Cullen/Frost Bankers (CFR) 10-Q 25 Oct 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 18, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	CFR
Entity Registrant Name	CULLEN/FROST BANKERS, INC.
Entity Central Index Key	39,263
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		63,944,066

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Cash and due from banks	$ 505,333	$ 545,542
Interest-bearing deposits	2,430,289	4,347,538
Federal funds sold and resell agreements	483,867	159,967
Total cash and cash equivalents	3,419,489	5,053,047
Securities held to maturity, at amortized cost	1,111,762	1,432,098
Securities available for sale, at estimated fair value	11,178,591	10,489,009
Trading account securities	22,238	21,098
Loans, net of unearned discounts	13,814,838	13,145,665
Less: Allowance for loan losses	(137,578)	(155,364)
Net loans	13,677,260	12,990,301
Premises and equipment, net	541,236	520,958
Goodwill	654,952	654,952
Other intangible assets, net	3,980	5,073
Cash surrender value of life insurance policies	182,603	180,477
Accrued interest receivable and other assets	431,291	400,867
Total assets	31,223,402	31,747,880
Liabilities:
Non-interest-bearing demand deposits	10,840,513	11,197,093
Interest-bearing deposits	15,508,341	15,675,296
Total deposits	26,348,854	26,872,389
Federal funds purchased and repurchase agreements	1,083,777	1,147,824
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,227	136,184
Subordinated notes, net of unamortized issuance costs	98,669	98,552
Accrued interest payable and other liabilities	247,629	195,068
Total liabilities	27,915,156	28,450,017
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, 2018 and December 31, 2017	144,486	144,486
Common stock, par value $0.01 per share; 210,000,000 shares authorized; 64,236,306 shares issued at both September 30, 2018 and December 31, 2017	642	642
Additional paid-in capital	962,917	953,361
Retained earnings	2,369,259	2,187,069
Accumulated other comprehensive income (loss), net of tax	(141,235)	79,512
Treasury stock, at cost; 313,490 shares at September 30, 2018 and 760,720 shares at December 31, 2017	(27,823)	(67,207)
Total shareholders’ equity	3,308,246	3,297,863
Total liabilities and shareholders’ equity	$ 31,223,402	$ 31,747,880

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
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	313,490	760,720
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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest income:
Loans, including fees	$ 172,333	$ 138,400	$ 487,668	$ 392,073
Securities:
Taxable	21,339	23,203	63,085	72,032
Tax-exempt	59,200	54,939	173,209	167,321
Interest-bearing deposits	14,445	10,800	42,456	26,712
Federal funds sold and resell agreements	1,399	244	3,575	514
Total interest income	268,716	227,586	769,993	658,652
Interest expense:
Deposits	22,201	5,668	50,414	9,709
Federal funds purchased and repurchase agreements	2,292	523	3,557	849
Junior subordinated deferrable interest debentures	1,394	1,020	3,847	2,890
Other long-term borrowings	1,164	1,164	3,492	2,696
Total interest expense	27,051	8,375	61,310	16,144
Net interest income	241,665	219,211	708,683	642,508
Provision for loan losses	2,650	10,980	17,846	27,358
Net interest income after provision for loan losses	239,015	208,231	690,837	615,150
Non-interest income:
Trust and investment management fees	30,801	27,493	89,509	81,690
Service charges on deposit accounts	21,569	20,967	63,554	62,934
Insurance commissions and fees	11,037	10,892	37,573	34,441
Interchange and debit card transaction fees	3,499	5,884	10,103	17,150
Other charges, commissions and fees	9,580	10,493	27,860	29,983
Net gain (loss) on securities transactions	(34)	(4,867)	(113)	(4,917)
Other	11,205	10,753	35,682	25,114
Total non-interest income	87,657	81,615	264,168	246,395
Non-interest expense:
Salaries and wages	87,547	84,388	259,434	247,895
Employee benefits	18,355	17,730	58,257	57,553
Net occupancy	19,894	19,391	59,089	57,781
Technology, furniture and equipment	21,004	18,743	61,142	54,983
Deposit insurance	4,694	4,862	14,178	15,347
Intangible amortization	336	405	1,093	1,301
Other	41,838	41,304	125,994	127,929
Total non-interest expense	193,668	186,823	579,187	562,789
Income before income taxes	133,004	103,023	375,818	298,756
Income taxes	15,160	9,892	40,153	35,131
Net income	117,844	93,131	335,665	263,625
Preferred stock dividends	2,016	2,016	6,047	6,047
Net income available to common shareholders	$ 115,828	$ 91,115	$ 329,618	$ 257,578
Earnings per common share:
Basic	$ 1.80	$ 1.43	$ 5.13	$ 4.02
Diluted	$ 1.78	$ 1.41	$ 5.08	$ 3.98

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net income	$ 117,844	$ 93,131	$ 335,665	$ 263,625
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss on securities available for sale during the period	(96,147)	7,082	(286,935)	131,283
Change in net unrealized gain on securities transferred to held to maturity	(3,764)	(3,514)	(8,424)	(13,660)
Reclassification adjustment for net (gains) losses included in net income	34	4,867	113	4,917
Total securities available for sale and transferred securities, before tax amount	(99,877)	8,435	(295,246)	122,540
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)	1,250	1,357	3,751	4,072
Total defined-benefit post-retirement benefit plans	1,250	1,357	3,751	4,072
Other comprehensive income (loss), before tax	(98,627)	9,792	(291,495)	126,612
Deferred tax expense (benefit) related to other comprehensive income	(20,711)	3,427	(61,213)	44,314
Other comprehensive income (loss), net of tax	(77,916)	6,365	(230,282)	82,298
Comprehensive income (loss)	$ 39,928	$ 99,496	$ 105,383	$ 345,923

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]
Total shareholders’ equity at beginning of period at Dec. 31, 2016	$ 3,002,528	$ 144,486	$ 637	$ 906,732	$ 1,985,569	$ (24,623)	$ (10,273)
Net income	263,625				263,625
Other comprehensive income (loss)	82,298					82,298
Stock option exercises/stock unit conversions (19,232 shares QTD and 447,831 shares YTD in 2018 and 22,724 shares QTD and 774,799 shares YTD in 2017)	45,422		5	36,148	(2,134)		11,403
Stock compensation expense recognized in earnings	9,013			9,013
Purchase of treasury stock (no shares QTD and 601 shares YTD in 2018 and 1,134,966 shares QTD and 1,135,435 shares YTD in 2017)	(100,042)						(100,042)
Cash dividends – preferred stock (approximately $0.34 per share QTD in both 2018 and in 2017 and $1.01 per share YTD in both 2018 and in 2017.)	(6,047)				(6,047)
Cash dividends – common stock ($0.67 per share QTD and $1.91 per share YTD in 2018 and $0.57 per share QTD and $1.68 per share YTD in 2017)	(107,754)				(107,754)
Total shareholders’ equity at end of period at Sep. 30, 2017	3,189,043	144,486	642	951,893	2,133,259	57,675	(98,912)
Total shareholders’ equity at beginning of period at Jun. 30, 2017	3,223,929	144,486	642	948,593	2,078,898	51,310	0
Net income	93,131				93,131
Other comprehensive income (loss)	6,365					6,365
Stock option exercises/stock unit conversions (19,232 shares QTD and 447,831 shares YTD in 2018 and 22,724 shares QTD and 774,799 shares YTD in 2017)	1,273			578	(393)		1,088
Stock compensation expense recognized in earnings	2,722			2,722
Purchase of treasury stock (no shares QTD and 601 shares YTD in 2018 and 1,134,966 shares QTD and 1,135,435 shares YTD in 2017)	(100,000)						(100,000)
Cash dividends – preferred stock (approximately $0.34 per share QTD in both 2018 and in 2017 and $1.01 per share YTD in both 2018 and in 2017.)	(2,016)				(2,016)
Cash dividends – common stock ($0.67 per share QTD and $1.91 per share YTD in 2018 and $0.57 per share QTD and $1.68 per share YTD in 2017)	(36,361)				(36,361)
Total shareholders’ equity at end of period at Sep. 30, 2017	3,189,043	144,486	642	951,893	2,133,259	57,675	(98,912)
Total shareholders' equity at beginning of period, as adjusted	3,295,578	144,486	642	953,361	2,184,784	79,512	(67,207)
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)
Cumulative effect of accounting change at Dec. 31, 2017	(2,285)				(2,285)
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)	9,535
Stock option exercises/stock unit conversions (19,232 shares QTD and 447,831 shares YTD in 2018 and 22,724 shares QTD and 774,799 shares YTD in 2017)	26,567				(12,887)		39,454
Stock compensation expense recognized in earnings	9,556			9,556
Purchase of treasury stock (no shares QTD and 601 shares YTD in 2018 and 1,134,966 shares QTD and 1,135,435 shares YTD in 2017)	(70)						(70)
Cash dividends – preferred stock (approximately $0.34 per share QTD in both 2018 and in 2017 and $1.01 per share YTD in both 2018 and in 2017.)	(6,047)				(6,047)
Cash dividends – common stock ($0.67 per share QTD and $1.91 per share YTD in 2018 and $0.57 per share QTD and $1.68 per share YTD in 2017)	(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 (19,232 shares QTD and 447,831 shares YTD in 2018 and 22,724 shares QTD and 774,799 shares YTD in 2017)	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 in both 2018 and in 2017 and $1.01 per share YTD in both 2018 and in 2017.)	(2,016)				(2,016)
Cash dividends – common stock ($0.67 per share QTD and $1.91 per share YTD in 2018 and $0.57 per share QTD and $1.68 per share YTD in 2017)	(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)

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Shareholders' Equity (Parenthetical) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Stockholders' Equity [Abstract]
Stock option exercises/stock unit conversions (shares)	19,232	22,724	447,831	774,799
Treasury stock, shares, acquired	0	1,134,966	601	1,135,435
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	$ 0.67	$ 0.57	$ 1.91	$ 1.68

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating Activities:
Net income	$ 335,665	$ 263,625
Adjustments to reconcile net income to net cash from operating activities:
Provision for loan losses	17,846	27,358
Deferred tax expense (benefit)	39,198	(9,505)
Accretion of loan discounts	(10,470)	(11,567)
Securities premium amortization (discount accretion), net	73,748	66,455
Net (gain) loss on securities transactions	113	4,917
Depreciation and amortization	37,452	35,819
Net (gain) loss on sale/write-down of assets/foreclosed assets	(6,122)	(2,045)
Stock-based compensation	9,556	9,013
Net tax benefit from stock-based compensation	3,378	5,844
Earnings on life insurance policies	(2,510)	(2,367)
Net change in:
Trading account securities	(1,094)	(3,018)
Accrued interest receivable and other assets	(19,047)	10,495
Accrued interest payable and other liabilities	(17,545)	(39,580)
Net cash from operating activities	460,168	355,444
Investing Activities:
Securities held-to-maturity - Purchases	(1,500)	0
Securities held-to-maturity - Sales	0	0
Securities held to maturity: Maturities, calls and principal repayments	300,340	780,562
Securities available for sale:
Purchases	(14,983,642)	(9,138,457)
Sales	13,838,566	8,993,963
Maturities, calls and principal repayments	182,028	283,278
Proceeds from sale of loans	21,318	0
Net change in loans	(718,551)	(745,702)
Benefits received on life insurance policies	384	462
Proceeds from sales of premises and equipment	12,859	1,553
Purchases of premises and equipment	(56,781)	(23,796)
Proceeds from sales of repossessed properties	1,106	517
Net cash from investing activities	(1,403,873)	152,380
Financing Activities:
Net change in deposits	(523,535)	591,694
Net change in short-term borrowings	(64,047)	20,927
Proceeds from issuance of subordinated notes	0	98,434
Principal payments on subordinated notes	0	(100,000)
Proceeds from stock option exercises	26,567	45,422
Purchase of treasury stock	(70)	(100,042)
Cash dividends paid on preferred stock	(6,047)	(6,047)
Cash dividends paid on common stock	(122,721)	(107,754)
Net cash from financing activities	(689,853)	442,634
Net change in cash and cash equivalents	(1,633,558)	950,458
Cash and equivalents at beginning of period	5,053,047	4,141,445
Cash and equivalents at end of period	$ 3,419,489	$ 5,091,903

Significant Accounting Policies

Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
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, 2017 , included in our Annual Report on Form 10-K filed with the SEC on February 7, 2018 (the “ 2017 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 2018 2017 Cash paid for interest $ 59,764 $ 15,611 Cash paid for income taxes 5,112 41,969 Significant non-cash transactions: Unsettled purchases/sales of securities 74,191 41,763 Loans foreclosed and transferred to other real estate owned and foreclosed assets 2,898 257 Accounting Changes, Reclassifications and Restatements. Certain items in prior financial statements have been reclassified to conform to the current presentation. In addition, we adopted ASU 2018-02, " Income Statement - Reporting Comprehensive Income (Topic 220) - Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income" as of January 1, 2018. In accordance with ASU 2018-02, we elected to reclassify certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act, which was enacted on December 22, 2017 (see Note 13 - Income Taxes), from accumulated other comprehensive income to retained earnings. Such amounts, which totaled $9.5 million , related to a net actuarial loss on defined benefit post-retirement plans and unrealized gains on securities available for sale and securities transferred to held to maturity. See Note 14 - Other Comprehensive Income. The effects of the Tax Cuts and Jobs Act on deferred taxes related to amounts initially recorded in accumulated other comprehensive income are provisional. As we finalize the accounting for the tax effects of the Tax Cuts and Jobs Act, additional reclassification adjustments may be recorded in future periods. See Note 13 - Income Taxes. Notwithstanding this election made in accordance with ASU 2018-02, our policy is to release such income tax effects only when the entire portfolio to which the underlying transactions relate is liquidated, sold or extinguished. We also adopted ASU 2014-09, "Revenue from Contracts with Customers (Topic 606)” as of January 1, 2018. Using a modified retrospective transition approach for contracts that were not complete as of our adoption, we recognized a cumulative effect reduction to beginning retained earnings totaling $2.3 million . The amount was related to certain revenue streams within trust and investment management fees. Additionally, based on our underlying contracts, ASU 2014-09 requires us to report network costs associated with debit card and ATM transactions netted against the related fee income from such transactions. Previously, such network costs were reported as a component of other non-interest expense. For the three and nine months ended September 30, 2018 , gross interchange and debit card transaction fees totaled $6.5 million and $19.0 million , respectively, while related network costs totaled $3.0 million and $8.9 million , respectively. On a net basis, we reported $3.5 million and $10.1 million as interchange and debit card transaction fees in the accompanying Consolidated Statement of Income for the three and nine months ended September 30, 2018 , respectively. For the three and nine months ended September 30, 2017 , we reported interchange and debit card transaction fees totaling $5.9 million and $17.2 million , respectively, on a gross basis in the accompanying Consolidated Statement of Income while related network costs totaling $3.0 million and $9.1 million were reported as a component of other non-interest expense for the three and nine months ended September 30, 2017 , respectively. ASU 2014-09 also required us to change the way we recognize certain recurring revenue streams reported as components of trust and investment management fees, insurance commissions and fees and other categories of non-interest income, however, such changes were not significant to our financial statements for the nine months ended September 30, 2018 . Under ASU 2014-09, we adopted new policies related to revenue recognition. In general, for revenue not associated with financial instruments, guarantees and lease contracts, we apply the following steps when recognizing revenue from contracts with customers: (i) identify the contract, (ii) identify the performance obligations, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations and (v) recognize revenue when performance obligation is satisfied. Our contracts with customers are generally short term in nature, typically due within one year or less or cancellable by us or our customer upon a short notice period. Performance obligations for our customer contracts are generally satisfied at a single point in time, typically when the transaction is complete, or over time. For performance obligations satisfied over time, we primarily use the output method, directly measuring the value of the products/services transferred to the customer, to determine when performance obligations have been satisfied. We typically receive payment from customers and recognize revenue concurrent with the satisfaction of our performance obligations. In most cases, this occurs within a single financial reporting period. For payments received in advance of the satisfaction of performance obligations, revenue recognition is deferred until such time the performance obligations have been satisfied. In cases where we have not received payment despite satisfaction of our performance obligations, we accrue an estimate of the amount due in the period our performance obligations have been satisfied. For contracts with variable components, only amounts for which collection is probable are accrued. We generally act in a principal capacity, on our own behalf, in most of our contracts with customers. In such transactions, we recognize revenue and the related costs to provide our services on a gross basis in our financial statements. In some cases, we act in an agent capacity, deriving revenue through assisting other entities in transactions with our customers. In such transactions, we recognized revenue and the related costs to provide our services on a net basis in our financial statements. These transactions primarily relate to insurance and brokerage commissions and fees derived from our customers' use of various interchange and ATM/debit card networks. In August 2018, the Securities and Exchange Commission (“SEC”) issued Final Rule Release No. 33-10532 - “Disclosure Update and Simplification.” This rule amends various SEC disclosure requirements that have been determined to be redundant, duplicative, overlapping, outdated, or superseded. The changes are generally expected to reduce or eliminate certain disclosures; however, the amendments did expand interim period disclosure requirements related to changes in shareholders' equity. Although the final rule does not become effective until November 5, 2018, we elected to expand our presentation of the Statement of Changes in Shareholders' Equity, as will be required under the new rule, in our financial statements as of and for the three and nine months ended September 30, 2018.

Securities

Securities	9 Months Ended
	Sep. 30, 2018
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, 2018 December 31, 2017 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 $ 3,040 $ 8 $ 94 $ 2,954 $ 3,610 $ 15 $ 38 $ 3,587 States and political subdivisions 1,107,222 7,232 2,057 1,112,397 1,428,488 26,462 2,746 1,452,204 Other 1,500 — 6 1,494 — — — — Total $ 1,111,762 $ 7,240 $ 2,157 $ 1,116,845 $ 1,432,098 $ 26,477 $ 2,784 $ 1,455,791 Available for Sale U.S. Treasury $ 3,454,899 $ — $ 52,020 $ 3,402,879 $ 3,453,391 $ 7,494 $ 15,732 $ 3,445,153 Residential mortgage-backed securities 756,649 11,254 11,532 756,371 648,288 19,048 2,250 665,086 States and political subdivisions 7,052,156 40,759 116,225 6,976,690 6,185,711 167,293 16,795 6,336,209 Other 42,651 — — 42,651 42,561 — — 42,561 Total $ 11,306,355 $ 52,013 $ 179,777 $ 11,178,591 $ 10,329,951 $ 193,835 $ 34,777 $ 10,489,009 All mortgage-backed securities included in the above table were issued by U.S. government agencies and corporations. At September 30, 2018 , approximately 98.4% of the securities in our municipal bond portfolio were issued by political subdivisions or agencies within the State of Texas, of which approximately 68.0% 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 $3.7 billion at September 30, 2018 and $3.8 billion at December 31, 2017 . 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, 2018 totaled $3.1 million ( $2.5 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, 2018 , 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 $ 19 $ 1,850 $ 75 $ 2,263 $ 94 States and political subdivisions 403,965 2,057 — — 403,965 2,057 Other 1,494 6 — — 1,494 6 Total $ 405,872 $ 2,082 $ 1,850 $ 75 $ 407,722 $ 2,157 Available for Sale U.S. Treasury $ 2,295,611 $ 34,315 $ 1,107,268 $ 17,705 $ 3,402,879 $ 52,020 Residential mortgage-backed securities 294,839 5,979 98,757 5,553 393,596 11,532 States and political subdivisions 3,585,212 65,581 810,732 50,644 4,395,944 116,225 Total $ 6,175,662 $ 105,875 $ 2,016,757 $ 73,902 $ 8,192,419 $ 179,777 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, 2018 , 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, 2018 , 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, 2018 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 $ 56,511 $ 57,181 $ 858,235 $ 851,163 Due after one year through five years 131,987 133,521 3,243,532 3,204,804 Due after five years through ten years 492,460 492,354 489,960 487,536 Due after ten years 427,764 430,835 5,915,328 5,836,066 Residential mortgage-backed securities 3,040 2,954 756,649 756,371 Equity securities — — 42,651 42,651 Total $ 1,111,762 $ 1,116,845 $ 11,306,355 $ 11,178,591 Sales of Securities. Sales of securities available for sale were as follows: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Proceeds from sales $ 2,948,178 $ 746,524 $ 13,838,566 $ 8,993,963 Gross realized gains — — 3 — Gross realized losses (34 ) (4,867 ) (116 ) (4,917 ) Tax (expense) benefit of securities gains/losses 7 1,703 24 1,721 Premiums and Discounts . Premium amortization and discount accretion included in interest income on securities was as follows: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Premium amortization $ (27,381 ) $ (24,586 ) $ (80,104 ) $ (72,733 ) Discount accretion 2,569 1,783 6,356 6,278 Net (premium amortization) discount accretion $ (24,812 ) $ (22,803 ) $ (73,748 ) $ (66,455 ) Trading Account Securities. Trading account securities, at estimated fair value, were as follows: September 30, December 31, U.S. Treasury $ 21,804 $ 19,210 States and political subdivisions 434 1,888 Total $ 22,238 $ 21,098 Net gains and losses on trading account securities were as follows: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Net gain on sales transactions $ 465 $ 414 $ 1,404 $ 1,018 Net mark-to-market gains (losses) (8 ) (8 ) (21 ) (51 ) Net gain (loss) on trading account securities $ 457 $ 406 $ 1,383 $ 967

Loans

Loans	9 Months Ended
	Sep. 30, 2018
Receivables [Abstract]
Loans	Loans Loans were as follows: September 30, Percentage of Total December 31, Percentage of Total Commercial and industrial $ 5,029,754 36.4 % $ 4,792,388 36.4 % Energy: Production 1,220,771 8.8 1,182,326 9.0 Service 164,889 1.2 171,795 1.3 Other 133,708 1.0 144,972 1.1 Total energy 1,519,368 11.0 1,499,093 11.4 Commercial real estate: Commercial mortgages 4,078,787 29.5 3,887,742 29.6 Construction 1,208,870 8.7 1,066,696 8.1 Land 315,384 2.3 331,986 2.5 Total commercial real estate 5,603,041 40.5 5,286,424 40.2 Consumer real estate: Home equity loans 352,292 2.5 355,342 2.7 Home equity lines of credit 326,876 2.4 291,950 2.2 Other 419,965 3.1 376,002 2.9 Total consumer real estate 1,099,133 8.0 1,023,294 7.8 Total real estate 6,702,174 48.5 6,309,718 48.0 Consumer and other 563,542 4.1 544,466 4.2 Total loans $ 13,814,838 100.0 % $ 13,145,665 100.0 % 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, 2018 , there were no concentrations of loans related to any single industry in excess of 10% of total loans other than energy loans, which totaled 11.0% of total loans. Unfunded commitments to extend credit and standby letters of credit issued to customers in the energy industry totaled $1.1 billion and $46.7 million , respectively, as of September 30, 2018 . 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, 2018 or December 31, 2017 . 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 $238.6 million at September 30, 2018 and $166.4 million at December 31, 2017 . Non-Accrual and Past Due Loans. Non-accrual loans, segregated by class of loans, were as follows: September 30, December 31, Commercial and industrial $ 12,278 $ 46,186 Energy 51,802 94,302 Commercial real estate: Buildings, land and other 15,913 7,589 Construction — — Consumer real estate 971 2,109 Consumer and other 1,637 128 Total $ 82,601 $ 150,314 Had non-accrual loans performed in accordance with their original contract terms, we would have recognized additional interest income, net of tax, of approximately $1.3 million and $4.2 million for the three and nine months ended September 30, 2018 , compared to $783 thousand and $2.4 million for the three and nine months ended September 30, 2017 . An age analysis of past due loans (including both accruing and non-accruing loans), segregated by class of loans, as of September 30, 2018 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 $ 33,985 $ 10,250 $ 44,235 $ 4,985,519 $ 5,029,754 $ 3,963 Energy 3,251 2,221 5,472 1,513,896 1,519,368 818 Commercial real estate: Buildings, land and other 21,596 2,980 24,576 4,369,595 4,394,171 2,606 Construction 784 1,042 1,826 1,207,044 1,208,870 1,042 Consumer real estate 7,743 1,773 9,516 1,089,617 1,099,133 1,432 Consumer and other 6,098 1,766 7,864 555,678 563,542 1,724 Total $ 73,457 $ 20,032 $ 93,489 $ 13,721,349 $ 13,814,838 $ 11,585 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, 2018 Commercial and industrial $ 19,857 $ 3,153 $ 6,574 $ 9,727 $ 4,622 Energy 68,087 7,942 43,614 51,556 12,672 Commercial real estate: Buildings, land and other 15,961 2,309 12,876 15,185 2,599 Construction — — — — — Consumer real estate 293 293 — 293 — Consumer and other 1,595 — 1,595 1,595 1,595 Total $ 105,793 $ 13,697 $ 64,659 $ 78,356 $ 21,488 December 31, 2017 Commercial and industrial $ 60,781 $ 28,038 $ 15,722 $ 43,760 $ 7,553 Energy 99,606 33,080 61,162 94,242 13,267 Commercial real estate: Buildings, land and other 10,795 6,394 — 6,394 — Construction — — — — — Consumer real estate 1,214 1,214 — 1,214 — Consumer and other — — — — — Total $ 172,396 $ 68,726 $ 76,884 $ 145,610 $ 20,820 The average recorded investment in impaired loans was as follows: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Commercial and industrial $ 13,447 $ 26,910 $ 21,025 $ 26,651 Energy 78,772 76,008 82,640 72,055 Commercial real estate: Buildings, land and other 14,306 5,553 12,328 6,106 Construction — — — — Consumer real estate 671 1,209 807 1,155 Consumer and other 1,073 — 805 13 Total $ 108,269 $ 109,680 $ 117,605 $ 105,980 Troubled Debt Restructurings . Troubled debt restructurings during the nine months ended September 30, 2018 and September 30, 2017 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 $ 2,203 $ — $ 4,026 $ 3,875 Energy 13,708 — 56,097 55,023 $ 15,911 $ — $ 60,123 $ 58,898 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 nine months ended September 30, 2018 and September 30, 2017 is set forth in the following table. September 30, 2018 September 30, 2017 Restructured loans past due in excess of 90 days at period-end: Number of loans — 1 Dollar amount of loans $ — $ 43,137 Restructured loans on non-accrual status at period end — 54,082 Charge-offs of restructured loans: Recognized in connection with restructuring — — Recognized on previously restructured loans 4,650 9,951 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 2017 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, 2018 December 31, 2017 Weighted Loans Weighted Loans Commercial and industrial: Risk grades 1-8 6.10 $ 4,755,739 6.06 $ 4,378,839 Risk grade 9 9.00 123,199 9.00 170,285 Risk grade 10 10.00 67,645 10.00 99,260 Risk grade 11 11.00 70,893 11.00 97,818 Risk grade 12 12.00 7,656 12.00 38,633 Risk grade 13 13.00 4,622 13.00 7,553 Total 6.31 $ 5,029,754 6.41 $ 4,792,388 Energy Risk grades 1-8 5.81 $ 1,367,235 6.01 $ 1,199,207 Risk grade 9 9.00 13,352 9.00 50,427 Risk grade 10 10.00 42,862 10.00 64,282 Risk grade 11 11.00 44,116 11.00 90,875 Risk grade 12 12.00 39,131 12.00 81,035 Risk grade 13 13.00 12,672 13.00 13,267 Total 6.32 $ 1,519,368 6.97 $ 1,499,093 Commercial real estate: Buildings, land and other Risk grades 1-8 6.77 $ 4,091,571 6.75 $ 3,868,659 Risk grade 9 9.00 135,521 9.00 151,487 Risk grade 10 10.00 60,169 10.00 129,391 Risk grade 11 11.00 90,997 11.00 62,602 Risk grade 12 12.00 13,314 12.00 7,589 Risk grade 13 13.00 2,599 13.00 — Total 6.99 $ 4,394,171 7.00 $ 4,219,728 Construction Risk grades 1-8 7.10 $ 1,129,492 7.11 $ 1,019,635 Risk grade 9 9.00 51,058 9.00 18,042 Risk grade 10 10.00 26,035 10.00 23,393 Risk grade 11 11.00 2,285 11.00 5,626 Risk grade 12 12.00 — 12.00 — Risk grade 13 13.00 — 13.00 — Total 7.25 $ 1,208,870 7.23 $ 1,066,696 Net (charge-offs)/recoveries, segregated by class of loans, were as follows: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Commercial and industrial $ (7,807 ) $ (4,565 ) $ (19,030 ) $ (12,155 ) Energy (5,347 ) 451 (10,272 ) (10,010 ) Commercial real estate: Buildings, land and other 33 266 (288 ) 768 Construction 3 2 11 8 Consumer real estate (388 ) (629 ) (1,078 ) (422 ) Consumer and other (1,792 ) (1,760 ) (4,975 ) (4,289 ) Total $ (15,298 ) $ (6,235 ) $ (35,632 ) $ (26,100 ) 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 2017 Form 10-K, totaled 129.4 at September 30, 2018 and 129.3 at December 31, 2017 . 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 2017 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, 2018 and December 31, 2017 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, 2018 Historical valuation allowances $ 25,582 $ 9,654 $ 20,441 $ 2,641 $ 6,815 $ 65,133 Specific valuation allowances 4,622 12,672 2,599 — 1,595 21,488 General valuation allowances 9,361 5,849 4,187 1,620 (114 ) 20,903 Macroeconomic valuation allowances 11,426 3,270 11,977 1,933 1,448 30,054 Total $ 50,991 $ 31,445 $ 39,204 $ 6,194 $ 9,744 $ 137,578 Allocated to loans: Individually evaluated $ 4,622 $ 12,672 $ 2,599 $ — $ 1,595 $ 21,488 Collectively evaluated 46,369 18,773 36,605 6,194 8,149 116,090 Total $ 50,991 $ 31,445 $ 39,204 $ 6,194 $ 9,744 $ 137,578 December 31, 2017 Historical valuation allowances $ 26,401 $ 22,073 $ 18,931 $ 2,473 $ 5,603 $ 75,481 Specific valuation allowances 7,553 13,267 — — — 20,820 General valuation allowances 9,112 7,964 4,165 2,133 (91 ) 23,283 Macroeconomic valuation allowances 16,548 8,224 7,852 1,051 2,105 35,780 Total $ 59,614 $ 51,528 $ 30,948 $ 5,657 $ 7,617 $ 155,364 Allocated to loans: Individually evaluated $ 7,553 $ 13,267 $ — $ — $ — $ 20,820 Collectively evaluated 52,061 38,261 30,948 5,657 7,617 134,544 Total $ 59,614 $ 51,528 $ 30,948 $ 5,657 $ 7,617 $ 155,364 Our recorded investment in loans as of September 30, 2018 and December 31, 2017 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, 2018 Individually evaluated $ 9,727 $ 51,556 $ 15,185 $ 293 $ 1,595 $ 78,356 Collectively evaluated 5,020,027 1,467,812 5,587,856 1,098,840 561,947 13,736,482 Total $ 5,029,754 $ 1,519,368 $ 5,603,041 $ 1,099,133 $ 563,542 $ 13,814,838 December 31, 2017 Individually evaluated $ 43,760 $ 94,242 $ 6,394 $ 1,214 $ — $ 145,610 Collectively evaluated 4,748,628 1,404,851 5,280,030 1,022,080 544,466 13,000,055 Total $ 4,792,388 $ 1,499,093 $ 5,286,424 $ 1,023,294 $ 544,466 $ 13,145,665 The following table details activity in the allowance for loan losses by portfolio segment for the three and nine months ended September 30, 2018 and 2017 . 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, 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 September 30, 2017 Beginning balance $ 48,906 $ 54,277 $ 33,002 $ 5,535 $ 7,838 $ 149,558 Provision for loan losses 4,096 (2,815 ) 4,805 1,969 2,925 10,980 Charge-offs (5,468 ) — — (766 ) (4,120 ) (10,354 ) Recoveries 903 451 268 137 2,360 4,119 Net charge-offs (4,565 ) 451 268 (629 ) (1,760 ) (6,235 ) Ending balance $ 48,437 $ 51,913 $ 38,075 $ 6,875 $ 9,003 $ 154,303 Nine months ended: 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 September 30, 2017 Beginning balance $ 52,915 $ 60,653 $ 30,213 $ 4,238 $ 5,026 $ 153,045 Provision for loan losses 7,677 1,270 7,086 3,059 8,266 27,358 Charge-offs (14,574 ) (10,595 ) (14 ) (779 ) (11,291 ) (37,253 ) Recoveries 2,419 585 790 357 7,002 11,153 Net charge-offs (12,155 ) (10,010 ) 776 (422 ) (4,289 ) (26,100 ) Ending balance $ 48,437 $ 51,913 $ 38,075 $ 6,875 $ 9,003 $ 154,303

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	9 Months Ended
	Sep. 30, 2018
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 $ 3,213 $ 4,044 Customer relationships 743 986 Non-compete agreements 24 43 $ 3,980 $ 5,073 The estimated aggregate future amortization expense for intangible assets remaining as of September 30, 2018 is as follows: Remainder of 2018 331 2019 1,167 2020 918 2021 697 2022 481 Thereafter 386 $ 3,980

Deposits

Deposits	9 Months Ended
	Sep. 30, 2018
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 $ 10,137,696 38.4 % $ 10,412,882 38.8 % Correspondent banks 187,589 0.7 222,648 0.8 Public funds 515,228 2.0 561,563 2.1 Total non-interest-bearing demand deposits 10,840,513 41.1 11,197,093 41.7 Interest-bearing deposits: Private accounts: Savings and interest checking 6,640,382 25.2 6,788,766 25.2 Money market accounts 7,694,628 29.2 7,624,471 28.4 Time accounts of $100,000 or more 484,236 1.8 453,668 1.7 Time accounts under $100,000 327,790 1.3 324,636 1.2 Total private accounts 15,147,036 57.5 15,191,541 56.5 Public funds: Savings and interest checking 303,150 1.2 410,140 1.5 Money market accounts 54,537 0.2 59,008 0.2 Time accounts of $100,000 or more 3,523 — 14,301 0.1 Time accounts under $100,000 95 — 306 — Total public funds 361,305 1.4 483,755 1.8 Total interest-bearing deposits 15,508,341 58.9 15,675,296 58.3 Total deposits $ 26,348,854 100.0 % $ 26,872,389 100.0 % The following table presents additional information about our deposits: September 30, December 31, Deposits from foreign sources (primarily Mexico) $ 739,424 $ 716,339 Deposits not covered by deposit insurance 12,403,549 13,281,040

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
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 2017 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 $ 8,284,873 $ 7,949,400 Standby letters of credit 244,743 236,595 Deferred standby letter of credit fees 1,657 1,843 Lease Commitments . We lease certain office facilities and office equipment under operating leases. Rent expense for all operating leases totaled $8.1 million and $24.4 million during the three and nine months ended September 30, 2018 and $7.7 million and $23.0 million during the three and nine months ended September 30, 2017 . There has been no significant change in our expected future minimum lease payments since December 31, 2017 . See the 2017 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, 2018
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, 2018 and December 31, 2017 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, 2018 or December 31, 2017 . 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, 2018 and December 31, 2017 . The following tables present actual and required capital ratios as of September 30, 2018 and December 31, 2017 for Cullen/Frost and Frost Bank under the Basel III Capital Rules. The minimum required capital amounts presented include the minimum required capital levels as of September 30, 2018 and December 31, 2017 based on the phase-in provisions of the Basel III Capital Rules and the minimum required capital levels as of January 1, 2019 when the Basel III Capital Rules have been 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 2017 Form 10-K for a more detailed discussion of the Basel III Capital Rules. 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 September 30, 2018 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,658,645 12.93 % $ 1,310,621 6.38 % $ 1,439,113 7.00 % $ 1,336,319 6.50 % Frost Bank 2,683,196 13.10 1,306,205 6.38 1,434,264 7.00 1,331,816 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 2,803,131 13.63 1,619,002 7.88 1,747,494 8.50 1,644,701 8.00 Frost Bank 2,683,196 13.10 1,613,547 7.88 1,741,606 8.50 1,639,159 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,174,209 15.44 2,030,177 9.88 2,158,670 10.50 2,055,876 10.00 Frost Bank 2,821,274 13.77 2,023,337 9.88 2,151,396 10.50 2,048,948 10.00 Leverage Ratio Cullen/Frost 2,803,131 9.19 1,220,627 4.00 1,220,627 4.00 1,525,784 5.00 Frost Bank 2,683,196 8.80 1,219,806 4.00 1,219,806 4.00 1,524,758 5.00 December 31, 2017 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,426,048 12.42 % $ 1,123,430 5.75 % $ 1,367,583 7.00 % $ 1,269,965 6.50 % Frost Bank 2,518,999 12.92 1,120,663 5.75 1,364,214 7.00 1,266,836 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 2,570,534 13.16 1,416,499 7.25 1,660,637 8.50 1,563,033 8.00 Frost Bank 2,518,999 12.92 1,413,010 7.25 1,656,546 8.50 1,559,183 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 2,959,326 15.15 1,807,257 9.25 2,051,375 10.50 1,953,792 10.00 Frost Bank 2,674,791 13.72 1,802,805 9.25 2,046,321 10.50 1,948,979 10.00 Leverage Ratio Cullen/Frost 2,570,534 8.46 1,215,227 4.00 1,215,186 4.00 1,519,034 5.00 Frost Bank 2,518,999 8.30 1,214,295 4.00 1,214,254 4.00 1,517,869 5.00 As of September 30, 2018 , capital levels at Cullen/Frost and Frost Bank exceed all capital adequacy requirements under the Basel III Capital Rules on a fully phased-in basis. Based on the ratios presented above, capital levels as of September 30, 2018 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, 2018 , 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. No shares were repurchased under this plan during 2018 or 2017. Under a prior plan, we repurchased 1,134,966 shares under the plan at a total cost of $100.0 million during the third quarter of 2017. 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. 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, 2018 , Frost Bank could pay aggregate dividends of up to $569.7 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, 2018
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 2017 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, 2018 and December 31, 2017 . September 30, 2018 December 31, 2017 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 $ 11,284 $ 314 $ 13,679 $ 242 Loan/lease interest rate swaps – liabilities 4,226 (201 ) 11,147 (593 ) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets 780,531 6,179 430,449 1,418 Loan/lease interest rate swaps – liabilities 341,987 (5,133 ) 541,496 (12,820 ) Loan/lease interest rate caps – assets 102,487 920 114,619 480 Customer counterparties: Loan/lease interest rate swaps – assets 341,987 7,017 541,496 17,882 Loan/lease interest rate swaps – liabilities 780,531 (20,613 ) 430,449 (4,861 ) Loan/lease interest rate caps – liabilities 102,487 (920 ) 114,619 (480 ) The weighted-average rates paid and received for interest rate swaps outstanding at September 30, 2018 were as follows: Weighted-Average Interest Rate Paid Interest Rate Received Interest rate swaps: Fair value hedge loan/lease interest rate swaps 2.45 % 2.16 % Non-hedging interest rate swaps – financial institution counterparties 4.09 % 3.73 % Non-hedging interest rate swaps – customer counterparties 3.73 % 4.09 % The weighted-average strike rate for outstanding interest rate caps was 2.96% at September 30, 2018 . 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, 2018 December 31, 2017 Notional Units Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Oil – assets Barrels 622 $ 917 253 $ 193 Oil – liabilities Barrels 2,889 (34,941 ) 2,731 (13,448 ) Natural gas – assets MMBTUs 7,939 427 5,927 1,399 Natural gas – liabilities MMBTUs 8,382 (598 ) 3,917 (326 ) Customer counterparties: Oil – assets Barrels 2,889 35,083 2,731 13,709 Oil – liabilities Barrels 622 (917 ) 253 (187 ) Natural gas – assets MMBTUs 9,588 682 3,917 340 Natural gas – liabilities MMBTUs 6,734 (392 ) 5,927 (1,366 ) 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, 2018 December 31, 2017 Notional Currency Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Forward contracts – assets EUR 1,097 $ 9 4,014 $ 77 Forward contracts – assets GBP 793 9 127 1 Forward contracts – liabilities EUR — — 4,846 (37 ) Forward contracts – liabilities CAD 22,971 (189 ) 25,413 (142 ) Forward contracts – liabilities GBP — — 1,178 (9 ) Forward contracts – liabilities AUD 54 — — — Customer counterparties: Forward contracts – assets EUR — — 3,867 58 Forward contracts – assets CAD 22,918 245 25,282 279 Forward contracts – liabilities EUR — — 4,041 (51 ) Forward contracts – liabilities GBP — — 127 — 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 2018 2017 2018 2017 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ 12 $ (149 ) $ 1 $ (592 ) Amount of (gain) loss included in other non-interest expense — (2 ) (1 ) (5 ) 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 2018 2017 2018 2017 Non-hedging interest rate derivatives: Other non-interest income $ 699 $ 1,085 $ 2,889 $ 2,062 Other non-interest expense 2 — (2 ) 1 Non-hedging commodity derivatives: Other non-interest income 456 231 492 387 Non-hedging foreign currency derivatives: Other non-interest income 53 83 203 101 Counterparty Credit Risk. Our credit exposure relating to interest rate swaps, commodity swaps/options and foreign currency forward contracts with bank customers was approximately $41.0 million at September 30, 2018 . 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 $431 thousand at September 30, 2018 . This amount was primarily related to 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. The aggregate fair value of securities we posted as collateral related to derivative contracts totaled $7.0 million at September 30, 2018 . At such date, we also had $26.7 million in cash collateral on deposit with other financial institution counterparties.

Balance Sheet Offsetting

Balance Sheet Offsetting	9 Months Ended
	Sep. 30, 2018
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, 2018 is presented in the following tables. Gross Amount Recognized Gross Amount Offset Net Amount Recognized September 30, 2018 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 7,413 $ — $ 7,413 Commodity swaps and options 1,344 — 1,344 Foreign currency forward contracts 18 — 18 Total derivatives 8,775 — 8,775 Resell agreements 12,142 — 12,142 Total $ 20,917 $ — $ 20,917 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 5,334 $ — $ 5,334 Commodity swaps and options 35,539 — 35,539 Foreign currency forward contracts 189 — 189 Total derivatives 41,062 — 41,062 Repurchase agreements 1,074,576 — 1,074,576 Total $ 1,115,638 $ — $ 1,115,638 Gross Amounts Not Offset Net Amount Recognized Financial Instruments Collateral Net Amount September 30, 2018 Financial assets: Derivatives: Counterparty A $ 1,477 $ (1,477 ) $ — $ — Counterparty B 1,963 (1,963 ) — — Counterparty C 75 (75 ) — — Other counterparties 5,260 (1,825 ) (3,223 ) 212 Total derivatives 8,775 (5,340 ) (3,223 ) 212 Resell agreements 12,142 — (12,142 ) — Total $ 20,917 $ (5,340 ) $ (15,365 ) $ 212 Financial liabilities: Derivatives: Counterparty A $ 3,840 $ (1,477 ) $ (2,363 ) $ — Counterparty B 6,627 (1,963 ) (4,453 ) 211 Counterparty C 806 (75 ) (731 ) — Other counterparties 29,789 (1,825 ) (25,908 ) 2,056 Total derivatives 41,062 (5,340 ) (33,455 ) 2,267 Repurchase agreements 1,074,576 — (1,074,576 ) — Total $ 1,115,638 $ (5,340 ) $ (1,108,031 ) $ 2,267 Information about financial instruments that are eligible for offset in the consolidated balance sheet as of December 31, 2017 is presented in the following tables. Gross Amount Recognized Gross Amount Offset Net Amount Recognized December 31, 2017 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 2,140 $ — $ 2,140 Commodity swaps and options 1,592 — 1,592 Foreign currency forward contracts 78 — 78 Total derivatives 3,810 — 3,810 Resell agreements 9,642 — 9,642 Total $ 13,452 $ — $ 13,452 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 13,413 $ — $ 13,413 Commodity swaps and options 13,774 — 13,774 Foreign currency forward contracts 188 — 188 Total derivatives 27,375 — 27,375 Repurchase agreements 1,117,199 — 1,117,199 Total $ 1,144,574 $ — $ 1,144,574 Gross Amounts Not Offset Net Amount Recognized Financial Instruments Collateral Net Amount December 31, 2017 Financial assets: Derivatives: Counterparty A $ 395 $ (395 ) $ — $ — Counterparty B 1,028 (1,028 ) — — Counterparty C 55 (55 ) — — Other counterparties 2,332 (1,830 ) (387 ) 115 Total derivatives 3,810 (3,308 ) (387 ) 115 Resell agreements 9,642 — (9,642 ) — Total $ 13,452 $ (3,308 ) $ (10,029 ) $ 115 Financial liabilities: Derivatives: Counterparty A $ 7,397 $ (395 ) $ (7,002 ) $ — Counterparty B 4,466 (1,028 ) (3,101 ) 337 Counterparty C 1,520 (55 ) (1,450 ) 15 Other counterparties 13,992 (1,830 ) (11,215 ) 947 Total derivatives 27,375 (3,308 ) (22,768 ) 1,299 Repurchase agreements 1,117,199 — (1,117,199 ) — Total $ 1,144,574 $ (3,308 ) $ (1,139,967 ) $ 1,299 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, 2018 and December 31, 2017 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, 2018 Repurchase agreements: U.S. Treasury $ 1,067,059 $ — $ — $ — $ 1,067,059 Residential mortgage-backed securities 7,517 — — — 7,517 Total borrowings $ 1,074,576 $ — $ — $ — $ 1,074,576 Gross amount of recognized liabilities for repurchase agreements $ 1,074,576 Amounts related to agreements not included in offsetting disclosures above $ — December 31, 2017 Repurchase agreements: U.S. Treasury $ 1,036,891 $ — $ — $ — $ 1,036,891 Residential mortgage-backed securities 80,308 — — — 80,308 Total borrowings $ 1,117,199 $ — $ — $ — $ 1,117,199 Gross amount of recognized liabilities for repurchase agreements $ 1,117,199 Amounts related to agreements not included in offsetting disclosures above $ —

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2018
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, 2018 , there were 1,415,135 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, 2018 53,008 $ 64.87 312,656 $ 81.71 80,103 $ 79.91 2,917,142 $ 63.34 Authorized — — — — — — — — Granted 6,576 109.58 — — — — — — Exercised/vested (10,674 ) 63.68 (2,470 ) 78.92 — — (434,687 ) 61.12 Forfeited/expired — — (4,836 ) 87.00 — — (49,125 ) 70.70 Balance, September 30, 2018 48,910 $ 71.14 305,350 $ 81.65 80,103 $ 79.91 2,433,330 $ 63.59 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 2018 2017 2018 2017 New shares issued from available authorized shares — 9,299 — 602,662 Issued from available treasury stock 19,232 13,425 447,831 172,137 Total 19,232 22,724 447,831 774,799 Proceeds from stock option exercises $ 1,119 $ 1,274 $ 26,567 $ 45,422 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 2018 2017 2018 2017 Stock options $ 960 $ 1,532 $ 3,064 $ 4,892 Non-vested stock awards/stock units 1,357 813 4,196 2,747 Director deferred stock units — — 720 519 Performance stock units 479 377 1,576 855 Total $ 2,796 $ 2,722 $ 9,556 $ 9,013 Income tax benefit $ 587 $ 953 $ 2,007 $ 3,155 Unrecognized stock-based compensation expense at September 30, 2018 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 $ 1,906 Non-vested stock awards/stock units 10,343 Performance stock units 3,553 Total $ 15,802

Earnings Per Common Share

Earnings Per Common Share	9 Months Ended
	Sep. 30, 2018
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 2017 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 2018 2017 2018 2017 Net income $ 117,844 $ 93,131 $ 335,665 $ 263,625 Less: Preferred stock dividends 2,016 2,016 6,047 6,047 Net income available to common shareholders 115,828 91,115 329,618 257,578 Less: Earnings allocated to participating securities 750 475 2,178 1,346 Net earnings allocated to common stock $ 115,078 $ 90,640 $ 327,440 $ 256,232 Distributed earnings allocated to common stock $ 42,811 $ 36,174 $ 121,907 $ 107,194 Undistributed earnings allocated to common stock 72,267 54,466 205,533 149,038 Net earnings allocated to common stock $ 115,078 $ 90,640 $ 327,440 $ 256,232 Weighted-average shares outstanding for basic earnings per common share 63,892,023 63,667,356 63,793,514 63,822,011 Dilutive effect of stock compensation 1,022,168 897,945 1,037,043 957,337 Weighted-average shares outstanding for diluted earnings per common share 64,914,191 64,565,301 64,830,557 64,779,348

Defined Benefit Plans

Defined Benefit Plans	9 Months Ended
	Sep. 30, 2018
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 2018 2017 2018 2017 Expected return on plan assets, net of expenses $ (2,979 ) $ (2,779 ) $ (8,937 ) $ (8,338 ) Interest cost on projected benefit obligation 1,475 1,547 4,424 4,642 Net amortization and deferral 1,250 1,357 3,751 4,072 Net periodic expense (benefit) $ (254 ) $ 125 $ (762 ) $ 376 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, 2018 . We do not expect to make any contributions to the qualified defined benefit plan during the remainder of 2018 .

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Income tax expense was as follows: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Current income tax expense (benefit) $ (1,152 ) $ 15,224 $ 955 $ 44,636 Deferred income tax expense (benefit) 16,312 (5,332 ) 39,198 (9,505 ) Income tax expense, as reported $ 15,160 $ 9,892 $ 40,153 $ 35,131 Effective tax rate 11.4 % 9.6 % 10.7 % 11.8 % Net deferred tax assets totaled $54.1 million at September 30, 2018 and $31.7 million at December 31, 2017 . No valuation allowance for deferred tax assets was recorded at September 30, 2018 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 2018 and 35% during 2017 primarily due to the effect of tax-exempt income from loans, securities and life insurance policies and the income tax effects associated with stock-based compensation. The effective income tax rates for the three and nine months ended September 30, 2017 were also impacted by the correction of an over-accrual of taxes that resulted from incorrectly classifying certain tax-exempt loans as taxable for federal income tax purposes since 2013. As a result, we recognized tax benefits totaling $3.7 million , which included $2.9 million related to the 2013 through 2016 tax years and $756 thousand related to the first and second quarters of 2017. 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 2015. Tax Cuts and Jobs Act. The Tax Cuts and Jobs Act was enacted on December 22, 2017 as more fully discussed in the 2017 Form 10-K. Among other things, the new law established a new, flat corporate federal statutory income tax rate of 21% . As a result, we remeasured our deferred tax assets and liabilities based on the new tax rate and recognized a provisional net tax benefit related to the remeasurement totaling $4.0 million . Notwithstanding the foregoing, we are still analyzing certain aspects of the new law and refining our calculations, which could affect the measurement of these assets and liabilities or give rise to new deferred tax amounts. Nonetheless, there has been no change to the provisional net tax benefit we recorded during the fourth quarter of 2017.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2018
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 $ (96,147 ) $ (20,190 ) $ (75,957 ) $ 7,082 $ 2,479 $ 4,603 Change in net unrealized gain on securities transferred to held to maturity (3,764 ) (790 ) (2,974 ) (3,514 ) (1,230 ) (2,284 ) Reclassification adjustment for net (gains) losses included in net income 34 7 27 4,867 1,703 3,164 Total securities available for sale and transferred securities (99,877 ) (20,973 ) (78,904 ) 8,435 2,952 5,483 Defined-benefit post-retirement benefit plans: Change in the net actuarial gain/loss — — — — — — Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) 1,250 262 988 1,357 475 882 Total defined-benefit post-retirement benefit plans 1,250 262 988 1,357 475 882 Total other comprehensive income (loss) $ (98,627 ) $ (20,711 ) $ (77,916 ) $ 9,792 $ 3,427 $ 6,365 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 $ (286,935 ) $ (60,256 ) $ (226,679 ) $ 131,283 $ 45,949 $ 85,334 Change in net unrealized gain on securities transferred to held to maturity (8,424 ) (1,769 ) (6,655 ) (13,660 ) (4,781 ) (8,879 ) Reclassification adjustment for net (gains) losses included in net income 113 24 89 4,917 1,721 3,196 Total securities available for sale and transferred securities (295,246 ) (62,001 ) (233,245 ) 122,540 42,889 79,651 Defined-benefit post-retirement benefit plans: Change in the net actuarial gain/loss — — — — — — Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) 3,751 788 2,963 4,072 1,425 2,647 Total defined-benefit post-retirement benefit plans 3,751 788 2,963 4,072 1,425 2,647 Total other comprehensive income (loss) $ (291,495 ) $ (61,213 ) $ (230,282 ) $ 126,612 $ 44,314 $ 82,298 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, 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 ) Balance January 1, 2017 $ 16,153 $ (40,776 ) $ (24,623 ) Other comprehensive income (loss) before reclassifications 76,455 — 76,455 Reclassification of amounts included in net income 3,196 2,647 5,843 Net other comprehensive income (loss) during period 79,651 2,647 82,298 Balance at September 30, 2017 $ 95,804 $ (38,129 ) $ 57,675

Operating Segments

Operating Segments	9 Months Ended
	Sep. 30, 2018
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 2017 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, 2018 $ 295,062 $ 36,868 $ (2,608 ) $ 329,322 September 30, 2017 266,582 36,529 (2,285 ) 300,826 Nine months ended: September 30, 2018 $ 874,056 $ 106,475 $ (7,680 ) $ 972,851 September 30, 2017 786,743 107,829 (5,669 ) 888,903 Net income (loss): Three months ended: September 30, 2018 $ 114,214 $ 6,442 $ (2,812 ) $ 117,844 September 30, 2017 88,368 6,417 (1,654 ) 93,131 Nine months ended: September 30, 2018 $ 327,131 $ 17,977 $ (9,443 ) $ 335,665 September 30, 2017 250,766 17,990 (5,131 ) 263,625

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
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 2017 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, 2018 and December 31, 2017 , 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, 2018 Securities available for sale: U.S. Treasury $ 3,402,879 $ — $ — $ 3,402,879 Residential mortgage-backed securities — 756,371 — 756,371 States and political subdivisions — 6,976,690 — 6,976,690 Other — 42,651 — 42,651 Trading account securities: U.S. Treasury 21,804 — — 21,804 States and political subdivisions — 434 — 434 Derivative assets: Interest rate swaps, caps and floors — 14,430 — 14,430 Commodity swaps and options — 35,813 1,296 37,109 Foreign currency forward contracts 263 — — 263 Derivative liabilities: Interest rate swaps, caps and floors — 26,867 — 26,867 Commodity swaps and options — 36,848 — 36,848 Foreign currency forward contracts 189 — — 189 December 31, 2017 Securities available for sale: U.S. Treasury $ 3,445,153 $ — $ — $ 3,445,153 Residential mortgage-backed securities — 665,086 — 665,086 States and political subdivisions — 6,336,209 — 6,336,209 Other — 42,561 — 42,561 Trading account securities: U.S. Treasury 19,210 — — 19,210 States and political subdivisions — 1,888 — 1,888 Derivative assets: Interest rate swaps, caps and floors — 20,022 — 20,022 Commodity swaps and options — 14,408 1,233 15,641 Foreign currency forward contracts 415 — — 415 Derivative liabilities: Interest rate swaps, caps and floors — 18,754 — 18,754 Commodity swaps and options — 15,327 — 15,327 Foreign currency forward contracts 239 — — 239 Derivative assets, measured at fair value on a recurring basis using significant unobservable (Level 3) inputs during the reported periods consist of commodity swaps sold to loan customers. The significant unobservable (Level 3) inputs used in the fair value measurement of these commodity swaps sold to loan customers primarily relate to the probability of default and loss severity in the event of default. The probability of default is determined by the underlying risk grade of the loan (see Note 3 - Loans) underlying the commodity swap in that the probability of default increases as a loan’s risk grade deteriorates, while the loss severity is estimated through an analysis of the collateral supporting both the underlying loan and commodity swap. Generally, a change in the assumption used for the probability of default is accompanied by a directionally similar change in the assumption used for the loss severity. The weighted-average risk grade of loans underlying commodity swaps measured at fair value using significant unobservable (Level 3) inputs was 12.0 at both September 30, 2018 and December 31, 2017 . The weighted-average loss severity in the event of default on the commodity swaps was 19.7% and 15.4% at September 30, 2018 and December 31, 2017 , respectively. A reconciliation of the beginning and ending balances of derivative assets measured at fair value on a recurring basis using significant unobservable (Level 3) inputs is not presented as such amounts were not significant during the reported periods. 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 $ 12,876 $ 28,638 $ — $ 64,287 Specific valuation allowance (allocations) reversals of prior allocations (2,599 ) 4,256 — (13,477 ) Fair value $ 10,277 $ 32,894 $ — $ 50,810 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 2018 2017 Foreclosed assets remeasured at initial recognition: Carrying value of foreclosed assets prior to remeasurement $ 2,898 $ — Charge-offs recognized in the allowance for loan losses — — Fair value $ 2,898 $ — Foreclosed assets remeasured subsequent to initial recognition: Carrying value of foreclosed assets prior to remeasurement $ 1,823 $ 89 Write-downs included in other non-interest expense (473 ) (16 ) Fair value $ 1,350 $ 73 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, 2018 December 31, 2017 Carrying Amount Estimated Fair Value Carrying Amount Estimated Fair Value Financial assets: Level 2 inputs: Cash and cash equivalents $ 3,419,489 $ 3,419,489 $ 5,053,047 $ 5,053,047 Securities held to maturity 1,111,762 1,116,845 1,432,098 1,455,791 Cash surrender value of life insurance policies 182,603 182,603 180,477 180,477 Accrued interest receivable 137,553 137,553 167,508 167,508 Level 3 inputs: Loans, net 13,677,260 13,621,005 12,990,301 12,981,165 Financial liabilities: Level 2 inputs: Deposits 26,348,854 26,342,071 26,872,389 26,866,676 Federal funds purchased and repurchase agreements 1,083,777 1,083,777 1,147,824 1,147,824 Junior subordinated deferrable interest debentures 136,227 137,115 136,184 137,115 Subordinated notes payable and other borrowings 98,669 98,875 98,552 105,311 Accrued interest payable 4,904 4,904 3,358 3,358 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, 2018
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 2017 Form 10-K for additional information related to previously issued accounting standards updates. Accounting Standards Update (“ASU”) 2014-09, “Revenue from Contracts with Customers (Topic 606).” ASU 2014-09 implements a common revenue standard that clarifies the principles for recognizing revenue. The core principle of ASU 2014-09 is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve that core principle, an entity should apply the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies a performance obligation. We adopted ASU 2014-09 effective January 1, 2018. See Note 1 - Significant Accounting Policies for additional information. ASU 2016-02, “Leases (Topic 842).” ASU 2016-02 will, among other things, require 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. ASU 2016-02 does not significantly change lease accounting requirements applicable to lessors; however, certain changes were made to align, where necessary, lessor accounting with the lessee accounting model and ASC Topic 606, “Revenue from Contracts with Customers .” ASU 2016-02 will be effective for us on January 1, 2019 and initially required transition using a modified retrospective approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. In July 2018, the Financial Accounting Standards Board issued ASU 2018-11 which, among other things, provides an additional transition method that would allow entities to not apply the guidance in ASU 2016-02 in the comparative periods presented in the financial statements and instead recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. We are currently implementing a third-party vendor solution to assist us in the application of ASU 2016-02. We currently estimate that the adoption of ASU 2016-02 will result in the recognition of right-of-use assets and related lease liabilities totaling approximately $400 million to $500 million . We expect to elect to apply certain practical expedients provided under ASU 2016-02 whereby we will 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 also do not expect to apply the recognition requirements of ASU 2016-02 to any short-term leases (as defined by related accounting guidance). We expect to account for lease and non-lease components separately because such amounts are readily determinable under our lease contracts and because we expect this election will result in a lower impact on our balance sheet. 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. ASU 2016-13 will be effective on January 1, 2020. We are currently evaluating the potential impact of ASU 2016-13 on our financial statements. In that regard, we have formed a cross-functional working group, under the direction of our Chief Financial Officer and our Chief Risk Officer. The working group is comprised of individuals from various functional areas including credit, risk management, finance and information technology, among others. We are currently working through our implementation plan which includes assessment and documentation of processes, internal controls and data sources; model development and documentation; 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 the ASU 2016-13. The adoption of the ASU 2016-13 could result in an increase in the allowance for loan losses as 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. While we are currently unable to reasonably estimate the impact of adopting ASU 2016-13, we expect that the impact of adoption 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 the adoption date. 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 does not change the accounting for callable debt securities held at a discount. ASU 2017-08 will be effective for us on January 1, 2019. The impact of adopting ASU 2017-08 will be significantly influenced by the composition of our securities portfolio as of that adoption date. Nonetheless, we currently expect to recognize a cumulative effect adjustment reducing retained earnings by approximately $11.3 million . Furthermore, we expect premium amortization expense for 2019 to be approximately $5.3 million higher than what would have been the case had we continued to amortize the affected securities to their respective maturity dates. ASU 2018-02, "Income Statement - Reporting Comprehensive Income (Topic 220) - Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income." Under ASU 2018-02, entities may elect to reclassify certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act, which was enacted on December 22, 2017, from accumulated other comprehensive income to retained earnings. ASU 2018-02 also requires certain accounting policy disclosures. We elected to adopt the provisions of ASU 2018-02 as of January 1, 2018 in advance of the required application date of January 1, 2019. See Note 1 - Significant Accounting Policies. ASU 2018-05, "Income Taxes (Topic 740) - Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin (SAB) No. 118." ASU 2018-05 amends the Accounting Standards Codification to incorporate various SEC paragraphs pursuant to the issuance of SAB 118. SAB 118 addresses the application of generally accepted accounting principles in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Cuts and Jobs Act. See Note 13 - Income Taxes. ASU 2018-13, “Fair Value Measurement (Topic 820) - Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement.” ASU 2018-13 modifies the disclosure requirements on fair value measurements in Topic 820. The amendments in this update remove disclosures that no longer are considered cost beneficial, modify/clarify the specific requirements of certain disclosures, and add disclosure requirements identified as relevant. ASU 2018-13 will be effective for us on January 1, 2020, with early adoption permitted, and is not expected to have a significant impact on our financial statements. ASU 2018-14, “Compensation - Retirement Benefits-Defined Benefit Plans-General (Subtopic 715-20).” ASU 2018-14 amends and modifies the disclosure requirements for employers that sponsor defined benefit pension or other post-retirement plans. The amendments in this update remove disclosures that no longer are considered cost beneficial, clarify the specific requirements of disclosures, and add disclosure requirements identified as relevant. ASU 2018-14 will be effective for us on January 1, 2021, with early adoption permitted, and is not expected to have a significant impact on our financial statements. ASU 2018-15, “Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40) - Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract.” ASU 2018-15 clarifies certain aspects of ASU 2015-05, “Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement,” which was issued in April 2015. Specifically, ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). ASU 2018-15 does not affect the accounting for the service element of a hosting arrangement that is a service contract. ASU 2018-15 will be effective for us on January 1, 2020, with early adoption permitted, and is not expected to have a significant impact on our financial statements.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
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, 2017 , included in our Annual Report on Form 10-K filed with the SEC on February 7, 2018 (the “ 2017 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. In addition, we adopted ASU 2018-02, " Income Statement - Reporting Comprehensive Income (Topic 220) - Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income" as of January 1, 2018. In accordance with ASU 2018-02, we elected to reclassify certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act, which was enacted on December 22, 2017 (see Note 13 - Income Taxes), from accumulated other comprehensive income to retained earnings. Such amounts, which totaled $9.5 million , related to a net actuarial loss on defined benefit post-retirement plans and unrealized gains on securities available for sale and securities transferred to held to maturity. See Note 14 - Other Comprehensive Income. The effects of the Tax Cuts and Jobs Act on deferred taxes related to amounts initially recorded in accumulated other comprehensive income are provisional. As we finalize the accounting for the tax effects of the Tax Cuts and Jobs Act, additional reclassification adjustments may be recorded in future periods. See Note 13 - Income Taxes. Notwithstanding this election made in accordance with ASU 2018-02, our policy is to release such income tax effects only when the entire portfolio to which the underlying transactions relate is liquidated, sold or extinguished.
New Accounting Pronouncements, Policy [Policy Text Block]	We also adopted ASU 2014-09, "Revenue from Contracts with Customers (Topic 606)” as of January 1, 2018. Using a modified retrospective transition approach for contracts that were not complete as of our adoption, we recognized a cumulative effect reduction to beginning retained earnings totaling $2.3 million . The amount was related to certain revenue streams within trust and investment management fees. Additionally, based on our underlying contracts, ASU 2014-09 requires us to report network costs associated with debit card and ATM transactions netted against the related fee income from such transactions. Previously, such network costs were reported as a component of other non-interest expense. For the three and nine months ended September 30, 2018 , gross interchange and debit card transaction fees totaled $6.5 million and $19.0 million , respectively, while related network costs totaled $3.0 million and $8.9 million , respectively. On a net basis, we reported $3.5 million and $10.1 million as interchange and debit card transaction fees in the accompanying Consolidated Statement of Income for the three and nine months ended September 30, 2018 , respectively. For the three and nine months ended September 30, 2017 , we reported interchange and debit card transaction fees totaling $5.9 million and $17.2 million , respectively, on a gross basis in the accompanying Consolidated Statement of Income while related network costs totaling $3.0 million and $9.1 million were reported as a component of other non-interest expense for the three and nine months ended September 30, 2017 , respectively. ASU 2014-09 also required us to change the way we recognize certain recurring revenue streams reported as components of trust and investment management fees, insurance commissions and fees and other categories of non-interest income, however, such changes were not significant to our financial statements for the nine months ended September 30, 2018 . Under ASU 2014-09, we adopted new policies related to revenue recognition. In general, for revenue not associated with financial instruments, guarantees and lease contracts, we apply the following steps when recognizing revenue from contracts with customers: (i) identify the contract, (ii) identify the performance obligations, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations and (v) recognize revenue when performance obligation is satisfied. Our contracts with customers are generally short term in nature, typically due within one year or less or cancellable by us or our customer upon a short notice period. Performance obligations for our customer contracts are generally satisfied at a single point in time, typically when the transaction is complete, or over time. For performance obligations satisfied over time, we primarily use the output method, directly measuring the value of the products/services transferred to the customer, to determine when performance obligations have been satisfied. We typically receive payment from customers and recognize revenue concurrent with the satisfaction of our performance obligations. In most cases, this occurs within a single financial reporting period. For payments received in advance of the satisfaction of performance obligations, revenue recognition is deferred until such time the performance obligations have been satisfied. In cases where we have not received payment despite satisfaction of our performance obligations, we accrue an estimate of the amount due in the period our performance obligations have been satisfied. For contracts with variable components, only amounts for which collection is probable are accrued. We generally act in a principal capacity, on our own behalf, in most of our contracts with customers. In such transactions, we recognize revenue and the related costs to provide our services on a gross basis in our financial statements. In some cases, we act in an agent capacity, deriving revenue through assisting other entities in transactions with our customers. In such transactions, we recognized revenue and the related costs to provide our services on a net basis in our financial statements. These transactions primarily relate to insurance and brokerage commissions and fees derived from our customers' use of various interchange and ATM/debit card networks. In August 2018, the Securities and Exchange Commission (“SEC”) issued Final Rule Release No. 33-10532 - “Disclosure Update and Simplification.” This rule amends various SEC disclosure requirements that have been determined to be redundant, duplicative, overlapping, outdated, or superseded. The changes are generally expected to reduce or eliminate certain disclosures; however, the amendments did expand interim period disclosure requirements related to changes in shareholders' equity. Although the final rule does not become effective until November 5, 2018, we elected to expand our presentation of the Statement of Changes in Shareholders' Equity, as will be required under the new rule, in our financial statements as of and for the three and nine months ended September 30, 2018. Accounting Standards Updates Information about certain recently issued accounting standards updates is presented below. Also refer to Note 20 - Accounting Standards Updates in our 2017 Form 10-K for additional information related to previously issued accounting standards updates. Accounting Standards Update (“ASU”) 2014-09, “Revenue from Contracts with Customers (Topic 606).” ASU 2014-09 implements a common revenue standard that clarifies the principles for recognizing revenue. The core principle of ASU 2014-09 is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve that core principle, an entity should apply the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies a performance obligation. We adopted ASU 2014-09 effective January 1, 2018. See Note 1 - Significant Accounting Policies for additional information. ASU 2016-02, “Leases (Topic 842).” ASU 2016-02 will, among other things, require 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. ASU 2016-02 does not significantly change lease accounting requirements applicable to lessors; however, certain changes were made to align, where necessary, lessor accounting with the lessee accounting model and ASC Topic 606, “Revenue from Contracts with Customers .” ASU 2016-02 will be effective for us on January 1, 2019 and initially required transition using a modified retrospective approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. In July 2018, the Financial Accounting Standards Board issued ASU 2018-11 which, among other things, provides an additional transition method that would allow entities to not apply the guidance in ASU 2016-02 in the comparative periods presented in the financial statements and instead recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. We are currently implementing a third-party vendor solution to assist us in the application of ASU 2016-02. We currently estimate that the adoption of ASU 2016-02 will result in the recognition of right-of-use assets and related lease liabilities totaling approximately $400 million to $500 million . We expect to elect to apply certain practical expedients provided under ASU 2016-02 whereby we will 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 also do not expect to apply the recognition requirements of ASU 2016-02 to any short-term leases (as defined by related accounting guidance). We expect to account for lease and non-lease components separately because such amounts are readily determinable under our lease contracts and because we expect this election will result in a lower impact on our balance sheet. 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. ASU 2016-13 will be effective on January 1, 2020. We are currently evaluating the potential impact of ASU 2016-13 on our financial statements. In that regard, we have formed a cross-functional working group, under the direction of our Chief Financial Officer and our Chief Risk Officer. The working group is comprised of individuals from various functional areas including credit, risk management, finance and information technology, among others. We are currently working through our implementation plan which includes assessment and documentation of processes, internal controls and data sources; model development and documentation; 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 the ASU 2016-13. The adoption of the ASU 2016-13 could result in an increase in the allowance for loan losses as 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. While we are currently unable to reasonably estimate the impact of adopting ASU 2016-13, we expect that the impact of adoption 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 the adoption date. 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 does not change the accounting for callable debt securities held at a discount. ASU 2017-08 will be effective for us on January 1, 2019. The impact of adopting ASU 2017-08 will be significantly influenced by the composition of our securities portfolio as of that adoption date. Nonetheless, we currently expect to recognize a cumulative effect adjustment reducing retained earnings by approximately $11.3 million . Furthermore, we expect premium amortization expense for 2019 to be approximately $5.3 million higher than what would have been the case had we continued to amortize the affected securities to their respective maturity dates. ASU 2018-02, "Income Statement - Reporting Comprehensive Income (Topic 220) - Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income." Under ASU 2018-02, entities may elect to reclassify certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act, which was enacted on December 22, 2017, from accumulated other comprehensive income to retained earnings. ASU 2018-02 also requires certain accounting policy disclosures. We elected to adopt the provisions of ASU 2018-02 as of January 1, 2018 in advance of the required application date of January 1, 2019. See Note 1 - Significant Accounting Policies. ASU 2018-05, "Income Taxes (Topic 740) - Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin (SAB) No. 118." ASU 2018-05 amends the Accounting Standards Codification to incorporate various SEC paragraphs pursuant to the issuance of SAB 118. SAB 118 addresses the application of generally accepted accounting principles in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Cuts and Jobs Act. See Note 13 - Income Taxes. ASU 2018-13, “Fair Value Measurement (Topic 820) - Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement.” ASU 2018-13 modifies the disclosure requirements on fair value measurements in Topic 820. The amendments in this update remove disclosures that no longer are considered cost beneficial, modify/clarify the specific requirements of certain disclosures, and add disclosure requirements identified as relevant. ASU 2018-13 will be effective for us on January 1, 2020, with early adoption permitted, and is not expected to have a significant impact on our financial statements. ASU 2018-14, “Compensation - Retirement Benefits-Defined Benefit Plans-General (Subtopic 715-20).” ASU 2018-14 amends and modifies the disclosure requirements for employers that sponsor defined benefit pension or other post-retirement plans. The amendments in this update remove disclosures that no longer are considered cost beneficial, clarify the specific requirements of disclosures, and add disclosure requirements identified as relevant. ASU 2018-14 will be effective for us on January 1, 2021, with early adoption permitted, and is not expected to have a significant impact on our financial statements. ASU 2018-15, “Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40) - Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract.” ASU 2018-15 clarifies certain aspects of ASU 2015-05, “Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement,” which was issued in April 2015. Specifically, ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). ASU 2018-15 does not affect the accounting for the service element of a hosting arrangement that is a service contract. ASU 2018-15 will be effective for us on January 1, 2020, with early adoption permitted, and is not expected to have a significant impact on our financial statements.

Accounting Standards Updates Ac

Accounting Standards Updates Accounting Standards Updates (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
New Accounting Pronouncements, Policy [Policy Text Block]	We also adopted ASU 2014-09, "Revenue from Contracts with Customers (Topic 606)” as of January 1, 2018. Using a modified retrospective transition approach for contracts that were not complete as of our adoption, we recognized a cumulative effect reduction to beginning retained earnings totaling $2.3 million . The amount was related to certain revenue streams within trust and investment management fees. Additionally, based on our underlying contracts, ASU 2014-09 requires us to report network costs associated with debit card and ATM transactions netted against the related fee income from such transactions. Previously, such network costs were reported as a component of other non-interest expense. For the three and nine months ended September 30, 2018 , gross interchange and debit card transaction fees totaled $6.5 million and $19.0 million , respectively, while related network costs totaled $3.0 million and $8.9 million , respectively. On a net basis, we reported $3.5 million and $10.1 million as interchange and debit card transaction fees in the accompanying Consolidated Statement of Income for the three and nine months ended September 30, 2018 , respectively. For the three and nine months ended September 30, 2017 , we reported interchange and debit card transaction fees totaling $5.9 million and $17.2 million , respectively, on a gross basis in the accompanying Consolidated Statement of Income while related network costs totaling $3.0 million and $9.1 million were reported as a component of other non-interest expense for the three and nine months ended September 30, 2017 , respectively. ASU 2014-09 also required us to change the way we recognize certain recurring revenue streams reported as components of trust and investment management fees, insurance commissions and fees and other categories of non-interest income, however, such changes were not significant to our financial statements for the nine months ended September 30, 2018 . Under ASU 2014-09, we adopted new policies related to revenue recognition. In general, for revenue not associated with financial instruments, guarantees and lease contracts, we apply the following steps when recognizing revenue from contracts with customers: (i) identify the contract, (ii) identify the performance obligations, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations and (v) recognize revenue when performance obligation is satisfied. Our contracts with customers are generally short term in nature, typically due within one year or less or cancellable by us or our customer upon a short notice period. Performance obligations for our customer contracts are generally satisfied at a single point in time, typically when the transaction is complete, or over time. For performance obligations satisfied over time, we primarily use the output method, directly measuring the value of the products/services transferred to the customer, to determine when performance obligations have been satisfied. We typically receive payment from customers and recognize revenue concurrent with the satisfaction of our performance obligations. In most cases, this occurs within a single financial reporting period. For payments received in advance of the satisfaction of performance obligations, revenue recognition is deferred until such time the performance obligations have been satisfied. In cases where we have not received payment despite satisfaction of our performance obligations, we accrue an estimate of the amount due in the period our performance obligations have been satisfied. For contracts with variable components, only amounts for which collection is probable are accrued. We generally act in a principal capacity, on our own behalf, in most of our contracts with customers. In such transactions, we recognize revenue and the related costs to provide our services on a gross basis in our financial statements. In some cases, we act in an agent capacity, deriving revenue through assisting other entities in transactions with our customers. In such transactions, we recognized revenue and the related costs to provide our services on a net basis in our financial statements. These transactions primarily relate to insurance and brokerage commissions and fees derived from our customers' use of various interchange and ATM/debit card networks. In August 2018, the Securities and Exchange Commission (“SEC”) issued Final Rule Release No. 33-10532 - “Disclosure Update and Simplification.” This rule amends various SEC disclosure requirements that have been determined to be redundant, duplicative, overlapping, outdated, or superseded. The changes are generally expected to reduce or eliminate certain disclosures; however, the amendments did expand interim period disclosure requirements related to changes in shareholders' equity. Although the final rule does not become effective until November 5, 2018, we elected to expand our presentation of the Statement of Changes in Shareholders' Equity, as will be required under the new rule, in our financial statements as of and for the three and nine months ended September 30, 2018. Accounting Standards Updates Information about certain recently issued accounting standards updates is presented below. Also refer to Note 20 - Accounting Standards Updates in our 2017 Form 10-K for additional information related to previously issued accounting standards updates. Accounting Standards Update (“ASU”) 2014-09, “Revenue from Contracts with Customers (Topic 606).” ASU 2014-09 implements a common revenue standard that clarifies the principles for recognizing revenue. The core principle of ASU 2014-09 is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve that core principle, an entity should apply the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies a performance obligation. We adopted ASU 2014-09 effective January 1, 2018. See Note 1 - Significant Accounting Policies for additional information. ASU 2016-02, “Leases (Topic 842).” ASU 2016-02 will, among other things, require 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. ASU 2016-02 does not significantly change lease accounting requirements applicable to lessors; however, certain changes were made to align, where necessary, lessor accounting with the lessee accounting model and ASC Topic 606, “Revenue from Contracts with Customers .” ASU 2016-02 will be effective for us on January 1, 2019 and initially required transition using a modified retrospective approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. In July 2018, the Financial Accounting Standards Board issued ASU 2018-11 which, among other things, provides an additional transition method that would allow entities to not apply the guidance in ASU 2016-02 in the comparative periods presented in the financial statements and instead recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. We are currently implementing a third-party vendor solution to assist us in the application of ASU 2016-02. We currently estimate that the adoption of ASU 2016-02 will result in the recognition of right-of-use assets and related lease liabilities totaling approximately $400 million to $500 million . We expect to elect to apply certain practical expedients provided under ASU 2016-02 whereby we will 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 also do not expect to apply the recognition requirements of ASU 2016-02 to any short-term leases (as defined by related accounting guidance). We expect to account for lease and non-lease components separately because such amounts are readily determinable under our lease contracts and because we expect this election will result in a lower impact on our balance sheet. 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. ASU 2016-13 will be effective on January 1, 2020. We are currently evaluating the potential impact of ASU 2016-13 on our financial statements. In that regard, we have formed a cross-functional working group, under the direction of our Chief Financial Officer and our Chief Risk Officer. The working group is comprised of individuals from various functional areas including credit, risk management, finance and information technology, among others. We are currently working through our implementation plan which includes assessment and documentation of processes, internal controls and data sources; model development and documentation; 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 the ASU 2016-13. The adoption of the ASU 2016-13 could result in an increase in the allowance for loan losses as 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. While we are currently unable to reasonably estimate the impact of adopting ASU 2016-13, we expect that the impact of adoption 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 the adoption date. 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 does not change the accounting for callable debt securities held at a discount. ASU 2017-08 will be effective for us on January 1, 2019. The impact of adopting ASU 2017-08 will be significantly influenced by the composition of our securities portfolio as of that adoption date. Nonetheless, we currently expect to recognize a cumulative effect adjustment reducing retained earnings by approximately $11.3 million . Furthermore, we expect premium amortization expense for 2019 to be approximately $5.3 million higher than what would have been the case had we continued to amortize the affected securities to their respective maturity dates. ASU 2018-02, "Income Statement - Reporting Comprehensive Income (Topic 220) - Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income." Under ASU 2018-02, entities may elect to reclassify certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act, which was enacted on December 22, 2017, from accumulated other comprehensive income to retained earnings. ASU 2018-02 also requires certain accounting policy disclosures. We elected to adopt the provisions of ASU 2018-02 as of January 1, 2018 in advance of the required application date of January 1, 2019. See Note 1 - Significant Accounting Policies. ASU 2018-05, "Income Taxes (Topic 740) - Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin (SAB) No. 118." ASU 2018-05 amends the Accounting Standards Codification to incorporate various SEC paragraphs pursuant to the issuance of SAB 118. SAB 118 addresses the application of generally accepted accounting principles in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Cuts and Jobs Act. See Note 13 - Income Taxes. ASU 2018-13, “Fair Value Measurement (Topic 820) - Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement.” ASU 2018-13 modifies the disclosure requirements on fair value measurements in Topic 820. The amendments in this update remove disclosures that no longer are considered cost beneficial, modify/clarify the specific requirements of certain disclosures, and add disclosure requirements identified as relevant. ASU 2018-13 will be effective for us on January 1, 2020, with early adoption permitted, and is not expected to have a significant impact on our financial statements. ASU 2018-14, “Compensation - Retirement Benefits-Defined Benefit Plans-General (Subtopic 715-20).” ASU 2018-14 amends and modifies the disclosure requirements for employers that sponsor defined benefit pension or other post-retirement plans. The amendments in this update remove disclosures that no longer are considered cost beneficial, clarify the specific requirements of disclosures, and add disclosure requirements identified as relevant. ASU 2018-14 will be effective for us on January 1, 2021, with early adoption permitted, and is not expected to have a significant impact on our financial statements. ASU 2018-15, “Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40) - Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract.” ASU 2018-15 clarifies certain aspects of ASU 2015-05, “Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement,” which was issued in April 2015. Specifically, ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). ASU 2018-15 does not affect the accounting for the service element of a hosting arrangement that is a service contract. ASU 2018-15 will be effective for us on January 1, 2020, with early adoption permitted, and is not expected to have a significant impact on our financial statements.

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Additional Cash Flow Information	Additional cash flow information was as follows: Nine Months Ended 2018 2017 Cash paid for interest $ 59,764 $ 15,611 Cash paid for income taxes 5,112 41,969 Significant non-cash transactions: Unsettled purchases/sales of securities 74,191 41,763 Loans foreclosed and transferred to other real estate owned and foreclosed assets 2,898 257

Securities (Tables)

Securities (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 December 31, 2017 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 $ 3,040 $ 8 $ 94 $ 2,954 $ 3,610 $ 15 $ 38 $ 3,587 States and political subdivisions 1,107,222 7,232 2,057 1,112,397 1,428,488 26,462 2,746 1,452,204 Other 1,500 — 6 1,494 — — — — Total $ 1,111,762 $ 7,240 $ 2,157 $ 1,116,845 $ 1,432,098 $ 26,477 $ 2,784 $ 1,455,791 Available for Sale U.S. Treasury $ 3,454,899 $ — $ 52,020 $ 3,402,879 $ 3,453,391 $ 7,494 $ 15,732 $ 3,445,153 Residential mortgage-backed securities 756,649 11,254 11,532 756,371 648,288 19,048 2,250 665,086 States and political subdivisions 7,052,156 40,759 116,225 6,976,690 6,185,711 167,293 16,795 6,336,209 Other 42,651 — — 42,651 42,561 — — 42,561 Total $ 11,306,355 $ 52,013 $ 179,777 $ 11,178,591 $ 10,329,951 $ 193,835 $ 34,777 $ 10,489,009
Securities, with Unrealized Losses Segregated by Length of Impairment	As of September 30, 2018 , 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 $ 19 $ 1,850 $ 75 $ 2,263 $ 94 States and political subdivisions 403,965 2,057 — — 403,965 2,057 Other 1,494 6 — — 1,494 6 Total $ 405,872 $ 2,082 $ 1,850 $ 75 $ 407,722 $ 2,157 Available for Sale U.S. Treasury $ 2,295,611 $ 34,315 $ 1,107,268 $ 17,705 $ 3,402,879 $ 52,020 Residential mortgage-backed securities 294,839 5,979 98,757 5,553 393,596 11,532 States and political subdivisions 3,585,212 65,581 810,732 50,644 4,395,944 116,225 Total $ 6,175,662 $ 105,875 $ 2,016,757 $ 73,902 $ 8,192,419 $ 179,777
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, 2018 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 $ 56,511 $ 57,181 $ 858,235 $ 851,163 Due after one year through five years 131,987 133,521 3,243,532 3,204,804 Due after five years through ten years 492,460 492,354 489,960 487,536 Due after ten years 427,764 430,835 5,915,328 5,836,066 Residential mortgage-backed securities 3,040 2,954 756,649 756,371 Equity securities — — 42,651 42,651 Total $ 1,111,762 $ 1,116,845 $ 11,306,355 $ 11,178,591
Sales of Securities Available for Sale	Sales of securities available for sale were as follows: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Proceeds from sales $ 2,948,178 $ 746,524 $ 13,838,566 $ 8,993,963 Gross realized gains — — 3 — Gross realized losses (34 ) (4,867 ) (116 ) (4,917 ) Tax (expense) benefit of securities gains/losses 7 1,703 24 1,721
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 2018 2017 2018 2017 Premium amortization $ (27,381 ) $ (24,586 ) $ (80,104 ) $ (72,733 ) Discount accretion 2,569 1,783 6,356 6,278 Net (premium amortization) discount accretion $ (24,812 ) $ (22,803 ) $ (73,748 ) $ (66,455 )
Trading Account Securities, at Estimated Fair Value	Trading account securities, at estimated fair value, were as follows: September 30, December 31, U.S. Treasury $ 21,804 $ 19,210 States and political subdivisions 434 1,888 Total $ 22,238 $ 21,098
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 2018 2017 2018 2017 Net gain on sales transactions $ 465 $ 414 $ 1,404 $ 1,018 Net mark-to-market gains (losses) (8 ) (8 ) (21 ) (51 ) Net gain (loss) on trading account securities $ 457 $ 406 $ 1,383 $ 967

Loans (Tables)

Loans (Tables)	9 Months Ended
	Sep. 30, 2018
Receivables [Abstract]
Loans	Loans were as follows: September 30, Percentage of Total December 31, Percentage of Total Commercial and industrial $ 5,029,754 36.4 % $ 4,792,388 36.4 % Energy: Production 1,220,771 8.8 1,182,326 9.0 Service 164,889 1.2 171,795 1.3 Other 133,708 1.0 144,972 1.1 Total energy 1,519,368 11.0 1,499,093 11.4 Commercial real estate: Commercial mortgages 4,078,787 29.5 3,887,742 29.6 Construction 1,208,870 8.7 1,066,696 8.1 Land 315,384 2.3 331,986 2.5 Total commercial real estate 5,603,041 40.5 5,286,424 40.2 Consumer real estate: Home equity loans 352,292 2.5 355,342 2.7 Home equity lines of credit 326,876 2.4 291,950 2.2 Other 419,965 3.1 376,002 2.9 Total consumer real estate 1,099,133 8.0 1,023,294 7.8 Total real estate 6,702,174 48.5 6,309,718 48.0 Consumer and other 563,542 4.1 544,466 4.2 Total loans $ 13,814,838 100.0 % $ 13,145,665 100.0 %
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 $ 12,278 $ 46,186 Energy 51,802 94,302 Commercial real estate: Buildings, land and other 15,913 7,589 Construction — — Consumer real estate 971 2,109 Consumer and other 1,637 128 Total $ 82,601 $ 150,314
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, 2018 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 $ 33,985 $ 10,250 $ 44,235 $ 4,985,519 $ 5,029,754 $ 3,963 Energy 3,251 2,221 5,472 1,513,896 1,519,368 818 Commercial real estate: Buildings, land and other 21,596 2,980 24,576 4,369,595 4,394,171 2,606 Construction 784 1,042 1,826 1,207,044 1,208,870 1,042 Consumer real estate 7,743 1,773 9,516 1,089,617 1,099,133 1,432 Consumer and other 6,098 1,766 7,864 555,678 563,542 1,724 Total $ 73,457 $ 20,032 $ 93,489 $ 13,721,349 $ 13,814,838 $ 11,585
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, 2018 Commercial and industrial $ 19,857 $ 3,153 $ 6,574 $ 9,727 $ 4,622 Energy 68,087 7,942 43,614 51,556 12,672 Commercial real estate: Buildings, land and other 15,961 2,309 12,876 15,185 2,599 Construction — — — — — Consumer real estate 293 293 — 293 — Consumer and other 1,595 — 1,595 1,595 1,595 Total $ 105,793 $ 13,697 $ 64,659 $ 78,356 $ 21,488 December 31, 2017 Commercial and industrial $ 60,781 $ 28,038 $ 15,722 $ 43,760 $ 7,553 Energy 99,606 33,080 61,162 94,242 13,267 Commercial real estate: Buildings, land and other 10,795 6,394 — 6,394 — Construction — — — — — Consumer real estate 1,214 1,214 — 1,214 — Consumer and other — — — — — Total $ 172,396 $ 68,726 $ 76,884 $ 145,610 $ 20,820
Average Recorded Investment In Impaired Loans	The average recorded investment in impaired loans was as follows: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Commercial and industrial $ 13,447 $ 26,910 $ 21,025 $ 26,651 Energy 78,772 76,008 82,640 72,055 Commercial real estate: Buildings, land and other 14,306 5,553 12,328 6,106 Construction — — — — Consumer real estate 671 1,209 807 1,155 Consumer and other 1,073 — 805 13 Total $ 108,269 $ 109,680 $ 117,605 $ 105,980
Troubled Debt Restructurings	Troubled debt restructurings during the nine months ended September 30, 2018 and September 30, 2017 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 $ 2,203 $ — $ 4,026 $ 3,875 Energy 13,708 — 56,097 55,023 $ 15,911 $ — $ 60,123 $ 58,898
Restructured Loans Past Due in Excess of 90 Days on Financing Receivables	Additional information related to restructured loans as of or for the nine months ended September 30, 2018 and September 30, 2017 is set forth in the following table. September 30, 2018 September 30, 2017 Restructured loans past due in excess of 90 days at period-end: Number of loans — 1 Dollar amount of loans $ — $ 43,137 Restructured loans on non-accrual status at period end — 54,082 Charge-offs of restructured loans: Recognized in connection with restructuring — — Recognized on previously restructured loans 4,650 9,951 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, 2018 December 31, 2017 Weighted Loans Weighted Loans Commercial and industrial: Risk grades 1-8 6.10 $ 4,755,739 6.06 $ 4,378,839 Risk grade 9 9.00 123,199 9.00 170,285 Risk grade 10 10.00 67,645 10.00 99,260 Risk grade 11 11.00 70,893 11.00 97,818 Risk grade 12 12.00 7,656 12.00 38,633 Risk grade 13 13.00 4,622 13.00 7,553 Total 6.31 $ 5,029,754 6.41 $ 4,792,388 Energy Risk grades 1-8 5.81 $ 1,367,235 6.01 $ 1,199,207 Risk grade 9 9.00 13,352 9.00 50,427 Risk grade 10 10.00 42,862 10.00 64,282 Risk grade 11 11.00 44,116 11.00 90,875 Risk grade 12 12.00 39,131 12.00 81,035 Risk grade 13 13.00 12,672 13.00 13,267 Total 6.32 $ 1,519,368 6.97 $ 1,499,093 Commercial real estate: Buildings, land and other Risk grades 1-8 6.77 $ 4,091,571 6.75 $ 3,868,659 Risk grade 9 9.00 135,521 9.00 151,487 Risk grade 10 10.00 60,169 10.00 129,391 Risk grade 11 11.00 90,997 11.00 62,602 Risk grade 12 12.00 13,314 12.00 7,589 Risk grade 13 13.00 2,599 13.00 — Total 6.99 $ 4,394,171 7.00 $ 4,219,728 Construction Risk grades 1-8 7.10 $ 1,129,492 7.11 $ 1,019,635 Risk grade 9 9.00 51,058 9.00 18,042 Risk grade 10 10.00 26,035 10.00 23,393 Risk grade 11 11.00 2,285 11.00 5,626 Risk grade 12 12.00 — 12.00 — Risk grade 13 13.00 — 13.00 — Total 7.25 $ 1,208,870 7.23 $ 1,066,696
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 2018 2017 2018 2017 Commercial and industrial $ (7,807 ) $ (4,565 ) $ (19,030 ) $ (12,155 ) Energy (5,347 ) 451 (10,272 ) (10,010 ) Commercial real estate: Buildings, land and other 33 266 (288 ) 768 Construction 3 2 11 8 Consumer real estate (388 ) (629 ) (1,078 ) (422 ) Consumer and other (1,792 ) (1,760 ) (4,975 ) (4,289 ) Total $ (15,298 ) $ (6,235 ) $ (35,632 ) $ (26,100 )
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, 2018 and December 31, 2017 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, 2018 Historical valuation allowances $ 25,582 $ 9,654 $ 20,441 $ 2,641 $ 6,815 $ 65,133 Specific valuation allowances 4,622 12,672 2,599 — 1,595 21,488 General valuation allowances 9,361 5,849 4,187 1,620 (114 ) 20,903 Macroeconomic valuation allowances 11,426 3,270 11,977 1,933 1,448 30,054 Total $ 50,991 $ 31,445 $ 39,204 $ 6,194 $ 9,744 $ 137,578 Allocated to loans: Individually evaluated $ 4,622 $ 12,672 $ 2,599 $ — $ 1,595 $ 21,488 Collectively evaluated 46,369 18,773 36,605 6,194 8,149 116,090 Total $ 50,991 $ 31,445 $ 39,204 $ 6,194 $ 9,744 $ 137,578 December 31, 2017 Historical valuation allowances $ 26,401 $ 22,073 $ 18,931 $ 2,473 $ 5,603 $ 75,481 Specific valuation allowances 7,553 13,267 — — — 20,820 General valuation allowances 9,112 7,964 4,165 2,133 (91 ) 23,283 Macroeconomic valuation allowances 16,548 8,224 7,852 1,051 2,105 35,780 Total $ 59,614 $ 51,528 $ 30,948 $ 5,657 $ 7,617 $ 155,364 Allocated to loans: Individually evaluated $ 7,553 $ 13,267 $ — $ — $ — $ 20,820 Collectively evaluated 52,061 38,261 30,948 5,657 7,617 134,544 Total $ 59,614 $ 51,528 $ 30,948 $ 5,657 $ 7,617 $ 155,364
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, 2018 and December 31, 2017 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, 2018 Individually evaluated $ 9,727 $ 51,556 $ 15,185 $ 293 $ 1,595 $ 78,356 Collectively evaluated 5,020,027 1,467,812 5,587,856 1,098,840 561,947 13,736,482 Total $ 5,029,754 $ 1,519,368 $ 5,603,041 $ 1,099,133 $ 563,542 $ 13,814,838 December 31, 2017 Individually evaluated $ 43,760 $ 94,242 $ 6,394 $ 1,214 $ — $ 145,610 Collectively evaluated 4,748,628 1,404,851 5,280,030 1,022,080 544,466 13,000,055 Total $ 4,792,388 $ 1,499,093 $ 5,286,424 $ 1,023,294 $ 544,466 $ 13,145,665
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, 2018 and 2017 . 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, 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 September 30, 2017 Beginning balance $ 48,906 $ 54,277 $ 33,002 $ 5,535 $ 7,838 $ 149,558 Provision for loan losses 4,096 (2,815 ) 4,805 1,969 2,925 10,980 Charge-offs (5,468 ) — — (766 ) (4,120 ) (10,354 ) Recoveries 903 451 268 137 2,360 4,119 Net charge-offs (4,565 ) 451 268 (629 ) (1,760 ) (6,235 ) Ending balance $ 48,437 $ 51,913 $ 38,075 $ 6,875 $ 9,003 $ 154,303 Nine months ended: 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 September 30, 2017 Beginning balance $ 52,915 $ 60,653 $ 30,213 $ 4,238 $ 5,026 $ 153,045 Provision for loan losses 7,677 1,270 7,086 3,059 8,266 27,358 Charge-offs (14,574 ) (10,595 ) (14 ) (779 ) (11,291 ) (37,253 ) Recoveries 2,419 585 790 357 7,002 11,153 Net charge-offs (12,155 ) (10,010 ) 776 (422 ) (4,289 ) (26,100 ) Ending balance $ 48,437 $ 51,913 $ 38,075 $ 6,875 $ 9,003 $ 154,303

Goodwill and Other Intangible_2

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	Goodwill is presented in the table below. September 30, 2018 December 31, 2017 Goodwill $ 654,952 $ 654,952
Schedule of Other Intangible Assets	Other intangible assets are presented in the table below. Other intangible assets: Core deposits $ 3,213 $ 4,044 Customer relationship 743 986 Non-compete agreements 24 43 $ 3,980 $ 5,073
Estimated Aggregate Future Amortization Expense for Intangible Assets	The estimated aggregate future amortization expense for intangible assets remaining as of September 30, 2018 is as follows: Remainder of 2018 331 2019 1,167 2020 918 2021 697 2022 481 Thereafter 386 $ 3,980

Deposits (Tables)

Deposits (Tables)	9 Months Ended
	Sep. 30, 2018
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 $ 10,137,696 38.4 % $ 10,412,882 38.8 % Correspondent banks 187,589 0.7 222,648 0.8 Public funds 515,228 2.0 561,563 2.1 Total non-interest-bearing demand deposits 10,840,513 41.1 11,197,093 41.7 Interest-bearing deposits: Private accounts: Savings and interest checking 6,640,382 25.2 6,788,766 25.2 Money market accounts 7,694,628 29.2 7,624,471 28.4 Time accounts of $100,000 or more 484,236 1.8 453,668 1.7 Time accounts under $100,000 327,790 1.3 324,636 1.2 Total private accounts 15,147,036 57.5 15,191,541 56.5 Public funds: Savings and interest checking 303,150 1.2 410,140 1.5 Money market accounts 54,537 0.2 59,008 0.2 Time accounts of $100,000 or more 3,523 — 14,301 0.1 Time accounts under $100,000 95 — 306 — Total public funds 361,305 1.4 483,755 1.8 Total interest-bearing deposits 15,508,341 58.9 15,675,296 58.3 Total deposits $ 26,348,854 100.0 % $ 26,872,389 100.0 %
Additional Information About Corporation's Deposits	The following table presents additional information about our deposits: September 30, December 31, Deposits from foreign sources (primarily Mexico) $ 739,424 $ 716,339 Deposits not covered by deposit insurance 12,403,549 13,281,040

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
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 $ 8,284,873 $ 7,949,400 Standby letters of credit 244,743 236,595 Deferred standby letter of credit fees 1,657 1,843

Capital and Regulatory Matters

Capital and Regulatory Matters (Tables)	9 Months Ended
	Sep. 30, 2018
Banking and Thrift [Abstract]
Actual and Required Capital Ratios	The following tables present actual and required capital ratios as of September 30, 2018 and December 31, 2017 for Cullen/Frost and Frost Bank under the Basel III Capital Rules. The minimum required capital amounts presented include the minimum required capital levels as of September 30, 2018 and December 31, 2017 based on the phase-in provisions of the Basel III Capital Rules and the minimum required capital levels as of January 1, 2019 when the Basel III Capital Rules have been 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 2017 Form 10-K for a more detailed discussion of the Basel III Capital Rules. 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 September 30, 2018 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,658,645 12.93 % $ 1,310,621 6.38 % $ 1,439,113 7.00 % $ 1,336,319 6.50 % Frost Bank 2,683,196 13.10 1,306,205 6.38 1,434,264 7.00 1,331,816 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 2,803,131 13.63 1,619,002 7.88 1,747,494 8.50 1,644,701 8.00 Frost Bank 2,683,196 13.10 1,613,547 7.88 1,741,606 8.50 1,639,159 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,174,209 15.44 2,030,177 9.88 2,158,670 10.50 2,055,876 10.00 Frost Bank 2,821,274 13.77 2,023,337 9.88 2,151,396 10.50 2,048,948 10.00 Leverage Ratio Cullen/Frost 2,803,131 9.19 1,220,627 4.00 1,220,627 4.00 1,525,784 5.00 Frost Bank 2,683,196 8.80 1,219,806 4.00 1,219,806 4.00 1,524,758 5.00 December 31, 2017 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,426,048 12.42 % $ 1,123,430 5.75 % $ 1,367,583 7.00 % $ 1,269,965 6.50 % Frost Bank 2,518,999 12.92 1,120,663 5.75 1,364,214 7.00 1,266,836 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 2,570,534 13.16 1,416,499 7.25 1,660,637 8.50 1,563,033 8.00 Frost Bank 2,518,999 12.92 1,413,010 7.25 1,656,546 8.50 1,559,183 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 2,959,326 15.15 1,807,257 9.25 2,051,375 10.50 1,953,792 10.00 Frost Bank 2,674,791 13.72 1,802,805 9.25 2,046,321 10.50 1,948,979 10.00 Leverage Ratio Cullen/Frost 2,570,534 8.46 1,215,227 4.00 1,215,186 4.00 1,519,034 5.00 Frost Bank 2,518,999 8.30 1,214,295 4.00 1,214,254 4.00 1,517,869 5.00

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 and December 31, 2017 . September 30, 2018 December 31, 2017 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 $ 11,284 $ 314 $ 13,679 $ 242 Loan/lease interest rate swaps – liabilities 4,226 (201 ) 11,147 (593 ) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets 780,531 6,179 430,449 1,418 Loan/lease interest rate swaps – liabilities 341,987 (5,133 ) 541,496 (12,820 ) Loan/lease interest rate caps – assets 102,487 920 114,619 480 Customer counterparties: Loan/lease interest rate swaps – assets 341,987 7,017 541,496 17,882 Loan/lease interest rate swaps – liabilities 780,531 (20,613 ) 430,449 (4,861 ) Loan/lease interest rate caps – liabilities 102,487 (920 ) 114,619 (480 )
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, 2018 were as follows: Weighted-Average Interest Rate Paid Interest Rate Received Interest rate swaps: Fair value hedge loan/lease interest rate swaps 2.45 % 2.16 % Non-hedging interest rate swaps – financial institution counterparties 4.09 % 3.73 % Non-hedging interest rate swaps – customer counterparties 3.73 % 4.09 %
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, 2018 December 31, 2017 Notional Units Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Oil – assets Barrels 622 $ 917 253 $ 193 Oil – liabilities Barrels 2,889 (34,941 ) 2,731 (13,448 ) Natural gas – assets MMBTUs 7,939 427 5,927 1,399 Natural gas – liabilities MMBTUs 8,382 (598 ) 3,917 (326 ) Customer counterparties: Oil – assets Barrels 2,889 35,083 2,731 13,709 Oil – liabilities Barrels 622 (917 ) 253 (187 ) Natural gas – assets MMBTUs 9,588 682 3,917 340 Natural gas – liabilities MMBTUs 6,734 (392 ) 5,927 (1,366 )
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, 2018 December 31, 2017 Notional Currency Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Forward contracts – assets EUR 1,097 $ 9 4,014 $ 77 Forward contracts – assets GBP 793 9 127 1 Forward contracts – liabilities EUR — — 4,846 (37 ) Forward contracts – liabilities CAD 22,971 (189 ) 25,413 (142 ) Forward contracts – liabilities GBP — — 1,178 (9 ) Forward contracts – liabilities AUD 54 — — — Customer counterparties: Forward contracts – assets EUR — — 3,867 58 Forward contracts – assets CAD 22,918 245 25,282 279 Forward contracts – liabilities EUR — — 4,041 (51 ) Forward contracts – liabilities GBP — — 127 —
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 2018 2017 2018 2017 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ 12 $ (149 ) $ 1 $ (592 ) Amount of (gain) loss included in other non-interest expense — (2 ) (1 ) (5 )
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 2018 2017 2018 2017 Non-hedging interest rate derivatives: Other non-interest income $ 699 $ 1,085 $ 2,889 $ 2,062 Other non-interest expense 2 — (2 ) 1 Non-hedging commodity derivatives: Other non-interest income 456 231 492 387 Non-hedging foreign currency derivatives: Other non-interest income 53 83 203 101

Balance Sheet Offsetting (Table

Balance Sheet Offsetting (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 is presented in the following tables. Gross Amount Recognized Gross Amount Offset Net Amount Recognized September 30, 2018 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 7,413 $ — $ 7,413 Commodity swaps and options 1,344 — 1,344 Foreign currency forward contracts 18 — 18 Total derivatives 8,775 — 8,775 Resell agreements 12,142 — 12,142 Total $ 20,917 $ — $ 20,917 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 5,334 $ — $ 5,334 Commodity swaps and options 35,539 — 35,539 Foreign currency forward contracts 189 — 189 Total derivatives 41,062 — 41,062 Repurchase agreements 1,074,576 — 1,074,576 Total $ 1,115,638 $ — $ 1,115,638 Information about financial instruments that are eligible for offset in the consolidated balance sheet as of December 31, 2017 is presented in the following tables. Gross Amount Recognized Gross Amount Offset Net Amount Recognized December 31, 2017 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 2,140 $ — $ 2,140 Commodity swaps and options 1,592 — 1,592 Foreign currency forward contracts 78 — 78 Total derivatives 3,810 — 3,810 Resell agreements 9,642 — 9,642 Total $ 13,452 $ — $ 13,452 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 13,413 $ — $ 13,413 Commodity swaps and options 13,774 — 13,774 Foreign currency forward contracts 188 — 188 Total derivatives 27,375 — 27,375 Repurchase agreements 1,117,199 — 1,117,199 Total $ 1,144,574 $ — $ 1,144,574
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 September 30, 2018 Financial assets: Derivatives: Counterparty A $ 1,477 $ (1,477 ) $ — $ — Counterparty B 1,963 (1,963 ) — — Counterparty C 75 (75 ) — — Other counterparties 5,260 (1,825 ) (3,223 ) 212 Total derivatives 8,775 (5,340 ) (3,223 ) 212 Resell agreements 12,142 — (12,142 ) — Total $ 20,917 $ (5,340 ) $ (15,365 ) $ 212 Financial liabilities: Derivatives: Counterparty A $ 3,840 $ (1,477 ) $ (2,363 ) $ — Counterparty B 6,627 (1,963 ) (4,453 ) 211 Counterparty C 806 (75 ) (731 ) — Other counterparties 29,789 (1,825 ) (25,908 ) 2,056 Total derivatives 41,062 (5,340 ) (33,455 ) 2,267 Repurchase agreements 1,074,576 — (1,074,576 ) — Total $ 1,115,638 $ (5,340 ) $ (1,108,031 ) $ 2,267 Gross Amounts Not Offset Net Amount Recognized Financial Instruments Collateral Net Amount December 31, 2017 Financial assets: Derivatives: Counterparty A $ 395 $ (395 ) $ — $ — Counterparty B 1,028 (1,028 ) — — Counterparty C 55 (55 ) — — Other counterparties 2,332 (1,830 ) (387 ) 115 Total derivatives 3,810 (3,308 ) (387 ) 115 Resell agreements 9,642 — (9,642 ) — Total $ 13,452 $ (3,308 ) $ (10,029 ) $ 115 Financial liabilities: Derivatives: Counterparty A $ 7,397 $ (395 ) $ (7,002 ) $ — Counterparty B 4,466 (1,028 ) (3,101 ) 337 Counterparty C 1,520 (55 ) (1,450 ) 15 Other counterparties 13,992 (1,830 ) (11,215 ) 947 Total derivatives 27,375 (3,308 ) (22,768 ) 1,299 Repurchase agreements 1,117,199 — (1,117,199 ) — Total $ 1,144,574 $ (3,308 ) $ (1,139,967 ) $ 1,299
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, 2018 and December 31, 2017 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, 2018 Repurchase agreements: U.S. Treasury $ 1,067,059 $ — $ — $ — $ 1,067,059 Residential mortgage-backed securities 7,517 — — — 7,517 Total borrowings $ 1,074,576 $ — $ — $ — $ 1,074,576 Gross amount of recognized liabilities for repurchase agreements $ 1,074,576 Amounts related to agreements not included in offsetting disclosures above $ — December 31, 2017 Repurchase agreements: U.S. Treasury $ 1,036,891 $ — $ — $ — $ 1,036,891 Residential mortgage-backed securities 80,308 — — — 80,308 Total borrowings $ 1,117,199 $ — $ — $ — $ 1,117,199 Gross amount of recognized liabilities for repurchase agreements $ 1,117,199 Amounts related to agreements not included in offsetting disclosures above $ —

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Activity in Corporation's Active Stock Plans	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, 2018 53,008 $ 64.87 312,656 $ 81.71 80,103 $ 79.91 2,917,142 $ 63.34 Authorized — — — — — — — — Granted 6,576 109.58 — — — — — — Exercised/vested (10,674 ) 63.68 (2,470 ) 78.92 — — (434,687 ) 61.12 Forfeited/expired — — (4,836 ) 87.00 — — (49,125 ) 70.70 Balance, September 30, 2018 48,910 $ 71.14 305,350 $ 81.65 80,103 $ 79.91 2,433,330 $ 63.59
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 2018 2017 2018 2017 New shares issued from available authorized shares — 9,299 — 602,662 Issued from available treasury stock 19,232 13,425 447,831 172,137 Total 19,232 22,724 447,831 774,799 Proceeds from stock option exercises $ 1,119 $ 1,274 $ 26,567 $ 45,422
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 2018 2017 2018 2017 Stock options $ 960 $ 1,532 $ 3,064 $ 4,892 Non-vested stock awards/stock units 1,357 813 4,196 2,747 Director deferred stock units — — 720 519 Performance stock units 479 377 1,576 855 Total $ 2,796 $ 2,722 $ 9,556 $ 9,013 Income tax benefit $ 587 $ 953 $ 2,007 $ 3,155
Unrecognized Stock-Based Compensation Expense	Unrecognized stock-based compensation expense at September 30, 2018 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 $ 1,906 Non-vested stock awards/stock units 10,343 Performance stock units 3,553 Total $ 15,802

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	9 Months Ended
	Sep. 30, 2018
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 2018 2017 2018 2017 Net income $ 117,844 $ 93,131 $ 335,665 $ 263,625 Less: Preferred stock dividends 2,016 2,016 6,047 6,047 Net income available to common shareholders 115,828 91,115 329,618 257,578 Less: Earnings allocated to participating securities 750 475 2,178 1,346 Net earnings allocated to common stock $ 115,078 $ 90,640 $ 327,440 $ 256,232 Distributed earnings allocated to common stock $ 42,811 $ 36,174 $ 121,907 $ 107,194 Undistributed earnings allocated to common stock 72,267 54,466 205,533 149,038 Net earnings allocated to common stock $ 115,078 $ 90,640 $ 327,440 $ 256,232 Weighted-average shares outstanding for basic earnings per common share 63,892,023 63,667,356 63,793,514 63,822,011 Dilutive effect of stock compensation 1,022,168 897,945 1,037,043 957,337 Weighted-average shares outstanding for diluted earnings per common share 64,914,191 64,565,301 64,830,557 64,779,348

Defined Benefit Plans (Tables)

Defined Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2018
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 2018 2017 2018 2017 Expected return on plan assets, net of expenses $ (2,979 ) $ (2,779 ) $ (8,937 ) $ (8,338 ) Interest cost on projected benefit obligation 1,475 1,547 4,424 4,642 Net amortization and deferral 1,250 1,357 3,751 4,072 Net periodic expense (benefit) $ (254 ) $ 125 $ (762 ) $ 376

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Tax Expense	Income tax expense was as follows: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Current income tax expense (benefit) $ (1,152 ) $ 15,224 $ 955 $ 44,636 Deferred income tax expense (benefit) 16,312 (5,332 ) 39,198 (9,505 ) Income tax expense, as reported $ 15,160 $ 9,892 $ 40,153 $ 35,131 Effective tax rate 11.4 % 9.6 % 10.7 % 11.8 %

Other Comprehensive Income (L_2

Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2018
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 $ (96,147 ) $ (20,190 ) $ (75,957 ) $ 7,082 $ 2,479 $ 4,603 Change in net unrealized gain on securities transferred to held to maturity (3,764 ) (790 ) (2,974 ) (3,514 ) (1,230 ) (2,284 ) Reclassification adjustment for net (gains) losses included in net income 34 7 27 4,867 1,703 3,164 Total securities available for sale and transferred securities (99,877 ) (20,973 ) (78,904 ) 8,435 2,952 5,483 Defined-benefit post-retirement benefit plans: Change in the net actuarial gain/loss — — — — — — Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) 1,250 262 988 1,357 475 882 Total defined-benefit post-retirement benefit plans 1,250 262 988 1,357 475 882 Total other comprehensive income (loss) $ (98,627 ) $ (20,711 ) $ (77,916 ) $ 9,792 $ 3,427 $ 6,365 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 $ (286,935 ) $ (60,256 ) $ (226,679 ) $ 131,283 $ 45,949 $ 85,334 Change in net unrealized gain on securities transferred to held to maturity (8,424 ) (1,769 ) (6,655 ) (13,660 ) (4,781 ) (8,879 ) Reclassification adjustment for net (gains) losses included in net income 113 24 89 4,917 1,721 3,196 Total securities available for sale and transferred securities (295,246 ) (62,001 ) (233,245 ) 122,540 42,889 79,651 Defined-benefit post-retirement benefit plans: Change in the net actuarial gain/loss — — — — — — Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) 3,751 788 2,963 4,072 1,425 2,647 Total defined-benefit post-retirement benefit plans 3,751 788 2,963 4,072 1,425 2,647 Total other comprehensive income (loss) $ (291,495 ) $ (61,213 ) $ (230,282 ) $ 126,612 $ 44,314 $ 82,298
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, 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 ) Balance January 1, 2017 $ 16,153 $ (40,776 ) $ (24,623 ) Other comprehensive income (loss) before reclassifications 76,455 — 76,455 Reclassification of amounts included in net income 3,196 2,647 5,843 Net other comprehensive income (loss) during period 79,651 2,647 82,298 Balance at September 30, 2017 $ 95,804 $ (38,129 ) $ 57,675

Operating Segments (Tables)

Operating Segments (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 $ 295,062 $ 36,868 $ (2,608 ) $ 329,322 September 30, 2017 266,582 36,529 (2,285 ) 300,826 Nine months ended: September 30, 2018 $ 874,056 $ 106,475 $ (7,680 ) $ 972,851 September 30, 2017 786,743 107,829 (5,669 ) 888,903 Net income (loss): Three months ended: September 30, 2018 $ 114,214 $ 6,442 $ (2,812 ) $ 117,844 September 30, 2017 88,368 6,417 (1,654 ) 93,131 Nine months ended: September 30, 2018 $ 327,131 $ 17,977 $ (9,443 ) $ 335,665 September 30, 2017 250,766 17,990 (5,131 ) 263,625

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 and December 31, 2017 , 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, 2018 Securities available for sale: U.S. Treasury $ 3,402,879 $ — $ — $ 3,402,879 Residential mortgage-backed securities — 756,371 — 756,371 States and political subdivisions — 6,976,690 — 6,976,690 Other — 42,651 — 42,651 Trading account securities: U.S. Treasury 21,804 — — 21,804 States and political subdivisions — 434 — 434 Derivative assets: Interest rate swaps, caps and floors — 14,430 — 14,430 Commodity swaps and options — 35,813 1,296 37,109 Foreign currency forward contracts 263 — — 263 Derivative liabilities: Interest rate swaps, caps and floors — 26,867 — 26,867 Commodity swaps and options — 36,848 — 36,848 Foreign currency forward contracts 189 — — 189 December 31, 2017 Securities available for sale: U.S. Treasury $ 3,445,153 $ — $ — $ 3,445,153 Residential mortgage-backed securities — 665,086 — 665,086 States and political subdivisions — 6,336,209 — 6,336,209 Other — 42,561 — 42,561 Trading account securities: U.S. Treasury 19,210 — — 19,210 States and political subdivisions — 1,888 — 1,888 Derivative assets: Interest rate swaps, caps and floors — 20,022 — 20,022 Commodity swaps and options — 14,408 1,233 15,641 Foreign currency forward contracts 415 — — 415 Derivative liabilities: Interest rate swaps, caps and floors — 18,754 — 18,754 Commodity swaps and options — 15,327 — 15,327 Foreign currency forward contracts 239 — — 239
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 $ 12,876 $ 28,638 $ — $ 64,287 Specific valuation allowance (allocations) reversals of prior allocations (2,599 ) 4,256 — (13,477 ) Fair value $ 10,277 $ 32,894 $ — $ 50,810
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 2018 2017 Foreclosed assets remeasured at initial recognition: Carrying value of foreclosed assets prior to remeasurement $ 2,898 $ — Charge-offs recognized in the allowance for loan losses — — Fair value $ 2,898 $ — Foreclosed assets remeasured subsequent to initial recognition: Carrying value of foreclosed assets prior to remeasurement $ 1,823 $ 89 Write-downs included in other non-interest expense (473 ) (16 ) Fair value $ 1,350 $ 73
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, 2018 December 31, 2017 Carrying Amount Estimated Fair Value Carrying Amount Estimated Fair Value Financial assets: Level 2 inputs: Cash and cash equivalents $ 3,419,489 $ 3,419,489 $ 5,053,047 $ 5,053,047 Securities held to maturity 1,111,762 1,116,845 1,432,098 1,455,791 Cash surrender value of life insurance policies 182,603 182,603 180,477 180,477 Accrued interest receivable 137,553 137,553 167,508 167,508 Level 3 inputs: Loans, net 13,677,260 13,621,005 12,990,301 12,981,165 Financial liabilities: Level 2 inputs: Deposits 26,348,854 26,342,071 26,872,389 26,866,676 Federal funds purchased and repurchase agreements 1,083,777 1,083,777 1,147,824 1,147,824 Junior subordinated deferrable interest debentures 136,227 137,115 136,184 137,115 Subordinated notes payable and other borrowings 98,669 98,875 98,552 105,311 Accrued interest payable 4,904 4,904 3,358 3,358

Significant Accounting Polici_4

Significant Accounting Policies - Additional Cash Flow Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Accounting Policies [Abstract]
Cash paid for interest	$ 59,764	$ 15,611
Cash paid for income taxes	5,112	41,969
Unsettled purchases/sales of securities	74,191	41,763
Loans foreclosed and transferred to other real estate owned and foreclosed assets	$ 2,898	$ 257

Significant Accounting Polici_5

Significant Accounting Policies Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
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
Cumulative Effect of New Accounting Principle in Period of Adoption					$ (2,285)
Gross interchange and debit card transaction fees	$ 6,475		19,029
Gross cost related to interchange and debit card transactions	2,976	$ 2,980	8,926	$ 9,094
Interchange and debit card transaction fees	$ 3,499	$ 5,884	$ 10,103	$ 17,150

Securities - Additional Informa

Securities - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	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	98.40%
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund	68.00%
Securities Pledged as Collateral	$ 3,700,000	$ 3,800,000
Available for Sale, Estimated Fair Value	11,178,591	$ 10,489,009
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	3,100
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	$ 2,500
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, 2018	Dec. 31, 2017
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,111,762	$ 1,432,098
Held-to-maturity Securities, Gross Unrealized Gains	7,240	26,477
Held-to-maturity Securities, Gross Unrealized Losses	2,157	2,784
Held to Maturity, Estimated Fair Value	1,116,845	1,455,791
Available for Sale, Amortized Cost	11,306,355	10,329,951
Available for Sale Securities, Gross Unrealized Gains	52,013	193,835
Available for Sale Securities, Gross Unrealized Losses	179,777	34,777
Available for Sale, Estimated Fair Value	11,178,591	10,489,009
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Amortized Cost	3,454,899	3,453,391
Available for Sale Securities, Gross Unrealized Gains	0	7,494
Available for Sale Securities, Gross Unrealized Losses	52,020	15,732
Available for Sale, Estimated Fair Value	3,402,879	3,445,153
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	3,040	3,610
Held-to-maturity Securities, Gross Unrealized Gains	8	15
Held-to-maturity Securities, Gross Unrealized Losses	94	38
Held to Maturity, Estimated Fair Value	2,954	3,587
Available for Sale, Amortized Cost	756,649	648,288
Available for Sale Securities, Gross Unrealized Gains	11,254	19,048
Available for Sale Securities, Gross Unrealized Losses	11,532	2,250
Available for Sale, Estimated Fair Value	756,371	665,086
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,107,222	1,428,488
Held-to-maturity Securities, Gross Unrealized Gains	7,232	26,462
Held-to-maturity Securities, Gross Unrealized Losses	2,057	2,746
Held to Maturity, Estimated Fair Value	1,112,397	1,452,204
Available for Sale, Amortized Cost	7,052,156	6,185,711
Available for Sale Securities, Gross Unrealized Gains	40,759	167,293
Available for Sale Securities, Gross Unrealized Losses	116,225	16,795
Available for Sale, Estimated Fair Value	6,976,690	6,336,209
Other [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,500	0
Held-to-maturity Securities, Gross Unrealized Gains	0	0
Held-to-maturity Securities, Gross Unrealized Losses	6	0
Held to Maturity, Estimated Fair Value	1,494	0
Available for Sale, Amortized Cost	42,651	42,561
Available for Sale Securities, Gross Unrealized Gains	0	0
Available for Sale Securities, Gross Unrealized Losses	0	0
Available for Sale, Estimated Fair Value	$ 42,651	$ 42,561

Securities - Securities, with U

Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 405,872
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,082
Held to Maturity, More than 12 Months, Estimated Fair Value	1,850
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	75
Held to Maturity, Total Estimated Fair Value	407,722
Held-to-maturity Securities, Gross Unrealized Losses	2,157	$ 2,784
Available for Sale, Less than 12 Months, Estimated Fair Value	6,175,662
Available for Sale, Less than 12 Months, Unrealized Losses	105,875
Available for Sale, More than 12 Months, Estimated Fair Value	2,016,757
Available for Sale, More than 12 Months, Unrealized Losses	73,902
Available for Sale, Total Estimated Fair Value	8,192,419
Available-for-sale Securities, Gross Unrealized Loss, before Tax	179,777	34,777
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	413
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	19
Held to Maturity, More than 12 Months, Estimated Fair Value	1,850
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	75
Held to Maturity, Total Estimated Fair Value	2,263
Held-to-maturity Securities, Gross Unrealized Losses	94	38
Available for Sale, Less than 12 Months, Estimated Fair Value	294,839
Available for Sale, Less than 12 Months, Unrealized Losses	5,979
Available for Sale, More than 12 Months, Estimated Fair Value	98,757
Available for Sale, More than 12 Months, Unrealized Losses	5,553
Available for Sale, Total Estimated Fair Value	393,596
Available-for-sale Securities, Gross Unrealized Loss, before Tax	11,532	2,250
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	403,965
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,057
Held to Maturity, More than 12 Months, Estimated Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Held to Maturity, Total Estimated Fair Value	403,965
Held-to-maturity Securities, Gross Unrealized Losses	2,057	2,746
Available for Sale, Less than 12 Months, Estimated Fair Value	3,585,212
Available for Sale, Less than 12 Months, Unrealized Losses	65,581
Available for Sale, More than 12 Months, Estimated Fair Value	810,732
Available for Sale, More than 12 Months, Unrealized Losses	50,644
Available for Sale, Total Estimated Fair Value	4,395,944
Available-for-sale Securities, Gross Unrealized Loss, before Tax	116,225	16,795
Other Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	1,494
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6
Held to Maturity, More than 12 Months, Estimated Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Held to Maturity, Total Estimated Fair Value	1,494
Held-to-maturity Securities, Gross Unrealized Losses	6	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	2,295,611
Available for Sale, Less than 12 Months, Unrealized Losses	34,315
Available for Sale, More than 12 Months, Estimated Fair Value	1,107,268
Available for Sale, More than 12 Months, Unrealized Losses	17,705
Available for Sale, Total Estimated Fair Value	3,402,879
Available-for-sale Securities, Gross Unrealized Loss, before Tax	$ 52,020	$ 15,732

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, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Held to Maturity, Due in one year or less, Amortized Cost	$ 56,511
Held to Maturity, Due after one year through five years, Amortized Cost	131,987
Held to Maturity, Due after five years through ten years, Amortized Cost	492,460
Held to Maturity, Due after ten years, Amortized Cost	427,764
Held to Maturity, Residential mortgage-backed securities, Amortized Cost	3,040
Held-to-maturity Securities, Equity Securities, Amortized cost	0
Held to Maturity, Amortized Cost	1,111,762	$ 1,432,098
Held to Maturity, Due in one year or less, Estimated Fair Value	57,181
Held to Maturity, Due after one year through five years, Estimated Fair Value	133,521
Held to Maturity, Due after five years through ten years, Estimated Fair Value	492,354
Held to Maturity, Due after ten years, Estimated Fair Value	430,835
Held to Maturity, Residential mortgage-backed securities, Estimated Fair Value	2,954
Held-to-maturity Securities, Equity Securities, Fair Value	0
Held to Maturity, Estimated Fair Value	1,116,845	1,455,791
Available-for-sale Securities, Due in one year or less, Amortized Cost	858,235
Available for Sale, Due after one year through five years, Amortized Cost	3,243,532
Available for Sale, Due after five years through ten years, Amortized Cost	489,960
Available for Sale, Due after ten years, Amortized Cost	5,915,328
Available for Sale, Residential mortgage-backed securities, Amortized Cost	756,649
Available-for-sale Securities, Equity Securities, Amortized cost	42,651
Available for Sale, Amortized Cost	11,306,355	10,329,951
Available for Sale, Due in one year or less, Estimated Fair Value	851,163
Available for Sale, Due after one year through five years, Estimated Fair Value	3,204,804
Available for Sale, Due after five years through ten years, Estimated Fair Value	487,536
Available for Sale, Due after ten years, Estimated Fair Value	5,836,066
Available for Sale, Residential mortgage-backed securities, Estimated Fair Value	756,371
Available For Sale Securities Equity Securities Fair Value	42,651
Available for Sale, Estimated Fair Value	$ 11,178,591	$ 10,489,009

Securities - Sales of Securitie

Securities - Sales of Securities Available for Sale (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 2,948,178	$ 746,524	$ 13,838,566	$ 8,993,963
Gross realized gains	0	0	3	0
Gross realized losses	(34)	(4,867)	(116)	(4,917)
Tax (expense)benefit of securities gains/losses	$ 7	$ 1,703	$ 24	$ 1,721

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Premium amortization	$ (27,381)	$ (24,586)	$ (80,104)	$ (72,733)
Discount accretion	2,569	1,783	6,356	6,278
Net (premium amortization) discount accretion	$ (24,812)	$ (22,803)	$ (73,748)	$ (66,455)

Securities - Trading Account Se

Securities - Trading Account Securities, at Estimated Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading account securities	$ 22,238	$ 21,098
U.S. Treasury [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading account securities	21,804	19,210
States and political subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading account securities	$ 434	$ 1,888

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Net gain on sales transactions	$ 465	$ 414	$ 1,404	$ 1,018
Net mark-to-market gains (losses)	(8)	(8)	(21)	(51)
Net gain (loss) on trading account securities	$ 457	$ 406	$ 1,383	$ 967

Loans - Additional Information

Loans - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income recognized subsequent to classification as impaired			$ 0		$ 0
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	11.00%		11.00%		11.40%
Loans and Leases Receivable, Related Parties	$ 238,600		$ 238,600		$ 166,400
Restructured loans on non-accrual status at period end			$ 0	$ 58,898
Texas Leading Index economic condition indicator	129.4		129.4		129.3
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end			$ 0	55,023
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end			0	54,082
Additional interest that would have been recognized on non-accrual loans, net of tax	$ 1,300	$ 783	4,200	$ 2,400
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,100,000		1,100,000
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 46,700		$ 46,700

Loans - Loans (Detail)

Loans - Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 5,029,754	$ 4,792,388
Energy:	1,519,368	1,499,093
Commercial mortgages	4,078,787	3,887,742
Construction	1,208,870	1,066,696
Land	315,384	331,986
Total commercial real estate	5,603,041	5,286,424
Home equity loans	352,292	355,342
Home equity lines of credit	326,876	291,950
Other	419,965	376,002
Total consumer real estate	1,099,133	1,023,294
Total real estate	6,702,174	6,309,718
Consumer and other	563,542	544,466
Total loans	$ 13,814,838	$ 13,145,665
Total commercial and industrial percentage of total loans	36.40%	36.40%
Percentage of total energy loans to total loans	11.00%	11.40%
Commercial mortgages percentage of total loans	29.50%	29.60%
Construction percentage of total loans	8.70%	8.10%
Land percentage of total loans	2.30%	2.50%
Total commercial real estate percentage of total loans	40.50%	40.20%
Home equity loans percentage of total loans	2.50%	2.70%
Home equity lines of credit percentage of total loans	2.40%	2.20%
Other percentage of total loans	3.10%	2.90%
Total consumer real estate percentage of total loans	8.00%	7.80%
Total real estate percentage of total loans	48.50%	48.00%
Total consumer and other percentage of total loans	4.10%	4.20%
Total loans percentage	100.00%	100.00%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 1,220,771	$ 1,182,326
Percentage of energy production loans to total loans	8.80%	9.00%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 164,889	$ 171,795
Percentage of energy service loans to total loans	1.20%	1.30%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 133,708	$ 144,972
Percentage of other energy loans to total loans	1.00%	1.10%

Loans - Non-Accrual Loans, Segr

Loans - Non-Accrual Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 82,601	$ 150,314
Commercial Portfolio Segment [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	12,278	46,186
Commercial and industrial, Energy [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	51,802	94,302
Commercial real estate, Buildings, land and other [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	15,913	7,589
Construction Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	0	0
Consumer Real Estate [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	971	2,109
Consumer and Other [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 1,637	$ 128

Loans - Age Analysis of Past Du

Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 93,489
Current Loans	13,721,349
Total loans	13,814,838	$ 13,145,665
Accruing Loans 90 or More Days Past Due	11,585
Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	44,235
Current Loans	4,985,519
Total loans	5,029,754
Accruing Loans 90 or More Days Past Due	3,963
Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	5,472
Current Loans	1,513,896
Total loans	1,519,368
Accruing Loans 90 or More Days Past Due	818
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	24,576
Current Loans	4,369,595
Total loans	4,394,171
Accruing Loans 90 or More Days Past Due	2,606
Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,826
Current Loans	1,207,044
Total loans	1,208,870
Accruing Loans 90 or More Days Past Due	1,042
Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	9,516
Current Loans	1,089,617
Total loans	1,099,133
Accruing Loans 90 or More Days Past Due	1,432
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	7,864
Current Loans	555,678
Total loans	563,542
Accruing Loans 90 or More Days Past Due	1,724
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	73,457
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	33,985
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	3,251
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	21,596
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	784
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	7,743
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	6,098
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	20,032
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	10,250
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	2,221
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	2,980
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	1,042
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	1,773
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	$ 1,766

Loans - Impaired Loans (Detail)

Loans - Impaired Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	$ 105,793	$ 172,396
Recorded Investment With No Allowance	13,697	68,726
Recorded Investment With Allowance	64,659	76,884
Total Recorded Investment	78,356	145,610
Related Allowance	21,488	20,820
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	19,857	60,781
Recorded Investment With No Allowance	3,153	28,038
Recorded Investment With Allowance	6,574	15,722
Total Recorded Investment	9,727	43,760
Related Allowance	4,622	7,553
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	68,087	99,606
Recorded Investment With No Allowance	7,942	33,080
Recorded Investment With Allowance	43,614	61,162
Total Recorded Investment	51,556	94,242
Related Allowance	12,672	13,267
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	15,961	10,795
Recorded Investment With No Allowance	2,309	6,394
Recorded Investment With Allowance	12,876	0
Total Recorded Investment	15,185	6,394
Related Allowance	2,599	0
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	0	0
Recorded Investment With No Allowance	0	0
Recorded Investment With Allowance	0	0
Total Recorded Investment	0	0
Related Allowance	0	0
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	293	1,214
Recorded Investment With No Allowance	293	1,214
Recorded Investment With Allowance	0	0
Total Recorded Investment	293	1,214
Related Allowance	0	0
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	1,595	0
Recorded Investment With No Allowance	0	0
Recorded Investment With Allowance	1,595	0
Total Recorded Investment	1,595	0
Related Allowance	$ 1,595	$ 0

Loans - Average Recorded Invest

Loans - Average Recorded Investment In Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 108,269	$ 109,680	$ 117,605	$ 105,980
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	13,447	26,910	21,025	26,651
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	78,772	76,008	82,640	72,055
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	14,306	5,553	12,328	6,106
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	0	0	0	0
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	671	1,209	807	1,155
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 1,073	$ 0	$ 805	$ 13

Loans - Troubled Debt Restructu

Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	$ 15,911	$ 60,123
Balance at period end	0	58,898
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	2,203	4,026
Balance at period end	0	3,875
Commercial and industrial, Energy [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	13,708	56,097
Balance at period end	$ 0	$ 55,023

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, 2018USD ($)Item	Sep. 30, 2017USD ($)Item
Financing Receivable, Modifications [Line Items]
Number Of Loans Restructured During The Last Year In Excess of 90 Days Past Due | Item	0	1
Dollar amount of previously restructured loans past due	$ 0	$ 43,137
Restructured loans on non-accrual status at period end	0	58,898
Charge-offs of restructured loans recognized in connection with restructuring	0	0
Charge-offs of restructured loans recognized on previously restructured loans	4,650	9,951
Proceeds from the sale of restructured loans	15,750	0
Non Accrual Loans [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans on non-accrual status at period end	$ 0	$ 54,082

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, 2018USD ($)Grade	Dec. 31, 2017USD ($)Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 5,029,754	$ 4,792,388
Loans	5,603,041	5,286,424
Loans	$ 1,208,870	$ 1,066,696
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.31	6.41
Loans	$ 5,029,754	$ 4,792,388
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.32	6.97
Loans	$ 1,519,368	$ 1,499,093
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.99	7
Loans	$ 4,394,171	$ 4,219,728
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.25	7.23
Loans	$ 1,208,870	$ 1,066,696
Risk grades 1-8 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.10	6.06
Loans	$ 4,755,739	$ 4,378,839
Risk grades 1-8 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	5.81	6.01
Loans	$ 1,367,235	$ 1,199,207
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	6.77	6.75
Loans	$ 4,091,571	$ 3,868,659
Risk grades 1-8 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.10	7.11
Loans	$ 1,129,492	$ 1,019,635
Risk grade 9 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9	9
Loans	$ 123,199	$ 170,285
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	$ 13,352	$ 50,427
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	$ 135,521	$ 151,487
Risk grade 9 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9	9
Loans	$ 51,058	$ 18,042
Risk grade 10 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10	10
Loans	$ 67,645	$ 99,260
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	$ 42,862	$ 64,282
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	$ 60,169	$ 129,391
Risk grade 10 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10	10
Loans	$ 26,035	$ 23,393
Risk grade 11 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11	11
Loans	$ 70,893	$ 97,818
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	$ 44,116	$ 90,875
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	$ 90,997	$ 62,602
Risk grade 11 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11	11
Loans	$ 2,285	$ 5,626
Risk grade 12 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12	12
Loans	$ 7,656	$ 38,633
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	$ 39,131	$ 81,035
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	$ 13,314	$ 7,589
Risk grade 12 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12	12
Loans	$ 0	$ 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	$ 4,622	$ 7,553
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	$ 12,672	$ 13,267
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	$ 2,599	$ 0
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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	$ (15,298)	$ (6,235)	$ (35,632)	$ (26,100)
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	(7,807)	(4,565)	(19,030)	(12,155)
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	(5,347)	451	(10,272)	(10,010)
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	33	266	(288)	768
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	3	2	11	8
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	(388)	(629)	(1,078)	(422)
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	$ (1,792)	$ (1,760)	$ (4,975)	$ (4,289)

Loans - Unallocated Portion of

Loans - Unallocated Portion of Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 137,578	$ 150,226	$ 155,364	$ 154,303	$ 149,558	$ 153,045
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	21,488		20,820
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	116,090		134,544
Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	65,133		75,481
Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	21,488		20,820
General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	20,903		23,283
Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	30,054		35,780
Commercial Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	50,991	57,713	59,614	48,437	48,906	52,915
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	4,622		7,553
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	46,369		52,061
Commercial Portfolio Segment [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	25,582		26,401
Commercial Portfolio Segment [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	4,622		7,553
Commercial Portfolio Segment [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	9,361		9,112
Commercial Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	11,426		16,548
Commercial and industrial, Energy [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	31,445	37,313	51,528	51,913	54,277	60,653
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	12,672		13,267
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	18,773		38,261
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	9,654		22,073
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	12,672		13,267
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	5,849		7,964
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	3,270		8,224
Commercial Real Estate Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	39,204	38,918	30,948	38,075	33,002	30,213
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	2,599		0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	36,605		30,948
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	20,441		18,931
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	2,599		0
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	4,187		4,165
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	11,977		7,852
Consumer Real Estate [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	6,194	6,336	5,657	6,875	5,535	4,238
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	6,194		5,657
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		2,473
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	1,620		2,133
Consumer Real Estate [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,933		1,051
Consumer and Other [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	9,744	$ 9,946	7,617	$ 9,003	$ 7,838	$ 5,026
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	1,595		0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	8,149		7,617
Consumer and Other [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	6,815		5,603
Consumer and Other [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,595		0
Consumer and Other [Member] | General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	(114)		(91)
Consumer and Other [Member] | Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 1,448		$ 2,105

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, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 5,029,754	$ 4,792,388
Energy:	1,519,368	1,499,093
Total commercial real estate	5,603,041	5,286,424
Total consumer real estate	1,099,133	1,023,294
Consumer and other	563,542	544,466
Total loans	13,814,838	13,145,665
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	9,727	43,760
Energy:	51,556	94,242
Total commercial real estate	15,185	6,394
Total consumer real estate	293	1,214
Consumer and other	1,595	0
Total loans	78,356	145,610
Loans Collectively Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	5,020,027	4,748,628
Energy:	1,467,812	1,404,851
Total commercial real estate	5,587,856	5,280,030
Total consumer real estate	1,098,840	1,022,080
Consumer and other	561,947	544,466
Total loans	$ 13,736,482	$ 13,000,055

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 150,226	$ 149,558	$ 155,364	$ 153,045
Provision for loan losses	2,650	10,980	17,846	27,358
Charge-offs	(18,596)	(10,354)	(47,326)	(37,253)
Recoveries	3,298	4,119	11,694	11,153
Net charge-offs	(15,298)	(6,235)	(35,632)	(26,100)
Ending balance	137,578	154,303	137,578	154,303
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	57,713	48,906	59,614	52,915
Provision for loan losses	1,085	4,096	10,407	7,677
Charge-offs	(8,491)	(5,468)	(21,896)	(14,574)
Recoveries	684	903	2,866	2,419
Net charge-offs	(7,807)	(4,565)	(19,030)	(12,155)
Ending balance	50,991	48,437	50,991	48,437
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	37,313	54,277	51,528	60,653
Provision for loan losses	(521)	(2,815)	(9,811)	1,270
Charge-offs	(5,400)	0	(10,939)	(10,595)
Recoveries	53	451	667	585
Net charge-offs	(5,347)	451	(10,272)	(10,010)
Ending balance	31,445	51,913	31,445	51,913
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	38,918	33,002	30,948	30,213
Provision for loan losses	250	4,805	8,533	7,086
Charge-offs	0	0	(619)	(14)
Recoveries	36	268	342	790
Net charge-offs	36	268	(277)	776
Ending balance	39,204	38,075	39,204	38,075
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,336	5,535	5,657	4,238
Provision for loan losses	246	1,969	1,615	3,059
Charge-offs	(431)	(766)	(1,632)	(779)
Recoveries	43	137	554	357
Net charge-offs	(388)	(629)	(1,078)	(422)
Ending balance	6,194	6,875	6,194	6,875
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	9,946	7,838	7,617	5,026
Provision for loan losses	1,590	2,925	7,102	8,266
Charge-offs	(4,274)	(4,120)	(12,240)	(11,291)
Recoveries	2,482	2,360	7,265	7,002
Net charge-offs	(1,792)	(1,760)	(4,975)	(4,289)
Ending balance	$ 9,744	$ 9,003	$ 9,744	$ 9,003

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Schedule of Goodwill (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
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, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	$ 3,980	$ 5,073
Core deposits [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	3,213	4,044
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	743	986
Non-compete agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	$ 24	$ 43

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Estimated Aggregate Future Amortization Expense for Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Remainder of 2018	$ 331
2,019	1,167
2,020	918
2,021	697
2,022	481
Thereafter	386
Future Amortization Expense, Total	$ 3,980	$ 5,073

Deposits - Schedule of Deposits

Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,840,513	$ 11,197,093
Total interest-bearing deposits	15,508,341	15,675,296
Total deposits	$ 26,348,854	$ 26,872,389
Total non-interest-bearing demand deposits, Percentage of Total	41.10%	41.70%
Total interest-bearing deposits, Percentage of Total	58.90%	58.30%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,137,696	$ 10,412,882
Total non-interest-bearing demand deposits, Percentage of Total	38.40%	38.80%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 187,589	$ 222,648
Total non-interest-bearing demand deposits, Percentage of Total	0.70%	0.80%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 6,640,382	$ 6,788,766
Money market accounts	7,694,628	7,624,471
Time accounts of $100,000 or More	484,236	453,668
Time accounts under $100,000	327,790	324,636
Total private accounts	$ 15,147,036	$ 15,191,541
Savings and interest checking, Percentage of Total	25.20%	25.20%
Money market accounts, Percentage of Total	29.20%	28.40%
Time accounts of $100,000 or more, Percentage of Total	1.80%	1.70%
Time accounts under $100,000, Percentage of Total	1.30%	1.20%
Total private accounts, Percentage of Total	57.50%	56.50%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 515,228	$ 561,563
Savings and interest checking	303,150	410,140
Money market accounts	54,537	59,008
Time accounts of $100,000 or More	3,523	14,301
Time accounts under $100,000	95	306
Total public funds	$ 361,305	$ 483,755
Total non-interest-bearing demand deposits, Percentage of Total	2.00%	2.10%
Savings and interest checking, Percentage of Total	1.20%	1.50%
Money market accounts, Percentage of Total	0.20%	0.20%
Time accounts of $100,000 or more, Percentage of Total	0.00%	0.10%
Time accounts under $100,000, Percentage of Total	0.00%	0.00%
Total public funds, Percentage of Total	1.40%	1.80%

Deposits - Additional Informati

Deposits - Additional Information About Corporation's Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Banking and Thrift [Abstract]
Deposits from foreign sources (primarily Mexico)	$ 739,424	$ 716,339
Deposits not covered by deposit insurance	$ 12,403,549	$ 13,281,040

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Rent expense for operating leases	$ 8.1	$ 7.7	$ 24.4	$ 23

Commitments and Contingencies_2

Commitments and Contingencies - Financial Instruments with Off-Balance-Sheet Risk (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Standby Letters of Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 244,743	$ 236,595
Deferred Standby Letter Of Credit Fees [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	1,657	1,843
Commitments to Extend Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 8,284,873	$ 7,949,400

Capital and Regulatory Matter_2

Capital and Regulatory Matters - Additional Information (Detail) $ in Thousands	Oct. 24, 2017USD ($)	Sep. 30, 2018USD ($)shares	Sep. 30, 2017USD ($)shares	Sep. 30, 2018USD ($)quartershares	Sep. 30, 2017USD ($)shares	Dec. 31, 2017USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares		0	1,134,966	601	1,135,435
Treasury Stock, Value, Acquired, Cost Method			$ 100,000	$ 70	$ 100,042
Preferred stock value issued include for regulatory tier one capital calculations		$ 144,500		$ 144,500		$ 144,500
Preferred stock dividend rate percentage				5.375%		5.375%
Qualified subordinated debt		98,669		$ 98,669		$ 98,552
Maximum dividends available without prior regulatory approval		569,700		$ 569,700
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
Stock Repurchase 2017 Plan
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Authorized Amount	$ 150,000
Stock Repurchase Program, Period in Force	2 years
Stock Repurchase 2017 Plan | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares				0	0
Stock Repurchase 2016 Plan | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares			1,134,966
Treasury Stock, Value, Acquired, Cost Method			$ 100,000
Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Qualified subordinated debt		$ 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, 2018	Dec. 31, 2017
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 2,658,645	$ 2,426,048
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.93%	12.42%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,310,621	$ 1,123,430
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	6.38%	5.75%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,439,113	$ 1,367,583
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	$ 1,336,319	$ 1,269,965
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	$ 2,803,131	$ 2,570,534
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.63%	13.16%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,619,002	$ 1,416,499
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	7.88%	7.25%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,747,494	$ 1,660,637
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	$ 1,644,701	$ 1,563,033
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	$ 3,174,209	$ 2,959,326
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.44%	15.15%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 2,030,177	$ 1,807,257
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio	9.88%	9.25%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,158,670	$ 2,051,375
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	$ 2,055,876	$ 1,953,792
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,803,131	$ 2,570,534
Leverage Ratio, Actual, Ratio	9.19%	8.46%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,220,627	$ 1,215,227
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	4.00%	4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,220,627	$ 1,215,186
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	$ 1,525,784	$ 1,519,034
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	$ 2,683,196	$ 2,518,999
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	13.10%	12.92%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,306,205	$ 1,120,663
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	6.38%	5.75%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,434,264	$ 1,364,214
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	$ 1,331,816	$ 1,266,836
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	$ 2,683,196	$ 2,518,999
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.10%	12.92%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,613,547	$ 1,413,010
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	7.88%	7.25%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,741,606	$ 1,656,546
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	$ 1,639,159	$ 1,559,183
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	$ 2,821,274	$ 2,674,791
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.77%	13.72%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 2,023,337	$ 1,802,805
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio	9.88%	9.25%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,151,396	$ 2,046,321
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	$ 2,048,948	$ 1,948,979
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,683,196	$ 2,518,999
Leverage Ratio, Actual, Ratio	8.80%	8.30%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,219,806	$ 1,214,295
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	4.00%	4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,219,806	$ 1,214,254
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	$ 1,524,758	$ 1,517,869
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, 2018	Dec. 31, 2017
Derivative [Line Items]
Weighted-average strike rate for outstanding interest rate caps	2.96%
Approximate credit exposure related to swaps with bank customers	$ 41,000
Aggregate fair value of securities posted as collateral for derivative contracts	7,000
Cash collateral for borrowed securities deposited with other financial institutions	26,700
Interest rate swaps with upstream financial institution counterparties [Member]
Derivative [Line Items]
Approximate credit exposure related to swaps with bank customers	431
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, 2018	Dec. 31, 2017
Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	$ 11,284	$ 13,679
Estimated Fair Value	314	242
Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	4,226	11,147
Estimated Fair Value	(201)	(593)
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	780,531	430,449
Estimated Fair Value	6,179	1,418
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	341,987	541,496
Estimated Fair Value	(5,133)	(12,820)
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest-rate caps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	102,487	114,619
Estimated Fair Value	920	480
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	341,987	541,496
Estimated Fair Value	7,017	17,882
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	780,531	430,449
Estimated Fair Value	(20,613)	(4,861)
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate caps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	102,487	114,619
Estimated Fair Value	$ (920)	$ (480)

Derivative Financial Instrume_5

Derivative Financial Instruments - Schedule of Weighted-Average Rates Paid and Received for Interest Rate Swaps Outstanding (Detail)	Sep. 30, 2018
Weighted Average Interest Rate Paid [Member]
Weighted Average Discount Rate [Line Items]
Fair value hedge loan/lease interest rate swaps	2.45%
Weighted-Average Interest Rate Received [Member]
Weighted Average Discount Rate [Line Items]
Fair value hedge loan/lease interest rate swaps	2.16%
Financial Institution Counterparties [Member] | Weighted Average Interest Rate Paid [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – financial institution counterparties	4.09%
Financial Institution Counterparties [Member] | Weighted-Average Interest Rate Received [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – financial institution counterparties	3.73%
Customer Counterparties [Member] | Weighted Average Interest Rate Paid [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – customer counterparties	3.73%
Customer Counterparties [Member] | Weighted-Average Interest Rate Received [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – customer counterparties	4.09%

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, 2018USD ($)MMBTUbbl	Dec. 31, 2017USD ($)MMBTUbbl
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	622	253
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	2,889	2,731
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 917	$ 193
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (34,941)	$ (13,448)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	7,939	5,927
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	8,382	3,917
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 427	$ 1,399
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (598)	$ (326)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	2,889	2,731
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	622	253
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 35,083	$ 13,709
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (917)	$ (187)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	9,588	3,917
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	6,734	5,927
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 682	$ 340
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (392)	$ (1,366)

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, $ in Thousands	Sep. 30, 2018USD ($)	Sep. 30, 2018CAD ($)	Sep. 30, 2018AUD ($)	Sep. 30, 2018EUR (€)	Sep. 30, 2018GBP (£)	Dec. 31, 2017USD ($)	Dec. 31, 2017CAD ($)	Dec. 31, 2017AUD ($)	Dec. 31, 2017EUR (€)	Dec. 31, 2017GBP (£)
Financial Institution Counterparties [Member] | Other Assets [Member] | Non Designated as Hedging Instrument [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Notional Amount				€ 1,097	£ 793				€ 4,014	£ 127
Financial Institution Counterparties [Member] | Other Liabilities [Member] | Non Designated as Hedging Instrument [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Notional Amount		$ 22,971	$ 54	0	0		$ 25,413	$ 0	4,846	1,178
Customer Counterparties [Member] | Other Assets [Member] | Non Designated as Hedging Instrument [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Notional Amount		$ 22,918		0			$ 25,282		3,867
Customer Counterparties [Member] | Other Liabilities [Member] | Non Designated as Hedging Instrument [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Notional Amount				€ 0	£ 0				€ 4,041	£ 127
Foreign Exchange Forward [Member] | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Estimated Fair Value	$ (189)					$ (142)
Foreign Exchange Forward [Member] | Customer Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value	245					279
GBP Foreign Exchange Forward [Member] | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value	9					1
Forward Contracts – Liability Estimated Fair Value	0					(9)
GBP Foreign Exchange Forward [Member] | Customer Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Estimated Fair Value	0					0
AUD Foreign Exchange Forward [Member] | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Estimated Fair Value	0					0
Eurodollar Future [Member] | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value	9					77
Forward Contracts – Liability Estimated Fair Value	0					(37)
Eurodollar Future [Member] | Customer Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value	0					58
Forward Contracts – Liability Estimated Fair Value	$ 0					$ (51)

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest Income on Loans [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ 12	$ (149)	$ 1	$ (592)
Other Non-Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ 0	$ (2)	$ (1)	$ (5)

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
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)	$ 699	$ 1,085	$ 2,889	$ 2,062
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)	2	0	(2)	1
Commodity Contract [Member] | Other Non-Interest Income [Member]
Other Non Interest Income Non Hedging Interest Rate Derivatives [Line Items]
Amount of gain (loss) included in income (expense)	456	231	492	387
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)	$ 53	$ 83	$ 203	$ 101

Balance Sheet Offsetting - Fina

Balance Sheet Offsetting - Financial Instruments Eligible for Offset Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	$ 8,775	$ 3,810
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	8,775	3,810
Gross Amount Recognized, Resell agreements, Financial assets	12,142	9,642
Gross Amount Offset, Resell agreements, Financial assets	0	0
Net Amount Recognized, Resell agreements, Financial assets	12,142	9,642
Gross Amount Recognized, Financial assets	20,917	13,452
Gross Amount Offset, Financial assets	0	0
Net Amount Recognized, Financial assets	20,917	13,452
Gross Amount Recognized, Derivatives, Financial Liabilities	41,062	27,375
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	41,062	27,375
Gross Amount Recognized, Repurchase agreements, Financial liabilities	1,074,576	1,117,199
Gross Amount Offset, Repurchase agreements, Financial liabilities	0	0
Net Amount Recognized, Repurchase agreements, Financial liabilities	1,074,576	1,117,199
Gross Amount Recognized, Financial liabilities	1,115,638	1,144,574
Gross Amount Offset, Financial liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,115,638	1,144,574
Loan/lease interest rate swaps and caps [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	7,413	2,140
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	7,413	2,140
Commodity swaps and options [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	1,344	1,592
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	1,344	1,592
Gross Amount Recognized, Derivatives, Financial Liabilities	35,539	13,774
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	35,539	13,774
Foreign currency forward contracts [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	18	78
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	18	78
Gross Amount Recognized, Derivatives, Financial Liabilities	189	188
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	189	188
Loan/lease interest rate swaps [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Liabilities	5,334	13,413
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	$ 5,334	$ 13,413

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, 2018	Dec. 31, 2017
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 8,775	$ 3,810
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(5,340)	(3,308)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(3,223)	(387)
Net Amount, Derivatives, Financial Assets	212	115
Net Amount Recognized, Resell agreements, Financial Assets	12,142	9,642
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(12,142)	(9,642)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	20,917	13,452
Gross Amounts Not Offset Collateral, Financial Assets	(15,365)	(10,029)
Net Amount, Financial Assets	212	115
Net Amount Recognized, Derivatives, Financial Liabilities	41,062	27,375
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(5,340)	(3,308)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(33,455)	(22,768)
Net Amount, Derivatives, Financial Liabilities	2,267	1,299
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	1,074,576	1,117,199
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(1,074,576)	(1,117,199)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,115,638	1,144,574
Gross Amounts Not Offset Collateral, Financial Liabilities	(1,108,031)	(1,139,967)
Net Amount, Financial Liabilities	2,267	1,299
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	1,477	395
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,477)	(395)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	3,840	7,397
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,477)	(395)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(2,363)	(7,002)
Net Amount, Derivatives, Financial Liabilities	0	0
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	1,963	1,028
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,963)	(1,028)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	6,627	4,466
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,963)	(1,028)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(4,453)	(3,101)
Net Amount, Derivatives, Financial Liabilities	211	337
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	75	55
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(75)	(55)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	806	1,520
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(75)	(55)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(731)	(1,450)
Net Amount, Derivatives, Financial Liabilities	0	15
Other Counterparties [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	5,260	2,332
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,825)	(1,830)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(3,223)	(387)
Net Amount, Derivatives, Financial Assets	212	115
Net Amount Recognized, Derivatives, Financial Liabilities	29,789	13,992
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,825)	(1,830)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(25,908)	(11,215)
Net Amount, Derivatives, Financial Liabilities	$ 2,056	$ 947

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, 2018	Dec. 31, 2017
Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase [Line Items]
Remaining contractual maturity of the agreements overnight and continuous	$ 1,074,576	$ 1,117,199
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	1,074,576	1,117,199
Securities Sold under Agreements to Repurchase, Gross	1,074,576	1,117,199
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	0	0
US Treasury Securities [Member]
Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase [Line Items]
Remaining contractual maturity of the agreements overnight and continuous	1,067,059	1,036,891
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	1,067,059	1,036,891
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	7,517	80,308
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	$ 7,517	$ 80,308

Stock-Based Compensation Stock-

Stock-Based Compensation Stock-Based Compensation - Narrative (Details)	Sep. 30, 2018shares
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	1,415,135

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options exercised, Number of Shares	(19,232)	(22,724)	(447,831)	(774,799)
Director deferred stock units outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares			53,008
Granted, Number of Units			6,576
Stock Awards Vested and Expected to Vest, Exercisable, Number	(10,674)		(10,674)
Ending Balance, Number of Shares	48,910		48,910
Beginning Balance, Director Deferred, Weighted-Average Grant-Date Fair Value			$ 64.87
Granted, Directors Deferred, Weighted-Average Grant-Date Fair Value			109.58
Ending Balance, Director Deferred, Weighted-Average Grant-Date Fair Value	$ 71.14		71.14
Stock Award Vested, Weighted Average Grant Date Fair Value			$ 63.68
Non-Vested Stock Awards/Stock Units Outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares			312,656
Stock Awards Vested and Expected to Vest, Exercisable, Number	(2,470)		(2,470)
Forfeited/expired, Number of Shares			(4,836)
Ending Balance, Number of Shares	305,350		305,350
Stock Award Vested, Weighted Average Grant Date Fair Value			$ 78.92
Beginning Balance, Weighted-Average Grant-Date Fair Value			81.71
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 81.65		81.65
Forfeited/expired, Weighted-Average Exercise Price			$ 87
Stock Options Outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares			2,917,142
Stock options exercised, Number of Shares			(434,687)
Forfeited/expired, Number of Shares			(49,125)
Ending Balance, Number of Shares	2,433,330		2,433,330
Beginning Balance, Weighted-Average Exercise Price			$ 63.34
Stock options exercised, Weighted-Average Exercise Price			61.12
Forfeited/expired, Weighted-Average Exercise Price			70.70
Ending Balance, Weighted-Average Exercise Price	$ 63.59		$ 63.59
Maximum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Number of Shares			80,103
Ending Balance, Number of Shares	80,103		80,103
Beginning Balance, Weighted-Average Grant-Date Fair Value			$ 79.91
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 79.91		$ 79.91

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
New shares issued from available authorized shares	0	9,299	0	602,662
Issued from available treasury stock	19,232	13,425	447,831	172,137
Total	19,232	22,724	447,831	774,799
Proceeds from stock option exercises	$ 1,119	$ 1,274	$ 26,567	$ 45,422

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock options	$ 960	$ 1,532	$ 3,064	$ 4,892
Non-vested stock awards/stock units	1,357	813	4,196	2,747
Director deferred stock units	0	0	720	519
Performance stock units	479	377	1,576	855
Total	2,796	2,722	9,556	9,013
Income tax benefit	$ 587	$ 953	$ 2,007	$ 3,155

Stock-Based Compensation - Unre

Stock-Based Compensation - Unrecognized Stock-Based Compensation Expense (Detail) $ in Thousands	Sep. 30, 2018USD ($)
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock options	$ 1,906
Non-vested stock awards/stock units	10,343
Performance stock units	3,553
Total	$ 15,802

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 117,844	$ 93,131	$ 335,665	$ 263,625
Less: Preferred stock dividends	2,016	2,016	6,047	6,047
Net income available to common shareholders	115,828	91,115	329,618	257,578
Less: Earnings allocated to participating securities	750	475	2,178	1,346
Distributed earnings allocated to common stock	42,811	36,174	121,907	107,194
Undistributed earnings allocated to common stock	72,267	54,466	205,533	149,038
Net earnings allocated to common stock	$ 115,078	$ 90,640	$ 327,440	$ 256,232
Weighted-average shares outstanding for basic earnings per common share	63,892,023	63,667,356	63,793,514	63,822,011
Dilutive effect of stock compensation	1,022,168	897,945	1,037,043	957,337
Weighted-average shares outstanding for diluted earnings per common share	64,914,191	64,565,301	64,830,557	64,779,348

Defined Benefit Plans - Net Per

Defined Benefit Plans - Net Periodic Cost (Benefit) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Retirement Benefits [Abstract]
Expected return on plan assets, net of expenses	$ (2,979)	$ (2,779)	$ (8,937)	$ (8,338)
Interest cost on projected benefit obligation	1,475	1,547	4,424	4,642
Net amortization and deferral	1,250	1,357	3,751	4,072
Net periodic expense (benefit)	$ (254)	$ 125	$ (762)	$ 376

Defined Benefit Plans Additiona

Defined Benefit Plans Additional Information (Details)	Sep. 30, 2018USD ($)
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 Thousands	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended	48 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Income Tax [Abstract]
Deferred Tax Assets, Net	$ 54,100			$ 54,100		$ 31,700
Valuation allowance	0			$ 0
U.S. federal statutory income tax rate, percent				21.00%	35.00%
Unrecognized tax benefits	0			$ 0
Deferred tax adjustment related to reduction in U.S. federal statutory income tax rate.						$ 4,000
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Current income tax expense (benefit)	$ (1,152)	$ 15,224		$ 955	$ 44,636
Error Correction [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Current income tax expense (benefit)			$ 756		$ 3,662		$ 2,906

Income Taxes - Income Tax Expen

Income Taxes - Income Tax Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Current income tax expense (benefit)	$ (1,152)	$ 15,224	$ 955	$ 44,636
Deferred income tax expense (benefit)	16,312	(5,332)	39,198	(9,505)
Income tax expense, as reported	$ 15,160	$ 9,892	$ 40,153	$ 35,131
Effective tax rate	11.40%	9.60%	10.70%	11.80%

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (96,147)	$ 7,082	$ (286,935)	$ 131,283
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(3,764)	(3,514)	(8,424)	(13,660)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	34	4,867	113	4,917
Total securities available for sale and transferred securities, before tax amount	(99,877)	8,435	(295,246)	122,540
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	1,250	1,357	3,751	4,072
Total defined-benefit post-retirement benefit plans	1,250	1,357	3,751	4,072
Total other comprehensive income (loss), before tax	(98,627)	9,792	(291,495)	126,612
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(20,190)	2,479	(60,256)	45,949
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(790)	(1,230)	(1,769)	(4,781)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	7	1,703	24	1,721
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(20,973)	2,952	(62,001)	42,889
Change in the net actuarial gain/loss,Tax Expense (Benefit)	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), Tax Expense (Benefit)	262	475	788	1,425
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	262	475	788	1,425
Total other comprehensive income (loss), Tax Expense (Benefit)	(20,711)	3,427	(61,213)	44,314
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(75,957)	4,603	(226,679)	85,334
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(2,974)	(2,284)	(6,655)	(8,879)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	27	3,164	89	3,196
Total securities available for sale and transferred securities, Net of Tax	(78,904)	5,483	(233,245)	79,651
Change in the net actuarial gain/loss, Net of Tax	0	0	0	0
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	988	882	2,963	2,647
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	988	882	2,963	2,647
Other comprehensive income (loss), net of tax	$ (77,916)	$ 6,365	$ (230,282)	$ 82,298

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 79,512	$ (24,623)
Other comprehensive income (loss) before reclassifications			(233,334)	76,455
Reclassification of amounts included in net income			3,052	5,843
Other comprehensive income (loss), net of tax	$ (77,916)	$ 6,365	(230,282)	82,298
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	(141,235)	57,675	(141,235)	57,675
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			117,230	16,153
Other comprehensive income (loss) before reclassifications			(233,334)	76,455
Reclassification of amounts included in net income			89	3,196
Other comprehensive income (loss), net of tax			(233,245)	79,651
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	(98,458)	95,804	(98,458)	95,804
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(37,718)	(40,776)
Other comprehensive income (loss) before reclassifications			0	0
Reclassification of amounts included in net income			2,963	2,647
Other comprehensive income (loss), net of tax			2,963	2,647
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	$ (42,777)	$ (38,129)	$ (42,777)	$ (38,129)

Operating Segments - Additional

Operating Segments - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018Segment
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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	$ 329,322	$ 300,826	$ 972,851	$ 888,903
Net income	117,844	93,131	335,665	263,625
Operating Segments [Member] | Banking [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	295,062	266,582	874,056	786,743
Net income	114,214	88,368	327,131	250,766
Operating Segments [Member] | Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	36,868	36,529	106,475	107,829
Net income	6,442	6,417	17,977	17,990
Operating Segments [Member] | Non-Banks [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	(2,608)	(2,285)	(7,680)	(5,669)
Net income	$ (2,812)	$ (1,654)	$ (9,443)	$ (5,131)

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) $ in Thousands	Sep. 30, 2018USD ($)Grade	Dec. 31, 2017USD ($)Grade
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments using fair value measurement option | $	$ 0	$ 0
Fair Value, Inputs, Level 3 [Member] | Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average risk grade of loans underlying swaps measured at fair value using significant unobservable inputs | Grade	12	12
Weighted average loss severity on default of swaps	19.70%	15.40%

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, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	$ 11,178,591	$ 10,489,009
Trading account securities	22,238	21,098
Derivative assets	8,775	3,810
Derivative liabilities	41,062	27,375
Interest rate swaps, caps and floors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	14,430	20,022
Derivative liabilities	26,867	18,754
Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	37,109	15,641
Derivative liabilities	36,848	15,327
Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	263	415
Derivative liabilities	189	239
U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	3,402,879	3,445,153
Trading account securities	21,804	19,210
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	756,371	665,086
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	6,976,690	6,336,209
Trading account securities	434	1,888
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	42,651	42,561
Level 1 [Member] | Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	263	415
Derivative liabilities	189	239
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	3,402,879	3,445,153
Trading account securities	21,804	19,210
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	14,430	20,022
Derivative liabilities	26,867	18,754
Level 2 [Member] | Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	35,813	14,408
Derivative liabilities	36,848	15,327
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	756,371	665,086
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	6,976,690	6,336,209
Trading account securities	434	1,888
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	42,651	42,561
Level 3 [Member] | Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 1,296	$ 1,233

Fair Value Measurements - Impai

Fair Value Measurements - Impaired Loans Remeasured and Reported at Fair Value of Underlying Collateral (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	$ 78,356	$ 145,610
Specific valuation allowance allocations	(21,488)	$ (20,820)
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	12,876		$ 0
Specific valuation allowance allocations	(2,599)		0
Loans receivable, fair value disclosure	10,277		0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	28,638		64,287
Specific valuation allowance reversal of prior allocation	4,256
Specific valuation allowance allocations			(13,477)
Loans receivable, fair value disclosure	$ 32,894		$ 50,810

Fair Value Measurements - Forec

Fair Value Measurements - Foreclosed Assets Remeasured and Reported at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
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	$ 2,898	$ 0
Foreclosed Assets Remeasured at Initial Recognition Charge-offs recognized in the allowance for loan losses	0	0
Fair Value of Foreclosed Assets Remeasured at Initial Recognition	2,898	0
Foreclosed Assets Remeasured Subsequent to initial Recognition Carrying value of foreclosed assets prior to remeasurement	1,823	89
Foreclosed Assets Remeasured Subsequent to Initial Recognition Write-downs included in other non-interest expense	(473)	(16)
Fair Value of Foreclosed Assets remeasured subsequent to initial recognition	$ 1,350	$ 73

Fair Value Measurements - Estim

Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 3,419,489	$ 5,053,047	$ 5,091,903	$ 4,141,445
Securities held to maturity	1,111,762	1,432,098
Loans, net	13,677,260	12,990,301
Deposits	26,348,854	26,872,389
Federal funds purchased and repurchase agreements	1,083,777	1,147,824
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,227	136,184
Subordinated notes payable and other borrowings	98,669	98,552
Carrying Amount [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,419,489	5,053,047
Securities held to maturity	1,111,762	1,432,098
Cash surrender value of life insurance policies	182,603	180,477
Accrued interest receivable	137,553	167,508
Deposits	26,348,854	26,872,389
Federal funds purchased and repurchase agreements	1,083,777	1,147,824
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,227	136,184
Subordinated notes payable and other borrowings	98,669	98,552
Accrued interest payable	4,904	3,358
Carrying Amount [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	13,677,260	12,990,301
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,419,489	5,053,047
Securities held to maturity	1,116,845	1,455,791
Cash surrender value of life insurance policies	182,603	180,477
Accrued interest receivable	137,553	167,508
Deposits	26,342,071	26,866,676
Federal funds purchased and repurchase agreements	1,083,777	1,147,824
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	137,115	137,115
Subordinated notes payable and other borrowings	98,875	105,311
Accrued interest payable	4,904	3,358
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 13,621,005	$ 12,981,165

Accounting Standards Updates _2

Accounting Standards Updates Accounting Standards Updates - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2019	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							$ (2,285)
Additional Premium Amortization Expense	$ 27,381	$ 24,586	$ 80,104	$ 72,733
Two Thousand And Nineteen [Member] | Scenario, Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption						$ (11,300)
Additional Premium Amortization Expense					$ 5,300
Two Thousand And Nineteen [Member] | Minimum [Member] | Scenario, Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption						400,000
Two Thousand And Nineteen [Member] | Maximum [Member] | Scenario, Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption						$ 500,000