YTD in 2019 and 0 shares QTD and 601 shares YTD in 2018)	(70)						(70)
Cash dividends – preferred stock (approximately $0.34 per share QTD and $1.01 per share YTD in both 2019 and in 2018)	(6,047)				(6,047)
Cash dividends – common stock ($0.71 per share QTD and $2.09 per share YTD in 2019 and $0.67 per share QTD and $1.91 per share YTD in 2018)	(122,721)				(122,721)
Total shareholders’ equity at end of period at Sep. 30, 2018	3,308,246	144,486	642	962,917	2,369,259	(141,235)	(27,823)
Total shareholders’ equity at beginning of period at Jun. 30, 2018	3,309,512	144,486	642	960,121	2,297,099	(63,319)	(29,517)
Net income	117,844				117,844
Other comprehensive income (loss)	(77,916)					(77,916)
Stock option exercises/stock unit conversions (101,284 shares QTD and 255,184 shares YTD in 2019 and 19,232 shares QTD and 447,831 shares YTD in 2018)	1,119				(575)		1,694
Stock compensation expense recognized in earnings	2,796			2,796
Cash dividends – preferred stock (approximately $0.34 per share QTD and $1.01 per share YTD in both 2019 and in 2018)	(2,016)				(2,016)
Cash dividends – common stock ($0.71 per share QTD and $2.09 per share YTD in 2019 and $0.67 per share QTD and $1.91 per share YTD in 2018)	(43,093)				(43,093)
Total shareholders’ equity at end of period at Sep. 30, 2018	3,308,246	144,486	642	962,917	2,369,259	(141,235)	(27,823)
Total shareholders’ equity at end of period at Dec. 31, 2018	3,368,917	144,486	642	967,304	2,440,002	(63,600)	(119,917)
Cumulative effect of new accounting principle in period of adoption	(14,672)				(14,672)
Total shareholders' equity at beginning of period, as adjusted at Dec. 31, 2018	3,354,245	144,486	642	967,304	2,425,330	(63,600)	(119,917)
Net income	339,918				339,918
Other comprehensive income (loss)	368,983					368,983
Stock option exercises/stock unit conversions (101,284 shares QTD and 255,184 shares YTD in 2019 and 19,232 shares QTD and 447,831 shares YTD in 2018)	13,506				(9,569)		23,075
Stock compensation expense recognized in earnings	10,693			10,693
Purchase of treasury stock (202,724 shares QTD and 704,382 shares YTD in 2019 and 0 shares QTD and 601 shares YTD in 2018)	(67,709)						(67,709)
Cash dividends – preferred stock (approximately $0.34 per share QTD and $1.01 per share YTD in both 2019 and in 2018)	(6,047)				(6,047)
Cash dividends – common stock ($0.71 per share QTD and $2.09 per share YTD in 2019 and $0.67 per share QTD and $1.91 per share YTD in 2018)	(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 (101,284 shares QTD and 255,184 shares YTD in 2019 and 19,232 shares QTD and 447,831 shares YTD in 2018)	5,665				(3,851)		9,516
Stock compensation expense recognized in earnings	2,675			2,675
Purchase of treasury stock (202,724 shares QTD and 704,382 shares YTD in 2019 and 0 shares QTD and 601 shares YTD in 2018)	(17,190)						(17,190)
Cash dividends – preferred stock (approximately $0.34 per share QTD and $1.01 per share YTD in both 2019 and in 2018)	(2,016)				(2,016)
Cash dividends – common stock ($0.71 per share QTD and $2.09 per share YTD in 2019 and $0.67 per share QTD and $1.91 per share YTD in 2018)	(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)

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Shareholders' Equity (Parenthetical) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Stockholders' Equity [Abstract]
Stock option exercises/stock unit conversions (shares)	101,284	19,232	255,184	447,831
Treasury stock, shares, acquired	202,724	0	704,382	601
Approximate cash dividends declared on preferred stock, per share	$ 0.34	$ 0.34	$ 1.01	$ 1.01
Cash dividends declared on common stock, per share	$ 0.71	$ 0.67	$ 2.09	$ 1.91

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Operating Activities:
Net income	$ 339,918	$ 335,665
Adjustments to reconcile net income to net cash from operating activities:
Provision for loan losses	25,404	17,846
Deferred tax expense (benefit)	(1,625)	39,198
Accretion of loan discounts	(11,364)	(10,470)
Securities premium amortization (discount accretion), net	84,963	73,748
Net (gain) loss on securities transactions	(265)	113
Depreciation and amortization	39,182	37,452
Net (gain) loss on sale/write-down of assets/foreclosed assets	(6,016)	(6,122)
Stock-based compensation	10,693	9,556
Net tax benefit from stock-based compensation	1,602	3,378
Earnings on life insurance policies	(2,744)	(2,510)
Net change in:
Trading account securities	(3,132)	(1,094)
Lease right-of-use assets	14,618	0
Accrued interest receivable and other assets	37,608	(19,047)
Accrued interest payable and other liabilities	56,354	(17,545)
Net cash from operating activities	585,196	460,168
Investing Activities:
Securities held-to-maturity - Purchases	0	(1,500)
Securities held to maturity: Maturities, calls and principal repayments	69,273	300,340
Securities available for sale:
Purchases	(9,815,936)	(14,983,642)
Sales	8,236,066	13,838,566
Maturities, calls and principal repayments	1,007,783	182,028
Proceeds from sale of loans	24,036	21,318
Net change in loans	(569,223)	(718,551)
Benefits received on life insurance policies	0	384
Proceeds from sales of premises and equipment	8,019	12,859
Purchases of premises and equipment	(168,326)	(56,781)
Proceeds from sales of repossessed properties	131	1,106
Net cash from investing activities	(1,208,177)	(1,403,873)
Financing Activities:
Net change in deposits	(65,622)	(523,535)
Net change in short-term borrowings	(166,893)	(64,047)
Proceeds from stock option exercises	13,506	26,567
Purchase of treasury stock	(67,709)	(70)
Cash dividends paid on preferred stock	(6,047)	(6,047)
Cash dividends paid on common stock	(132,132)	(122,721)
Net cash from financing activities	(424,897)	(689,853)
Net change in cash and cash equivalents	(1,047,878)	(1,633,558)
Cash and equivalents at beginning of period	3,955,779	5,053,047
Cash and equivalents at end of period	$ 2,907,901	$ 3,419,489

Significant Accounting Policies

Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
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, 2018 , included in our Annual Report on Form 10-K filed with the SEC on February 6, 2019 (the “ 2018 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 2019 2018 Cash paid for interest $ 100,041 $ 59,764 Cash paid for income taxes 42,352 5,112 Significant non-cash transactions: Unsettled purchases/sales of securities 82,064 74,191 Loans foreclosed and transferred to other real estate owned and foreclosed assets 1,302 2,898 Loans to facilitate the sale of other real estate owned 847 — Right-of-use lease assets obtained in exchange for lessee operating lease liabilities 315,542 — Accounting Changes, Reclassifications and Restatements. Certain items in prior financial statements have been reclassified to conform to the current presentation. In addition, as of January 1, 2019, we adopted certain accounting standard updates related to accounting for leases (Topic 842 - Leases), primarily Accounting Standards Update (“ASU”) 2016-02 and subsequent updates. Among other things, these updates require lessees to recognize a lease liability, measured on a discounted basis, related to the lessee's obligation to make lease payments arising under a lease contract; and a right-of-use asset related to the lessee’s right to use, or control the use of, a specified asset for the lease term. The updates did not significantly change lease accounting requirements applicable to lessors and did not significantly impact our financial statements in relation to contracts whereby we act as a lessor. We adopted the updates 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 January 1, 2019. We elected to apply certain practical adoption expedients provided under the updates whereby we did not reassess (i) whether any expired or existing contracts are or contain leases, (ii) the lease classification for any expired or existing leases and (iii) initial direct costs for any existing leases. We did not elect to apply the recognition requirements of the updates to any short-term leases (as discussed below). As of September 30, 2019 , right-of-use lease assets and related lease liabilities totaled $299.5 million and $325.3 million , respectively. During the second quarter of 2019, we recognized a right-of-use asset totaling $121.7 million and a related lease liability totaling $121.7 million in connection with the commencement of the lease of our new corporate headquarters facility in downtown San Antonio. See Note 6 - Commitments and Contingencies. We lease certain office facilities and office equipment under operating leases. We also own certain office facilities which we lease to outside parties under operating lessor leases; however, such leases are not significant. We do not apply the recognition requirements of Topic 842 - Leases to short-term operating leases. A short-term operating lease has an original term of 12 months or less and does not have a purchase option that is likely to be exercised. For non-short-term operating leases, we recognized lease right-of-use assets and related lease liabilities on our balance sheet upon commencement of the lease in accordance with Topic 842 - Leases. In recognizing lease right-of-use assets and related lease liabilities, we account for lease and non-lease components (such as taxes, insurance, and common area maintenance costs) separately as such amounts are generally readily determinable under our lease contracts. Lease payments over the expected term are discounted using our incremental borrowing rate referenced to the Federal Home Loan Bank Secure Connect advance rates for borrowings of similar term. We also consider renewal and termination options in the determination of the term of the lease. If it is reasonably certain that a renewal or termination option will be exercised, the effects of such options are included in the determination of the expected lease term. Generally, we cannot be reasonably certain about whether or not we will renew a lease until such time the lease is within the last two years of the existing lease term. However, renewal options related to our regional headquarter facilities or operations centers are evaluated on a case-by-case basis, typically in advance of such time frame. When we are reasonably certain that a renewal option will be exercised, we measure/remeasure the right-of-use asset and related lease liability using the lease payments specified for the renewal period or, if such amounts are unspecified, we generally assume an increase (evaluated on a case-by-case basis in light of prevailing market conditions) in the lease payment over the final period of the existing lease term. We also adopted ASU 2017-08 “Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities” as of January 1, 2019. ASU 2017-08 shortens the amortization period for certain callable debt securities held at a premium to require such premiums to be amortized to the earliest call date unless applicable guidance related to certain pools of securities is applied to consider estimated prepayments. Under prior guidance, entities were generally required to amortize premiums on individual, non-pooled callable debt securities as a yield adjustment over the contractual life of the security. ASU 2017-08 does not change the accounting for callable debt securities held at a discount. Upon adoption, using a modified retrospective transition adoption approach, we recognized a cumulative effect reduction to retained earnings totaling $14.7 million . We previously reported a cumulative effect reduction of $12.6 million . During the third quarter of 2019, we retroactively corrected this amount and reduced retained earnings by an additional $2.1 million . All financial information related to prior periods has been revised to reflect this adjustment. We expect premium amortization expense for 2019 will be approximately $5.2 million higher than what would have been the case had we continued to amortize the affected securities to their respective maturity dates.

Securities

Securities	9 Months Ended
	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Securities	Securities Securities. A summary of the amortized cost and estimated fair value of securities, excluding trading securities, is presented below. September 30, 2019 December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Held to Maturity Residential mortgage-backed securities $ 2,472 $ 25 $ 2 $ 2,495 $ 2,737 $ 8 $ 85 $ 2,660 States and political subdivisions 1,016,866 26,563 — 1,043,429 1,101,820 11,525 552 1,112,793 Other 1,500 — 1 1,499 1,500 — — 1,500 Total $ 1,020,838 $ 26,588 $ 3 $ 1,047,423 $ 1,106,057 $ 11,533 $ 637 $ 1,116,953 Available for Sale U.S. Treasury $ 2,806,802 $ 20,725 $ 1,731 $ 2,825,796 $ 3,455,417 $ 1,772 $ 29,500 $ 3,427,689 Residential mortgage-backed securities 2,198,466 42,766 780 2,240,452 823,208 13,079 6,547 829,740 States and political subdivisions 6,931,673 380,113 413 7,311,373 7,089,132 70,760 72,690 7,087,202 Other 42,825 — — 42,825 42,690 — — 42,690 Total $ 11,979,766 $ 443,604 $ 2,924 $ 12,420,446 $ 11,410,447 $ 85,611 $ 108,737 $ 11,387,321 All mortgage-backed securities included in the above table were issued by U.S. government agencies and corporations. At September 30, 2019 , approximately 99.7% of the securities in our municipal bond portfolio were issued by political subdivisions or agencies within the State of Texas, of which approximately 69.3% are either guaranteed by the Texas Permanent School Fund, which has a “triple A” insurer financial strength rating, or are secured by U.S. Treasury securities via defeasance of the debt by the issuers. Securities with limited marketability and that do not have readily determinable fair values are carried at cost minus impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for identical or similar securities of the same issuer. These securities include stock in the Federal Reserve Bank and the Federal Home Loan Bank and are reported as other available for sale securities in the table above. The carrying value of securities pledged to secure public funds, trust deposits, repurchase agreements and for other purposes, as required or permitted by law was $3.6 billion at September 30, 2019 and $3.8 billion at December 31, 2018 . During the fourth quarter of 2012, we reclassified certain securities from available for sale to held to maturity. The securities had an aggregate fair value of $2.3 billion with an aggregate net unrealized gain of $165.7 million ( $107.7 million , net of tax) on the date of the transfer. The net unamortized, unrealized gain on the remaining transferred securities included in accumulated other comprehensive income in the accompanying balance sheet as of September 30, 2019 totaled $1.8 million ( $1.4 million , net of tax). 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. Unrealized Losses. As of September 30, 2019 , securities with unrealized losses, segregated by length of impairment, were as follows: Less than 12 Months More than 12 Months Total Estimated Fair Value Unrealized Losses Estimated Fair Value Unrealized Losses Estimated Fair Value Unrealized Losses Held to Maturity Residential mortgage-backed securities $ — $ — $ 413 $ 2 $ 413 $ 2 Other 1,499 1 — — 1,499 1 Total $ 1,499 $ 1 $ 413 $ 2 $ 1,912 $ 3 Available for Sale U.S. Treasury $ — $ — $ 1,722,158 $ 1,731 $ 1,722,158 $ 1,731 Residential mortgage-backed securities 49,657 290 44,458 490 94,115 780 States and political subdivisions 41,619 413 — — 41,619 413 Total $ 91,276 $ 703 $ 1,766,616 $ 2,221 $ 1,857,892 $ 2,924 Declines in the fair value of held-to-maturity and available-for-sale securities below their cost that are deemed to be other than temporary are reflected in earnings as realized losses to the extent the impairment is related to credit losses. The amount of the impairment related to other factors is recognized in other comprehensive income. In estimating other-than-temporary impairment losses, management considers, among other things, (i) the length of time and the extent to which the fair value has been less than cost, (ii) the financial condition and near-term prospects of the issuer, and (iii) the intent and our ability to retain our investment in the issuer for a period of time sufficient to allow for any anticipated recovery in cost. Management has the ability and intent to hold the securities classified as held to maturity in the table above until they mature, at which time we expect to receive full value for the securities. Furthermore, as of September 30, 2019 , 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. Any 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. Management does not believe any of the securities are impaired due to reasons of credit quality. Accordingly, as of September 30, 2019 , management believes the impairments detailed in the table above are temporary and no impairment loss has been realized in our consolidated income statement. Contractual Maturities. The amortized cost and estimated fair value of securities, excluding trading securities, at September 30, 2019 are presented below by contractual maturity. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Residential mortgage-backed securities and equity securities are shown separately since they are not due at a single maturity date. Held to Maturity Available for Sale Amortized Cost Estimated Fair Value Amortized Cost Estimated Fair Value Due in one year or less $ 25,254 $ 25,418 $ 1,882,023 $ 1,881,435 Due after one year through five years 166,412 170,675 1,328,841 1,356,833 Due after five years through ten years 530,808 543,464 453,192 480,892 Due after ten years 295,892 305,371 6,074,419 6,418,009 Residential mortgage-backed securities 2,472 2,495 2,198,466 2,240,452 Equity securities — — 42,825 42,825 Total $ 1,020,838 $ 1,047,423 $ 11,979,766 $ 12,420,446 Sales of Securities. Sales of securities available for sale were as follows: Three Months Ended Nine Months Ended 2019 2018 2019 2018 Proceeds from sales $ 4,944,537 $ 2,948,178 $ 8,236,066 $ 13,838,566 Gross realized gains 96 — 899 3 Gross realized losses — (34 ) (634 ) (116 ) Tax (expense) benefit of securities gains/losses (21 ) 7 (56 ) 24 Premiums and Discounts . Premium amortization and discount accretion included in interest income on securities was as follows: Three Months Ended Nine Months Ended 2019 2018 2019 2018 Premium amortization $ (29,511 ) $ (27,381 ) $ (88,798 ) $ (80,104 ) Discount accretion 1,274 2,569 3,835 6,356 Net (premium amortization) discount accretion $ (28,237 ) $ (24,812 ) $ (84,963 ) $ (73,748 ) Trading Account Securities. Trading account securities, at estimated fair value, were as follows: September 30, December 31, U.S. Treasury $ 23,747 $ 21,928 States and political subdivisions 3,922 2,158 Total $ 27,669 $ 24,086 Net gains and losses on trading account securities were as follows: Three Months Ended Nine Months Ended 2019 2018 2019 2018 Net gain on sales transactions $ 731 $ 465 $ 1,789 $ 1,404 Net mark-to-market gains (losses) (201 ) (8 ) (170 ) (21 ) Net gain (loss) on trading account securities $ 530 $ 457 $ 1,619 $ 1,383

Loans

Loans	9 Months Ended
	Sep. 30, 2019
Receivables [Abstract]
Loans	Loans Loans were as follows: September 30, Percentage of Total December 31, Percentage of Total Commercial and industrial $ 5,179,780 35.4 % $ 5,111,957 36.3 % Energy: Production 1,282,180 8.7 1,309,314 9.3 Service 189,350 1.3 168,775 1.2 Other 75,388 0.5 124,509 0.9 Total energy 1,546,918 10.5 1,602,598 11.4 Commercial real estate: Commercial mortgages 4,555,171 31.1 4,121,966 29.2 Construction 1,370,206 9.4 1,267,717 9.0 Land 290,706 2.0 306,755 2.2 Total commercial real estate 6,216,083 42.5 5,696,438 40.4 Consumer real estate: Home equity loans 363,224 2.5 353,924 2.5 Home equity lines of credit 350,461 2.4 337,168 2.4 Other 458,618 3.1 427,898 3.0 Total consumer real estate 1,172,303 8.0 1,118,990 7.9 Total real estate 7,388,386 50.5 6,815,428 48.3 Consumer and other 519,844 3.6 569,750 4.0 Total loans $ 14,634,928 100.0 % $ 14,099,733 100.0 % 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, 2019 , there were no concentrations of loans related to any single industry in excess of 10% of total loans other than energy loans, which totaled 10.5% of total loans. Unfunded commitments to extend credit and standby letters of credit issued to customers in the energy industry totaled $1.3 billion and $66.1 million , respectively, as of September 30, 2019 . 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, 2019 or December 31, 2018 . 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 $326.1 million at September 30, 2019 and $256.1 million at December 31, 2018 . Non-Accrual and Past Due Loans. Non-accrual loans, segregated by class of loans, were as follows: September 30, December 31, Commercial and industrial $ 13,929 $ 9,239 Energy 70,883 46,932 Commercial real estate: Buildings, land and other 9,240 15,268 Construction 697 — Consumer real estate 527 892 Consumer and other 2,170 1,408 Total $ 97,446 $ 73,739 Had non-accrual loans performed in accordance with their original contract terms, we would have recognized additional interest income, net of tax, of approximately $0.9 million and $3.0 million for the three and nine months ended September 30, 2019 , compared to $1.3 million and $4.2 million for the three and nine months ended September 30, 2018 . An age analysis of past due loans (including both accruing and non-accruing loans), segregated by class of loans, as of September 30, 2019 was as follows: Loans 30-89 Days Past Due Loans 90 or More Days Past Due Total Past Due Loans Current Loans Total Loans Accruing Loans 90 or More Days Past Due Commercial and industrial $ 39,540 $ 13,622 $ 53,162 $ 5,126,618 $ 5,179,780 $ 3,574 Energy 3,350 21,836 25,186 1,521,732 1,546,918 530 Commercial real estate: Buildings, land and other 29,147 8,279 37,426 4,808,451 4,845,877 6,513 Construction 1,412 1,471 2,883 1,367,323 1,370,206 774 Consumer real estate 8,305 2,443 10,748 1,161,555 1,172,303 2,150 Consumer and other 7,815 2,419 10,234 509,610 519,844 855 Total $ 89,569 $ 50,070 $ 139,639 $ 14,495,289 $ 14,634,928 $ 14,396 Impaired Loans. Impaired loans are set forth in the following table. No interest income was recognized on impaired loans subsequent to their classification as impaired. Unpaid Contractual Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance September 30, 2019 Commercial and industrial $ 15,020 $ 4,874 $ 7,280 $ 12,154 $ 5,785 Energy 88,531 4,777 65,800 70,577 19,722 Commercial real estate: Buildings, land and other 9,553 3,864 5,015 8,879 1,483 Construction 697 697 — 697 — Consumer real estate 293 293 — 293 — Consumer and other 2,171 — 2,170 2,170 2,170 Total $ 116,265 $ 14,505 $ 80,265 $ 94,770 $ 29,160 December 31, 2018 Commercial and industrial $ 9,094 $ 2,842 $ 4,287 $ 7,129 $ 2,558 Energy 67,900 6,817 39,890 46,707 9,671 Commercial real estate: Buildings, land and other 15,774 2,168 12,517 14,685 2,599 Construction — — — — — Consumer real estate 293 293 — 293 — Consumer and other 1,475 — 1,407 1,407 1,407 Total $ 94,536 $ 12,120 $ 58,101 $ 70,221 $ 16,235 The average recorded investment in impaired loans was as follows: Three Months Ended Nine Months Ended 2019 2018 2019 2018 Commercial and industrial $ 14,438 $ 13,447 $ 12,552 $ 21,025 Energy 55,195 78,772 50,642 82,640 Commercial real estate: Buildings, land and other 9,418 14,306 14,961 12,328 Construction 349 — 277 — Consumer real estate 293 671 541 807 Consumer and other 1,795 1,073 1,605 805 Total $ 81,488 $ 108,269 $ 80,578 $ 117,605 Troubled Debt Restructurings . Troubled debt restructurings during the nine months ended September 30, 2019 and September 30, 2018 are set forth in the following table. Nine Months Ended Nine Months Ended Balance at Restructure Balance at Period-End Balance at Restructure Balance at Period-End Commercial and industrial $ 3,845 $ 2,188 $ 2,203 $ — Energy — — 13,708 — Commercial real estate: Buildings, land and other 9,456 9,494 — — Consumer real estate 124 123 — — $ 13,425 $ 11,805 $ 15,911 $ — 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 loan losses. Additional information related to restructured loans as of or for the three months ended September 30, 2019 and September 30, 2018 is set forth in the following table. September 30, 2019 September 30, 2018 Restructured loans past due in excess of 90 days at period-end: Number of loans 4 — Dollar amount of loans $ 3,244 $ — Restructured loans on non-accrual status at period end 5,645 — Charge-offs of restructured loans: Recognized in connection with restructuring 1,500 — Recognized on previously restructured loans — 4,650 Proceeds from sale of restructured loans — 15,750 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 (see details above), (iv) net charge-offs, (v) 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 2018 Form 10-K. In monitoring credit quality trends in the context of assessing the appropriate level of the allowance for loan losses, we monitor portfolio credit quality by the weighted-average risk grade of each class of commercial loan. Individual relationship managers 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. September 30, 2019 December 31, 2018 Weighted Loans Weighted Loans Commercial and industrial: Risk grades 1-8 6.19 $ 4,856,712 6.12 $ 4,862,275 Risk grade 9 9.00 165,195 9.00 112,431 Risk grade 10 10.00 75,276 10.00 58,328 Risk grade 11 11.00 68,668 11.00 69,684 Risk grade 12 12.00 8,144 12.00 6,681 Risk grade 13 13.00 5,785 13.00 2,558 Total 6.42 $ 5,179,780 6.30 $ 5,111,957 Energy Risk grades 1-8 5.85 $ 1,379,566 5.76 $ 1,451,673 Risk grade 9 9.00 80,170 9.00 35,565 Risk grade 10 10.00 1,105 10.00 43,001 Risk grade 11 11.00 15,194 11.00 25,427 Risk grade 12 12.00 51,161 12.00 37,261 Risk grade 13 13.00 19,722 13.00 9,671 Total 6.36 $ 1,546,918 6.22 $ 1,602,598 Commercial real estate: Buildings, land and other Risk grades 1-8 6.78 $ 4,528,963 6.76 $ 4,143,264 Risk grade 9 9.00 117,739 9.00 109,660 Risk grade 10 10.00 97,083 10.00 62,353 Risk grade 11 11.00 92,852 11.00 98,176 Risk grade 12 12.00 7,757 12.00 12,669 Risk grade 13 13.00 1,483 13.00 2,599 Total 6.99 $ 4,845,877 6.98 $ 4,428,721 Construction Risk grades 1-8 7.19 $ 1,314,076 7.13 $ 1,177,260 Risk grade 9 9.00 31,506 9.00 60,754 Risk grade 10 10.00 21,686 10.00 24,877 Risk grade 11 11.00 2,241 11.00 4,826 Risk grade 12 12.00 697 12.00 — Risk grade 13 13.00 — 13.00 — Total 7.28 $ 1,370,206 7.29 $ 1,267,717 Net (charge-offs)/recoveries, segregated by class of loans, were as follows: Three Months Ended Nine Months Ended 2019 2018 2019 2018 Commercial and industrial $ (1,520 ) $ (7,807 ) $ (5,912 ) $ (19,030 ) Energy (1,260 ) (5,347 ) (3,184 ) (10,272 ) Commercial real estate: Buildings, land and other 38 33 (466 ) (288 ) Construction 8 3 14 11 Consumer real estate (103 ) (388 ) (2,078 ) (1,078 ) Consumer and other (3,534 ) (1,792 ) (9,351 ) (4,975 ) Total $ (6,371 ) $ (15,298 ) $ (20,977 ) $ (35,632 ) 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 2018 Form 10-K, totaled 128.1 at September 30, 2019 and 126.4 at December 31, 2018 . A higher TLI value implies more favorable economic conditions. Allowance for Loan Losses . The allowance for loan losses is a reserve established through a provision for loan losses charged to expense, which represents management’s best estimate of inherent losses that have been incurred within the existing portfolio of loans. The allowance, in the judgment of management, is necessary to reserve for estimated loan losses and risks inherent in the loan portfolio. Our allowance for loan loss methodology, which is more fully described in our 2018 Form 10-K, follows the accounting guidance set forth in U.S. generally accepted accounting principles and the Interagency Policy Statement on the Allowance for Loan and Lease Losses, which was jointly issued by U.S. bank regulatory agencies. The level of the allowance reflects management’s continuing evaluation of industry concentrations, specific credit risks, loan loss and recovery experience, current loan portfolio quality, present economic, political and regulatory conditions and unidentified losses inherent in the current loan portfolio. 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. The following table presents details of the allowance for loan losses allocated to each portfolio segment as of September 30, 2019 and December 31, 2018 and detailed on the basis of the impairment evaluation methodology we used: Commercial and Industrial Energy Commercial Real Estate Consumer Real Estate Consumer and Other Total September 30, 2019 Historical valuation allowances $ 28,872 $ 7,422 $ 22,051 $ 2,641 $ 7,545 $ 68,531 Specific valuation allowances 5,785 19,722 1,483 — 2,170 29,160 General valuation allowances 10,866 4,120 4,349 1,172 (325 ) 20,182 Macroeconomic valuation allowances 7,144 2,082 7,470 1,071 919 18,686 Total $ 52,667 $ 33,346 $ 35,353 $ 4,884 $ 10,309 $ 136,559 Allocated to loans: Individually evaluated $ 5,785 $ 19,722 $ 1,483 $ — $ 2,170 $ 29,160 Collectively evaluated 46,882 13,624 33,870 4,884 8,139 107,399 Total $ 52,667 $ 33,346 $ 35,353 $ 4,884 $ 10,309 $ 136,559 December 31, 2018 Historical valuation allowances $ 25,351 $ 9,697 $ 20,817 $ 2,688 $ 6,845 $ 65,398 Specific valuation allowances 2,558 9,671 2,599 — 1,407 16,235 General valuation allowances 10,062 6,014 4,366 1,671 (13 ) 22,100 Macroeconomic valuation allowances 10,609 3,670 10,995 1,744 1,381 28,399 Total $ 48,580 $ 29,052 $ 38,777 $ 6,103 $ 9,620 $ 132,132 Allocated to loans: Individually evaluated $ 2,558 $ 9,671 $ 2,599 $ — $ 1,407 $ 16,235 Collectively evaluated 46,022 19,381 36,178 6,103 8,213 115,897 Total $ 48,580 $ 29,052 $ 38,777 $ 6,103 $ 9,620 $ 132,132 Our recorded investment in loans as of September 30, 2019 and December 31, 2018 related to each balance in the allowance for loan losses by portfolio segment and detailed on the basis of the impairment methodology we used was as follows: Commercial and Industrial Energy Commercial Consumer Consumer Total September 30, 2019 Individually evaluated $ 12,154 $ 70,577 $ 9,576 $ 293 $ 2,170 $ 94,770 Collectively evaluated 5,167,626 1,476,341 6,206,507 1,172,010 517,674 14,540,158 Total $ 5,179,780 $ 1,546,918 $ 6,216,083 $ 1,172,303 $ 519,844 $ 14,634,928 December 31, 2018 Individually evaluated $ 7,129 $ 46,707 $ 14,685 $ 293 $ 1,407 $ 70,221 Collectively evaluated 5,104,828 1,555,891 5,681,753 1,118,697 568,343 14,029,512 Total $ 5,111,957 $ 1,602,598 $ 5,696,438 $ 1,118,990 $ 569,750 $ 14,099,733 The following table details activity in the allowance for loan losses by portfolio segment for the three and nine months ended September 30, 2019 and 2018 . Allocation of a portion of the allowance to one category of loans does not preclude its availability to absorb losses in other categories. Commercial and Industrial Energy Commercial Real Estate Consumer Real Estate Consumer and Other Total Three months ended: September 30, 2019 Beginning balance $ 57,714 $ 25,818 $ 35,914 $ 5,637 $ 9,846 $ 134,929 Provision for loan losses (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 September 30, 2018 Beginning balance $ 57,713 $ 37,313 $ 38,918 $ 6,336 $ 9,946 $ 150,226 Provision for loan losses 1,085 (521 ) 250 246 1,590 2,650 Charge-offs (8,491 ) (5,400 ) — (431 ) (4,274 ) (18,596 ) Recoveries 684 53 36 43 2,482 3,298 Net charge-offs (7,807 ) (5,347 ) 36 (388 ) (1,792 ) (15,298 ) Ending balance $ 50,991 $ 31,445 $ 39,204 $ 6,194 $ 9,744 $ 137,578 Nine months ended: September 30, 2019 Beginning balance $ 48,580 $ 29,052 $ 38,777 $ 6,103 $ 9,620 $ 132,132 Provision for loan losses 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 September 30, 2018 Beginning balance $ 59,614 $ 51,528 $ 30,948 $ 5,657 $ 7,617 $ 155,364 Provision for loan losses 10,407 (9,811 ) 8,533 1,615 7,102 17,846 Charge-offs (21,896 ) (10,939 ) (619 ) (1,632 ) (12,240 ) (47,326 ) Recoveries 2,866 667 342 554 7,265 11,694 Net charge-offs (19,030 ) (10,272 ) (277 ) (1,078 ) (4,975 ) (35,632 ) Ending balance $ 50,991 $ 31,445 $ 39,204 $ 6,194 $ 9,744 $ 137,578

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	9 Months Ended
	Sep. 30, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets Goodwill and other intangible assets are presented in the table below. September 30, December 31, Goodwill $ 654,952 $ 654,952 Other intangible assets: Core deposits $ 2,254 $ 2,959 Customer relationships 491 672 Non-compete agreements — 18 $ 2,745 $ 3,649 The estimated aggregate future amortization expense for intangible assets remaining as of September 30, 2019 is as follows: Remainder of 2019 $ 263 2020 919 2021 697 2022 481 2023 283 Thereafter 102 $ 2,745

Deposits

Deposits	9 Months Ended
	Sep. 30, 2019
Banking and Thrift [Abstract]
Deposits	Deposits Deposits were as follows: September 30, Percentage of Total December 31, Percentage of Total Non-interest-bearing demand deposits: Commercial and individual $ 10,070,621 37.2 % $ 10,305,850 37.9 % Correspondent banks 258,981 1.0 235,748 0.9 Public funds 420,642 1.5 455,896 1.7 Total non-interest-bearing demand deposits 10,750,244 39.7 10,997,494 40.5 Interest-bearing deposits: Private accounts: Savings and interest checking 6,741,340 24.9 6,977,813 25.7 Money market accounts 7,960,223 29.4 7,777,470 28.6 Time accounts of $100,000 or more 707,657 2.6 526,789 2.0 Time accounts under $100,000 346,440 1.3 331,511 1.2 Total private accounts 15,755,660 58.2 15,613,583 57.5 Public funds: Savings and interest checking 483,303 1.8 473,754 1.8 Money market accounts 68,321 0.2 59,953 0.2 Time accounts of $100,000 or more 25,922 0.1 4,332 — Time accounts under $100,000 132 — 88 — Total public funds 577,678 2.1 538,127 2.0 Total interest-bearing deposits 16,333,338 60.3 16,151,710 59.5 Total deposits $ 27,083,582 100.0 % $ 27,149,204 100.0 % The following table presents additional information about our deposits: September 30, December 31, Deposits from the Certificate of Deposit Account Registry Service (CDARS) deposits $ 358 $ — Deposits from foreign sources (primarily Mexico) 776,490 752,658 Deposits not covered by deposit insurance 12,681,664 13,111,210

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments 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 2018 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 $ 9,036,441 $ 8,369,721 Standby letters of credit 283,821 271,575 Deferred standby letter of credit fees 1,547 2,069 Lease Commitments . We lease certain office facilities and office equipment under operating leases. Rent expense for all operating leases totaled $11.7 million and $30.0 million during the three and nine months ended September 30, 2019 and $8.1 million and $24.4 million during the three and nine months ended September 30, 2018 . On January 1, 2019, we adopted a new accounting standard which required the recognition of our operating leases on our balance sheet. See Note 1 - Significant Accounting Policies. As of September 30, 2019 , right-of-use lease assets and related lease liabilities totaled $299.5 million and $325.3 million , respectively, and are included with premises and equipment and accrued interest payable and other liabilities, respectively, on our accompanying consolidated balance sheet. During the second quarter of 2019, we recognized a right-of-use asset totaling $121.7 million and a related lease liability totaling $121.7 million in connection with the commencement of the lease of our new corporate headquarters facility in downtown San Antonio. There has been no significant change in our expected future minimum lease payments since December 31, 2018 . See the 2018 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.

Capital and Regulatory Matters

Capital and Regulatory Matters	9 Months Ended
	Sep. 30, 2019
Banking and Thrift [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. 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, and subject to transition provisions. 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. For Cullen/Frost, additional Tier 1 capital at September 30, 2019 and December 31, 2018 includes $144.5 million of 5.375% non-cumulative perpetual preferred stock. Frost Bank did not have any additional Tier 1 capital beyond Common Equity Tier 1 at September 30, 2019 or December 31, 2018 . 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 allowance for loan losses. 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, 2019 and December 31, 2018 . The following tables present actual and required capital ratios as of September 30, 2019 and December 31, 2018 for Cullen/Frost and Frost Bank under the Basel III Capital Rules. The Basel III Capital Rules became fully phased-in on January 1, 2019. The minimum required capital amounts presented as of December 31, 2018 include the minimum required capital levels applicable as of that date as well as the minimum required capital levels as of January 1, 2019 when the Basel III Capital Rules became fully phased-in. 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 2018 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, 2018 have been revised to reflect these reclassifications. Actual Minimum Capital Required - Basel III Required to be Capital Ratio Capital Ratio Capital Ratio September 30, 2019 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,788,313 12.35 % $ 1,580,199 7.00 % $ 1,467,328 6.50 % Frost Bank 2,910,446 12.92 1,576,393 7.00 1,463,794 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 2,932,799 12.99 1,918,813 8.50 1,805,942 8.00 Frost Bank 2,910,446 12.92 1,914,192 8.50 1,801,593 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,302,858 14.63 2,370,298 10.50 2,257,427 10.00 Frost Bank 3,047,505 13.53 2,364,590 10.50 2,251,991 10.00 Leverage Ratio Cullen/Frost 2,932,798 9.36 1,252,781 4.00 1,565,976 5.00 Frost Bank 2,910,446 9.30 1,251,713 4.00 1,564,642 5.00 Actual Minimum Capital Required - Basel III Phase-In Schedule Minimum Capital Required - Basel III Fully Phased-In Required to be Capital Ratio Capital Ratio Capital Ratio Capital Ratio December 31, 2018 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,642,475 12.27 % $ 1,372,573 6.375 % $ 1,507,139 7.00 % $ 1,358,171 6.50 % Frost Bank 2,743,973 12.78 1,368,701 6.375 1,502,887 7.00 1,354,222 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 2,786,961 12.94 1,695,532 7.875 1,830,098 8.50 1,671,595 8.00 Frost Bank 2,743,973 12.78 1,690,748 7.875 1,824,934 8.50 1,666,735 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,152,593 14.64 2,126,143 9.875 2,260,709 10.50 2,089,494 10.00 Frost Bank 2,876,605 13.40 2,120,144 9.875 2,254,331 10.50 2,083,419 10.00 Leverage Ratio Cullen/Frost 2,786,961 9.06 1,231,028 4.00 1,231,028 4.00 1,538,785 5.00 Frost Bank 2,743,973 8.93 1,229,650 4.00 1,229,650 4.00 1,537,062 5.00 As of September 30, 2019 , capital levels at Cullen/Frost and Frost Bank exceed all capital adequacy requirements under the fully phased-in Basel III Capital Rules. Based on the ratios presented above, capital levels as of September 30, 2019 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, 2019 , that Cullen/Frost and Frost Bank meet all capital adequacy requirements to which they are subject. 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 October 24, 2017 , our board of directors authorized a $150.0 million stock repurchase program, allowing us to repurchase shares of our common stock over a two -year period from time to time at various prices in the open market or through private transactions. We repurchased 496,307 shares at a total cost of $50.0 million under this plan during the second quarter of 2019 and 1,027,292 shares at a total cost of $100.0 million during the fourth quarter of 2018. 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 -year period from time to time at various prices in the open market or through private transactions. We repurchased 202,724 shares at a total cost of $ 17.2 million under this plan during the third quarter of 2019. Under the Basel III Capital Rules, Cullen/Frost may not repurchase or redeem any of its preferred stock or subordinated notes and, in some cases, its common stock without the prior approval of the Federal Reserve Board. 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, 2019 , Frost Bank could pay aggregate dividends of up to $633.8 million to Cullen/Frost without prior regulatory approval. Under the terms of the junior subordinated deferrable interest debentures that Cullen/Frost has issued to Cullen/Frost Capital Trust II 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. Under the terms of our Series A Preferred Stock, in the event that we do not declare and pay dividends on our Series A Preferred Stock for the most recent dividend period, we may not, with certain exceptions, declare or pay dividends on, or purchase, redeem or otherwise acquire, shares of our common stock or any of our securities that rank junior to our Series A Preferred Stock.

Derivative Financial Instrument

Derivative Financial Instruments	9 Months Ended
	Sep. 30, 2019
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 2018 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, 2019 and December 31, 2018 . September 30, 2019 December 31, 2018 Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Derivatives designated as hedges of fair value: Financial institution counterparties: Loan/lease interest rate swaps – assets $ 5,583 $ 15 $ 10,941 $ 207 Loan/lease interest rate swaps – liabilities 6,356 (179 ) 3,885 (199 ) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets 94,855 65 496,887 2,384 Loan/lease interest rate swaps – liabilities 1,008,173 (24,268 ) 691,143 (8,921 ) Loan/lease interest rate caps – assets 102,634 233 122,791 509 Customer counterparties: Loan/lease interest rate swaps – assets 1,008,173 55,529 691,143 16,706 Loan/lease interest rate swaps – liabilities 94,855 (175 ) 496,887 (8,891 ) Loan/lease interest rate caps – liabilities 102,634 (233 ) 122,791 (509 ) The weighted-average rates paid and received for interest rate swaps outstanding at September 30, 2019 were as follows: Weighted-Average Interest Rate Paid Interest Rate Received Interest rate swaps: Fair value hedge loan/lease interest rate swaps 2.33 % 2.05 % Non-hedging interest rate swaps – financial institution counterparties 4.17 3.71 Non-hedging interest rate swaps – customer counterparties 3.71 4.17 The weighted-average strike rate for outstanding interest rate caps was 3.16% at September 30, 2019 . 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, 2019 December 31, 2018 Notional Units Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Oil – assets Barrels 2,280 $ 12,884 2,416 $ 24,332 Oil – liabilities Barrels 727 (1,363 ) 415 (646 ) Natural gas – assets MMBTUs 8,716 2,053 5,745 417 Natural gas – liabilities MMBTUs 3,439 (191 ) 9,314 (1,272 ) Customer counterparties: Oil – assets Barrels 730 1,392 415 646 Oil – liabilities Barrels 2,277 (12,572 ) 2,416 (24,009 ) Natural gas – assets MMBTUs 3,543 205 10,236 1,373 Natural gas – liabilities MMBTUs 8,613 (1,960 ) 4,823 (393 ) 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, 2019 December 31, 2018 Notional Currency Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Forward contracts – liabilities CAD 5,283 $ (11 ) 11,003 $ (13 ) Forward contracts – liabilities GBP — — 142 (2 ) Forward contracts – liabilities MXN — — 3,015 (132 ) Customer counterparties: Forward contracts – assets CAD 5,271 24 10,979 40 Forward contracts – assets GBP — — 145 4 Forward contracts – assets MXN — — 3,000 149 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 2019 2018 2019 2018 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ 24 $ 12 $ 78 $ 1 Amount of (gain) loss included in other non-interest expense (1 ) — — (1 ) As stated above, we enter into non-hedge related derivative positions primarily to accommodate the business needs of our customers. Upon the origination of a derivative contract with a customer, we simultaneously enter into an offsetting derivative contract with a third party financial institution. We recognize immediate income based upon the difference in the bid/ask spread of the underlying transactions with our customers and the third party. Because we act only as an intermediary for our customer, subsequent changes in the fair value of the underlying derivative contracts for the most part offset each other and do not significantly impact our results of operations. Amounts included in the consolidated statements of income related to non-hedging interest rate, commodity and foreign currency derivative instruments are presented in the table below. Three Months Ended Nine Months Ended 2019 2018 2019 2018 Non-hedging interest rate derivatives: Other non-interest income $ 215 $ 699 $ 1,188 $ 2,889 Other non-interest expense — 2 — (2 ) Non-hedging commodity derivatives: Other non-interest income 109 456 322 492 Non-hedging foreign currency derivatives: Other non-interest income 12 53 41 203 Counterparty Credit Risk. Our credit exposure relating to interest rate swaps, commodity swaps/options and foreign currency forward contracts with bank customers was approximately $55.4 million at September 30, 2019 . 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 $18.6 million at September 30, 2019 . 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, 2019 , we had $32.1 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, 2019
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, 2019 is presented in the following tables. Gross Amount Recognized Gross Amount Offset Net Amount Recognized September 30, 2019 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 313 $ — $ 313 Commodity swaps and options 14,937 — 14,937 Foreign currency forward contracts — — — Total derivatives 15,250 — 15,250 Resell agreements 11,299 — 11,299 Total $ 26,549 $ — $ 26,549 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 24,447 $ — $ 24,447 Commodity swaps and options 1,554 — 1,554 Foreign currency forward contracts 11 — 11 Total derivatives 26,012 — 26,012 Repurchase agreements 1,187,355 — 1,187,355 Total $ 1,213,367 $ — $ 1,213,367 Gross Amounts Not Offset Net Amount Recognized Financial Instruments Collateral Net Amount September 30, 2019 Financial assets: Derivatives: Counterparty A $ 38 $ (38 ) $ — $ — Counterparty B 4,344 (4,344 ) — — Counterparty C 8 (8 ) — — Other counterparties 10,860 (5,503 ) (2,880 ) 2,477 Total derivatives 15,250 (9,893 ) (2,880 ) 2,477 Resell agreements 11,299 — (11,299 ) — Total $ 26,549 $ (9,893 ) $ (14,179 ) $ 2,477 Financial liabilities: Derivatives: Counterparty A $ 6,192 $ (38 ) $ (6,154 ) $ — Counterparty B 7,810 (4,344 ) (3,466 ) — Counterparty C 164 (8 ) (156 ) — Other counterparties 11,846 (5,503 ) (6,259 ) 84 Total derivatives 26,012 (9,893 ) (16,035 ) 84 Repurchase agreements 1,187,355 — (1,187,355 ) — Total $ 1,213,367 $ (9,893 ) $ (1,203,390 ) $ 84 Information about financial instruments that are eligible for offset in the consolidated balance sheet as of December 31, 2018 is presented in the following tables. Gross Amount Recognized Gross Amount Offset Net Amount Recognized December 31, 2018 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 3,100 $ — $ 3,100 Commodity swaps and options 24,749 — 24,749 Foreign currency forward contracts — — — Total derivatives 27,849 — 27,849 Resell agreements 11,642 — 11,642 Total $ 39,491 $ — $ 39,491 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 9,120 $ — $ 9,120 Commodity swaps and options 1,918 — 1,918 Foreign currency forward contracts 147 — 147 Total derivatives 11,185 — 11,185 Repurchase agreements 1,360,298 — 1,360,298 Total $ 1,371,483 $ — $ 1,371,483 Gross Amounts Not Offset Net Amount Recognized Financial Instruments Collateral Net Amount December 31, 2018 Financial assets: Derivatives: Counterparty A $ 598 $ (598 ) $ — $ — Counterparty B 7,255 (3,380 ) (3,875 ) — Counterparty C 81 (81 ) — — Other counterparties 19,915 (2,084 ) (17,776 ) 55 Total derivatives 27,849 (6,143 ) (21,651 ) 55 Resell agreements 11,642 — (11,642 ) — Total $ 39,491 $ (6,143 ) $ (33,293 ) $ 55 Financial liabilities: Derivatives: Counterparty A $ 4,293 $ (598 ) $ (3,651 ) $ 44 Counterparty B 3,380 (3,380 ) — — Counterparty C 326 (81 ) (245 ) — Other counterparties 3,186 (2,084 ) (725 ) 377 Total derivatives 11,185 (6,143 ) (4,621 ) 421 Repurchase agreements 1,360,298 — (1,360,298 ) — Total $ 1,371,483 $ (6,143 ) $ (1,364,919 ) $ 421 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, 2019 and December 31, 2018 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, 2019 Repurchase agreements: U.S. Treasury $ 162,566 $ 2,550 $ — $ — $ 165,116 Residential mortgage-backed securities 1,022,239 — — — 1,022,239 Total borrowings $ 1,184,805 $ 2,550 $ — $ — $ 1,187,355 Gross amount of recognized liabilities for repurchase agreements $ 1,187,355 Amounts related to agreements not included in offsetting disclosures above $ — December 31, 2018 Repurchase agreements: U.S. Treasury $ 1,334,063 $ — $ — $ — $ 1,334,063 Residential mortgage-backed securities 26,235 — — — 26,235 Total borrowings $ 1,360,298 $ — $ — $ — $ 1,360,298 Gross amount of recognized liabilities for repurchase agreements $ 1,360,298 Amounts related to agreements not included in offsetting disclosures above $ —

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [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, 2019 , there were 1,277,578 shares remaining available for grant for future stock-based compensation awards. Director Deferred Stock Units Outstanding Non-Vested Stock Awards/Stock Units Outstanding Performance Stock Units Outstanding Stock Options Outstanding Number of Units Weighted- Average Fair Value at Grant Number of Shares/Units Weighted- Average Fair Value at Grant Number of Units Weighted- Average Fair Value at Grant Number of Shares Weighted- Average Exercise Price Balance, January 1, 2019 48,910 $ 71.14 383,797 $ 85.59 125,809 $ 82.55 2,352,008 $ 63.55 Authorized — — — — — — — — Granted 7,592 102.70 1,957 91.96 — — — — Exercised/vested (1,132 ) 106.03 (17,800 ) 65.11 — — (236,252 ) 57.17 Forfeited/expired — — (11,850 ) 89.29 — — (11,000 ) 65.11 Balance, September 30, 2019 55,370 $ 74.76 356,104 $ 86.53 125,809 $ 82.55 2,104,756 $ 64.26 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 2019 2018 2019 2018 New shares issued from available authorized shares — — — — Issued from available treasury stock 71,284 19,232 225,184 447,831 Total 71,284 19,232 225,184 447,831 Proceeds from stock option exercises $ 5,665 $ 1,119 $ 13,506 $ 26,567 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 2019 2018 2019 2018 Stock options $ 330 $ 960 $ 1,091 $ 3,064 Non-vested stock awards/stock units 1,536 1,357 5,727 4,196 Director deferred stock units — — 780 720 Performance stock units 809 479 3,095 1,576 Total $ 2,675 $ 2,796 $ 10,693 $ 9,556 Income tax benefit $ 452 $ 587 $ 1,768 $ 2,007 Unrecognized stock-based compensation expense at September 30, 2019 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. Stock options $ 96 Non-vested stock awards/stock units 11,196 Performance stock units 3,432 Total $ 14,724

Earnings Per Common Share

Earnings Per Common Share	9 Months Ended
	Sep. 30, 2019
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 2018 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 2019 2018 2019 2018 Net income $ 111,836 $ 117,844 $ 339,918 $ 335,665 Less: Preferred stock dividends 2,016 2,016 6,047 6,047 Net income available to common shareholders 109,820 115,828 333,871 329,618 Less: Earnings allocated to participating securities 876 750 2,761 2,178 Net earnings allocated to common stock $ 108,944 $ 115,078 $ 331,110 $ 327,440 Distributed earnings allocated to common stock $ 44,370 $ 42,811 $ 131,069 $ 121,907 Undistributed earnings allocated to common stock 64,574 72,267 200,041 205,533 Net earnings allocated to common stock $ 108,944 $ 115,078 $ 331,110 $ 327,440 Weighted-average shares outstanding for basic earnings per common share 62,566,128 63,892,023 62,786,501 63,793,514 Dilutive effect of stock compensation 592,866 1,022,168 725,911 1,037,043 Weighted-average shares outstanding for diluted earnings per common share 63,158,994 64,914,191 63,512,412 64,830,557

Defined Benefit Plans

Defined Benefit Plans	9 Months Ended
	Sep. 30, 2019
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 2019 2018 2019 2018 Expected return on plan assets, net of expenses $ (2,693 ) $ (2,979 ) $ (8,079 ) $ (8,937 ) Interest cost on projected benefit obligation 1,618 1,475 4,854 4,424 Net amortization and deferral 1,406 1,250 4,218 3,751 Net periodic expense (benefit) $ 331 $ (254 ) $ 993 $ (762 ) 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, 2019 . We do not expect to make any contributions to the qualified defined benefit plan during the remainder of 2019 .

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Income tax expense was as follows: Three Months Ended Nine Months Ended 2019 2018 2019 2018 Current income tax expense (benefit) $ 14,480 $ (1,152 ) $ 43,984 $ 955 Deferred income tax expense (benefit) (950 ) 16,312 (1,625 ) 39,198 Income tax expense, as reported $ 13,530 $ 15,160 $ 42,359 $ 40,153 Effective tax rate 10.8 % 11.4 % 11.1 % 10.7 % We had a net deferred tax liability totaling $76.7 million at September 30, 2019 and a net deferred tax asset totaling $19.8 million at December 31, 2018 . The change in net deferred taxes was primarily related to unrealized gains on available-for-sale securities during the nine months ended September 30, 2019 . No valuation allowance for deferred tax assets was recorded at September 30, 2019 as management believes it is more likely than not that all of the deferred tax assets will be realized against deferred tax liabilities and projected future taxable income. The effective income tax rates differed from the U.S. statutory federal income tax rates of 21% during the comparable periods primarily due to the effect of tax-exempt income from loans, securities and life insurance policies and the income tax effects associated with stock-based compensation. There were no unrecognized tax benefits during any of the reported periods. Interest and/or penalties related to income taxes are reported as a component of income tax expense. Such amounts were not significant during the reported periods. We file income tax returns in the U.S. federal jurisdiction. We are no longer subject to U.S. federal income tax examinations by tax authorities for years before 2016.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Other Comprehensive Income (Loss)	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 Amount Tax Expense, (Benefit) Net of Tax Amount Before Tax Amount Tax Expense, (Benefit) Net of Tax Amount Securities available for sale and transferred securities: Change in net unrealized gain/loss during the period $ 107,786 $ 22,635 $ 85,151 $ (96,147 ) $ (20,190 ) $ (75,957 ) Change in net unrealized gain on securities transferred to held to maturity (305 ) (64 ) (241 ) (3,764 ) (790 ) (2,974 ) Reclassification adjustment for net (gains) losses included in net income (97 ) (21 ) (76 ) 34 7 27 Total securities available for sale and transferred securities 107,384 22,550 84,834 (99,877 ) (20,973 ) (78,904 ) 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) 1,406 295 1,111 1,250 262 988 Total defined-benefit post-retirement benefit plans 1,406 295 1,111 1,250 262 988 Total other comprehensive income (loss) $ 108,790 $ 22,845 $ 85,945 $ (98,627 ) $ (20,711 ) $ (77,916 ) Nine Months Ended Nine Months Ended Before Tax Amount Tax Expense, (Benefit) Net of Tax Amount Before Tax Amount Tax Expense, (Benefit) Net of Tax Amount Securities available for sale and transferred securities: Change in net unrealized gain/loss during the period $ 464,072 $ 97,455 $ 366,617 $ (286,935 ) $ (60,256 ) $ (226,679 ) Change in net unrealized gain on securities transferred to held to maturity (957 ) (201 ) (756 ) (8,424 ) (1,769 ) (6,655 ) Reclassification adjustment for net (gains) losses included in net income (266 ) (56 ) (210 ) 113 24 89 Total securities available for sale and transferred securities 462,849 97,198 365,651 (295,246 ) (62,001 ) (233,245 ) Defined-benefit post-retirement benefit plans: Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) 4,218 886 3,332 3,751 788 2,963 Total defined-benefit post-retirement benefit plans 4,218 886 3,332 3,751 788 2,963 Total other comprehensive income (loss) $ 467,067 $ 98,084 $ 368,983 $ (291,495 ) $ (61,213 ) $ (230,282 ) Activity in accumulated other comprehensive income (loss), net of tax, was as follows: Securities Available For Sale Defined Benefit Plans Accumulated Other Comprehensive Income 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 Balance January 1, 2018 $ 117,230 $ (37,718 ) $ 79,512 Other comprehensive income (loss) before reclassifications (233,334 ) — (233,334 ) Reclassification of amounts included in net income 89 2,963 3,052 Net other comprehensive income (loss) during period (233,245 ) 2,963 (230,282 ) Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings 17,557 (8,022 ) 9,535 Balance at September 30, 2018 $ (98,458 ) $ (42,777 ) $ (141,235 )

Operating Segments

Operating Segments	9 Months Ended
	Sep. 30, 2019
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 2018 Form 10-K for additional information regarding our operating segments. Summarized operating results by segment were as follows: Banking Frost Wealth Advisors Non-Banks Consolidated Revenues from (expenses to) external customers: Three months ended: September 30, 2019 $ 307,312 $ 37,549 $ (2,630 ) $ 342,231 September 30, 2018 295,062 36,868 (2,608 ) 329,322 Nine months ended: September 30, 2019 $ 918,672 $ 111,064 $ (8,182 ) $ 1,021,554 September 30, 2018 874,056 106,475 (7,680 ) 972,851 Net income (loss): Three months ended: September 30, 2019 $ 110,583 $ 4,106 $ (2,853 ) $ 111,836 September 30, 2018 114,214 6,442 (2,812 ) 117,844 Nine months ended: September 30, 2019 $ 334,393 $ 15,403 $ (9,878 ) $ 339,918 September 30, 2018 327,131 17,977 (9,443 ) 335,665

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The fair value of an asset or liability is the price that would be received to sell that asset or paid to transfer that liability in an orderly transaction occurring in the principal market (or most advantageous market in the absence of a principal market) for such asset or liability. In estimating fair value, we utilize valuation techniques that are consistent with the market approach, the income approach and/or the cost approach. Such valuation techniques are consistently applied. Inputs to valuation techniques include the assumptions that market participants would use in pricing an asset or liability. ASC Topic 820 establishes a three-level fair value hierarchy for valuation inputs that gives the highest priority to quoted prices in active markets for identical assets or liabilities and the lowest priority to unobservable inputs. See our 2018 Form 10-K for additional information regarding the fair value hierarchy and a description of our valuation techniques. Financial Assets and Financial Liabilities. The table below summarizes financial assets and financial liabilities measured at fair value on a recurring basis as of September 30, 2019 and December 31, 2018 , segregated by the level of the valuation inputs within the fair value hierarchy of ASC Topic 820 utilized to measure fair value. Level 1 Level 2 Level 3 Total Fair September 30, 2019 Securities available for sale: U.S. Treasury $ 2,825,796 $ — $ — $ 2,825,796 Residential mortgage-backed securities — 2,240,452 — 2,240,452 States and political subdivisions — 7,311,373 — 7,311,373 Other — 42,825 — 42,825 Trading account securities: U.S. Treasury 23,747 — — 23,747 States and political subdivisions — 3,922 — 3,922 Derivative assets: Interest rate swaps, caps and floors — 55,842 — 55,842 Commodity swaps and options — 16,534 — 16,534 Foreign currency forward contracts 24 — — 24 Derivative liabilities: Interest rate swaps, caps and floors — 24,855 — 24,855 Commodity swaps and options — 16,086 — 16,086 Foreign currency forward contracts 11 — — 11 December 31, 2018 Securities available for sale: U.S. Treasury $ 3,427,689 $ — $ — $ 3,427,689 Residential mortgage-backed securities — 829,740 — 829,740 States and political subdivisions — 7,087,202 — 7,087,202 Other — 42,690 — 42,690 Trading account securities: U.S. Treasury 21,928 — — 21,928 States and political subdivisions — 2,158 — 2,158 Derivative assets: Interest rate swaps, caps and floors — 19,806 — 19,806 Commodity swaps and options — 26,768 — 26,768 Foreign currency forward contracts 193 — — 193 Derivative liabilities: Interest rate swaps, caps and floors — 18,520 — 18,520 Commodity swaps and options — 26,320 — 26,320 Foreign currency forward contracts 147 — — 147 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 (for example, when there is evidence of impairment). Financial assets measured at fair value on a non-recurring basis during the reported periods include certain impaired loans reported at the fair value of the underlying collateral if repayment is expected solely from the collateral. The following table presents impaired loans that were remeasured and reported at fair value through a specific valuation allowance allocation of the allowance for loan losses 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 of impaired loans before allocations $ 2,416 $ 60,010 $ 12,876 $ 28,638 Specific valuation allowance (allocations) reversals of prior allocations 1,116 (12,437 ) (2,599 ) 4,256 Fair value $ 3,532 $ 47,573 $ 10,277 $ 32,894 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 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 2019 2018 Foreclosed assets remeasured at initial recognition: Carrying value of foreclosed assets prior to remeasurement $ 1,302 $ 2,898 Charge-offs recognized in the allowance for loan losses (76 ) — Fair value $ 1,226 $ 2,898 Foreclosed assets remeasured subsequent to initial recognition: Carrying value of foreclosed assets prior to remeasurement $ — $ 1,823 Write-downs included in other non-interest expense — (473 ) Fair value $ — $ 1,350 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, 2019 December 31, 2018 Carrying Amount Estimated Fair Value Carrying Amount Estimated Fair Value Financial assets: Level 2 inputs: Cash and cash equivalents $ 2,907,901 $ 2,907,901 $ 3,955,779 $ 3,955,779 Securities held to maturity 1,020,838 1,047,423 1,106,057 1,116,953 Cash surrender value of life insurance policies 186,217 186,217 183,473 183,473 Accrued interest receivable 133,950 133,950 188,989 188,989 Level 3 inputs: Loans, net 14,498,369 14,542,918 13,967,601 13,933,239 Financial liabilities: Level 2 inputs: Deposits 27,083,582 27,084,765 27,149,204 27,143,572 Federal funds purchased and repurchase agreements 1,200,655 1,200,655 1,367,548 1,367,548 Junior subordinated deferrable interest debentures 136,285 137,115 136,242 137,115 Subordinated notes payable and other borrowings 98,826 106,447 98,708 98,458 Accrued interest payable 10,177 10,177 7,394 7,394 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, 2019
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 2018 Form 10-K for additional information related to previously issued accounting standards updates. ASU 2016-02, “Leases (Topic 842).” ASU 2016-02, among other things, requires lessees to recognize a lease liability, which is a lessee's obligation to make lease payments arising from a lease, measured on a discounted basis; and a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. We adopted ASU 2016-02, along with several other subsequent codification updates related to lease accounting, as of January 1, 2019. See Note 1 - Significant Accounting Policies for additional information. 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. In addition, ASU 2016-13 amends the accounting for credit losses on available-for-sale debt securities and purchased financial assets with credit deterioration. In April 2019, ASU 2019-04, “Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments,” was issued to address certain codification improvements and to provide certain accounting policy electives related to accrued interest as well as disclosure related to credit losses, among other things. In May 2019, ASU 2019-05, “Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief,” was issued to provide transition relief in connection with the adoption of ASU 2016-03 whereby entities would have the option to irrevocably elect the fair value option for certain financial assets previously measured at amortized cost basis. ASU 2016-13, as updated, will be effective on January 1, 2020. We are currently working through our implementation plan with our cross-functional working group, under the direction of our Chief Financial Officer and our Chief Credit Officer. The working group is comprised of individuals from various functional areas including credit, risk management, finance and information technology, among others. Our implementation plan includes assessment and documentation of processes, internal controls and data sources; model development, documentation and validation; and system configuration, among other things. We are also in the process of implementing a third-party vendor solution to assist us in the application of ASU 2016-13. Based upon our preliminary parallel run as of September 30, 2019, we currently expect the adoption of ASU 2016-13 will result in a combined 15.0% to 25.0% increase in our allowance for loan losses and our reserves for unfunded commitments. This increase is a result of changing from an “incurred loss” model, which encompasses allowances for current known and inherent losses within the portfolio, to an “expected loss” model, which encompasses allowances for losses expected to be incurred over the life of the portfolio. Furthermore, ASU 2016-13 will necessitate that we establish an allowance for expected credit losses for certain debt securities and other financial assets; however, we do not expect these allowances to be significant. Additionally, the adoption of ASU 2016-13 is not expected to have a significant impact on our regulatory capital ratios. The ultimate impact of adoption on January 1, 2020 could be significantly different than our current expectation as our modeling processes will be significantly influenced by the composition, characteristics and quality of our loan and securities portfolios as well as the prevailing economic conditions and forecasts as of that date, notwithstanding any further refinements to our expected credit loss models. ASU 2017-08, “Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities.” ASU 2017-08 shortens the amortization period for certain callable debt securities held at a premium to require such premiums to be amortized to the earliest call date unless applicable guidance related to certain pools of securities is applied to consider estimated prepayments. Under prior guidance, entities were generally required to amortize premiums on individual, non-pooled callable debt securities as a yield adjustment over the contractual life of the security. ASU 2017-08 did not change the accounting for callable debt securities held at a discount. We adopted ASU 2017-08 effective January 1, 2019 and recognized a cumulative effect adjustment reducing retained earnings by $14.7 million . See Note 1 - Significant Accounting Policies. ASU 2017-12, “Derivatives and Hedging (Topic 815) - Targeted Improvements to Accounting for Hedging Activities.” ASU 2017-12 amends the hedge accounting recognition and presentation requirements in ASC 815 to improve the transparency and understandability of information conveyed to financial statement users about an entity’s risk management activities to better align the entity’s financial reporting for hedging relationships with those risk management activities and to reduce the complexity of and simplify the application of hedge accounting. ASU 2017-12 became effective for us on January 1, 2019 and did not have a significant impact on our financial statements. In April 2019, ASU 2019-04 was issued to clarify certain aspects of accounting for hedging activities addressed by ASU 2017-12, among other things. ASU 2018-16, “Derivatives and Hedging (Topic 815) - Inclusion of the Secured Overnight Financing Rate (“SOFR”) Overnight Index Swap (“OIS”) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes.” The amendments in this update permit use of the OIS rate based on SOFR as a U.S. benchmark interest rate for hedge accounting purposes under Topic 815 in addition to the interest rates on direct U.S. Treasury obligations, the LIBOR swap rate, the OIS rate based on the Fed Funds Effective Rate and the Securities Industry and Financial Markets Association (“SIFMA”) Municipal Swap Rate. ASU 2018-16 became effective for us on January 1, 2019 and did not have a significant impact on our financial statements.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
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, 2018 , included in our Annual Report on Form 10-K filed with the SEC on February 6, 2019 (the “ 2018 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]	Reclassifications and Restatements. Certain items in prior financial statements have been reclassified to conform to the current presentation.
New Accounting Pronouncements, Policy [Policy Text Block]	In addition, as of January 1, 2019, we adopted certain accounting standard updates related to accounting for leases (Topic 842 - Leases), primarily Accounting Standards Update (“ASU”) 2016-02 and subsequent updates. Among other things, these updates require lessees to recognize a lease liability, measured on a discounted basis, related to the lessee's obligation to make lease payments arising under a lease contract; and a right-of-use asset related to the lessee’s right to use, or control the use of, a specified asset for the lease term. The updates did not significantly change lease accounting requirements applicable to lessors and did not significantly impact our financial statements in relation to contracts whereby we act as a lessor. We adopted the updates 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 January 1, 2019. We elected to apply certain practical adoption expedients provided under the updates whereby we did not reassess (i) whether any expired or existing contracts are or contain leases, (ii) the lease classification for any expired or existing leases and (iii) initial direct costs for any existing leases. We did not elect to apply the recognition requirements of the updates to any short-term leases (as discussed below). As of September 30, 2019 , right-of-use lease assets and related lease liabilities totaled $299.5 million and $325.3 million , respectively. During the second quarter of 2019, we recognized a right-of-use asset totaling $121.7 million and a related lease liability totaling $121.7 million in connection with the commencement of the lease of our new corporate headquarters facility in downtown San Antonio. See Note 6 - Commitments and Contingencies. We lease certain office facilities and office equipment under operating leases. We also own certain office facilities which we lease to outside parties under operating lessor leases; however, such leases are not significant. We do not apply the recognition requirements of Topic 842 - Leases to short-term operating leases. A short-term operating lease has an original term of 12 months or less and does not have a purchase option that is likely to be exercised. For non-short-term operating leases, we recognized lease right-of-use assets and related lease liabilities on our balance sheet upon commencement of the lease in accordance with Topic 842 - Leases. In recognizing lease right-of-use assets and related lease liabilities, we account for lease and non-lease components (such as taxes, insurance, and common area maintenance costs) separately as such amounts are generally readily determinable under our lease contracts. Lease payments over the expected term are discounted using our incremental borrowing rate referenced to the Federal Home Loan Bank Secure Connect advance rates for borrowings of similar term. We also consider renewal and termination options in the determination of the term of the lease. If it is reasonably certain that a renewal or termination option will be exercised, the effects of such options are included in the determination of the expected lease term. Generally, we cannot be reasonably certain about whether or not we will renew a lease until such time the lease is within the last two years of the existing lease term. However, renewal options related to our regional headquarter facilities or operations centers are evaluated on a case-by-case basis, typically in advance of such time frame. When we are reasonably certain that a renewal option will be exercised, we measure/remeasure the right-of-use asset and related lease liability using the lease payments specified for the renewal period or, if such amounts are unspecified, we generally assume an increase (evaluated on a case-by-case basis in light of prevailing market conditions) in the lease payment over the final period of the existing lease term. We also adopted ASU 2017-08 “Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities” as of January 1, 2019. ASU 2017-08 shortens the amortization period for certain callable debt securities held at a premium to require such premiums to be amortized to the earliest call date unless applicable guidance related to certain pools of securities is applied to consider estimated prepayments. Under prior guidance, entities were generally required to amortize premiums on individual, non-pooled callable debt securities as a yield adjustment over the contractual life of the security. ASU 2017-08 does not change the accounting for callable debt securities held at a discount. Upon adoption, using a modified retrospective transition adoption approach, we recognized a cumulative effect reduction to retained earnings totaling $14.7 million . We previously reported a cumulative effect reduction of $12.6 million . During the third quarter of 2019, we retroactively corrected this amount and reduced retained earnings by an additional $2.1 million . All financial information related to prior periods has been revised to reflect this adjustment. We expect premium amortization expense for 2019 will be approximately $5.2 million higher than what would have been the case had we continued to amortize the affected securities to their respective maturity dates. Information about certain recently issued accounting standards updates is presented below. Also refer to Note 20 - Accounting Standards Updates in our 2018 Form 10-K for additional information related to previously issued accounting standards updates. ASU 2016-02, “Leases (Topic 842).” ASU 2016-02, among other things, requires lessees to recognize a lease liability, which is a lessee's obligation to make lease payments arising from a lease, measured on a discounted basis; and a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. We adopted ASU 2016-02, along with several other subsequent codification updates related to lease accounting, as of January 1, 2019. See Note 1 - Significant Accounting Policies for additional information. 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. In addition, ASU 2016-13 amends the accounting for credit losses on available-for-sale debt securities and purchased financial assets with credit deterioration. In April 2019, ASU 2019-04, “Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments,” was issued to address certain codification improvements and to provide certain accounting policy electives related to accrued interest as well as disclosure related to credit losses, among other things. In May 2019, ASU 2019-05, “Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief,” was issued to provide transition relief in connection with the adoption of ASU 2016-03 whereby entities would have the option to irrevocably elect the fair value option for certain financial assets previously measured at amortized cost basis. ASU 2016-13, as updated, will be effective on January 1, 2020. We are currently working through our implementation plan with our cross-functional working group, under the direction of our Chief Financial Officer and our Chief Credit Officer. The working group is comprised of individuals from various functional areas including credit, risk management, finance and information technology, among others. Our implementation plan includes assessment and documentation of processes, internal controls and data sources; model development, documentation and validation; and system configuration, among other things. We are also in the process of implementing a third-party vendor solution to assist us in the application of ASU 2016-13. Based upon our preliminary parallel run as of September 30, 2019, we currently expect the adoption of ASU 2016-13 will result in a combined 15.0% to 25.0% increase in our allowance for loan losses and our reserves for unfunded commitments. This increase is a result of changing from an “incurred loss” model, which encompasses allowances for current known and inherent losses within the portfolio, to an “expected loss” model, which encompasses allowances for losses expected to be incurred over the life of the portfolio. Furthermore, ASU 2016-13 will necessitate that we establish an allowance for expected credit losses for certain debt securities and other financial assets; however, we do not expect these allowances to be significant. Additionally, the adoption of ASU 2016-13 is not expected to have a significant impact on our regulatory capital ratios. The ultimate impact of adoption on January 1, 2020 could be significantly different than our current expectation as our modeling processes will be significantly influenced by the composition, characteristics and quality of our loan and securities portfolios as well as the prevailing economic conditions and forecasts as of that date, notwithstanding any further refinements to our expected credit loss models. ASU 2017-08, “Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities.” ASU 2017-08 shortens the amortization period for certain callable debt securities held at a premium to require such premiums to be amortized to the earliest call date unless applicable guidance related to certain pools of securities is applied to consider estimated prepayments. Under prior guidance, entities were generally required to amortize premiums on individual, non-pooled callable debt securities as a yield adjustment over the contractual life of the security. ASU 2017-08 did not change the accounting for callable debt securities held at a discount. We adopted ASU 2017-08 effective January 1, 2019 and recognized a cumulative effect adjustment reducing retained earnings by $14.7 million . See Note 1 - Significant Accounting Policies. ASU 2017-12, “Derivatives and Hedging (Topic 815) - Targeted Improvements to Accounting for Hedging Activities.” ASU 2017-12 amends the hedge accounting recognition and presentation requirements in ASC 815 to improve the transparency and understandability of information conveyed to financial statement users about an entity’s risk management activities to better align the entity’s financial reporting for hedging relationships with those risk management activities and to reduce the complexity of and simplify the application of hedge accounting. ASU 2017-12 became effective for us on January 1, 2019 and did not have a significant impact on our financial statements. In April 2019, ASU 2019-04 was issued to clarify certain aspects of accounting for hedging activities addressed by ASU 2017-12, among other things. ASU 2018-16, “Derivatives and Hedging (Topic 815) - Inclusion of the Secured Overnight Financing Rate (“SOFR”) Overnight Index Swap (“OIS”) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes.” The amendments in this update permit use of the OIS rate based on SOFR as a U.S. benchmark interest rate for hedge accounting purposes under Topic 815 in addition to the interest rates on direct U.S. Treasury obligations, the LIBOR swap rate, the OIS rate based on the Fed Funds Effective Rate and the Securities Industry and Financial Markets Association (“SIFMA”) Municipal Swap Rate. ASU 2018-16 became effective for us on January 1, 2019 and did not have a significant impact on our financial statements.

Accounting Standards Updates Ac

Accounting Standards Updates Accounting Standards Updates (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
New Accounting Pronouncements, Policy [Policy Text Block]	In addition, as of January 1, 2019, we adopted certain accounting standard updates related to accounting for leases (Topic 842 - Leases), primarily Accounting Standards Update (“ASU”) 2016-02 and subsequent updates. Among other things, these updates require lessees to recognize a lease liability, measured on a discounted basis, related to the lessee's obligation to make lease payments arising under a lease contract; and a right-of-use asset related to the lessee’s right to use, or control the use of, a specified asset for the lease term. The updates did not significantly change lease accounting requirements applicable to lessors and did not significantly impact our financial statements in relation to contracts whereby we act as a lessor. We adopted the updates 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 January 1, 2019. We elected to apply certain practical adoption expedients provided under the updates whereby we did not reassess (i) whether any expired or existing contracts are or contain leases, (ii) the lease classification for any expired or existing leases and (iii) initial direct costs for any existing leases. We did not elect to apply the recognition requirements of the updates to any short-term leases (as discussed below). As of September 30, 2019 , right-of-use lease assets and related lease liabilities totaled $299.5 million and $325.3 million , respectively. During the second quarter of 2019, we recognized a right-of-use asset totaling $121.7 million and a related lease liability totaling $121.7 million in connection with the commencement of the lease of our new corporate headquarters facility in downtown San Antonio. See Note 6 - Commitments and Contingencies. We lease certain office facilities and office equipment under operating leases. We also own certain office facilities which we lease to outside parties under operating lessor leases; however, such leases are not significant. We do not apply the recognition requirements of Topic 842 - Leases to short-term operating leases. A short-term operating lease has an original term of 12 months or less and does not have a purchase option that is likely to be exercised. For non-short-term operating leases, we recognized lease right-of-use assets and related lease liabilities on our balance sheet upon commencement of the lease in accordance with Topic 842 - Leases. In recognizing lease right-of-use assets and related lease liabilities, we account for lease and non-lease components (such as taxes, insurance, and common area maintenance costs) separately as such amounts are generally readily determinable under our lease contracts. Lease payments over the expected term are discounted using our incremental borrowing rate referenced to the Federal Home Loan Bank Secure Connect advance rates for borrowings of similar term. We also consider renewal and termination options in the determination of the term of the lease. If it is reasonably certain that a renewal or termination option will be exercised, the effects of such options are included in the determination of the expected lease term. Generally, we cannot be reasonably certain about whether or not we will renew a lease until such time the lease is within the last two years of the existing lease term. However, renewal options related to our regional headquarter facilities or operations centers are evaluated on a case-by-case basis, typically in advance of such time frame. When we are reasonably certain that a renewal option will be exercised, we measure/remeasure the right-of-use asset and related lease liability using the lease payments specified for the renewal period or, if such amounts are unspecified, we generally assume an increase (evaluated on a case-by-case basis in light of prevailing market conditions) in the lease payment over the final period of the existing lease term. We also adopted ASU 2017-08 “Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities” as of January 1, 2019. ASU 2017-08 shortens the amortization period for certain callable debt securities held at a premium to require such premiums to be amortized to the earliest call date unless applicable guidance related to certain pools of securities is applied to consider estimated prepayments. Under prior guidance, entities were generally required to amortize premiums on individual, non-pooled callable debt securities as a yield adjustment over the contractual life of the security. ASU 2017-08 does not change the accounting for callable debt securities held at a discount. Upon adoption, using a modified retrospective transition adoption approach, we recognized a cumulative effect reduction to retained earnings totaling $14.7 million . We previously reported a cumulative effect reduction of $12.6 million . During the third quarter of 2019, we retroactively corrected this amount and reduced retained earnings by an additional $2.1 million . All financial information related to prior periods has been revised to reflect this adjustment. We expect premium amortization expense for 2019 will be approximately $5.2 million higher than what would have been the case had we continued to amortize the affected securities to their respective maturity dates. Information about certain recently issued accounting standards updates is presented below. Also refer to Note 20 - Accounting Standards Updates in our 2018 Form 10-K for additional information related to previously issued accounting standards updates. ASU 2016-02, “Leases (Topic 842).” ASU 2016-02, among other things, requires lessees to recognize a lease liability, which is a lessee's obligation to make lease payments arising from a lease, measured on a discounted basis; and a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. We adopted ASU 2016-02, along with several other subsequent codification updates related to lease accounting, as of January 1, 2019. See Note 1 - Significant Accounting Policies for additional information. 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. In addition, ASU 2016-13 amends the accounting for credit losses on available-for-sale debt securities and purchased financial assets with credit deterioration. In April 2019, ASU 2019-04, “Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments,” was issued to address certain codification improvements and to provide certain accounting policy electives related to accrued interest as well as disclosure related to credit losses, among other things. In May 2019, ASU 2019-05, “Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief,” was issued to provide transition relief in connection with the adoption of ASU 2016-03 whereby entities would have the option to irrevocably elect the fair value option for certain financial assets previously measured at amortized cost basis. ASU 2016-13, as updated, will be effective on January 1, 2020. We are currently working through our implementation plan with our cross-functional working group, under the direction of our Chief Financial Officer and our Chief Credit Officer. The working group is comprised of individuals from various functional areas including credit, risk management, finance and information technology, among others. Our implementation plan includes assessment and documentation of processes, internal controls and data sources; model development, documentation and validation; and system configuration, among other things. We are also in the process of implementing a third-party vendor solution to assist us in the application of ASU 2016-13. Based upon our preliminary parallel run as of September 30, 2019, we currently expect the adoption of ASU 2016-13 will result in a combined 15.0% to 25.0% increase in our allowance for loan losses and our reserves for unfunded commitments. This increase is a result of changing from an “incurred loss” model, which encompasses allowances for current known and inherent losses within the portfolio, to an “expected loss” model, which encompasses allowances for losses expected to be incurred over the life of the portfolio. Furthermore, ASU 2016-13 will necessitate that we establish an allowance for expected credit losses for certain debt securities and other financial assets; however, we do not expect these allowances to be significant. Additionally, the adoption of ASU 2016-13 is not expected to have a significant impact on our regulatory capital ratios. The ultimate impact of adoption on January 1, 2020 could be significantly different than our current expectation as our modeling processes will be significantly influenced by the composition, characteristics and quality of our loan and securities portfolios as well as the prevailing economic conditions and forecasts as of that date, notwithstanding any further refinements to our expected credit loss models. ASU 2017-08, “Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities.” ASU 2017-08 shortens the amortization period for certain callable debt securities held at a premium to require such premiums to be amortized to the earliest call date unless applicable guidance related to certain pools of securities is applied to consider estimated prepayments. Under prior guidance, entities were generally required to amortize premiums on individual, non-pooled callable debt securities as a yield adjustment over the contractual life of the security. ASU 2017-08 did not change the accounting for callable debt securities held at a discount. We adopted ASU 2017-08 effective January 1, 2019 and recognized a cumulative effect adjustment reducing retained earnings by $14.7 million . See Note 1 - Significant Accounting Policies. ASU 2017-12, “Derivatives and Hedging (Topic 815) - Targeted Improvements to Accounting for Hedging Activities.” ASU 2017-12 amends the hedge accounting recognition and presentation requirements in ASC 815 to improve the transparency and understandability of information conveyed to financial statement users about an entity’s risk management activities to better align the entity’s financial reporting for hedging relationships with those risk management activities and to reduce the complexity of and simplify the application of hedge accounting. ASU 2017-12 became effective for us on January 1, 2019 and did not have a significant impact on our financial statements. In April 2019, ASU 2019-04 was issued to clarify certain aspects of accounting for hedging activities addressed by ASU 2017-12, among other things. ASU 2018-16, “Derivatives and Hedging (Topic 815) - Inclusion of the Secured Overnight Financing Rate (“SOFR”) Overnight Index Swap (“OIS”) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes.” The amendments in this update permit use of the OIS rate based on SOFR as a U.S. benchmark interest rate for hedge accounting purposes under Topic 815 in addition to the interest rates on direct U.S. Treasury obligations, the LIBOR swap rate, the OIS rate based on the Fed Funds Effective Rate and the Securities Industry and Financial Markets Association (“SIFMA”) Municipal Swap Rate. ASU 2018-16 became effective for us on January 1, 2019 and did not have a significant impact on our financial statements.

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Additional Cash Flow Information	Additional cash flow information was as follows: Nine Months Ended 2019 2018 Cash paid for interest $ 100,041 $ 59,764 Cash paid for income taxes 42,352 5,112 Significant non-cash transactions: Unsettled purchases/sales of securities 82,064 74,191 Loans foreclosed and transferred to other real estate owned and foreclosed assets 1,302 2,898 Loans to facilitate the sale of other real estate owned 847 — Right-of-use lease assets obtained in exchange for lessee operating lease liabilities 315,542 —

Securities (Tables)

Securities (Tables)	9 Months Ended
	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities	A summary of the amortized cost and estimated fair value of securities, excluding trading securities, is presented below. September 30, 2019 December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Held to Maturity Residential mortgage-backed securities $ 2,472 $ 25 $ 2 $ 2,495 $ 2,737 $ 8 $ 85 $ 2,660 States and political subdivisions 1,016,866 26,563 — 1,043,429 1,101,820 11,525 552 1,112,793 Other 1,500 — 1 1,499 1,500 — — 1,500 Total $ 1,020,838 $ 26,588 $ 3 $ 1,047,423 $ 1,106,057 $ 11,533 $ 637 $ 1,116,953 Available for Sale U.S. Treasury $ 2,806,802 $ 20,725 $ 1,731 $ 2,825,796 $ 3,455,417 $ 1,772 $ 29,500 $ 3,427,689 Residential mortgage-backed securities 2,198,466 42,766 780 2,240,452 823,208 13,079 6,547 829,740 States and political subdivisions 6,931,673 380,113 413 7,311,373 7,089,132 70,760 72,690 7,087,202 Other 42,825 — — 42,825 42,690 — — 42,690 Total $ 11,979,766 $ 443,604 $ 2,924 $ 12,420,446 $ 11,410,447 $ 85,611 $ 108,737 $ 11,387,321
Securities, with Unrealized Losses Segregated by Length of Impairment	As of September 30, 2019 , securities with unrealized losses, segregated by length of impairment, were as follows: Less than 12 Months More than 12 Months Total Estimated Fair Value Unrealized Losses Estimated Fair Value Unrealized Losses Estimated Fair Value Unrealized Losses Held to Maturity Residential mortgage-backed securities $ — $ — $ 413 $ 2 $ 413 $ 2 Other 1,499 1 — — 1,499 1 Total $ 1,499 $ 1 $ 413 $ 2 $ 1,912 $ 3 Available for Sale U.S. Treasury $ — $ — $ 1,722,158 $ 1,731 $ 1,722,158 $ 1,731 Residential mortgage-backed securities 49,657 290 44,458 490 94,115 780 States and political subdivisions 41,619 413 — — 41,619 413 Total $ 91,276 $ 703 $ 1,766,616 $ 2,221 $ 1,857,892 $ 2,924
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity	The amortized cost and estimated fair value of securities, excluding trading securities, at September 30, 2019 are presented below by contractual maturity. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Residential mortgage-backed securities and equity securities are shown separately since they are not due at a single maturity date. Held to Maturity Available for Sale Amortized Cost Estimated Fair Value Amortized Cost Estimated Fair Value Due in one year or less $ 25,254 $ 25,418 $ 1,882,023 $ 1,881,435 Due after one year through five years 166,412 170,675 1,328,841 1,356,833 Due after five years through ten years 530,808 543,464 453,192 480,892 Due after ten years 295,892 305,371 6,074,419 6,418,009 Residential mortgage-backed securities 2,472 2,495 2,198,466 2,240,452 Equity securities — — 42,825 42,825 Total $ 1,020,838 $ 1,047,423 $ 11,979,766 $ 12,420,446
Sales of Securities Available for Sale	Sales of securities available for sale were as follows: Three Months Ended Nine Months Ended 2019 2018 2019 2018 Proceeds from sales $ 4,944,537 $ 2,948,178 $ 8,236,066 $ 13,838,566 Gross realized gains 96 — 899 3 Gross realized losses — (34 ) (634 ) (116 ) Tax (expense) benefit of securities gains/losses (21 ) 7 (56 ) 24
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 2019 2018 2019 2018 Premium amortization $ (29,511 ) $ (27,381 ) $ (88,798 ) $ (80,104 ) Discount accretion 1,274 2,569 3,835 6,356 Net (premium amortization) discount accretion $ (28,237 ) $ (24,812 ) $ (84,963 ) $ (73,748 )
Trading Account Securities, at Estimated Fair Value	Trading account securities, at estimated fair value, were as follows: September 30, December 31, U.S. Treasury $ 23,747 $ 21,928 States and political subdivisions 3,922 2,158 Total $ 27,669 $ 24,086
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 2019 2018 2019 2018 Net gain on sales transactions $ 731 $ 465 $ 1,789 $ 1,404 Net mark-to-market gains (losses) (201 ) (8 ) (170 ) (21 ) Net gain (loss) on trading account securities $ 530 $ 457 $ 1,619 $ 1,383

Loans (Tables)

Loans (Tables)	9 Months Ended
	Sep. 30, 2019
Receivables [Abstract]
Loans	Loans were as follows: September 30, Percentage of Total December 31, Percentage of Total Commercial and industrial $ 5,179,780 35.4 % $ 5,111,957 36.3 % Energy: Production 1,282,180 8.7 1,309,314 9.3 Service 189,350 1.3 168,775 1.2 Other 75,388 0.5 124,509 0.9 Total energy 1,546,918 10.5 1,602,598 11.4 Commercial real estate: Commercial mortgages 4,555,171 31.1 4,121,966 29.2 Construction 1,370,206 9.4 1,267,717 9.0 Land 290,706 2.0 306,755 2.2 Total commercial real estate 6,216,083 42.5 5,696,438 40.4 Consumer real estate: Home equity loans 363,224 2.5 353,924 2.5 Home equity lines of credit 350,461 2.4 337,168 2.4 Other 458,618 3.1 427,898 3.0 Total consumer real estate 1,172,303 8.0 1,118,990 7.9 Total real estate 7,388,386 50.5 6,815,428 48.3 Consumer and other 519,844 3.6 569,750 4.0 Total loans $ 14,634,928 100.0 % $ 14,099,733 100.0 %
Non-Accrual Loans, Segregated by Class of Loans	Non-accrual loans, segregated by class of loans, were as follows: September 30, December 31, Commercial and industrial $ 13,929 $ 9,239 Energy 70,883 46,932 Commercial real estate: Buildings, land and other 9,240 15,268 Construction 697 — Consumer real estate 527 892 Consumer and other 2,170 1,408 Total $ 97,446 $ 73,739
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, 2019 was as follows: Loans 30-89 Days Past Due Loans 90 or More Days Past Due Total Past Due Loans Current Loans Total Loans Accruing Loans 90 or More Days Past Due Commercial and industrial $ 39,540 $ 13,622 $ 53,162 $ 5,126,618 $ 5,179,780 $ 3,574 Energy 3,350 21,836 25,186 1,521,732 1,546,918 530 Commercial real estate: Buildings, land and other 29,147 8,279 37,426 4,808,451 4,845,877 6,513 Construction 1,412 1,471 2,883 1,367,323 1,370,206 774 Consumer real estate 8,305 2,443 10,748 1,161,555 1,172,303 2,150 Consumer and other 7,815 2,419 10,234 509,610 519,844 855 Total $ 89,569 $ 50,070 $ 139,639 $ 14,495,289 $ 14,634,928 $ 14,396
Impaired Loans	Impaired loans are set forth in the following table. No interest income was recognized on impaired loans subsequent to their classification as impaired. Unpaid Contractual Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance September 30, 2019 Commercial and industrial $ 15,020 $ 4,874 $ 7,280 $ 12,154 $ 5,785 Energy 88,531 4,777 65,800 70,577 19,722 Commercial real estate: Buildings, land and other 9,553 3,864 5,015 8,879 1,483 Construction 697 697 — 697 — Consumer real estate 293 293 — 293 — Consumer and other 2,171 — 2,170 2,170 2,170 Total $ 116,265 $ 14,505 $ 80,265 $ 94,770 $ 29,160 December 31, 2018 Commercial and industrial $ 9,094 $ 2,842 $ 4,287 $ 7,129 $ 2,558 Energy 67,900 6,817 39,890 46,707 9,671 Commercial real estate: Buildings, land and other 15,774 2,168 12,517 14,685 2,599 Construction — — — — — Consumer real estate 293 293 — 293 — Consumer and other 1,475 — 1,407 1,407 1,407 Total $ 94,536 $ 12,120 $ 58,101 $ 70,221 $ 16,235
Average Recorded Investment In Impaired Loans	The average recorded investment in impaired loans was as follows: Three Months Ended Nine Months Ended 2019 2018 2019 2018 Commercial and industrial $ 14,438 $ 13,447 $ 12,552 $ 21,025 Energy 55,195 78,772 50,642 82,640 Commercial real estate: Buildings, land and other 9,418 14,306 14,961 12,328 Construction 349 — 277 — Consumer real estate 293 671 541 807 Consumer and other 1,795 1,073 1,605 805 Total $ 81,488 $ 108,269 $ 80,578 $ 117,605
Troubled Debt Restructurings	Troubled debt restructurings during the nine months ended September 30, 2019 and September 30, 2018 are set forth in the following table. Nine Months Ended Nine Months Ended Balance at Restructure Balance at Period-End Balance at Restructure Balance at Period-End Commercial and industrial $ 3,845 $ 2,188 $ 2,203 $ — Energy — — 13,708 — Commercial real estate: Buildings, land and other 9,456 9,494 — — Consumer real estate 124 123 — — $ 13,425 $ 11,805 $ 15,911 $ —
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, 2019 and September 30, 2018 is set forth in the following table. September 30, 2019 September 30, 2018 Restructured loans past due in excess of 90 days at period-end: Number of loans 4 — Dollar amount of loans $ 3,244 $ — Restructured loans on non-accrual status at period end 5,645 — Charge-offs of restructured loans: Recognized in connection with restructuring 1,500 — Recognized on previously restructured loans — 4,650 Proceeds from sale of restructured loans — 15,750
Weighted Average Risk Grades for All Commercial Loans by Class	The following tables present weighted-average risk grades for all commercial loans by class. September 30, 2019 December 31, 2018 Weighted Loans Weighted Loans Commercial and industrial: Risk grades 1-8 6.19 $ 4,856,712 6.12 $ 4,862,275 Risk grade 9 9.00 165,195 9.00 112,431 Risk grade 10 10.00 75,276 10.00 58,328 Risk grade 11 11.00 68,668 11.00 69,684 Risk grade 12 12.00 8,144 12.00 6,681 Risk grade 13 13.00 5,785 13.00 2,558 Total 6.42 $ 5,179,780 6.30 $ 5,111,957 Energy Risk grades 1-8 5.85 $ 1,379,566 5.76 $ 1,451,673 Risk grade 9 9.00 80,170 9.00 35,565 Risk grade 10 10.00 1,105 10.00 43,001 Risk grade 11 11.00 15,194 11.00 25,427 Risk grade 12 12.00 51,161 12.00 37,261 Risk grade 13 13.00 19,722 13.00 9,671 Total 6.36 $ 1,546,918 6.22 $ 1,602,598 Commercial real estate: Buildings, land and other Risk grades 1-8 6.78 $ 4,528,963 6.76 $ 4,143,264 Risk grade 9 9.00 117,739 9.00 109,660 Risk grade 10 10.00 97,083 10.00 62,353 Risk grade 11 11.00 92,852 11.00 98,176 Risk grade 12 12.00 7,757 12.00 12,669 Risk grade 13 13.00 1,483 13.00 2,599 Total 6.99 $ 4,845,877 6.98 $ 4,428,721 Construction Risk grades 1-8 7.19 $ 1,314,076 7.13 $ 1,177,260 Risk grade 9 9.00 31,506 9.00 60,754 Risk grade 10 10.00 21,686 10.00 24,877 Risk grade 11 11.00 2,241 11.00 4,826 Risk grade 12 12.00 697 12.00 — Risk grade 13 13.00 — 13.00 — Total 7.28 $ 1,370,206 7.29 $ 1,267,717
Net (Charge-Offs)/Recoveries, Segregated by Class of Loans	Net (charge-offs)/recoveries, segregated by class of loans, were as follows: Three Months Ended Nine Months Ended 2019 2018 2019 2018 Commercial and industrial $ (1,520 ) $ (7,807 ) $ (5,912 ) $ (19,030 ) Energy (1,260 ) (5,347 ) (3,184 ) (10,272 ) Commercial real estate: Buildings, land and other 38 33 (466 ) (288 ) Construction 8 3 14 11 Consumer real estate (103 ) (388 ) (2,078 ) (1,078 ) Consumer and other (3,534 ) (1,792 ) (9,351 ) (4,975 ) Total $ (6,371 ) $ (15,298 ) $ (20,977 ) $ (35,632 )
Unallocated Portion of Allowance for Loan Losses	The following table presents details of the allowance for loan losses allocated to each portfolio segment as of September 30, 2019 and December 31, 2018 and detailed on the basis of the impairment evaluation methodology we used: Commercial and Industrial Energy Commercial Real Estate Consumer Real Estate Consumer and Other Total September 30, 2019 Historical valuation allowances $ 28,872 $ 7,422 $ 22,051 $ 2,641 $ 7,545 $ 68,531 Specific valuation allowances 5,785 19,722 1,483 — 2,170 29,160 General valuation allowances 10,866 4,120 4,349 1,172 (325 ) 20,182 Macroeconomic valuation allowances 7,144 2,082 7,470 1,071 919 18,686 Total $ 52,667 $ 33,346 $ 35,353 $ 4,884 $ 10,309 $ 136,559 Allocated to loans: Individually evaluated $ 5,785 $ 19,722 $ 1,483 $ — $ 2,170 $ 29,160 Collectively evaluated 46,882 13,624 33,870 4,884 8,139 107,399 Total $ 52,667 $ 33,346 $ 35,353 $ 4,884 $ 10,309 $ 136,559 December 31, 2018 Historical valuation allowances $ 25,351 $ 9,697 $ 20,817 $ 2,688 $ 6,845 $ 65,398 Specific valuation allowances 2,558 9,671 2,599 — 1,407 16,235 General valuation allowances 10,062 6,014 4,366 1,671 (13 ) 22,100 Macroeconomic valuation allowances 10,609 3,670 10,995 1,744 1,381 28,399 Total $ 48,580 $ 29,052 $ 38,777 $ 6,103 $ 9,620 $ 132,132 Allocated to loans: Individually evaluated $ 2,558 $ 9,671 $ 2,599 $ — $ 1,407 $ 16,235 Collectively evaluated 46,022 19,381 36,178 6,103 8,213 115,897 Total $ 48,580 $ 29,052 $ 38,777 $ 6,103 $ 9,620 $ 132,132
Investment in Loans Related to Allowance for Loan Losses by Portfolio Segment Disaggregated Based on Impairment Methodology	Our recorded investment in loans as of September 30, 2019 and December 31, 2018 related to each balance in the allowance for loan losses by portfolio segment and detailed on the basis of the impairment methodology we used was as follows: Commercial and Industrial Energy Commercial Consumer Consumer Total September 30, 2019 Individually evaluated $ 12,154 $ 70,577 $ 9,576 $ 293 $ 2,170 $ 94,770 Collectively evaluated 5,167,626 1,476,341 6,206,507 1,172,010 517,674 14,540,158 Total $ 5,179,780 $ 1,546,918 $ 6,216,083 $ 1,172,303 $ 519,844 $ 14,634,928 December 31, 2018 Individually evaluated $ 7,129 $ 46,707 $ 14,685 $ 293 $ 1,407 $ 70,221 Collectively evaluated 5,104,828 1,555,891 5,681,753 1,118,697 568,343 14,029,512 Total $ 5,111,957 $ 1,602,598 $ 5,696,438 $ 1,118,990 $ 569,750 $ 14,099,733
Activity in Allowance for Loan Losses by Portfolio Segment	The following table details activity in the allowance for loan losses by portfolio segment for the three and nine months ended September 30, 2019 and 2018 . Allocation of a portion of the allowance to one category of loans does not preclude its availability to absorb losses in other categories. Commercial and Industrial Energy Commercial Real Estate Consumer Real Estate Consumer and Other Total Three months ended: September 30, 2019 Beginning balance $ 57,714 $ 25,818 $ 35,914 $ 5,637 $ 9,846 $ 134,929 Provision for loan losses (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 September 30, 2018 Beginning balance $ 57,713 $ 37,313 $ 38,918 $ 6,336 $ 9,946 $ 150,226 Provision for loan losses 1,085 (521 ) 250 246 1,590 2,650 Charge-offs (8,491 ) (5,400 ) — (431 ) (4,274 ) (18,596 ) Recoveries 684 53 36 43 2,482 3,298 Net charge-offs (7,807 ) (5,347 ) 36 (388 ) (1,792 ) (15,298 ) Ending balance $ 50,991 $ 31,445 $ 39,204 $ 6,194 $ 9,744 $ 137,578 Nine months ended: September 30, 2019 Beginning balance $ 48,580 $ 29,052 $ 38,777 $ 6,103 $ 9,620 $ 132,132 Provision for loan losses 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 September 30, 2018 Beginning balance $ 59,614 $ 51,528 $ 30,948 $ 5,657 $ 7,617 $ 155,364 Provision for loan losses 10,407 (9,811 ) 8,533 1,615 7,102 17,846 Charge-offs (21,896 ) (10,939 ) (619 ) (1,632 ) (12,240 ) (47,326 ) Recoveries 2,866 667 342 554 7,265 11,694 Net charge-offs (19,030 ) (10,272 ) (277 ) (1,078 ) (4,975 ) (35,632 ) Ending balance $ 50,991 $ 31,445 $ 39,204 $ 6,194 $ 9,744 $ 137,578

Goodwill and Other Intangible_2

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	<div style="font-family:Times New Roman;font-size:10pt;"><div style="line-height:120%;text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">Goodwill is presented in the table below.</font></div><div style="line-height:120%;text-align:center;font-size:10pt;"><div style="padding-left:0px;text-indent:0px;line-height:normal;padding-top:10px;"><table cellpadding="0" cellspacing="0" style="font-family:Times New Roman;font-size:10pt;margin-left:auto;margin-right:auto;width:100%;border-collapse:collapse;text-align:left;"><tr><td colspan="8" rowspan="1"></td></tr><tr><td style="width:63%;" rowspan="1" colspan="1"></td><td style="width:1%;" rowspan="1" colspan="1"></td><td style="width:16%;" rowspan="1" colspan="1"></td><td style="width:1%;" rowspan="1" colspan="1"></td><td style="width:1%;" rowspan="1" colspan="1"></td><td style="width:1%;" rowspan="1" colspan="1"></td><td style="width:16%;" rowspan="1" colspan="1"></td><td style="width:1%;" rowspan="1" colspan="1"></td></tr><tr><td style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="overflow:hidden;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"> </font></div></td><td colspan="3" style="vertical-align:bottom;border-bottom:1px solid #000000;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1"><div style="text-align:center;font-size:8pt;"><font style="font-family:inherit;font-size:8pt;font-weight:bold;">September 30, 2019</font></div></td><td style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="overflow:hidden;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"> </font></div></td><td colspan="3" style="vertical-align:bottom;border-bottom:1px solid #000000;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1"><div style="text-align:center;font-size:8pt;"><font style="font-family:inherit;font-size:8pt;font-weight:bold;">December 31, 2018</font></div></td></tr><tr><td style="vertical-align:top;background-color:#cceeff;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">Goodwill</font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;padding-left:2px;padding-top:2px;padding-bottom:2px;background-color:#cceeff;border-top:1px solid #000000;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">$</font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;background-color:#cceeff;padding-top:2px;padding-bottom:2px;border-top:1px solid #000000;" rowspan="1" colspan="1"><div style="text-align:right;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">654,952</font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;background-color:#cceeff;border-top:1px solid #000000;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"><br clear="none"/></font></div></td><td style="vertical-align:bottom;background-color:#cceeff;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="overflow:hidden;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"> </font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;padding-left:2px;padding-top:2px;padding-bottom:2px;background-color:#cceeff;border-top:1px solid #000000;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">$</font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;background-color:#cceeff;padding-top:2px;padding-bottom:2px;border-top:1px solid #000000;" rowspan="1" colspan="1"><div style="text-align:right;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">654,952</font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;background-color:#cceeff;border-top:1px solid #000000;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"><br clear="none"/></font></div></td></tr></table></div></div></div>
Schedule of Other Intangible Assets	<div style="font-family:Times New Roman;font-size:10pt;"><div style="line-height:120%;text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">Other intangible assets are presented in the table below.</font></div><div style="line-height:120%;text-align:center;font-size:10pt;"><div style="padding-left:0px;text-indent:0px;line-height:normal;padding-top:10px;"><table cellpadding="0" cellspacing="0" style="font-family:Times New Roman;font-size:10pt;margin-left:auto;margin-right:auto;width:100%;border-collapse:collapse;text-align:left;"><tr><td colspan="8" rowspan="1"></td></tr><tr><td style="width:63%;" rowspan="1" colspan="1"></td><td style="width:1%;" rowspan="1" colspan="1"></td><td style="width:16%;" rowspan="1" colspan="1"></td><td style="width:1%;" rowspan="1" colspan="1"></td><td style="width:1%;" rowspan="1" colspan="1"></td><td style="width:1%;" rowspan="1" colspan="1"></td><td style="width:16%;" rowspan="1" colspan="1"></td><td style="width:1%;" rowspan="1" colspan="1"></td></tr><tr><td style="vertical-align:top;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">Other intangible assets:</font></div></td><td colspan="3" style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1"><div style="overflow:hidden;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"> </font></div></td><td style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="overflow:hidden;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"> </font></div></td><td colspan="3" style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1"><div style="overflow:hidden;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"> </font></div></td></tr><tr><td style="vertical-align:top;background-color:#cceeff;padding-left:28px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">Core deposits</font></div></td><td style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;background-color:#cceeff;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">$</font></div></td><td style="vertical-align:bottom;background-color:#cceeff;padding-top:2px;padding-bottom:2px;" rowspan="1" colspan="1"><div style="text-align:right;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">2,254</font></div></td><td style="vertical-align:bottom;background-color:#cceeff;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"><br clear="none"/></font></div></td><td style="vertical-align:bottom;background-color:#cceeff;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="overflow:hidden;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"> </font></div></td><td style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;background-color:#cceeff;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">$</font></div></td><td style="vertical-align:bottom;background-color:#cceeff;padding-top:2px;padding-bottom:2px;" rowspan="1" colspan="1"><div style="text-align:right;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">2,959</font></div></td><td style="vertical-align:bottom;background-color:#cceeff;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"><br clear="none"/></font></div></td></tr><tr><td style="vertical-align:top;padding-left:28px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">Customer relationship</font></div></td><td colspan="2" style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;" rowspan="1"><div style="text-align:right;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">491</font></div></td><td style="vertical-align:bottom;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"><br clear="none"/></font></div></td><td style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="overflow:hidden;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"> </font></div></td><td colspan="2" style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;" rowspan="1"><div style="text-align:right;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">672</font></div></td><td style="vertical-align:bottom;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"><br clear="none"/></font></div></td></tr><tr><td style="vertical-align:top;background-color:#cceeff;padding-left:28px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">Non-compete agreements</font></div></td><td colspan="2" style="vertical-align:bottom;border-bottom:1px solid #000000;background-color:#cceeff;padding-left:2px;padding-top:2px;padding-bottom:2px;" rowspan="1"><div style="text-align:right;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">—</font></div></td><td style="vertical-align:bottom;border-bottom:1px solid #000000;background-color:#cceeff;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"><br clear="none"/></font></div></td><td style="vertical-align:bottom;background-color:#cceeff;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="overflow:hidden;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"> </font></div></td><td colspan="2" style="vertical-align:bottom;border-bottom:1px solid #000000;background-color:#cceeff;padding-left:2px;padding-top:2px;padding-bottom:2px;" rowspan="1"><div style="text-align:right;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">18</font></div></td><td style="vertical-align:bottom;border-bottom:1px solid #000000;background-color:#cceeff;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"><br clear="none"/></font></div></td></tr><tr><td style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="overflow:hidden;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"> </font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;padding-left:2px;padding-top:2px;padding-bottom:2px;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">$</font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;padding-top:2px;padding-bottom:2px;" rowspan="1" colspan="1"><div style="text-align:right;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">2,745</font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"><br clear="none"/></font></div></td><td style="vertical-align:bottom;padding-left:2px;padding-top:2px;padding-bottom:2px;padding-right:2px;" rowspan="1" colspan="1"><div style="overflow:hidden;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"> </font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;padding-left:2px;padding-top:2px;padding-bottom:2px;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">$</font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;padding-top:2px;padding-bottom:2px;" rowspan="1" colspan="1"><div style="text-align:right;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">3,649</font></div></td><td style="vertical-align:bottom;border-bottom:3px double #000000;" rowspan="1" colspan="1"><div style="text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;"><br clear="none"/></font></div></td></tr></table></div></div></div>
Estimated Aggregate Future Amortization Expense for Intangible Assets	The estimated aggregate future amortization expense for intangible assets remaining as of September 30, 2019 is as follows: Remainder of 2019 $ 263 2020 919 2021 697 2022 481 2023 283 Thereafter 102 $ 2,745

Deposits (Tables)

Deposits (Tables)	9 Months Ended
	Sep. 30, 2019
Banking and Thrift [Abstract]
Schedule of Deposits	Deposits were as follows: September 30, Percentage of Total December 31, Percentage of Total Non-interest-bearing demand deposits: Commercial and individual $ 10,070,621 37.2 % $ 10,305,850 37.9 % Correspondent banks 258,981 1.0 235,748 0.9 Public funds 420,642 1.5 455,896 1.7 Total non-interest-bearing demand deposits 10,750,244 39.7 10,997,494 40.5 Interest-bearing deposits: Private accounts: Savings and interest checking 6,741,340 24.9 6,977,813 25.7 Money market accounts 7,960,223 29.4 7,777,470 28.6 Time accounts of $100,000 or more 707,657 2.6 526,789 2.0 Time accounts under $100,000 346,440 1.3 331,511 1.2 Total private accounts 15,755,660 58.2 15,613,583 57.5 Public funds: Savings and interest checking 483,303 1.8 473,754 1.8 Money market accounts 68,321 0.2 59,953 0.2 Time accounts of $100,000 or more 25,922 0.1 4,332 — Time accounts under $100,000 132 — 88 — Total public funds 577,678 2.1 538,127 2.0 Total interest-bearing deposits 16,333,338 60.3 16,151,710 59.5 Total deposits $ 27,083,582 100.0 % $ 27,149,204 100.0 %
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 $ 358 $ — Deposits from foreign sources (primarily Mexico) 776,490 752,658 Deposits not covered by deposit insurance 12,681,664 13,111,210

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2019
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 $ 9,036,441 $ 8,369,721 Standby letters of credit 283,821 271,575 Deferred standby letter of credit fees 1,547 2,069

Capital and Regulatory Matters

Capital and Regulatory Matters (Tables)	9 Months Ended
	Sep. 30, 2019
Banking and Thrift [Abstract]
Actual and Required Capital Ratios	The following tables present actual and required capital ratios as of September 30, 2019 and December 31, 2018 for Cullen/Frost and Frost Bank under the Basel III Capital Rules. The Basel III Capital Rules became fully phased-in on January 1, 2019. The minimum required capital amounts presented as of December 31, 2018 include the minimum required capital levels applicable as of that date as well as the minimum required capital levels as of January 1, 2019 when the Basel III Capital Rules became fully phased-in. 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 2018 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, 2018 have been revised to reflect these reclassifications. Actual Minimum Capital Required - Basel III Required to be Capital Ratio Capital Ratio Capital Ratio September 30, 2019 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,788,313 12.35 % $ 1,580,199 7.00 % $ 1,467,328 6.50 % Frost Bank 2,910,446 12.92 1,576,393 7.00 1,463,794 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 2,932,799 12.99 1,918,813 8.50 1,805,942 8.00 Frost Bank 2,910,446 12.92 1,914,192 8.50 1,801,593 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,302,858 14.63 2,370,298 10.50 2,257,427 10.00 Frost Bank 3,047,505 13.53 2,364,590 10.50 2,251,991 10.00 Leverage Ratio Cullen/Frost 2,932,798 9.36 1,252,781 4.00 1,565,976 5.00 Frost Bank 2,910,446 9.30 1,251,713 4.00 1,564,642 5.00 Actual Minimum Capital Required - Basel III Phase-In Schedule Minimum Capital Required - Basel III Fully Phased-In Required to be Capital Ratio Capital Ratio Capital Ratio Capital Ratio December 31, 2018 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,642,475 12.27 % $ 1,372,573 6.375 % $ 1,507,139 7.00 % $ 1,358,171 6.50 % Frost Bank 2,743,973 12.78 1,368,701 6.375 1,502,887 7.00 1,354,222 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 2,786,961 12.94 1,695,532 7.875 1,830,098 8.50 1,671,595 8.00 Frost Bank 2,743,973 12.78 1,690,748 7.875 1,824,934 8.50 1,666,735 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,152,593 14.64 2,126,143 9.875 2,260,709 10.50 2,089,494 10.00 Frost Bank 2,876,605 13.40 2,120,144 9.875 2,254,331 10.50 2,083,419 10.00 Leverage Ratio Cullen/Frost 2,786,961 9.06 1,231,028 4.00 1,231,028 4.00 1,538,785 5.00 Frost Bank 2,743,973 8.93 1,229,650 4.00 1,229,650 4.00 1,537,062 5.00

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2019
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, 2019 and December 31, 2018 . September 30, 2019 December 31, 2018 Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Derivatives designated as hedges of fair value: Financial institution counterparties: Loan/lease interest rate swaps – assets $ 5,583 $ 15 $ 10,941 $ 207 Loan/lease interest rate swaps – liabilities 6,356 (179 ) 3,885 (199 ) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets 94,855 65 496,887 2,384 Loan/lease interest rate swaps – liabilities 1,008,173 (24,268 ) 691,143 (8,921 ) Loan/lease interest rate caps – assets 102,634 233 122,791 509 Customer counterparties: Loan/lease interest rate swaps – assets 1,008,173 55,529 691,143 16,706 Loan/lease interest rate swaps – liabilities 94,855 (175 ) 496,887 (8,891 ) Loan/lease interest rate caps – liabilities 102,634 (233 ) 122,791 (509 )
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, 2019 were as follows: Weighted-Average Interest Rate Paid Interest Rate Received Interest rate swaps: Fair value hedge loan/lease interest rate swaps 2.33 % 2.05 % Non-hedging interest rate swaps – financial institution counterparties 4.17 3.71 Non-hedging interest rate swaps – customer counterparties 3.71 4.17
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, 2019 December 31, 2018 Notional Units Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Oil – assets Barrels 2,280 $ 12,884 2,416 $ 24,332 Oil – liabilities Barrels 727 (1,363 ) 415 (646 ) Natural gas – assets MMBTUs 8,716 2,053 5,745 417 Natural gas – liabilities MMBTUs 3,439 (191 ) 9,314 (1,272 ) Customer counterparties: Oil – assets Barrels 730 1,392 415 646 Oil – liabilities Barrels 2,277 (12,572 ) 2,416 (24,009 ) Natural gas – assets MMBTUs 3,543 205 10,236 1,373 Natural gas – liabilities MMBTUs 8,613 (1,960 ) 4,823 (393 )
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, 2019 December 31, 2018 Notional Currency Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Forward contracts – liabilities CAD 5,283 $ (11 ) 11,003 $ (13 ) Forward contracts – liabilities GBP — — 142 (2 ) Forward contracts – liabilities MXN — — 3,015 (132 ) Customer counterparties: Forward contracts – assets CAD 5,271 24 10,979 40 Forward contracts – assets GBP — — 145 4 Forward contracts – assets MXN — — 3,000 149
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 2019 2018 2019 2018 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ 24 $ 12 $ 78 $ 1 Amount of (gain) loss included in other non-interest expense (1 ) — — (1 )
Schedule of Amounts Related to Non-Hedging Interest Rate and Commodity Derivatives	Amounts included in the consolidated statements of income related to non-hedging interest rate, commodity and foreign currency derivative instruments are presented in the table below. Three Months Ended Nine Months Ended 2019 2018 2019 2018 Non-hedging interest rate derivatives: Other non-interest income $ 215 $ 699 $ 1,188 $ 2,889 Other non-interest expense — 2 — (2 ) Non-hedging commodity derivatives: Other non-interest income 109 456 322 492 Non-hedging foreign currency derivatives: Other non-interest income 12 53 41 203

Balance Sheet Offsetting (Table

Balance Sheet Offsetting (Tables)	9 Months Ended
	Sep. 30, 2019
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, 2019 is presented in the following tables. Gross Amount Recognized Gross Amount Offset Net Amount Recognized September 30, 2019 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 313 $ — $ 313 Commodity swaps and options 14,937 — 14,937 Foreign currency forward contracts — — — Total derivatives 15,250 — 15,250 Resell agreements 11,299 — 11,299 Total $ 26,549 $ — $ 26,549 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 24,447 $ — $ 24,447 Commodity swaps and options 1,554 — 1,554 Foreign currency forward contracts 11 — 11 Total derivatives 26,012 — 26,012 Repurchase agreements 1,187,355 — 1,187,355 Total $ 1,213,367 $ — $ 1,213,367 Information about financial instruments that are eligible for offset in the consolidated balance sheet as of December 31, 2018 is presented in the following tables. Gross Amount Recognized Gross Amount Offset Net Amount Recognized December 31, 2018 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 3,100 $ — $ 3,100 Commodity swaps and options 24,749 — 24,749 Foreign currency forward contracts — — — Total derivatives 27,849 — 27,849 Resell agreements 11,642 — 11,642 Total $ 39,491 $ — $ 39,491 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 9,120 $ — $ 9,120 Commodity swaps and options 1,918 — 1,918 Foreign currency forward contracts 147 — 147 Total derivatives 11,185 — 11,185 Repurchase agreements 1,360,298 — 1,360,298 Total $ 1,371,483 $ — $ 1,371,483
Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset	Gross Amounts Not Offset Net Amount Recognized Financial Instruments Collateral Net Amount December 31, 2018 Financial assets: Derivatives: Counterparty A $ 598 $ (598 ) $ — $ — Counterparty B 7,255 (3,380 ) (3,875 ) — Counterparty C 81 (81 ) — — Other counterparties 19,915 (2,084 ) (17,776 ) 55 Total derivatives 27,849 (6,143 ) (21,651 ) 55 Resell agreements 11,642 — (11,642 ) — Total $ 39,491 $ (6,143 ) $ (33,293 ) $ 55 Financial liabilities: Derivatives: Counterparty A $ 4,293 $ (598 ) $ (3,651 ) $ 44 Counterparty B 3,380 (3,380 ) — — Counterparty C 326 (81 ) (245 ) — Other counterparties 3,186 (2,084 ) (725 ) 377 Total derivatives 11,185 (6,143 ) (4,621 ) 421 Repurchase agreements 1,360,298 — (1,360,298 ) — Total $ 1,371,483 $ (6,143 ) $ (1,364,919 ) $ 421 Gross Amounts Not Offset Net Amount Recognized Financial Instruments Collateral Net Amount September 30, 2019 Financial assets: Derivatives: Counterparty A $ 38 $ (38 ) $ — $ — Counterparty B 4,344 (4,344 ) — — Counterparty C 8 (8 ) — — Other counterparties 10,860 (5,503 ) (2,880 ) 2,477 Total derivatives 15,250 (9,893 ) (2,880 ) 2,477 Resell agreements 11,299 — (11,299 ) — Total $ 26,549 $ (9,893 ) $ (14,179 ) $ 2,477 Financial liabilities: Derivatives: Counterparty A $ 6,192 $ (38 ) $ (6,154 ) $ — Counterparty B 7,810 (4,344 ) (3,466 ) — Counterparty C 164 (8 ) (156 ) — Other counterparties 11,846 (5,503 ) (6,259 ) 84 Total derivatives 26,012 (9,893 ) (16,035 ) 84 Repurchase agreements 1,187,355 — (1,187,355 ) — Total $ 1,213,367 $ (9,893 ) $ (1,203,390 ) $ 84
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, 2019 and December 31, 2018 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, 2019 Repurchase agreements: U.S. Treasury $ 162,566 $ 2,550 $ — $ — $ 165,116 Residential mortgage-backed securities 1,022,239 — — — 1,022,239 Total borrowings $ 1,184,805 $ 2,550 $ — $ — $ 1,187,355 Gross amount of recognized liabilities for repurchase agreements $ 1,187,355 Amounts related to agreements not included in offsetting disclosures above $ — December 31, 2018 Repurchase agreements: U.S. Treasury $ 1,334,063 $ — $ — $ — $ 1,334,063 Residential mortgage-backed securities 26,235 — — — 26,235 Total borrowings $ 1,360,298 $ — $ — $ — $ 1,360,298 Gross amount of recognized liabilities for repurchase agreements $ 1,360,298 Amounts related to agreements not included in offsetting disclosures above $ —

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [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, 2019 , there were 1,277,578 shares remaining available for grant for future stock-based compensation awards. Director Deferred Stock Units Outstanding Non-Vested Stock Awards/Stock Units Outstanding Performance Stock Units Outstanding Stock Options Outstanding Number of Units Weighted- Average Fair Value at Grant Number of Shares/Units Weighted- Average Fair Value at Grant Number of Units Weighted- Average Fair Value at Grant Number of Shares Weighted- Average Exercise Price Balance, January 1, 2019 48,910 $ 71.14 383,797 $ 85.59 125,809 $ 82.55 2,352,008 $ 63.55 Authorized — — — — — — — — Granted 7,592 102.70 1,957 91.96 — — — — Exercised/vested (1,132 ) 106.03 (17,800 ) 65.11 — — (236,252 ) 57.17 Forfeited/expired — — (11,850 ) 89.29 — — (11,000 ) 65.11 Balance, September 30, 2019 55,370 $ 74.76 356,104 $ 86.53 125,809 $ 82.55 2,104,756 $ 64.26
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 2019 2018 2019 2018 New shares issued from available authorized shares — — — — Issued from available treasury stock 71,284 19,232 225,184 447,831 Total 71,284 19,232 225,184 447,831 Proceeds from stock option exercises $ 5,665 $ 1,119 $ 13,506 $ 26,567
Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block]	Stock-based compensation expense and the related income tax benefit is presented in the following table. Three Months Ended Nine Months Ended 2019 2018 2019 2018 Stock options $ 330 $ 960 $ 1,091 $ 3,064 Non-vested stock awards/stock units 1,536 1,357 5,727 4,196 Director deferred stock units — — 780 720 Performance stock units 809 479 3,095 1,576 Total $ 2,675 $ 2,796 $ 10,693 $ 9,556 Income tax benefit $ 452 $ 587 $ 1,768 $ 2,007
Unrecognized Stock-Based Compensation Expense	Unrecognized stock-based compensation expense at September 30, 2019 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. Stock options $ 96 Non-vested stock awards/stock units 11,196 Performance stock units 3,432 Total $ 14,724

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	9 Months Ended
	Sep. 30, 2019
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 2019 2018 2019 2018 Net income $ 111,836 $ 117,844 $ 339,918 $ 335,665 Less: Preferred stock dividends 2,016 2,016 6,047 6,047 Net income available to common shareholders 109,820 115,828 333,871 329,618 Less: Earnings allocated to participating securities 876 750 2,761 2,178 Net earnings allocated to common stock $ 108,944 $ 115,078 $ 331,110 $ 327,440 Distributed earnings allocated to common stock $ 44,370 $ 42,811 $ 131,069 $ 121,907 Undistributed earnings allocated to common stock 64,574 72,267 200,041 205,533 Net earnings allocated to common stock $ 108,944 $ 115,078 $ 331,110 $ 327,440 Weighted-average shares outstanding for basic earnings per common share 62,566,128 63,892,023 62,786,501 63,793,514 Dilutive effect of stock compensation 592,866 1,022,168 725,911 1,037,043 Weighted-average shares outstanding for diluted earnings per common share 63,158,994 64,914,191 63,512,412 64,830,557

Defined Benefit Plans (Tables)

Defined Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2019
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 2019 2018 2019 2018 Expected return on plan assets, net of expenses $ (2,693 ) $ (2,979 ) $ (8,079 ) $ (8,937 ) Interest cost on projected benefit obligation 1,618 1,475 4,854 4,424 Net amortization and deferral 1,406 1,250 4,218 3,751 Net periodic expense (benefit) $ 331 $ (254 ) $ 993 $ (762 )

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income Tax Expense	Income tax expense was as follows: Three Months Ended Nine Months Ended 2019 2018 2019 2018 Current income tax expense (benefit) $ 14,480 $ (1,152 ) $ 43,984 $ 955 Deferred income tax expense (benefit) (950 ) 16,312 (1,625 ) 39,198 Income tax expense, as reported $ 13,530 $ 15,160 $ 42,359 $ 40,153 Effective tax rate 10.8 % 11.4 % 11.1 % 10.7 %

Other Comprehensive Income (L_2

Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Component of 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 Amount Tax Expense, (Benefit) Net of Tax Amount Before Tax Amount Tax Expense, (Benefit) Net of Tax Amount Securities available for sale and transferred securities: Change in net unrealized gain/loss during the period $ 107,786 $ 22,635 $ 85,151 $ (96,147 ) $ (20,190 ) $ (75,957 ) Change in net unrealized gain on securities transferred to held to maturity (305 ) (64 ) (241 ) (3,764 ) (790 ) (2,974 ) Reclassification adjustment for net (gains) losses included in net income (97 ) (21 ) (76 ) 34 7 27 Total securities available for sale and transferred securities 107,384 22,550 84,834 (99,877 ) (20,973 ) (78,904 ) 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) 1,406 295 1,111 1,250 262 988 Total defined-benefit post-retirement benefit plans 1,406 295 1,111 1,250 262 988 Total other comprehensive income (loss) $ 108,790 $ 22,845 $ 85,945 $ (98,627 ) $ (20,711 ) $ (77,916 ) Nine Months Ended Nine Months Ended Before Tax Amount Tax Expense, (Benefit) Net of Tax Amount Before Tax Amount Tax Expense, (Benefit) Net of Tax Amount Securities available for sale and transferred securities: Change in net unrealized gain/loss during the period $ 464,072 $ 97,455 $ 366,617 $ (286,935 ) $ (60,256 ) $ (226,679 ) Change in net unrealized gain on securities transferred to held to maturity (957 ) (201 ) (756 ) (8,424 ) (1,769 ) (6,655 ) Reclassification adjustment for net (gains) losses included in net income (266 ) (56 ) (210 ) 113 24 89 Total securities available for sale and transferred securities 462,849 97,198 365,651 (295,246 ) (62,001 ) (233,245 ) Defined-benefit post-retirement benefit plans: Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) 4,218 886 3,332 3,751 788 2,963 Total defined-benefit post-retirement benefit plans 4,218 886 3,332 3,751 788 2,963 Total other comprehensive income (loss) $ 467,067 $ 98,084 $ 368,983 $ (291,495 ) $ (61,213 ) $ (230,282 )
Schedule of Accumulated Other Comprehensive Income, Net of Tax	Activity in accumulated other comprehensive income (loss), net of tax, was as follows: Securities Available For Sale Defined Benefit Plans Accumulated Other Comprehensive Income 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 Balance January 1, 2018 $ 117,230 $ (37,718 ) $ 79,512 Other comprehensive income (loss) before reclassifications (233,334 ) — (233,334 ) Reclassification of amounts included in net income 89 2,963 3,052 Net other comprehensive income (loss) during period (233,245 ) 2,963 (230,282 ) Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings 17,557 (8,022 ) 9,535 Balance at September 30, 2018 $ (98,458 ) $ (42,777 ) $ (141,235 )

Operating Segments (Tables)

Operating Segments (Tables)	9 Months Ended
	Sep. 30, 2019
Segment Reporting [Abstract]
Summary of Operating Results by Segment	Summarized operating results by segment were as follows: Banking Frost Wealth Advisors Non-Banks Consolidated Revenues from (expenses to) external customers: Three months ended: September 30, 2019 $ 307,312 $ 37,549 $ (2,630 ) $ 342,231 September 30, 2018 295,062 36,868 (2,608 ) 329,322 Nine months ended: September 30, 2019 $ 918,672 $ 111,064 $ (8,182 ) $ 1,021,554 September 30, 2018 874,056 106,475 (7,680 ) 972,851 Net income (loss): Three months ended: September 30, 2019 $ 110,583 $ 4,106 $ (2,853 ) $ 111,836 September 30, 2018 114,214 6,442 (2,812 ) 117,844 Nine months ended: September 30, 2019 $ 334,393 $ 15,403 $ (9,878 ) $ 339,918 September 30, 2018 327,131 17,977 (9,443 ) 335,665

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis	The table below summarizes financial assets and financial liabilities measured at fair value on a recurring basis as of September 30, 2019 and December 31, 2018 , segregated by the level of the valuation inputs within the fair value hierarchy of ASC Topic 820 utilized to measure fair value. Level 1 Level 2 Level 3 Total Fair September 30, 2019 Securities available for sale: U.S. Treasury $ 2,825,796 $ — $ — $ 2,825,796 Residential mortgage-backed securities — 2,240,452 — 2,240,452 States and political subdivisions — 7,311,373 — 7,311,373 Other — 42,825 — 42,825 Trading account securities: U.S. Treasury 23,747 — — 23,747 States and political subdivisions — 3,922 — 3,922 Derivative assets: Interest rate swaps, caps and floors — 55,842 — 55,842 Commodity swaps and options — 16,534 — 16,534 Foreign currency forward contracts 24 — — 24 Derivative liabilities: Interest rate swaps, caps and floors — 24,855 — 24,855 Commodity swaps and options — 16,086 — 16,086 Foreign currency forward contracts 11 — — 11 December 31, 2018 Securities available for sale: U.S. Treasury $ 3,427,689 $ — $ — $ 3,427,689 Residential mortgage-backed securities — 829,740 — 829,740 States and political subdivisions — 7,087,202 — 7,087,202 Other — 42,690 — 42,690 Trading account securities: U.S. Treasury 21,928 — — 21,928 States and political subdivisions — 2,158 — 2,158 Derivative assets: Interest rate swaps, caps and floors — 19,806 — 19,806 Commodity swaps and options — 26,768 — 26,768 Foreign currency forward contracts 193 — — 193 Derivative liabilities: Interest rate swaps, caps and floors — 18,520 — 18,520 Commodity swaps and options — 26,320 — 26,320 Foreign currency forward contracts 147 — — 147
Impaired Loans Remeasured And Reported At Fair Value Specific Valuation Allowance Allocation Method Of Underlying Collateral [Table Text Block]	The following table presents impaired loans that were remeasured and reported at fair value through a specific valuation allowance allocation of the allowance for loan losses 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 of impaired loans before allocations $ 2,416 $ 60,010 $ 12,876 $ 28,638 Specific valuation allowance (allocations) reversals of prior allocations 1,116 (12,437 ) (2,599 ) 4,256 Fair value $ 3,532 $ 47,573 $ 10,277 $ 32,894
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 2019 2018 Foreclosed assets remeasured at initial recognition: Carrying value of foreclosed assets prior to remeasurement $ 1,302 $ 2,898 Charge-offs recognized in the allowance for loan losses (76 ) — Fair value $ 1,226 $ 2,898 Foreclosed assets remeasured subsequent to initial recognition: Carrying value of foreclosed assets prior to remeasurement $ — $ 1,823 Write-downs included in other non-interest expense — (473 ) Fair value $ — $ 1,350
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, 2019 December 31, 2018 Carrying Amount Estimated Fair Value Carrying Amount Estimated Fair Value Financial assets: Level 2 inputs: Cash and cash equivalents $ 2,907,901 $ 2,907,901 $ 3,955,779 $ 3,955,779 Securities held to maturity 1,020,838 1,047,423 1,106,057 1,116,953 Cash surrender value of life insurance policies 186,217 186,217 183,473 183,473 Accrued interest receivable 133,950 133,950 188,989 188,989 Level 3 inputs: Loans, net 14,498,369 14,542,918 13,967,601 13,933,239 Financial liabilities: Level 2 inputs: Deposits 27,083,582 27,084,765 27,149,204 27,143,572 Federal funds purchased and repurchase agreements 1,200,655 1,200,655 1,367,548 1,367,548 Junior subordinated deferrable interest debentures 136,285 137,115 136,242 137,115 Subordinated notes payable and other borrowings 98,826 106,447 98,708 98,458 Accrued interest payable 10,177 10,177 7,394 7,394

Significant Accounting Polici_4

Significant Accounting Policies - Additional Cash Flow Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Accounting Policies [Abstract]
Cash paid for interest	$ 100,041	$ 59,764
Cash paid for income taxes	42,352	5,112
Unsettled purchases/sales of securities	82,064	74,191
Loans foreclosed and transferred to other real estate owned and foreclosed assets	1,302	2,898
Loans to facilitate the sale of other real estate owned	847	0
Right-of-use lease assets obtained in exchange for lessee operating lease liabilities	$ 315,542	$ 0

Significant Accounting Polici_5

Significant Accounting Policies Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2019	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Right-of-Use Asset			$ 299,500	$ 170,500
Operating Lease, Liability			$ 325,300	$ 174,400
Recognition of right of use asset related to the new corporate headquarters facility	$ 121,700
Recognition of operating lease liability related to the new corporate headquarters facility	$ 121,700
Cumulative effect of new accounting principle in period of adoption					$ (14,672)	$ (2,285)
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption					(14,672)	$ (2,285)
Previously reported cumulative effect reduction to retained earnings					(12,600)
Retroactive adjustment to cumulative effect for new accounting principle					$ (2,100)
Two Thousand And Nineteen [Member] | Scenario, Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Additional premium amortization expense on securities		$ 5,200

Securities - Additional Informa

Securities - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2012
Schedule Of Marketable Securities [Line Items]
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas	99.70%
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund	69.30%
Carry Value of Securities Pledged	$ 3,600,000	$ 3,800,000
Available for Sale, Estimated Fair Value	12,420,446	$ 11,387,321
Available for sale securities reclassified to held to maturity, unrealized gain			$ 165,700
Available for sale securities reclassified to held to maturity, unrealized gain net of tax			107,700
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income Pre Tax	1,800
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	$ 1,400
Reclassified To Held To Maturity [Member]
Schedule Of Marketable Securities [Line Items]
Available for Sale, Estimated Fair Value			$ 2,300,000

Securities - Amortized Cost and

Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 1,020,838	$ 1,106,057
Held-to-maturity Securities, Gross Unrealized Gains	26,588	11,533
Held-to-maturity Securities, Gross Unrealized Losses	3	637
Held to Maturity, Estimated Fair Value	1,047,423	1,116,953
Available for Sale, Amortized Cost	11,979,766	11,410,447
Available for Sale Securities, Gross Unrealized Gains	443,604	85,611
Available for Sale Securities, Gross Unrealized Losses	2,924	108,737
Available for Sale, Estimated Fair Value	12,420,446	11,387,321
U.S. Treasury [Member]
Schedule of Investments [Line Items]
Available for Sale, Amortized Cost	2,806,802	3,455,417
Available for Sale Securities, Gross Unrealized Gains	20,725	1,772
Available for Sale Securities, Gross Unrealized Losses	1,731	29,500
Available for Sale, Estimated Fair Value	2,825,796	3,427,689
Residential mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	2,472	2,737
Held-to-maturity Securities, Gross Unrealized Gains	25	8
Held-to-maturity Securities, Gross Unrealized Losses	2	85
Held to Maturity, Estimated Fair Value	2,495	2,660
Available for Sale, Amortized Cost	2,198,466	823,208
Available for Sale Securities, Gross Unrealized Gains	42,766	13,079
Available for Sale Securities, Gross Unrealized Losses	780	6,547
Available for Sale, Estimated Fair Value	2,240,452	829,740
States and political subdivisions [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,016,866	1,101,820
Held-to-maturity Securities, Gross Unrealized Gains	26,563	11,525
Held-to-maturity Securities, Gross Unrealized Losses	0	552
Held to Maturity, Estimated Fair Value	1,043,429	1,112,793
Available for Sale, Amortized Cost	6,931,673	7,089,132
Available for Sale Securities, Gross Unrealized Gains	380,113	70,760
Available for Sale Securities, Gross Unrealized Losses	413	72,690
Available for Sale, Estimated Fair Value	7,311,373	7,087,202
Other [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,500	1,500
Held-to-maturity Securities, Gross Unrealized Gains	0	0
Held-to-maturity Securities, Gross Unrealized Losses	1	0
Held to Maturity, Estimated Fair Value	1,499	1,500
Available for Sale, Amortized Cost	42,825	42,690
Available for Sale Securities, Gross Unrealized Gains	0	0
Available for Sale Securities, Gross Unrealized Losses	0	0
Available for Sale, Estimated Fair Value	$ 42,825	$ 42,690

Securities - Securities, with U

Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 1,499
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1
Held to Maturity, More than 12 Months, Estimated Fair Value	413
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2
Held to Maturity, Total Estimated Fair Value	1,912
Held-to-maturity Securities, Gross Unrealized Losses	3	$ 637
Available for Sale, Less than 12 Months, Estimated Fair Value	91,276
Available for Sale, Less than 12 Months, Unrealized Losses	703
Available for Sale, More than 12 Months, Estimated Fair Value	1,766,616
Available for Sale, More than 12 Months, Unrealized Losses	2,221
Available for Sale, Total Estimated Fair Value	1,857,892
Available-for-sale Securities, Gross Unrealized Loss, before Tax	2,924	108,737
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0
Held to Maturity, More than 12 Months, Estimated Fair Value	413
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2
Held to Maturity, Total Estimated Fair Value	413
Held-to-maturity Securities, Gross Unrealized Losses	2	85
Available for Sale, Less than 12 Months, Estimated Fair Value	49,657
Available for Sale, Less than 12 Months, Unrealized Losses	290
Available for Sale, More than 12 Months, Estimated Fair Value	44,458
Available for Sale, More than 12 Months, Unrealized Losses	490
Available for Sale, Total Estimated Fair Value	94,115
Available-for-sale Securities, Gross Unrealized Loss, before Tax	780	6,547
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held-to-maturity Securities, Gross Unrealized Losses	0	552
Available for Sale, Less than 12 Months, Estimated Fair Value	41,619
Available for Sale, Less than 12 Months, Unrealized Losses	413
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	41,619
Available-for-sale Securities, Gross Unrealized Loss, before Tax	413	72,690
Other Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	1,499
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1
Held to Maturity, More than 12 Months, Estimated Fair Value	0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Held to Maturity, Total Estimated Fair Value	1,499
Held-to-maturity Securities, Gross Unrealized Losses	1	0
Available-for-sale Securities, Gross Unrealized Loss, before Tax	0	0
US Treasury Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	0
Available for Sale, Less than 12 Months, Unrealized Losses	0
Available for Sale, More than 12 Months, Estimated Fair Value	1,722,158
Available for Sale, More than 12 Months, Unrealized Losses	1,731
Available for Sale, Total Estimated Fair Value	1,722,158
Available-for-sale Securities, Gross Unrealized Loss, before Tax	$ 1,731	$ 29,500

Securities - Amortized Cost a_2

Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Held to Maturity, Due in one year or less, Amortized Cost	$ 25,254
Held to Maturity, Due after one year through five years, Amortized Cost	166,412
Held to Maturity, Due after five years through ten years, Amortized Cost	530,808
Held to Maturity, Due after ten years, Amortized Cost	295,892
Held to Maturity, Residential mortgage-backed securities, Amortized Cost	2,472
Held-to-maturity Securities, Equity Securities, Amortized cost	0
Held to Maturity, Amortized Cost	1,020,838	$ 1,106,057
Held to Maturity, Due in one year or less, Estimated Fair Value	25,418
Held to Maturity, Due after one year through five years, Estimated Fair Value	170,675
Held to Maturity, Due after five years through ten years, Estimated Fair Value	543,464
Held to Maturity, Due after ten years, Estimated Fair Value	305,371
Held to Maturity, Residential mortgage-backed securities, Estimated Fair Value	2,495
Held-to-maturity Securities, Equity Securities, Fair Value	0
Held to Maturity, Estimated Fair Value	1,047,423	1,116,953
Available-for-sale Securities, Due in one year or less, Amortized Cost	1,882,023
Available for Sale, Due after one year through five years, Amortized Cost	1,328,841
Available for Sale, Due after five years through ten years, Amortized Cost	453,192
Available for Sale, Due after ten years, Amortized Cost	6,074,419
Available for Sale, Residential mortgage-backed securities, Amortized Cost	2,198,466
Available-for-sale Securities, Equity Securities, Amortized cost	42,825
Available for Sale, Amortized Cost	11,979,766	11,410,447
Available for Sale, Due in one year or less, Estimated Fair Value	1,881,435
Available for Sale, Due after one year through five years, Estimated Fair Value	1,356,833
Available for Sale, Due after five years through ten years, Estimated Fair Value	480,892
Available for Sale, Due after ten years, Estimated Fair Value	6,418,009
Available for Sale, Residential mortgage-backed securities, Estimated Fair Value	2,240,452
Available For Sale Securities Equity Securities Fair Value	42,825
Available for Sale, Estimated Fair Value	$ 12,420,446	$ 11,387,321

Securities - Sales of Securitie

Securities - Sales of Securities Available for Sale (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 4,944,537	$ 2,948,178	$ 8,236,066	$ 13,838,566
Gross realized gains	96	0	899	3
Gross realized losses	0	(34)	(634)	(116)
Tax (expense)benefit of securities gains/losses	$ (21)	$ 7	$ (56)	$ 24

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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Premium amortization	$ (29,511)	$ (27,381)	$ (88,798)	$ (80,104)
Discount accretion	1,274	2,569	3,835	6,356
Net (premium amortization) discount accretion	$ (28,237)	$ (24,812)	$ (84,963)	$ (73,748)

Securities - Trading Account Se

Securities - Trading Account Securities, at Estimated Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	$ 27,669	$ 24,086
U.S. Treasury [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	23,747	21,928
States and political subdivisions [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	$ 3,922	$ 2,158

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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Net gain on sales transactions	$ 731	$ 465	$ 1,789	$ 1,404
Net mark-to-market gains (losses)	(201)	(8)	(170)	(21)
Net gain (loss) on trading account securities	$ 530	$ 457	$ 1,619	$ 1,383

Loans - Additional Information

Loans - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
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%
Percentage of total energy loans to total loans	10.50%		10.50%		11.40%
Loans and Leases Receivable, Related Parties	$ 326.1		$ 326.1		$ 256.1
Interest income recognized subsequent to classification as impaired			$ 0		$ 0
Texas Leading Index economic condition indicator	128.1		128.1		126.4
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	$ 0.9	$ 1.3	$ 3	$ 4.2
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,300		1,300
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 66.1		$ 66.1

Loans - Loans (Detail)

Loans - Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 5,179,780	$ 5,111,957
Energy:	1,546,918	1,602,598
Commercial mortgages	4,555,171	4,121,966
Construction	1,370,206	1,267,717
Land	290,706	306,755
Total commercial real estate	6,216,083	5,696,438
Home equity loans	363,224	353,924
Home equity lines of credit	350,461	337,168
Other	458,618	427,898
Total consumer real estate	1,172,303	1,118,990
Total real estate	7,388,386	6,815,428
Consumer and other	519,844	569,750
Total loans	$ 14,634,928	$ 14,099,733
Total commercial and industrial percentage of total loans	35.40%	36.30%
Percentage of total energy loans to total loans	10.50%	11.40%
Commercial mortgages percentage of total loans	31.10%	29.20%
Construction percentage of total loans	9.40%	9.00%
Land percentage of total loans	2.00%	2.20%
Total commercial real estate percentage of total loans	42.50%	40.40%
Home equity loans percentage of total loans	2.50%	2.50%
Home equity lines of credit percentage of total loans	2.40%	2.40%
Other percentage of total loans	3.10%	3.00%
Total consumer real estate percentage of total loans	8.00%	7.90%
Total real estate percentage of total loans	50.50%	48.30%
Total consumer and other percentage of total loans	3.60%	4.00%
Total loans percentage	100.00%	100.00%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 1,282,180	$ 1,309,314
Percentage of energy production loans to total loans	8.70%	9.30%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 189,350	$ 168,775
Percentage of energy service loans to total loans	1.30%	1.20%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 75,388	$ 124,509
Percentage of other energy loans to total loans	0.50%	0.90%

Loans - Non-Accrual Loans, Segr

Loans - Non-Accrual Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 97,446	$ 73,739
Commercial Portfolio Segment [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	13,929	9,239
Commercial and industrial, Energy [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	70,883	46,932
Commercial real estate, Buildings, land and other [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	9,240	15,268
Construction Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	697	0
Consumer Real Estate [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	527	892
Consumer and Other [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 2,170	$ 1,408

Loans - Age Analysis of Past Du

Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 139,639
Current Loans	14,495,289
Total loans	14,634,928	$ 14,099,733
Accruing Loans 90 or More Days Past Due	14,396
Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	53,162
Current Loans	5,126,618
Total loans	5,179,780
Accruing Loans 90 or More Days Past Due	3,574
Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	25,186
Current Loans	1,521,732
Total loans	1,546,918
Accruing Loans 90 or More Days Past Due	530
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	37,426
Current Loans	4,808,451
Total loans	4,845,877
Accruing Loans 90 or More Days Past Due	6,513
Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,883
Current Loans	1,367,323
Total loans	1,370,206
Accruing Loans 90 or More Days Past Due	774
Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	10,748
Current Loans	1,161,555
Total loans	1,172,303
Accruing Loans 90 or More Days Past Due	2,150
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	10,234
Current Loans	509,610
Total loans	519,844
Accruing Loans 90 or More Days Past Due	855
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	89,569
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	39,540
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	3,350
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	29,147
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,412
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	8,305
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	7,815
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	50,070
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	13,622
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	21,836
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	8,279
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,471
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,443
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 2,419

Loans - Impaired Loans (Detail)

Loans - Impaired Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	$ 116,265	$ 94,536
Recorded Investment With No Allowance	14,505	12,120
Recorded Investment With Allowance	80,265	58,101
Total Recorded Investment	94,770	70,221
Related Allowance	29,160	16,235
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	15,020	9,094
Recorded Investment With No Allowance	4,874	2,842
Recorded Investment With Allowance	7,280	4,287
Total Recorded Investment	12,154	7,129
Related Allowance	5,785	2,558
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	88,531	67,900
Recorded Investment With No Allowance	4,777	6,817
Recorded Investment With Allowance	65,800	39,890
Total Recorded Investment	70,577	46,707
Related Allowance	19,722	9,671
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	9,553	15,774
Recorded Investment With No Allowance	3,864	2,168
Recorded Investment With Allowance	5,015	12,517
Total Recorded Investment	8,879	14,685
Related Allowance	1,483	2,599
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	697	0
Recorded Investment With No Allowance	697	0
Recorded Investment With Allowance	0	0
Total Recorded Investment	697	0
Related Allowance	0	0
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	293	293
Recorded Investment With No Allowance	293	293
Recorded Investment With Allowance	0	0
Total Recorded Investment	293	293
Related Allowance	0	0
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	2,171	1,475
Recorded Investment With No Allowance	0	0
Recorded Investment With Allowance	2,170	1,407
Total Recorded Investment	2,170	1,407
Related Allowance	$ 2,170	$ 1,407

Loans - Average Recorded Invest

Loans - Average Recorded Investment In Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 81,488	$ 108,269	$ 80,578	$ 117,605
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	14,438	13,447	12,552	21,025
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	55,195	78,772	50,642	82,640
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	9,418	14,306	14,961	12,328
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	349	0	277	0
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	293	671	541	807
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 1,795	$ 1,073	$ 1,605	$ 805

Loans - Troubled Debt Restructu

Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	$ 13,425	$ 15,911
Balance at period end	11,805	0
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	3,845	2,203
Balance at period end	2,188	0
Commercial and industrial, Energy [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	0	13,708
Balance at period end	0	0
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	9,456	0
Balance at period end	9,494	0
Consumer Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	124	0
Balance at period end	$ 123	$ 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, 2019USD ($)Item	Sep. 30, 2018USD ($)Item
Financing Receivable, Modifications [Line Items]
Number Of Loans Restructured During The Last Year In Excess of 90 Days Past Due | Item	4	0
Dollar amount of previously restructured loans past due	$ 3,244	$ 0
Restructured loans on non-accrual status at period end	5,645	0
Charge-offs of restructured loans recognized in connection with restructuring	1,500	0
Charge-offs of restructured loans recognized on previously restructured loans	0	4,650
Proceeds from the sale of restructured loans	$ 0	$ 15,750

Loans - Weighted Average Risk G

Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019USD ($)Grade	Dec. 31, 2018USD ($)Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 5,179,780	$ 5,111,957
Loans	6,216,083	5,696,438
Loans	$ 1,370,206	$ 1,267,717
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.42	6.30
Loans	$ 5,179,780	$ 5,111,957
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.36	6.22
Loans	$ 1,546,918	$ 1,602,598
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.99	6.98
Loans	$ 4,845,877	$ 4,428,721
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.28	7.29
Loans	$ 1,370,206	$ 1,267,717
Risk grades 1-8 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.19	6.12
Loans	$ 4,856,712	$ 4,862,275
Risk grades 1-8 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	5.85	5.76
Loans	$ 1,379,566	$ 1,451,673
Risk grades 1-8 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.78	6.76
Loans	$ 4,528,963	$ 4,143,264
Risk grades 1-8 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.19	7.13
Loans	$ 1,314,076	$ 1,177,260
Risk grade 9 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9	9
Loans	$ 165,195	$ 112,431
Risk grade 9 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9	9
Loans	$ 80,170	$ 35,565
Risk grade 9 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9	9
Loans	$ 117,739	$ 109,660
Risk grade 9 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9	9
Loans	$ 31,506	$ 60,754
Risk grade 10 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10	10
Loans	$ 75,276	$ 58,328
Risk grade 10 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10	10
Loans	$ 1,105	$ 43,001
Risk grade 10 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10	10
Loans	$ 97,083	$ 62,353
Risk grade 10 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10	10
Loans	$ 21,686	$ 24,877
Risk grade 11 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11	11
Loans	$ 68,668	$ 69,684
Risk grade 11 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11	11
Loans	$ 15,194	$ 25,427
Risk grade 11 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11	11
Loans	$ 92,852	$ 98,176
Risk grade 11 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11	11
Loans	$ 2,241	$ 4,826
Risk grade 12 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12	12
Loans	$ 8,144	$ 6,681
Risk grade 12 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12	12
Loans	$ 51,161	$ 37,261
Risk grade 12 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12	12
Loans	$ 7,757	$ 12,669
Risk grade 12 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12	12
Loans	$ 697	$ 0
Risk grade 13 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13	13
Loans	$ 5,785	$ 2,558
Risk grade 13 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13	13
Loans	$ 19,722	$ 9,671
Risk grade 13 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13	13
Loans	$ 1,483	$ 2,599
Risk grade 13 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13	13
Loans	$ 0	$ 0

Loans - Net (Charge-Offs)_Recov

Loans - Net (Charge-Offs)/Recoveries, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	$ (6,371)	$ (15,298)	$ (20,977)	$ (35,632)
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	(1,520)	(7,807)	(5,912)	(19,030)
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	(1,260)	(5,347)	(3,184)	(10,272)
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	38	33	(466)	(288)
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	8	3	14	11
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	(103)	(388)	(2,078)	(1,078)
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	$ (3,534)	$ (1,792)	$ (9,351)	$ (4,975)

Loans - Unallocated Portion of

Loans - Unallocated Portion of Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 136,559	$ 134,929	$ 132,132	$ 137,578	$ 150,226	$ 155,364
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	29,160		16,235
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	107,399		115,897
Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	68,531		65,398
Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	29,160		16,235
General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	20,182		22,100
Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	18,686		28,399
Commercial Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	52,667	57,714	48,580	50,991	57,713	59,614
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	5,785		2,558
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	46,882		46,022
Commercial Portfolio Segment [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	28,872		25,351
Commercial Portfolio Segment [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	5,785		2,558
Commercial Portfolio Segment [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	10,866		10,062
Commercial Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	7,144		10,609
Commercial and industrial, Energy [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	33,346	25,818	29,052	31,445	37,313	51,528
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	19,722		9,671
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	13,624		19,381
Commercial and industrial, Energy [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	7,422		9,697
Commercial and industrial, Energy [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	19,722		9,671
Commercial and industrial, Energy [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	4,120		6,014
Commercial and industrial, Energy [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,082		3,670
Commercial Real Estate Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	35,353	35,914	38,777	39,204	38,918	30,948
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	1,483		2,599
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	33,870		36,178
Commercial Real Estate Portfolio Segment [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	22,051		20,817
Commercial Real Estate Portfolio Segment [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,483		2,599
Commercial Real Estate Portfolio Segment [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	4,349		4,366
Commercial Real Estate Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	7,470		10,995
Consumer Real Estate [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	4,884	5,637	6,103	6,194	6,336	5,657
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	4,884		6,103
Consumer Real Estate [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,641		2,688
Consumer Real Estate [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer Real Estate [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,172		1,671
Consumer Real Estate [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,071		1,744
Consumer and Other [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	10,309	$ 9,846	9,620	$ 9,744	$ 9,946	$ 7,617
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	2,170		1,407
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	8,139		8,213
Consumer and Other [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	7,545		6,845
Consumer and Other [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,170		1,407
Consumer and Other [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	(325)		(13)
Consumer and Other [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 919		$ 1,381

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, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 5,179,780	$ 5,111,957
Energy:	1,546,918	1,602,598
Total commercial real estate	6,216,083	5,696,438
Total consumer real estate	1,172,303	1,118,990
Consumer and other	519,844	569,750
Total loans	14,634,928	14,099,733
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	12,154	7,129
Energy:	70,577	46,707
Total commercial real estate	9,576	14,685
Total consumer real estate	293	293
Consumer and other	2,170	1,407
Total loans	94,770	70,221
Loans Collectively Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	5,167,626	5,104,828
Energy:	1,476,341	1,555,891
Total commercial real estate	6,206,507	5,681,753
Total consumer real estate	1,172,010	1,118,697
Consumer and other	517,674	568,343
Total loans	$ 14,540,158	$ 14,029,512

Loans - Activity in Allowance f

Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 134,929	$ 150,226	$ 132,132	$ 155,364
Provision for loan losses	8,001	2,650	25,404	17,846
Charge-offs	(11,619)	(18,596)	(33,492)	(47,326)
Recoveries	5,248	3,298	12,515	11,694
Net charge-offs	(6,371)	(15,298)	(20,977)	(35,632)
Ending balance	136,559	137,578	136,559	137,578
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	57,714	57,713	48,580	59,614
Provision for loan losses	(3,527)	1,085	9,999	10,407
Charge-offs	(2,705)	(8,491)	(8,782)	(21,896)
Recoveries	1,185	684	2,870	2,866
Net charge-offs	(1,520)	(7,807)	(5,912)	(19,030)
Ending balance	52,667	50,991	52,667	50,991
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	25,818	37,313	29,052	51,528
Provision for loan losses	8,788	(521)	7,478	(9,811)
Charge-offs	(2,000)	(5,400)	(4,000)	(10,939)
Recoveries	740	53	816	667
Net charge-offs	(1,260)	(5,347)	(3,184)	(10,272)
Ending balance	33,346	31,445	33,346	31,445
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	35,914	38,918	38,777	30,948
Provision for loan losses	(607)	250	(2,972)	8,533
Charge-offs	0	0	(617)	(619)
Recoveries	46	36	165	342
Net charge-offs	46	36	(452)	(277)
Ending balance	35,353	39,204	35,353	39,204
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,637	6,336	6,103	5,657
Provision for loan losses	(650)	246	859	1,615
Charge-offs	(557)	(431)	(2,936)	(1,632)
Recoveries	454	43	858	554
Net charge-offs	(103)	(388)	(2,078)	(1,078)
Ending balance	4,884	6,194	4,884	6,194
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	9,846	9,946	9,620	7,617
Provision for loan losses	3,997	1,590	10,040	7,102
Charge-offs	(6,357)	(4,274)	(17,157)	(12,240)
Recoveries	2,823	2,482	7,806	7,265
Net charge-offs	(3,534)	(1,792)	(9,351)	(4,975)
Ending balance	$ 10,309	$ 9,744	$ 10,309	$ 9,744

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Schedule of Goodwill (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
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, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	$ 2,745	$ 3,649
Core deposits [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	2,254	2,959
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	491	672
Non-compete agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	$ 0	$ 18

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Estimated Aggregate Future Amortization Expense for Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Remainder of 2019	$ 263
2020	919
2021	697
2022	481
2023	283
Thereafter	102
Future Amortization Expense, Total	$ 2,745	$ 3,649

Deposits - Schedule of Deposits

Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,750,244	$ 10,997,494
Total interest-bearing deposits	16,333,338	16,151,710
Total deposits	$ 27,083,582	$ 27,149,204
Total non-interest-bearing demand deposits, Percentage of Total	39.70%	40.50%
Total interest-bearing deposits, Percentage of Total	60.30%	59.50%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,070,621	$ 10,305,850
Total non-interest-bearing demand deposits, Percentage of Total	37.20%	37.90%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 258,981	$ 235,748
Total non-interest-bearing demand deposits, Percentage of Total	1.00%	0.90%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 6,741,340	$ 6,977,813
Money market accounts	7,960,223	7,777,470
Time accounts of $100,000 or More	707,657	526,789
Time accounts under $100,000	346,440	331,511
Total private accounts	$ 15,755,660	$ 15,613,583
Savings and interest checking, Percentage of Total	24.90%	25.70%
Money market accounts, Percentage of Total	29.40%	28.60%
Time accounts of $100,000 or more, Percentage of Total	2.60%	2.00%
Time accounts under $100,000, Percentage of Total	1.30%	1.20%
Total private accounts, Percentage of Total	58.20%	57.50%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 420,642	$ 455,896
Savings and interest checking	483,303	473,754
Money market accounts	68,321	59,953
Time accounts of $100,000 or More	25,922	4,332
Time accounts under $100,000	132	88
Total public funds	$ 577,678	$ 538,127
Total non-interest-bearing demand deposits, Percentage of Total	1.50%	1.70%
Savings and interest checking, Percentage of Total	1.80%	1.80%
Money market accounts, Percentage of Total	0.20%	0.20%
Time accounts of $100,000 or more, Percentage of Total	0.10%	0.00%
Time accounts under $100,000, Percentage of Total	0.00%	0.00%
Total public funds, Percentage of Total	2.10%	2.00%

Deposits - Additional Informati

Deposits - Additional Information About Corporation's Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Banking and Thrift [Abstract]
Certificate Of Deposit Account Registry Service Cdars Deposits Held	$ 358	$ 0
Deposits from foreign sources (primarily Mexico)	776,490	752,658
Deposits not covered by deposit insurance	$ 12,681,664	$ 13,111,210

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019
Rent expense for operating leases	$ 11.7		$ 8.1	$ 30	$ 24.4
Operating Lease, Right-of-Use Asset	299.5			299.5		$ 170.5
Operating Lease, Liability	$ 325.3			$ 325.3		$ 174.4
Recognition of right of use asset related to the new corporate headquarters facility		$ 121.7
Recognition of operating lease liability related to the new corporate headquarters facility		$ 121.7

Commitments and Contingencies_2

Commitments and Contingencies - Financial Instruments with Off-Balance-Sheet Risk (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Standby Letters of Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 283,821	$ 271,575
Deferred Standby Letter Of Credit Fees [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	1,547	2,069
Commitments to Extend Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 9,036,441	$ 8,369,721

Capital and Regulatory Matter_2

Capital and Regulatory Matters - Additional Information (Detail) $ in Thousands	Jul. 24, 2019USD ($)	Oct. 24, 2017USD ($)	Sep. 30, 2019USD ($)shares	Jun. 30, 2019USD ($)shares	Dec. 31, 2018USD ($)shares	Sep. 30, 2018shares	Sep. 30, 2019USD ($)quartershares	Sep. 30, 2018USD ($)shares	Dec. 31, 2018USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Authorized Amount	$ 100,000	$ 150,000
Stock Repurchase Program, Period in Force	1 year	2 years
Treasury stock, shares, acquired | shares			202,724			0	704,382	601
Treasury Stock, Value, Acquired, Cost Method			$ (17,190)				$ (67,709)	$ (70)
Qualified subordinated debt			98,826		$ 98,708		98,826		$ 98,708
Maximum dividends available without prior regulatory approval			633,800				$ 633,800
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter							20
Trust Preferred Securities Tier One Excluded Portion [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities							$ 133,000		133,000
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock value issued include for regulatory tier one capital calculations			144,500		144,500		$ 144,500		$ 144,500
Preferred stock dividend rate percentage							5.375%		5.375%
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		$ 0
Stock Repurchase 2017 Plan | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares				496,307	1,027,292
Treasury Stock, Value, Acquired, Cost Method				$ (50,000)	$ (100,000)
Stock Repurchase 2019 plan [Member] | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares			202,724
Treasury Stock, Value, Acquired, Cost Method			$ (17,200)
Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Qualified subordinated debt			$ 100,000		$ 100,000		$ 100,000		$ 100,000

Capital and Regulatory Matter_3

Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 2,788,313	$ 2,642,475
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.35%	12.27%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount		$ 1,372,573
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio		6.375%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,580,199	$ 1,507,139
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	$ 1,467,328	$ 1,358,171
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	$ 2,932,799	$ 2,786,961
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.99%	12.94%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount		$ 1,695,532
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio		7.875%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,918,813	$ 1,830,098
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	$ 1,805,942	$ 1,671,595
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	$ 3,302,858	$ 3,152,593
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.63%	14.64%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount		$ 2,126,143
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio		9.875%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,370,298	$ 2,260,709
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	$ 2,257,427	$ 2,089,494
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,932,798	$ 2,786,961
Leverage Ratio, Actual, Ratio	9.36%	9.06%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount		$ 1,231,028
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio		4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,252,781	$ 1,231,028
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	$ 1,565,976	$ 1,538,785
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	$ 2,910,446	$ 2,743,973
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.92%	12.78%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount		$ 1,368,701
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio		6.375%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,576,393	$ 1,502,887
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	$ 1,463,794	$ 1,354,222
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	$ 2,910,446	$ 2,743,973
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.92%	12.78%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount		$ 1,690,748
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio		7.875%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,914,192	$ 1,824,934
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	$ 1,801,593	$ 1,666,735
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	$ 3,047,505	$ 2,876,605
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.53%	13.40%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount		$ 2,120,144
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio		9.875%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,364,590	$ 2,254,331
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	$ 2,251,991	$ 2,083,419
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,910,446	$ 2,743,973
Leverage Ratio, Actual, Ratio	9.30%	8.93%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount		$ 1,229,650
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio		4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,251,713	$ 1,229,650
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	$ 1,564,642	$ 1,537,062
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	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Weighted-average strike rate for outstanding interest rate caps	3.16%
Approximate credit exposure related to swaps with bank customers	$ 55,400
Cash collateral for borrowed securities deposited with other financial institutions	32,100
Interest rate swaps with upstream financial institution counterparties [Member]
Derivative [Line Items]
Approximate credit exposure related to swaps with bank customers	18,600
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

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, 2019	Dec. 31, 2018
Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	$ 5,583	$ 10,941
Estimated Fair Value	15	207
Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	6,356	3,885
Estimated Fair Value	(179)	(199)
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	94,855	496,887
Estimated Fair Value	65	2,384
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	1,008,173	691,143
Estimated Fair Value	(24,268)	(8,921)
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest-rate caps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	102,634	122,791
Estimated Fair Value	233	509
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	1,008,173	691,143
Estimated Fair Value	55,529	16,706
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	94,855	496,887
Estimated Fair Value	(175)	(8,891)
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate caps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	102,634	122,791
Estimated Fair Value	$ (233)	$ (509)

Derivative Financial Instrume_5

Derivative Financial Instruments - Schedule of Weighted-Average Rates Paid and Received for Interest Rate Swaps Outstanding (Detail)	Sep. 30, 2019
Weighted Average Interest Rate Paid [Member]
Weighted Average Discount Rate [Line Items]
Fair value hedge loan/lease interest rate swaps	2.33%
Weighted-Average Interest Rate Received [Member]
Weighted Average Discount Rate [Line Items]
Fair value hedge loan/lease interest rate swaps	2.05%
Financial Institution Counterparties [Member] | Weighted Average Interest Rate Paid [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – financial institution counterparties	4.17%
Financial Institution Counterparties [Member] | Weighted-Average Interest Rate Received [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – financial institution counterparties	3.71%
Customer Counterparties [Member] | Weighted Average Interest Rate Paid [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – customer counterparties	3.71%
Customer Counterparties [Member] | Weighted-Average Interest Rate Received [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – customer counterparties	4.17%

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, 2019USD ($)MMBTUbbl	Dec. 31, 2018USD ($)MMBTUbbl
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	2,280	2,416
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	727	415
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 12,884	$ 24,332
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (1,363)	$ (646)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	8,716	5,745
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	3,439	9,314
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 2,053	$ 417
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (191)	$ (1,272)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	730	415
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	2,277	2,416
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 1,392	$ 646
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (12,572)	$ (24,009)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	3,543	10,236
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	8,613	4,823
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 205	$ 1,373
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (1,960)	$ (393)

Derivative Financial Instrume_7

Derivative Financial Instruments - Notional Amount and Fair Value of Foreign Currency Forward Contract (Detail) £ in Thousands, $ in Thousands, $ in Thousands, $ in Thousands	Sep. 30, 2019USD ($)	Sep. 30, 2019CAD ($)	Sep. 30, 2019MXN ($)	Sep. 30, 2019GBP (£)	Dec. 31, 2018USD ($)	Dec. 31, 2018CAD ($)	Dec. 31, 2018MXN ($)	Dec. 31, 2018GBP (£)
Financial Institution Counterparties [Member] | Other Liabilities [Member] | Non Designated as Hedging Instrument [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Notional Amount		$ 5,283	$ 0	£ 0		$ 11,003	$ 3,015	£ 142
Customer Counterparties [Member] | Other Assets [Member] | Non Designated as Hedging Instrument [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Notional Amount		$ 5,271	$ 0	£ 0		$ 10,979	$ 3,000	£ 145
Foreign Exchange Forward [Member] | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Estimated Fair Value	$ (11)				$ (13)
Foreign Exchange Forward [Member] | Customer Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value	24				40
GBP Foreign Exchange Forward [Member] | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Estimated Fair Value	0				(2)
GBP Foreign Exchange Forward [Member] | Customer Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value	0				4
MXN Foreign Exchange Forwards [Member] | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Estimated Fair Value	0				(132)
MXN Foreign Exchange Forwards [Member] | Customer Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value	$ 0				$ 149

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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Interest Income on Loans [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ 24	$ 12	$ 78	$ 1
Other Non-Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ (1)	$ 0	$ 0	$ (1)

Derivative Financial Instrume_9

Derivative Financial Instruments - Schedule of Amounts Related to Non-Hedging Interest Rate and Commodity Derivatives (Detail) - Non Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
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)	$ 215	$ 699	$ 1,188	$ 2,889
Interest Rate Contract [Member] | Other Non-Interest Expense [Member]
Other Non Interest Income Non Hedging Interest Rate Derivatives [Line Items]
Amount of gain (loss) included in income (expense)	0	2	0	(2)
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)	109	456	322	492
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)	$ 12	$ 53	$ 41	$ 203

Balance Sheet Offsetting - Fina

Balance Sheet Offsetting - Financial Instruments Eligible for Offset Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	$ 15,250	$ 27,849
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	15,250	27,849
Gross Amount Recognized, Resell agreements, Financial assets	11,299	11,642
Gross Amount Offset, Resell agreements, Financial assets	0	0
Net Amount Recognized, Resell agreements, Financial assets	11,299	11,642
Gross Amount Recognized, Financial assets	26,549	39,491
Gross Amount Offset, Financial assets	0	0
Net Amount Recognized, Financial assets	26,549	39,491
Gross Amount Recognized, Derivatives, Financial Liabilities	26,012	11,185
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	26,012	11,185
Gross Amount Recognized, Repurchase agreements, Financial liabilities	1,187,355	1,360,298
Gross Amount Offset, Repurchase agreements, Financial liabilities	0	0
Net Amount Recognized, Repurchase agreements, Financial liabilities	1,187,355	1,360,298
Gross Amount Recognized, Financial liabilities	1,213,367	1,371,483
Gross Amount Offset, Financial liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,213,367	1,371,483
Loan/lease interest rate swaps and caps [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	313	3,100
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	313	3,100
Commodity swaps and options [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	14,937	24,749
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	14,937	24,749
Gross Amount Recognized, Derivatives, Financial Liabilities	1,554	1,918
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	1,554	1,918
Foreign currency forward contracts [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	0	0
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	0	0
Gross Amount Recognized, Derivatives, Financial Liabilities	11	147
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	11	147
Loan/lease interest rate swaps [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Liabilities	24,447	9,120
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	$ 24,447	$ 9,120

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, 2019	Dec. 31, 2018
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 15,250	$ 27,849
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(9,893)	(6,143)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(2,880)	(21,651)
Net Amount, Derivatives, Financial Assets	2,477	55
Net Amount Recognized, Resell agreements, Financial Assets	11,299	11,642
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(11,299)	(11,642)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	26,549	39,491
Gross Amounts Not Offset Collateral, Financial Assets	(14,179)	(33,293)
Net Amount, Financial Assets	2,477	55
Net Amount Recognized, Derivatives, Financial Liabilities	26,012	11,185
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(9,893)	(6,143)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(16,035)	(4,621)
Net Amount, Derivatives, Financial Liabilities	84	421
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	1,187,355	1,360,298
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(1,187,355)	(1,360,298)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,213,367	1,371,483
Gross Amounts Not Offset Collateral, Financial Liabilities	(1,203,390)	(1,364,919)
Net Amount, Financial Liabilities	84	421
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	38	598
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(38)	(598)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	6,192	4,293
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(38)	(598)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(6,154)	(3,651)
Net Amount, Derivatives, Financial Liabilities	0	44
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	4,344	7,255
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(4,344)	(3,380)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	(3,875)
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	7,810	3,380
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(4,344)	(3,380)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(3,466)	0
Net Amount, Derivatives, Financial Liabilities	0	0
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	8	81
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(8)	(81)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	164	326
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(8)	(81)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(156)	(245)
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	10,860	19,915
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(5,503)	(2,084)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(2,880)	(17,776)
Net Amount, Derivatives, Financial Assets	2,477	55
Net Amount Recognized, Derivatives, Financial Liabilities	11,846	3,186
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(5,503)	(2,084)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(6,259)	(725)
Net Amount, Derivatives, Financial Liabilities	$ 84	$ 377

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, 2019	Dec. 31, 2018
Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase [Line Items]
Remaining contractual maturity of the agreements overnight and continuous	$ 1,184,805	$ 1,360,298
Remaining contractual maturity of the agreements up to 30 Days	2,550	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	1,187,355	1,360,298
Securities Sold under Agreements to Repurchase, Gross	1,187,355	1,360,298
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	162,566	1,334,063
Remaining contractual maturity of the agreements up to 30 Days	2,550	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	165,116	1,334,063
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	1,022,239	26,235
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	$ 1,022,239	$ 26,235

Stock-Based Compensation Stock-

Stock-Based Compensation Stock-Based Compensation - Narrative (Details)	Sep. 30, 2019shares
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	1,277,578

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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options exercised, Number of Shares	(101,284)	(19,232)	(255,184)	(447,831)
Director deferred stock units outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares/Units			48,910
Granted, Number of Units			7,592
Stock Awards exercised/vested, Number of Shares			(1,132)
Ending Balance, Number of Shares/Units	55,370		55,370
Beginning Balance, Weighted-Average Grant-Date Fair Value			$ 71.14
Granted Weighted-Average Grant-Date Fair Value			102.70
Stock Award Exercised/Vested, Weighted Average Grant Date Fair Value			106.03
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 74.76		$ 74.76
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			383,797
Granted, Number of Units			1,957
Stock Awards exercised/vested, Number of Shares			(17,800)
Forfeited/expired, Number of Shares/Units			(11,850)
Ending Balance, Number of Shares/Units	356,104		356,104
Beginning Balance, Weighted-Average Grant-Date Fair Value			$ 85.59
Granted Weighted-Average Grant-Date Fair Value			91.96
Stock Award Exercised/Vested, Weighted Average Grant Date Fair Value			65.11
Forfeited/Expired, Weighted-Average Price			89.29
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 86.53		$ 86.53
Stock Options Outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares			2,352,008
Stock options exercised, Number of Shares			(236,252)
Forfeited/expired, Number of Shares			(11,000)
Ending Balance, Number of Shares	2,104,756		2,104,756
Beginning Balance, Weighted-Average Exercise Price			$ 63.55
Stock options exercised, Weighted-Average Exercise Price			57.17
Forfeited/expired, Weighted-Average Exercise Price			65.11
Ending Balance, Weighted-Average Exercise Price	$ 64.26		$ 64.26
Maximum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares/Units			125,809
Ending Balance, Number of Shares/Units	125,809		125,809
Beginning Balance, Weighted-Average Grant-Date Fair Value			$ 82.55
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 82.55		$ 82.55

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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
New shares issued from available authorized shares	0	0	0	0
Issued from available treasury stock	71,284	19,232	225,184	447,831
Total	71,284	19,232	225,184	447,831
Proceeds from stock option exercises	$ 5,665	$ 1,119	$ 13,506	$ 26,567

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock options	$ 330	$ 960	$ 1,091	$ 3,064
Non-vested stock awards/stock units	1,536	1,357	5,727	4,196
Director deferred stock units	0	0	780	720
Performance stock units	809	479	3,095	1,576
Total	2,675	2,796	10,693	9,556
Income tax benefit	$ 452	$ 587	$ 1,768	$ 2,007

Stock-Based Compensation - Unre

Stock-Based Compensation - Unrecognized Stock-Based Compensation Expense (Detail) $ in Thousands	Sep. 30, 2019USD ($)
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock options	$ 96
Non-vested stock awards/stock units	11,196
Performance stock units	3,432
Total	$ 14,724

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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income	$ 111,836	$ 117,844	$ 339,918	$ 335,665
Less: Preferred stock dividends	2,016	2,016	6,047	6,047
Net income available to common shareholders	109,820	115,828	333,871	329,618
Less: Earnings allocated to participating securities	876	750	2,761	2,178
Distributed earnings allocated to common stock	44,370	42,811	131,069	121,907
Undistributed earnings allocated to common stock	64,574	72,267	200,041	205,533
Net earnings allocated to common stock	$ 108,944	$ 115,078	$ 331,110	$ 327,440
Weighted-average shares outstanding for basic earnings per common share	62,566,128	63,892,023	62,786,501	63,793,514
Dilutive effect of stock compensation	592,866	1,022,168	725,911	1,037,043
Weighted-average shares outstanding for diluted earnings per common share	63,158,994	64,914,191	63,512,412	64,830,557

Defined Benefit Plans - Net Per

Defined Benefit Plans - Net Periodic Cost (Benefit) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Retirement Benefits [Abstract]
Expected return on plan assets, net of expenses	$ (2,693)	$ (2,979)	$ (8,079)	$ (8,937)
Interest cost on projected benefit obligation	1,618	1,475	4,854	4,424
Net amortization and deferral	1,406	1,250	4,218	3,751
Net periodic expense (benefit)	$ 331	$ (254)	$ 993	$ (762)

Defined Benefit Plans Additiona

Defined Benefit Plans Additional Information (Details)	Sep. 30, 2019USD ($)
Retirement Benefits [Abstract]
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Income Tax [Abstract]
Deferred Tax Liabilities, Net	$ 76.7
Deferred Tax Assets, Net		$ 19.8
Valuation allowance	$ 0
U.S. federal statutory income tax rate, percent	21.00%
Unrecognized tax benefits	$ 0

Income Taxes - Income Tax Expen

Income Taxes - Income Tax Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Current income tax expense (benefit)	$ 14,480	$ (1,152)	$ 43,984	$ 955
Deferred income tax expense (benefit)	(950)	16,312	(1,625)	39,198
Income tax expense, as reported	$ 13,530	$ 15,160	$ 42,359	$ 40,153
Effective tax rate	10.80%	11.40%	11.10%	10.70%

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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 107,786	$ (96,147)	$ 464,072	$ (286,935)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(305)	(3,764)	(957)	(8,424)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	(97)	34	(266)	113
Total securities available for sale and transferred securities, before tax amount	107,384	(99,877)	462,849	(295,246)
Change in the net actuarial gain/loss,Before Tax Amount	0	0	0	0
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	1,406	1,250	4,218	3,751
Total defined-benefit post-retirement benefit plans	1,406	1,250	4,218	3,751
Total other comprehensive income (loss), before tax	108,790	(98,627)	467,067	(291,495)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	22,635	(20,190)	97,455	(60,256)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(64)	(790)	(201)	(1,769)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	(21)	7	(56)	24
Total securities available for sale and transferred securities, Tax Expense (Benefit)	22,550	(20,973)	97,198	(62,001)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	295	262	886	788
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	295	262	886	788
Total other comprehensive income (loss), Tax Expense (Benefit)	22,845	(20,711)	98,084	(61,213)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	85,151	(75,957)	366,617	(226,679)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(241)	(2,974)	(756)	(6,655)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	(76)	27	(210)	89
Total securities available for sale and transferred securities, Net of Tax	84,834	(78,904)	365,651	(233,245)
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	1,111	988	3,332	2,963
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	1,111	988	3,332	2,963
Other comprehensive income (loss), net of tax	$ 85,945	$ (77,916)	$ 368,983	$ (230,282)

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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ (63,600)
Other comprehensive income (loss), net of tax	$ 85,945	$ (77,916)	368,983	$ (230,282)
Ending Balance	305,383		305,383
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(16,103)	117,230
Other comprehensive income (loss) before reclassifications			365,861	(233,334)
Reclassification of amounts included in net income			(210)	89
Other comprehensive income (loss), net of tax			365,651	(233,245)
Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings				17,557
Ending Balance	349,548	(98,458)	349,548	(98,458)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(47,497)	(37,718)
Other comprehensive income (loss) before reclassifications			0	0
Reclassification of amounts included in net income			3,332	2,963
Other comprehensive income (loss), net of tax			3,332	2,963
Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings				(8,022)
Ending Balance	(44,165)	(42,777)	(44,165)	(42,777)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(63,600)	79,512
Other comprehensive income (loss) before reclassifications			365,861	(233,334)
Reclassification of amounts included in net income			3,122	3,052
Other comprehensive income (loss), net of tax			368,983	(230,282)
Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings				9,535
Ending Balance	$ 305,383	$ (141,235)	$ 305,383	$ (141,235)

Operating Segments - Additional

Operating Segments - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2019Segment
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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	$ 342,231	$ 329,322	$ 1,021,554	$ 972,851
Net income	111,836	117,844	339,918	335,665
Operating Segments [Member] | Banking [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	307,312	295,062	918,672	874,056
Net income	110,583	114,214	334,393	327,131
Operating Segments [Member] | Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	37,549	36,868	111,064	106,475
Net income	4,106	6,442	15,403	17,977
Operating Segments [Member] | Non-Banks [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	(2,630)	(2,608)	(8,182)	(7,680)
Net income	$ (2,853)	$ (2,812)	$ (9,878)	$ (9,443)

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
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, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	$ 12,420,446	$ 11,387,321
Trading account securities	27,669	24,086
Derivative assets	15,250	27,849
Derivative liabilities	26,012	11,185
Interest rate swaps, caps and floors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	55,842	19,806
Derivative liabilities	24,855	18,520
Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	16,534	26,768
Derivative liabilities	16,086	26,320
Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	24	193
Derivative liabilities	11	147
U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	2,825,796	3,427,689
Trading account securities	23,747	21,928
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	2,240,452	829,740
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	7,311,373	7,087,202
Trading account securities	3,922	2,158
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	42,825	42,690
Level 1 [Member] | Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	24	193
Derivative liabilities	11	147
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	2,825,796	3,427,689
Trading account securities	23,747	21,928
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	55,842	19,806
Derivative liabilities	24,855	18,520
Level 2 [Member] | Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	16,534	26,768
Derivative liabilities	16,086	26,320
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	2,240,452	829,740
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	7,311,373	7,087,202
Trading account securities	3,922	2,158
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	$ 42,825	$ 42,690

Fair Value Measurements - Impai

Fair Value Measurements - Impaired Loans Remeasured and Reported at Fair Value of Underlying Collateral (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	$ 94,770	$ 70,221
Specific valuation allowance allocations	(29,160)	$ (16,235)
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	2,416		$ 12,876
Specific valuation allowance allocations	1,116		(2,599)
Loans receivable, fair value disclosure	3,532		10,277
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	60,010		28,638
Specific valuation allowance allocations	(12,437)		4,256
Loans receivable, fair value disclosure	$ 47,573		$ 32,894

Fair Value Measurements - Forec

Fair Value Measurements - Foreclosed Assets Remeasured and Reported at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
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	$ 1,302	$ 2,898
Foreclosed Assets Remeasured at Initial Recognition Charge-offs recognized in the allowance for loan losses	(76)	0
Fair Value of Foreclosed Assets Remeasured at Initial Recognition	1,226	2,898
Foreclosed Assets Remeasured Subsequent to initial Recognition Carrying value of foreclosed assets prior to remeasurement	0	1,823
Foreclosed Assets Remeasured Subsequent to Initial Recognition Write-downs included in other non-interest expense	0	(473)
Fair Value of Foreclosed Assets remeasured subsequent to initial recognition	$ 0	$ 1,350

Fair Value Measurements - Estim

Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 2,907,901	$ 3,955,779	$ 3,419,489	$ 5,053,047
Securities held to maturity	1,020,838	1,106,057
Loans, net	14,498,369	13,967,601
Deposits	27,083,582	27,149,204
Federal funds purchased and repurchase agreements	1,200,655	1,367,548
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,285	136,242
Subordinated notes payable and other borrowings	98,826	98,708
Carrying Amount [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,907,901	3,955,779
Securities held to maturity	1,020,838	1,106,057
Cash surrender value of life insurance policies	186,217	183,473
Accrued interest receivable	133,950	188,989
Deposits	27,083,582	27,149,204
Federal funds purchased and repurchase agreements	1,200,655	1,367,548
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,285	136,242
Subordinated notes payable and other borrowings	98,826	98,708
Accrued interest payable	10,177	7,394
Carrying Amount [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	14,498,369	13,967,601
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,907,901	3,955,779
Securities held to maturity	1,047,423	1,116,953
Cash surrender value of life insurance policies	186,217	183,473
Accrued interest receivable	133,950	188,989
Deposits	27,084,765	27,143,572
Federal funds purchased and repurchase agreements	1,200,655	1,367,548
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	137,115	137,115
Subordinated notes payable and other borrowings	106,447	98,458
Accrued interest payable	10,177	7,394
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 14,542,918	$ 13,933,239

Accounting Standards Updates _2

Accounting Standards Updates Accounting Standards Updates - Additional Information (Details) - USD ($) $ in Thousands	Jan. 01, 2020	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ (14,672)	$ (2,285)
Minimum [Member] | Scenario, Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage increase in allowance for loan losses and reserve for unfunded commitments	15.00%
Maximum [Member] | Scenario, Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage increase in allowance for loan losses and reserve for unfunded commitments	25.00%
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ (14,672)	$ (2,285)