Cullen/Frost Bankers (CFR) 10-Q 26 Apr 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	Apr. 19, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
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		63,797,196

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Cash and due from banks	$ 434,155	$ 545,542
Interest-bearing deposits	3,907,051	4,347,538
Federal funds sold and resell agreements	339,742	159,967
Total cash and cash equivalents	4,680,948	5,053,047
Securities held to maturity, at amortized cost	1,247,154	1,432,098
Securities available for sale, at estimated fair value	10,536,532	10,489,009
Trading account securities	19,772	21,098
Loans, net of unearned discounts	13,364,029	13,145,665
Less: Allowance for loan losses	(149,885)	(155,364)
Net loans	13,214,144	12,990,301
Premises and equipment, net	521,202	520,958
Goodwill	654,952	654,952
Other intangible assets, net	4,685	5,073
Cash surrender value of life insurance policies	181,297	180,477
Accrued interest receivable and other assets	398,546	400,867
Total assets	31,459,232	31,747,880
Liabilities:
Non-interest-bearing demand deposits	10,934,162	11,197,093
Interest-bearing deposits	15,743,616	15,675,296
Total deposits	26,677,778	26,872,389
Federal funds purchased and repurchase agreements	1,032,221	1,147,824
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,198	136,184
Subordinated notes, net of unamortized issuance costs	98,591	98,552
Accrued interest payable and other liabilities	271,014	195,068
Total liabilities	28,215,802	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 March 31, 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 March 31, 2018 and December 31, 2017	642	642
Additional paid-in capital	956,536	953,361
Retained earnings	2,234,301	2,187,069
Accumulated other comprehensive income, net of tax	(53,354)	79,512
Treasury stock, at cost; 442,610 shares at March 31, 2018 and 760,720 shares at December 31, 2017	(39,181)	(67,207)
Total shareholders’ equity	3,243,430	3,297,863
Total liabilities and shareholders’ equity	$ 31,459,232	$ 31,747,880

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($)	Mar. 31, 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	442,610	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
	Mar. 31, 2018	Mar. 31, 2017
Interest income:
Loans, including fees	$ 151,202	$ 122,600
Securities:
Taxable	20,558	25,302
Tax-exempt	56,711	56,947
Interest-bearing deposits	14,094	6,836
Federal funds sold and resell agreements	761	107
Total interest income	243,326	211,792
Interest expense:
Deposits	10,638	1,868
Federal funds purchased and repurchase agreements	634	139
Junior subordinated deferrable interest debentures	1,142	908
Other long-term borrowings	1,164	368
Total interest expense	13,578	3,283
Net interest income	229,748	208,509
Provision for loan losses	6,945	7,952
Net interest income after provision for loan losses	222,803	200,557
Non-interest income:
Trust and investment management fees	29,587	26,470
Service charges on deposit accounts	20,843	20,769
Insurance commissions and fees	15,980	13,821
Interchange and debit card transaction fees	3,158	5,574
Other charges, commissions and fees	9,007	9,592
Net gain (loss) on securities transactions	(19)	0
Other	12,889	7,474
Total non-interest income	91,445	83,700
Non-interest expense:
Salaries and wages	86,683	82,512
Employee benefits	21,995	21,625
Net occupancy	19,740	19,237
Technology, furniture and equipment	19,679	17,990
Deposit insurance	4,879	4,915
Intangible amortization	388	458
Other	43,247	41,178
Total non-interest expense	196,611	187,915
Income before income taxes	117,637	96,342
Income taxes	11,157	11,401
Net income	106,480	84,941
Preferred stock dividends	2,016	2,016
Net income available to common shareholders	$ 104,464	$ 82,925
Earnings per common share:
Basic	$ 1.63	$ 1.29
Diluted	$ 1.61	$ 1.28

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net income	$ 106,480	$ 84,941
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss on securities available for sale during the period	(178,904)	33,811
Change in net unrealized gain on securities transferred to held to maturity	(2,619)	(6,286)
Reclassification adjustment for net (gains) losses included in net income	19	0
Total securities available for sale and transferred securities, before tax amount	(181,504)	27,525
Defined-benefit post-retirement benefit plans:
Change in the net actuarial gain/loss	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
Total defined-benefit post-retirement benefit plans	1,250	1,357
Other comprehensive income (loss), before tax	(180,254)	28,882
Deferred tax expense (benefit) related to other comprehensive income	(37,853)	10,109
Other comprehensive income (loss), net of tax	(142,401)	18,773
Comprehensive income (loss)	$ (35,921)	$ 103,714

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity $ in Thousands	USD ($)
Total shareholders’ equity at beginning of period at Dec. 31, 2016	$ 3,002,528
Cumulative effect of accounting change at Dec. 31, 2016	0
Total shareholders' equity at beginning of period, as adjusted at Dec. 31, 2016	3,002,528
Net income	84,941
Other comprehensive income (loss)	18,773
Stock option exercises/stock unit conversions (318,110 shares in 2018 and 442,054 shares in 2017)	24,747
Stock compensation expense recognized in earnings	3,103
Purchase of treasury stock (469 shares in 2017)	(42)
Cash dividends – preferred stock (approximately $0.34 per share in both 2018 and in 2017)	(2,016)
Cash dividends – common stock ($0.57 per share in 2018 and $0.54 per share in 2017)	(34,656)
Total shareholders’ equity at end of period at Mar. 31, 2017	3,097,378
Total shareholders’ equity at beginning of period at Dec. 31, 2017	3,297,863
Cumulative effect of accounting change at Dec. 31, 2017	(2,285)
Total shareholders' equity at beginning of period, as adjusted at Dec. 31, 2017	3,295,578
Net income	106,480
Other comprehensive income (loss)	(142,401)
Stock option exercises/stock unit conversions (318,110 shares in 2018 and 442,054 shares in 2017)	19,165
Stock compensation expense recognized in earnings	3,175
Purchase of treasury stock (469 shares in 2017)	0
Cash dividends – preferred stock (approximately $0.34 per share in both 2018 and in 2017)	(2,016)
Cash dividends – common stock ($0.57 per share in 2018 and $0.54 per share in 2017)	(36,551)
Total shareholders’ equity at end of period at Mar. 31, 2018	$ 3,243,430

Consolidated Statements of Cha7

Consolidated Statements of Changes in Shareholders' Equity (Parenthetical) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Stockholders' Equity [Abstract]
Stock option exercises/stock unit conversions (shares)	318,110	442,054
Treasury stock, shares, acquired	0	469
Approximate cash dividends declared on preferred stock, per share	$ 0.34	$ 0.34
Cash dividends declared on common stock, per share	$ 0.57	$ 0.54

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating Activities:
Net income	$ 106,480	$ 84,941
Adjustments to reconcile net income to net cash from operating activities:
Provision for loan losses	6,945	7,952
Deferred tax expense (benefit)	10,411	(4,301)
Accretion of loan discounts	(3,378)	(3,913)
Securities premium amortization (discount accretion), net	24,260	21,638
Net (gain) loss on securities transactions	19	0
Depreciation and amortization	12,130	12,121
Net (gain) loss on sale/write-down of assets/foreclosed assets	(3,982)	(533)
Stock-based compensation	3,175	3,103
Net tax benefit from stock-based compensation	2,211	3,515
Earnings on life insurance policies	(820)	(783)
Net change in:
Trading account securities	1,326	(1,088)
Accrued interest receivable and other assets	23,555	55,705
Accrued interest payable and other liabilities	28,376	(48,702)
Net cash from operating activities	210,708	129,655
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	179,149	599,457
Securities available for sale:
Purchases	(3,245,923)	(466,004)
Sales	2,984,867	0
Maturities, calls and principal repayments	62,768	107,586
Proceeds from sale of loans	0	0
Net change in loans	(227,417)	(214,281)
Benefits received on life insurance policies	0	461
Proceeds from sales of premises and equipment	11,317	1,544
Purchases of premises and equipment	(16,759)	(6,311)
Proceeds from sales of repossessed properties	307	345
Net cash from investing activities	(253,191)	22,797
Financing Activities:
Net change in deposits	(194,611)	330,589
Net change in short-term borrowings	(115,603)	(81,792)
Proceeds from issuance of subordinated notes	0	98,446
Principal payments on subordinated notes	0	(100,000)
Proceeds from stock option exercises	19,165	24,747
Purchase of treasury stock	0	(42)
Cash dividends paid on preferred stock	(2,016)	(2,016)
Cash dividends paid on common stock	(36,551)	(34,656)
Net cash from financing activities	(329,616)	235,276
Net change in cash and cash equivalents	(372,099)	387,728
Cash and equivalents at beginning of period	5,053,047	4,141,445
Cash and equivalents at end of period	$ 4,680,948	$ 4,529,173

Significant Accounting Policies

Significant Accounting Policies	3 Months Ended
	Mar. 31, 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: Three Months Ended 2018 2017 Cash paid for interest $ 13,740 $ 3,257 Cash paid for income taxes — — Significant non-cash transactions: Unsettled purchases/sales of securities 47,723 33,466 Loans foreclosed and transferred to other real estate owned and foreclosed assets 7 — 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, 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 fees from such transactions. Previously, such network costs were reported as a component of other non-interest expense. For the three months ended March 31, 2018 , gross interchange and debit card transaction fees totaled $6.1 million while related network costs totaled $2.9 million . On a net basis, we reported $3.2 million as interchange and debit card transaction fees in the accompanying Consolidated Statement of Income for the three months ended March 31, 2018 . For the three months ended March 31, 2017 , we reported interchange and debit card transaction fees totaling $5.6 million on a gross basis in the accompanying Consolidated Statement of Income while related network costs totaling $3.2 million were reported as a component of other non-interest expense. 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 three months ended March 31, 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.

Securities

Securities	3 Months Ended
	Mar. 31, 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. March 31, 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 $ 3,503 $ 12 $ 57 $ 3,458 $ 3,610 $ 15 $ 38 $ 3,587 States and political subdivisions 1,242,151 13,910 3,126 1,252,935 1,428,488 26,462 2,746 1,452,204 Other 1,500 — 7 1,493 — — — — Total $ 1,247,154 $ 13,922 $ 3,190 $ 1,257,886 $ 1,432,098 $ 26,477 $ 2,784 $ 1,455,791 Available for Sale U.S. Treasury $ 3,453,887 $ 373 $ 36,113 $ 3,418,147 $ 3,453,391 $ 7,494 $ 15,732 $ 3,445,153 Residential mortgage-backed securities 636,498 16,012 6,815 645,695 648,288 19,048 2,250 665,086 States and political subdivisions 6,423,389 71,360 64,644 6,430,105 6,185,711 167,293 16,795 6,336,209 Other 42,585 — — 42,585 42,561 — — 42,561 Total $ 10,556,359 $ 87,745 $ 107,572 $ 10,536,532 $ 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 March 31, 2018 , approximately 98.2% of the securities in our municipal bond portfolio were issued by political subdivisions or agencies within the State of Texas, of which approximately 67.7% 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 $3.5 billion at March 31, 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 March 31, 2018 totaled $8.9 million ( $7.1 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 March 31, 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 $ 1,120 $ 23 $ 1,516 $ 34 $ 2,636 $ 57 States and political subdivisions 231,906 900 44,869 2,226 276,775 3,126 Other 1,493 7 — — 1,493 7 Total $ 234,519 $ 930 $ 46,385 $ 2,260 $ 280,904 $ 3,190 Available for Sale U.S. Treasury $ 2,616,630 $ 27,797 $ 514,691 $ 8,316 $ 3,131,321 $ 36,113 Residential mortgage-backed securities 219,091 4,508 45,424 2,307 264,515 6,815 States and political subdivisions 2,004,045 26,466 815,756 38,178 2,819,801 64,644 Total $ 4,839,766 $ 58,771 $ 1,375,871 $ 48,801 $ 6,215,637 $ 107,572 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 March 31, 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 March 31, 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 March 31, 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 $ 105,054 $ 106,904 $ 118,442 $ 119,061 Due after one year through five years 165,355 168,916 4,010,195 3,980,391 Due after five years through ten years 432,654 433,756 486,543 491,153 Due after ten years 540,588 544,852 5,262,096 5,257,647 Residential mortgage-backed securities 3,503 3,458 636,498 645,695 Equity securities — — 42,585 42,585 Total $ 1,247,154 $ 1,257,886 $ 10,556,359 $ 10,536,532 Sales of Securities. Sales of securities available for sale were as follows: Three Months Ended 2018 2017 Proceeds from sales $ 2,984,867 $ — Gross realized gains — — Gross realized losses (19 ) — Tax (expense) benefit of securities gains/losses 4 — Premiums and Discounts . Premium amortization and discount accretion included in interest income on securities was as follows: Three Months Ended 2018 2017 Premium amortization $ (26,036 ) $ (24,028 ) Discount accretion 1,776 2,390 Net (premium amortization) discount accretion $ (24,260 ) $ (21,638 ) Trading Account Securities. Trading account securities, at estimated fair value, were as follows: March 31, December 31, U.S. Treasury $ 19,772 $ 19,210 States and political subdivisions — 1,888 Total $ 19,772 $ 21,098 Net gains and losses on trading account securities were as follows: Three Months Ended 2018 2017 Net gain on sales transactions $ 505 $ 311 Net mark-to-market gains (losses) (36 ) 13 Net gain (loss) on trading account securities $ 469 $ 324

Loans

Loans	3 Months Ended
	Mar. 31, 2018
Receivables [Abstract]
Loans	Loans Loans were as follows: March 31, Percentage of Total December 31, Percentage of Total Commercial and industrial $ 4,876,523 36.5 % $ 4,792,388 36.4 % Energy: Production 1,125,321 8.4 1,182,326 9.0 Service 192,115 1.4 171,795 1.3 Other 129,552 0.9 144,972 1.1 Total energy 1,446,988 10.7 1,499,093 11.4 Commercial real estate: Commercial mortgages 4,060,946 30.4 3,887,742 29.6 Construction 1,076,785 8.1 1,066,696 8.1 Land 317,189 2.4 331,986 2.5 Total commercial real estate 5,454,920 40.9 5,286,424 40.2 Consumer real estate: Home equity loans 355,715 2.7 355,342 2.7 Home equity lines of credit 295,677 2.2 291,950 2.2 Other 388,271 2.9 376,002 2.9 Total consumer real estate 1,039,663 7.8 1,023,294 7.8 Total real estate 6,494,583 48.7 6,309,718 48.0 Consumer and other 545,935 4.1 544,466 4.2 Total loans $ 13,364,029 100.0 % $ 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 March 31, 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 10.7% of total loans. Unfunded commitments to extend credit and standby letters of credit issued to customers in the energy industry totaled $1.2 billion and $44.8 million , respectively, as of March 31, 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 March 31, 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 $180.7 million at March 31, 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: March 31, December 31, Commercial and industrial $ 17,314 $ 46,186 Energy 93,097 94,302 Commercial real estate: Buildings, land and other 10,858 7,589 Construction — — Consumer real estate 1,878 2,109 Consumer and other 5 128 Total $ 123,152 $ 150,314 As of March 31, 2018 , non-accrual loans reported in the table above included $2.2 million related to loans that were restructured as “troubled debt restructurings” during 2018 . See the section captioned “Troubled Debt Restructurings” elsewhere in this note. Had non-accrual loans performed in accordance with their original contract terms, we would have recognized additional interest income, net of tax, of approximately $1.5 million for the three months ended March 31, 2018 , compared to $851 thousand for three months ended March 31, 2017 . An age analysis of past due loans (including both accruing and non-accruing loans), segregated by class of loans, as of March 31, 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 $ 25,701 $ 23,946 $ 49,647 $ 4,826,876 $ 4,876,523 $ 10,357 Energy 24,404 4,207 28,611 1,418,377 1,446,988 3,137 Commercial real estate: Buildings, land and other 36,130 5,470 41,600 4,336,535 4,378,135 3,889 Construction 9,497 — 9,497 1,067,288 1,076,785 — Consumer real estate 8,225 3,164 11,389 1,028,274 1,039,663 1,680 Consumer and other 7,486 218 7,704 538,231 545,935 213 Total $ 111,443 $ 37,005 $ 148,448 $ 13,215,581 $ 13,364,029 $ 19,276 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 March 31, 2018 Commercial and industrial $ 38,136 $ 1,802 $ 13,430 $ 15,232 $ 7,930 Energy 123,351 41,569 63,502 105,071 14,772 Commercial real estate: Buildings, land and other 13,078 7,577 2,047 9,624 708 Construction — — — — — Consumer real estate 1,427 1,427 — 1,427 — Consumer and other — — — — — Total $ 175,992 $ 52,375 $ 78,979 $ 131,354 $ 23,410 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 2018 2017 Commercial and industrial $ 29,496 $ 26,393 Energy 99,657 68,101 Commercial real estate: Buildings, land and other 8,009 6,660 Construction — — Consumer real estate 1,321 1,102 Consumer and other — 27 Total $ 138,483 $ 102,283 Troubled Debt Restructurings . Troubled debt restructurings during the three months ended March 31, 2018 and March 31, 2017 are set forth in the following table. Three Months Ended Three Months Ended Balance at Restructure Balance at Period-End Balance at Restructure Balance at Period-End Commercial and industrial $ 2,203 $ 2,171 $ — $ — Energy 13,708 12,058 11,262 11,212 $ 15,911 $ 14,229 $ 11,262 $ 11,212 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 was as follows: March 31, 2018 March 31, 2017 Restructured loans past due in excess of 90 days at period-end: Number of loans — 1 Dollar amount of loans $ — $ 747 Restructured loans on non-accrual status at period end 2,171 11,212 Charge-offs of restructured loans: Recognized in connection with restructuring — — Recognized on previously restructured loans 1,650 2,000 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. March 31, 2018 December 31, 2017 Weighted Loans Weighted Loans Commercial and industrial: Risk grades 1-8 6.05 $ 4,505,491 6.06 $ 4,378,839 Risk grade 9 9.00 151,084 9.00 170,285 Risk grade 10 10.00 121,457 10.00 99,260 Risk grade 11 11.00 81,148 11.00 97,818 Risk grade 12 12.00 9,412 12.00 38,633 Risk grade 13 13.00 7,931 13.00 7,553 Total 6.35 $ 4,876,523 6.41 $ 4,792,388 Energy Risk grades 1-8 6.12 $ 1,173,702 6.01 $ 1,199,207 Risk grade 9 9.00 50,364 9.00 50,427 Risk grade 10 10.00 37,670 10.00 64,282 Risk grade 11 11.00 92,155 11.00 90,875 Risk grade 12 12.00 78,325 12.00 81,035 Risk grade 13 13.00 14,772 13.00 13,267 Total 7.02 $ 1,446,988 6.97 $ 1,499,093 Commercial real estate: Buildings, land and other Risk grades 1-8 6.77 $ 4,051,952 6.75 $ 3,868,659 Risk grade 9 9.00 124,048 9.00 151,487 Risk grade 10 10.00 120,399 10.00 129,391 Risk grade 11 11.00 70,878 11.00 62,602 Risk grade 12 12.00 10,150 12.00 7,589 Risk grade 13 13.00 708 13.00 — Total 7.00 $ 4,378,135 7.00 $ 4,219,728 Construction Risk grades 1-8 7.16 $ 1,040,071 7.11 $ 1,019,635 Risk grade 9 9.00 15,541 9.00 18,042 Risk grade 10 10.00 17,361 10.00 23,393 Risk grade 11 11.00 3,812 11.00 5,626 Risk grade 12 12.00 — 12.00 — Risk grade 13 13.00 — 13.00 — Total 7.25 $ 1,076,785 7.23 $ 1,066,696 Net (charge-offs)/recoveries, segregated by class of loans, were as follows: Three Months Ended 2018 2017 Commercial and industrial $ (7,675 ) $ (2,729 ) Energy (2,849 ) (4,225 ) Commercial real estate: Buildings, land and other 81 42 Construction 2 3 Consumer real estate (526 ) 96 Consumer and other (1,457 ) (1,128 ) Total $ (12,424 ) $ (7,941 ) 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 131.5 at February 28, 2018 (most recent date available) and 129.6 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 March 31, 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 March 31, 2018 Historical valuation allowances $ 26,658 $ 12,266 $ 19,870 $ 2,470 $ 6,729 $ 67,993 Specific valuation allowances 7,930 14,772 708 — — 23,410 General valuation allowances 8,053 8,015 4,025 1,601 (95 ) 21,599 Macroeconomic valuation allowances 15,092 3,986 13,871 2,278 1,656 36,883 Total $ 57,733 $ 39,039 $ 38,474 $ 6,349 $ 8,290 $ 149,885 Allocated to loans: Individually evaluated $ 7,930 $ 14,772 $ 708 $ — $ — $ 23,410 Collectively evaluated 49,803 24,267 37,766 6,349 8,290 126,475 Total $ 57,733 $ 39,039 $ 38,474 $ 6,349 $ 8,290 $ 149,885 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 March 31, 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 March 31, 2018 Individually evaluated $ 15,232 $ 105,071 $ 9,624 $ 1,427 $ — $ 131,354 Collectively evaluated 4,861,291 1,341,917 5,445,296 1,038,236 545,935 13,232,675 Total $ 4,876,523 $ 1,446,988 $ 5,454,920 $ 1,039,663 $ 545,935 $ 13,364,029 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 months ended March 31, 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: March 31, 2018 Beginning balance $ 59,614 $ 51,528 $ 30,948 $ 5,657 $ 7,617 $ 155,364 Provision for loan losses 5,794 (9,640 ) 7,443 1,218 2,130 6,945 Charge-offs (9,252 ) (2,850 ) (5 ) (719 ) (3,972 ) (16,798 ) Recoveries 1,577 1 88 193 2,515 4,374 Net charge-offs (7,675 ) (2,849 ) 83 (526 ) (1,457 ) (12,424 ) Ending balance $ 57,733 $ 39,039 $ 38,474 $ 6,349 $ 8,290 $ 149,885 March 31, 2017 Beginning balance $ 52,915 $ 60,653 $ 30,213 $ 4,238 $ 5,026 $ 153,045 Provision for loan losses (4,603 ) 5,365 3,751 489 2,950 7,952 Charge-offs (3,527 ) (4,278 ) — (11 ) (3,548 ) (11,364 ) Recoveries 798 53 45 107 2,420 3,423 Net charge-offs (2,729 ) (4,225 ) 45 96 (1,128 ) (7,941 ) Ending balance $ 45,583 $ 61,793 $ 34,009 $ 4,823 $ 6,848 $ 153,056

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	3 Months Ended
	Mar. 31, 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. March 31, December 31, Goodwill $ 654,952 $ 654,952 Other intangible assets: Core deposits $ 3,748 $ 4,044 Customer relationships 900 986 Non-compete agreements 37 43 $ 4,685 $ 5,073 The estimated aggregate future amortization expense for intangible assets remaining as of March 31, 2018 is as follows: Remainder of 2018 $ 1,036 2019 1,167 2020 918 2021 697 2022 481 Thereafter 386 $ 4,685

Deposits

Deposits	3 Months Ended
	Mar. 31, 2018
Banking and Thrift [Abstract]
Deposits	Deposits Deposits were as follows: March 31, Percentage of Total December 31, Percentage of Total Non-interest-bearing demand deposits: Commercial and individual $ 10,294,977 38.6 % $ 10,412,882 38.8 % Correspondent banks 212,199 0.8 222,648 0.8 Public funds 426,986 1.6 561,563 2.1 Total non-interest-bearing demand deposits 10,934,162 41.0 11,197,093 41.7 Interest-bearing deposits: Private accounts: Savings and interest checking 6,869,783 25.8 6,788,766 25.2 Money market accounts 7,706,531 28.9 7,624,471 28.4 Time accounts of $100,000 or more 458,103 1.7 453,668 1.7 Time accounts under $100,000 322,105 1.2 324,636 1.2 Total private accounts 15,356,522 57.6 15,191,541 56.5 Public funds: Savings and interest checking 308,609 1.1 410,140 1.5 Money market accounts 63,155 0.2 59,008 0.2 Time accounts of $100,000 or more 14,892 0.1 14,301 0.1 Time accounts under $100,000 438 — 306 — Total public funds 387,094 1.4 483,755 1.8 Total interest-bearing deposits 15,743,616 59.0 15,675,296 58.3 Total deposits $ 26,677,778 100.0 % $ 26,872,389 100.0 % The following table presents additional information about our deposits: March 31, December 31, Deposits from foreign sources (primarily Mexico) $ 736,786 $ 716,339 Deposits not covered by deposit insurance 13,104,781 13,281,040

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 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: March 31, December 31, Commitments to extend credit $ 7,939,929 $ 7,949,400 Standby letters of credit 220,294 236,595 Deferred standby letter of credit fees 1,586 1,843 Lease Commitments . We lease certain office facilities and office equipment under operating leases. Rent expense for all operating leases totaled $8.2 million and $7.7 million during the three months ended March 31, 2018 and 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	3 Months Ended
	Mar. 31, 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 March 31, 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 March 31, 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 March 31, 2018 and December 31, 2017 . The following tables present actual and required capital ratios as of March 31, 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 March 31, 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 March 31, 2018 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,504,319 12.69 % $ 1,258,491 6.375 % $ 1,381,873 7.00 % $ 1,283,167 6.50 % Frost Bank 2,560,828 13.00 1,255,352 6.375 1,378,426 7.00 1,279,967 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 2,648,805 13.42 1,554,607 7.875 1,677,988 8.50 1,579,283 8.00 Frost Bank 2,560,828 13.00 1,550,729 7.875 1,673,803 8.50 1,575,344 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,032,190 15.36 1,949,427 9.875 2,072,809 10.50 1,974,104 10.00 Frost Bank 2,711,213 13.77 1,944,565 9.875 2,067,639 10.50 1,969,180 10.00 Leverage Ratio Cullen/Frost 2,648,805 8.62 1,228,567 4.00 1,228,567 4.00 1,535,709 5.00 Frost Bank 2,560,828 8.34 1,227,539 4.00 1,227,539 4.00 1,534,424 5.00 Actual Minimum Capital Required - Basel III Phase-In Schedule Minimum Capital Required - Basel III Fully Phased-In Required to be Capital Ratio Capital Ratio Capital Ratio Capital Ratio December 31, 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 March 31, 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 March 31, 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 March 31, 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 March 31, 2018 , Frost Bank could pay aggregate dividends of up to $446.7 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	3 Months Ended
	Mar. 31, 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 March 31, 2018 and December 31, 2017 . March 31, 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 $ 11,904 $ 286 $ 13,679 $ 242 Loan/lease interest rate swaps – liabilities 10,141 (445 ) 11,147 (593 ) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets 615,433 3,416 430,449 1,418 Loan/lease interest rate swaps – liabilities 520,466 (8,461 ) 541,496 (12,820 ) Loan/lease interest rate caps – assets 121,976 906 114,619 480 Customer counterparties: Loan/lease interest rate swaps – assets 491,786 12,204 541,496 17,882 Loan/lease interest rate swaps – liabilities 596,473 (12,846 ) 430,449 (4,861 ) Loan/lease interest rate caps – liabilities 121,976 (906 ) 114,619 (480 ) The weighted-average rates paid and received for interest rate swaps outstanding at March 31, 2018 were as follows: Weighted-Average Interest Rate Paid Interest Rate Received Interest rate swaps: Fair value hedge loan/lease interest rate swaps 3.27 % 1.77 % Non-hedging interest rate swaps – financial institution counterparties 3.96 % 3.28 % Non-hedging interest rate swaps – customer counterparties 3.35 % 4.06 % The weighted-average strike rate for outstanding interest rate caps was 3.47% at March 31, 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. March 31, 2018 December 31, 2017 Notional Units Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Oil – assets Barrels 462 $ 1,676 253 $ 193 Oil – liabilities Barrels 2,663 (19,122 ) 2,731 (13,448 ) Natural gas – assets MMBTUs 6,963 984 5,927 1,399 Natural gas – liabilities MMBTUs 6,230 (591 ) 3,917 (326 ) Customer counterparties: Oil – assets Barrels 2,699 19,301 2,731 13,709 Oil – liabilities Barrels 426 (1,668 ) 253 (187 ) Natural gas – assets MMBTUs 6,133 602 3,917 340 Natural gas – liabilities MMBTUs 7,060 (975 ) 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: March 31, 2018 December 31, 2017 Notional Currency Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Forward contracts – assets EUR 5,110 $ 167 4,014 $ 77 Forward contracts – assets GBP 832 8 127 1 Forward contracts – assets AUD 59 1 — — Forward contracts – liabilities EUR — — 4,846 (37 ) Forward contracts – liabilities CAD 23,228 (68 ) 25,413 (142 ) Forward contracts – liabilities GBP 453 (5 ) 1,178 (9 ) Forward contracts – liabilities AUD 58 — — — Customer counterparties: Forward contracts – assets EUR — — 3,867 58 Forward contracts – assets CAD 23,170 136 25,282 279 Forward contracts – assets GBP 88 1 — — Forward contracts – liabilities EUR 4,041 (135 ) 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 2018 2017 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ (42 ) $ (245 ) Amount of (gain) loss included in other non-interest expense — (1 ) As stated above, we enter into non-hedge related derivative positions primarily to accommodate the business needs of our customers. Upon the origination of a derivative contract with a customer, we simultaneously enter into an offsetting derivative contract with a third party financial institution. We recognize immediate income based upon the difference in the bid/ask spread of the underlying transactions with our customers and the third party. Because we act only as an intermediary for our customer, subsequent changes in the fair value of the underlying derivative contracts for the most part offset each other and do not significantly impact our results of operations. Amounts included in the consolidated statements of income related to non-hedging interest rate, commodity and foreign currency derivative instruments are presented in the table below. Three Months Ended 2018 2017 Non-hedging interest rate derivatives: Other non-interest income $ 1,488 $ 370 Other non-interest expense (21 ) (1 ) Non-hedging commodity derivatives: Other non-interest income 90 52 Non-hedging foreign currency derivatives: Other non-interest income 59 9 Counterparty Credit Risk. Our credit exposure relating to interest rate swaps, commodity swaps/options and foreign currency forward contracts with bank customers was approximately $29.3 million at March 31, 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 $9.2 million at March 31, 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 $6.6 million at March 31, 2018 . At such date, we also had $25.8 million in cash collateral on deposit with other financial institution counterparties.

Balance Sheet Offsetting

Balance Sheet Offsetting	3 Months Ended
	Mar. 31, 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 March 31, 2018 is presented in the following tables. Gross Amount Recognized Gross Amount Offset Net Amount Recognized March 31, 2018 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 4,608 $ — $ 4,608 Commodity swaps and options 2,660 — 2,660 Foreign currency forward contracts 176 — 176 Total derivatives 7,444 — 7,444 Resell agreements 9,642 — 9,642 Total $ 17,086 $ — $ 17,086 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 8,906 $ — $ 8,906 Commodity swaps and options 19,713 — 19,713 Foreign currency forward contracts 73 — 73 Total derivatives 28,692 — 28,692 Repurchase agreements 1,015,021 — 1,015,021 Total $ 1,043,713 $ — $ 1,043,713 Gross Amounts Not Offset Net Amount Recognized Financial Instruments Collateral Net Amount March 31, 2018 Financial assets: Derivatives: Counterparty A $ 1,012 $ (1,012 ) $ — $ — Counterparty B 2,152 (2,152 ) — — Counterparty C 84 (84 ) — — Other counterparties 4,196 (1,217 ) (2,751 ) 228 Total derivatives 7,444 (4,465 ) (2,751 ) 228 Resell agreements 9,642 — (9,642 ) — Total $ 17,086 $ (4,465 ) $ (12,393 ) $ 228 Financial liabilities: Derivatives: Counterparty A $ 5,706 $ (1,012 ) $ (4,694 ) $ — Counterparty B 4,103 (2,152 ) (1,258 ) 693 Counterparty C 1,467 (84 ) (1,383 ) — Other counterparties 17,416 (1,217 ) (16,099 ) 100 Total derivatives 28,692 (4,465 ) (23,434 ) 793 Repurchase agreements 1,015,021 — (1,015,021 ) — Total $ 1,043,713 $ (4,465 ) $ (1,038,455 ) $ 793 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 March 31, 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 March 31, 2018 Repurchase agreements: U.S. Treasury $ 999,320 $ — $ — $ — $ 999,320 Residential mortgage-backed securities 15,701 — — — 15,701 Total borrowings $ 1,015,021 $ — $ — $ — $ 1,015,021 Gross amount of recognized liabilities for repurchase agreements $ 1,015,021 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	3 Months Ended
	Mar. 31, 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 March 31, 2018 , there were 1,400,402 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 — — — — — — — — Exercised/vested — — — — — — (318,110 ) 60.25 Forfeited/expired — — (3,527 ) 86.54 — — (29,125 ) 69.67 Balance, March 31, 2018 53,008 $ 64.87 309,129 $ 81.66 80,103 $ 79.91 2,569,907 $ 63.65 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 2018 2017 New shares issued from available authorized shares — 283,342 Issued from available treasury stock 318,110 158,712 Total 318,110 442,054 Proceeds from stock option exercises $ 19,165 $ 24,747 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 2018 2017 Stock options $ 1,085 $ 1,787 Non-vested stock awards/stock units 1,468 1,033 Director deferred stock units — — Performance stock units 622 283 Total $ 3,175 $ 3,103 Income tax benefit $ 667 $ 1,086 Unrecognized stock-based compensation expense at March 31, 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 $ 4,148 Non-vested stock awards/stock units 13,218 Performance stock units 4,507 Total $ 21,873

Earnings Per Common Share

Earnings Per Common Share	3 Months Ended
	Mar. 31, 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 2018 2017 Net income $ 106,480 $ 84,941 Less: Preferred stock dividends 2,016 2,016 Net income available to common shareholders 104,464 82,925 Less: Earnings allocated to participating securities 702 435 Net earnings allocated to common stock $ 103,762 $ 82,490 Distributed earnings allocated to common stock $ 36,306 $ 34,475 Undistributed earnings allocated to common stock 67,456 48,015 Net earnings allocated to common stock $ 103,762 $ 82,490 Weighted-average shares outstanding for basic earnings per common share 63,649,198 63,738,191 Dilutive effect of stock compensation 1,012,997 999,194 Weighted-average shares outstanding for diluted earnings per common share 64,662,195 64,737,385

Defined Benefit Plans

Defined Benefit Plans	3 Months Ended
	Mar. 31, 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 2018 2017 Expected return on plan assets, net of expenses $ (2,979 ) $ (2,779 ) Interest cost on projected benefit obligation 1,475 1,547 Net amortization and deferral 1,250 1,357 Net periodic expense (benefit) $ (254 ) $ 125 Our non-qualified defined benefit pension plan is not funded. No contributions to the qualified defined benefit pension plan were made during the three months ended March 31, 2018 . We do not expect to make any contributions to the qualified defined benefit plan during the remainder of 2018 .

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Income tax expense was as follows: Three Months Ended 2018 2017 Current income tax expense $ 746 $ 15,702 Deferred income tax expense (benefit) 10,411 (4,301 ) Income tax expense, as reported $ 11,157 $ 11,401 Effective tax rate 9.5 % 11.8 % Net deferred tax assets totaled $59.6 million at March 31, 2018 and $31.7 million at December 31, 2017 . No valuation allowance for deferred tax assets was recorded at March 31, 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. 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 2014. 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)	3 Months Ended
	Mar. 31, 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 $ (178,904 ) $ (37,570 ) $ (141,334 ) $ 33,811 $ 11,834 $ 21,977 Change in net unrealized gain on securities transferred to held to maturity (2,619 ) (550 ) (2,069 ) (6,286 ) (2,200 ) (4,086 ) Reclassification adjustment for net (gains) losses included in net income 19 4 15 — — — Total securities available for sale and transferred securities (181,504 ) (38,116 ) (143,388 ) 27,525 9,634 17,891 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) 1,250 263 987 1,357 475 882 Total defined-benefit post-retirement benefit plans 1,250 263 987 1,357 475 882 Total other comprehensive income (loss) $ (180,254 ) $ (37,853 ) $ (142,401 ) $ 28,882 $ 10,109 $ 18,773 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 (143,403 ) — (143,403 ) Reclassification of amounts included in net income 15 987 1,002 Net other comprehensive income (loss) during period (143,388 ) 987 (142,401 ) 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 March 31, 2018 $ (8,601 ) $ (44,753 ) $ (53,354 ) Balance January 1, 2017 $ 16,153 $ (40,776 ) $ (24,623 ) Other comprehensive income (loss) before reclassifications 17,891 — 17,891 Reclassification of amounts included in net income — 882 882 Net other comprehensive income (loss) during period 17,891 882 18,773 Balance at March 31, 2017 $ 34,044 $ (39,894 ) $ (5,850 )

Operating Segments

Operating Segments	3 Months Ended
	Mar. 31, 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: March 31, 2018 $ 288,561 $ 35,081 $ (2,449 ) $ 321,193 March 31, 2017 258,911 34,588 (1,290 ) 292,209 Net income (loss): Three months ended: March 31, 2018 $ 103,641 $ 5,634 $ (2,795 ) $ 106,480 March 31, 2017 80,869 5,294 (1,222 ) 84,941

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 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 March 31, 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 March 31, 2018 Securities available for sale: U.S. Treasury $ 3,418,147 $ — $ — $ 3,418,147 Residential mortgage-backed securities — 645,695 — 645,695 States and political subdivisions — 6,430,105 — 6,430,105 Other — 42,585 — 42,585 Trading account securities: U.S. Treasury 19,772 — — 19,772 Derivative assets: Interest rate swaps, caps and floors — 16,812 — 16,812 Commodity swaps and options — 20,844 1,719 22,563 Foreign currency forward contracts 313 — — 313 Derivative liabilities: Interest rate swaps, caps and floors — 22,658 — 22,658 Commodity swaps and options — 22,356 — 22,356 Foreign currency forward contracts 208 — — 208 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 for both periods ended March 31, 2018 and December 31, 2017 . The weighted-average loss severity in the event of default on the commodity swaps was 15.4% for both periods ended March 31, 2018 and December 31, 2017 . 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. Three Months Ended Three Months Ended Level 2 Level 3 Level 2 Level 3 Carrying value of impaired loans before allocations $ 365 $ 56,788 $ — $ 24,171 Specific valuation allowance (allocations) reversals of prior allocations (64 ) (1,520 ) — (1,340 ) Fair value $ 301 $ 55,268 $ — $ 22,831 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: Three Months Ended 2018 2017 Foreclosed assets remeasured at initial recognition: Carrying value of foreclosed assets prior to remeasurement $ 7 $ — Charge-offs recognized in the allowance for loan losses — — Fair value $ 7 $ — 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: March 31, 2018 December 31, 2017 Carrying Amount Estimated Fair Value Carrying Amount Estimated Fair Value Financial assets: Level 2 inputs: Cash and cash equivalents $ 4,680,948 $ 4,680,948 $ 5,053,047 $ 5,053,047 Securities held to maturity 1,247,154 1,257,886 1,432,098 1,455,791 Cash surrender value of life insurance policies 181,297 181,297 180,477 180,477 Accrued interest receivable 125,485 125,485 167,508 167,508 Level 3 inputs: Loans, net 13,214,144 13,184,663 12,990,301 12,981,165 Financial liabilities: Level 2 inputs: Deposits 26,677,778 26,671,005 26,872,389 26,866,676 Federal funds purchased and repurchase agreements 1,032,221 1,032,221 1,147,824 1,147,824 Junior subordinated deferrable interest debentures 136,198 137,115 136,184 137,115 Subordinated notes payable and other borrowings 98,591 103,409 98,552 105,311 Accrued interest payable 3,196 3,196 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	3 Months Ended
	Mar. 31, 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 21 - 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 will require 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. Notwithstanding the foregoing, in January 2018, the Financial Accounting Standards Board issued a proposal to provide 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 evaluating the potential impact of ASU 2016-02 on our financial statements. In that regard, we are currently implementing a third-party vendor solution to assist us in the application of ASU 2016-02. 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 developing an implementation plan to include assessment of processes, portfolio segmentation, model development, system requirements and the identification of data and resource needs, among other things. We have recently begun to implement 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, with early adoption permitted. While we are currently evaluating the potential impact of ASU 2017-08 on our financial statements, we expect that the impact of adoption will be significantly influenced by the composition of our securities portfolio as of the adoption date. 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, 2107, 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.

Significant Accounting Polici26

Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 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, 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 fees from such transactions. Previously, such network costs were reported as a component of other non-interest expense. For the three months ended March 31, 2018 , gross interchange and debit card transaction fees totaled $6.1 million while related network costs totaled $2.9 million . On a net basis, we reported $3.2 million as interchange and debit card transaction fees in the accompanying Consolidated Statement of Income for the three months ended March 31, 2018 . For the three months ended March 31, 2017 , we reported interchange and debit card transaction fees totaling $5.6 million on a gross basis in the accompanying Consolidated Statement of Income while related network costs totaling $3.2 million were reported as a component of other non-interest expense. 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 three months ended March 31, 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. Accounting Standards Updates Information about certain recently issued accounting standards updates is presented below. Also refer to Note 21 - 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 will require 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. Notwithstanding the foregoing, in January 2018, the Financial Accounting Standards Board issued a proposal to provide 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 evaluating the potential impact of ASU 2016-02 on our financial statements. In that regard, we are currently implementing a third-party vendor solution to assist us in the application of ASU 2016-02. 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 developing an implementation plan to include assessment of processes, portfolio segmentation, model development, system requirements and the identification of data and resource needs, among other things. We have recently begun to implement 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, with early adoption permitted. While we are currently evaluating the potential impact of ASU 2017-08 on our financial statements, we expect that the impact of adoption will be significantly influenced by the composition of our securities portfolio as of the adoption date. 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, 2107, 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.

Accounting Standards Updates Ac

Accounting Standards Updates Accounting Standards Updates (Policies)	3 Months Ended
	Mar. 31, 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 fees from such transactions. Previously, such network costs were reported as a component of other non-interest expense. For the three months ended March 31, 2018 , gross interchange and debit card transaction fees totaled $6.1 million while related network costs totaled $2.9 million . On a net basis, we reported $3.2 million as interchange and debit card transaction fees in the accompanying Consolidated Statement of Income for the three months ended March 31, 2018 . For the three months ended March 31, 2017 , we reported interchange and debit card transaction fees totaling $5.6 million on a gross basis in the accompanying Consolidated Statement of Income while related network costs totaling $3.2 million were reported as a component of other non-interest expense. 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 three months ended March 31, 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. Accounting Standards Updates Information about certain recently issued accounting standards updates is presented below. Also refer to Note 21 - 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 will require 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. Notwithstanding the foregoing, in January 2018, the Financial Accounting Standards Board issued a proposal to provide 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 evaluating the potential impact of ASU 2016-02 on our financial statements. In that regard, we are currently implementing a third-party vendor solution to assist us in the application of ASU 2016-02. 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 developing an implementation plan to include assessment of processes, portfolio segmentation, model development, system requirements and the identification of data and resource needs, among other things. We have recently begun to implement 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, with early adoption permitted. While we are currently evaluating the potential impact of ASU 2017-08 on our financial statements, we expect that the impact of adoption will be significantly influenced by the composition of our securities portfolio as of the adoption date. 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, 2107, 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.

Significant Accounting Polici28

Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Additional Cash Flow Information	Additional cash flow information was as follows: Three Months Ended 2018 2017 Cash paid for interest $ 13,740 $ 3,257 Cash paid for income taxes — — Significant non-cash transactions: Unsettled purchases/sales of securities 47,723 33,466 Loans foreclosed and transferred to other real estate owned and foreclosed assets 7 —

Securities (Tables)

Securities (Tables)	3 Months Ended
	Mar. 31, 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. March 31, 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 $ 3,503 $ 12 $ 57 $ 3,458 $ 3,610 $ 15 $ 38 $ 3,587 States and political subdivisions 1,242,151 13,910 3,126 1,252,935 1,428,488 26,462 2,746 1,452,204 Other 1,500 — 7 1,493 — — — — Total $ 1,247,154 $ 13,922 $ 3,190 $ 1,257,886 $ 1,432,098 $ 26,477 $ 2,784 $ 1,455,791 Available for Sale U.S. Treasury $ 3,453,887 $ 373 $ 36,113 $ 3,418,147 $ 3,453,391 $ 7,494 $ 15,732 $ 3,445,153 Residential mortgage-backed securities 636,498 16,012 6,815 645,695 648,288 19,048 2,250 665,086 States and political subdivisions 6,423,389 71,360 64,644 6,430,105 6,185,711 167,293 16,795 6,336,209 Other 42,585 — — 42,585 42,561 — — 42,561 Total $ 10,556,359 $ 87,745 $ 107,572 $ 10,536,532 $ 10,329,951 $ 193,835 $ 34,777 $ 10,489,009
Securities, with Unrealized Losses Segregated by Length of Impairment	As of March 31, 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 $ 1,120 $ 23 $ 1,516 $ 34 $ 2,636 $ 57 States and political subdivisions 231,906 900 44,869 2,226 276,775 3,126 Other 1,493 7 — — 1,493 7 Total $ 234,519 $ 930 $ 46,385 $ 2,260 $ 280,904 $ 3,190 Available for Sale U.S. Treasury $ 2,616,630 $ 27,797 $ 514,691 $ 8,316 $ 3,131,321 $ 36,113 Residential mortgage-backed securities 219,091 4,508 45,424 2,307 264,515 6,815 States and political subdivisions 2,004,045 26,466 815,756 38,178 2,819,801 64,644 Total $ 4,839,766 $ 58,771 $ 1,375,871 $ 48,801 $ 6,215,637 $ 107,572
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 March 31, 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 $ 105,054 $ 106,904 $ 118,442 $ 119,061 Due after one year through five years 165,355 168,916 4,010,195 3,980,391 Due after five years through ten years 432,654 433,756 486,543 491,153 Due after ten years 540,588 544,852 5,262,096 5,257,647 Residential mortgage-backed securities 3,503 3,458 636,498 645,695 Equity securities — — 42,585 42,585 Total $ 1,247,154 $ 1,257,886 $ 10,556,359 $ 10,536,532
Sales of Securities Available for Sale	Sales of securities available for sale were as follows: Three Months Ended 2018 2017 Proceeds from sales $ 2,984,867 $ — Gross realized gains — — Gross realized losses (19 ) — Tax (expense) benefit of securities gains/losses 4 —
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 2018 2017 Premium amortization $ (26,036 ) $ (24,028 ) Discount accretion 1,776 2,390 Net (premium amortization) discount accretion $ (24,260 ) $ (21,638 )
Trading Account Securities, at Estimated Fair Value	Trading account securities, at estimated fair value, were as follows: March 31, December 31, U.S. Treasury $ 19,772 $ 19,210 States and political subdivisions — 1,888 Total $ 19,772 $ 21,098
Net Gains and Losses on Trading Account Securities	Net gains and losses on trading account securities were as follows: Three Months Ended 2018 2017 Net gain on sales transactions $ 505 $ 311 Net mark-to-market gains (losses) (36 ) 13 Net gain (loss) on trading account securities $ 469 $ 324

Loans (Tables)

Loans (Tables)	3 Months Ended
	Mar. 31, 2018
Receivables [Abstract]
Loans	Loans were as follows: March 31, Percentage of Total December 31, Percentage of Total Commercial and industrial $ 4,876,523 36.5 % $ 4,792,388 36.4 % Energy: Production 1,125,321 8.4 1,182,326 9.0 Service 192,115 1.4 171,795 1.3 Other 129,552 0.9 144,972 1.1 Total energy 1,446,988 10.7 1,499,093 11.4 Commercial real estate: Commercial mortgages 4,060,946 30.4 3,887,742 29.6 Construction 1,076,785 8.1 1,066,696 8.1 Land 317,189 2.4 331,986 2.5 Total commercial real estate 5,454,920 40.9 5,286,424 40.2 Consumer real estate: Home equity loans 355,715 2.7 355,342 2.7 Home equity lines of credit 295,677 2.2 291,950 2.2 Other 388,271 2.9 376,002 2.9 Total consumer real estate 1,039,663 7.8 1,023,294 7.8 Total real estate 6,494,583 48.7 6,309,718 48.0 Consumer and other 545,935 4.1 544,466 4.2 Total loans $ 13,364,029 100.0 % $ 13,145,665 100.0 %
Non-Accrual Loans, Segregated by Class of Loans	Non-accrual loans, segregated by class of loans, were as follows: March 31, December 31, Commercial and industrial $ 17,314 $ 46,186 Energy 93,097 94,302 Commercial real estate: Buildings, land and other 10,858 7,589 Construction — — Consumer real estate 1,878 2,109 Consumer and other 5 128 Total $ 123,152 $ 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 March 31, 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 $ 25,701 $ 23,946 $ 49,647 $ 4,826,876 $ 4,876,523 $ 10,357 Energy 24,404 4,207 28,611 1,418,377 1,446,988 3,137 Commercial real estate: Buildings, land and other 36,130 5,470 41,600 4,336,535 4,378,135 3,889 Construction 9,497 — 9,497 1,067,288 1,076,785 — Consumer real estate 8,225 3,164 11,389 1,028,274 1,039,663 1,680 Consumer and other 7,486 218 7,704 538,231 545,935 213 Total $ 111,443 $ 37,005 $ 148,448 $ 13,215,581 $ 13,364,029 $ 19,276
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 March 31, 2018 Commercial and industrial $ 38,136 $ 1,802 $ 13,430 $ 15,232 $ 7,930 Energy 123,351 41,569 63,502 105,071 14,772 Commercial real estate: Buildings, land and other 13,078 7,577 2,047 9,624 708 Construction — — — — — Consumer real estate 1,427 1,427 — 1,427 — Consumer and other — — — — — Total $ 175,992 $ 52,375 $ 78,979 $ 131,354 $ 23,410 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 2018 2017 Commercial and industrial $ 29,496 $ 26,393 Energy 99,657 68,101 Commercial real estate: Buildings, land and other 8,009 6,660 Construction — — Consumer real estate 1,321 1,102 Consumer and other — 27 Total $ 138,483 $ 102,283
Troubled Debt Restructurings	Troubled debt restructurings during the three months ended March 31, 2018 and March 31, 2017 are set forth in the following table. Three Months Ended Three Months Ended Balance at Restructure Balance at Period-End Balance at Restructure Balance at Period-End Commercial and industrial $ 2,203 $ 2,171 $ — $ — Energy 13,708 12,058 11,262 11,212 $ 15,911 $ 14,229 $ 11,262 $ 11,212
Restructured Loans Past Due in Excess of 90 Days on Financing Receivables	Additional information related to restructured loans was as follows: March 31, 2018 March 31, 2017 Restructured loans past due in excess of 90 days at period-end: Number of loans — 1 Dollar amount of loans $ — $ 747 Restructured loans on non-accrual status at period end 2,171 11,212 Charge-offs of restructured loans: Recognized in connection with restructuring — — Recognized on previously restructured loans 1,650 2,000
Weighted Average Risk Grades for All Commercial Loans by Class	The following tables present weighted-average risk grades for all commercial loans by class. March 31, 2018 December 31, 2017 Weighted Loans Weighted Loans Commercial and industrial: Risk grades 1-8 6.05 $ 4,505,491 6.06 $ 4,378,839 Risk grade 9 9.00 151,084 9.00 170,285 Risk grade 10 10.00 121,457 10.00 99,260 Risk grade 11 11.00 81,148 11.00 97,818 Risk grade 12 12.00 9,412 12.00 38,633 Risk grade 13 13.00 7,931 13.00 7,553 Total 6.35 $ 4,876,523 6.41 $ 4,792,388 Energy Risk grades 1-8 6.12 $ 1,173,702 6.01 $ 1,199,207 Risk grade 9 9.00 50,364 9.00 50,427 Risk grade 10 10.00 37,670 10.00 64,282 Risk grade 11 11.00 92,155 11.00 90,875 Risk grade 12 12.00 78,325 12.00 81,035 Risk grade 13 13.00 14,772 13.00 13,267 Total 7.02 $ 1,446,988 6.97 $ 1,499,093 Commercial real estate: Buildings, land and other Risk grades 1-8 6.77 $ 4,051,952 6.75 $ 3,868,659 Risk grade 9 9.00 124,048 9.00 151,487 Risk grade 10 10.00 120,399 10.00 129,391 Risk grade 11 11.00 70,878 11.00 62,602 Risk grade 12 12.00 10,150 12.00 7,589 Risk grade 13 13.00 708 13.00 — Total 7.00 $ 4,378,135 7.00 $ 4,219,728 Construction Risk grades 1-8 7.16 $ 1,040,071 7.11 $ 1,019,635 Risk grade 9 9.00 15,541 9.00 18,042 Risk grade 10 10.00 17,361 10.00 23,393 Risk grade 11 11.00 3,812 11.00 5,626 Risk grade 12 12.00 — 12.00 — Risk grade 13 13.00 — 13.00 — Total 7.25 $ 1,076,785 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 2018 2017 Commercial and industrial $ (7,675 ) $ (2,729 ) Energy (2,849 ) (4,225 ) Commercial real estate: Buildings, land and other 81 42 Construction 2 3 Consumer real estate (526 ) 96 Consumer and other (1,457 ) (1,128 ) Total $ (12,424 ) $ (7,941 )
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 March 31, 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 March 31, 2018 Historical valuation allowances $ 26,658 $ 12,266 $ 19,870 $ 2,470 $ 6,729 $ 67,993 Specific valuation allowances 7,930 14,772 708 — — 23,410 General valuation allowances 8,053 8,015 4,025 1,601 (95 ) 21,599 Macroeconomic valuation allowances 15,092 3,986 13,871 2,278 1,656 36,883 Total $ 57,733 $ 39,039 $ 38,474 $ 6,349 $ 8,290 $ 149,885 Allocated to loans: Individually evaluated $ 7,930 $ 14,772 $ 708 $ — $ — $ 23,410 Collectively evaluated 49,803 24,267 37,766 6,349 8,290 126,475 Total $ 57,733 $ 39,039 $ 38,474 $ 6,349 $ 8,290 $ 149,885 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 March 31, 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 March 31, 2018 Individually evaluated $ 15,232 $ 105,071 $ 9,624 $ 1,427 $ — $ 131,354 Collectively evaluated 4,861,291 1,341,917 5,445,296 1,038,236 545,935 13,232,675 Total $ 4,876,523 $ 1,446,988 $ 5,454,920 $ 1,039,663 $ 545,935 $ 13,364,029 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 months ended March 31, 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: March 31, 2018 Beginning balance $ 59,614 $ 51,528 $ 30,948 $ 5,657 $ 7,617 $ 155,364 Provision for loan losses 5,794 (9,640 ) 7,443 1,218 2,130 6,945 Charge-offs (9,252 ) (2,850 ) (5 ) (719 ) (3,972 ) (16,798 ) Recoveries 1,577 1 88 193 2,515 4,374 Net charge-offs (7,675 ) (2,849 ) 83 (526 ) (1,457 ) (12,424 ) Ending balance $ 57,733 $ 39,039 $ 38,474 $ 6,349 $ 8,290 $ 149,885 March 31, 2017 Beginning balance $ 52,915 $ 60,653 $ 30,213 $ 4,238 $ 5,026 $ 153,045 Provision for loan losses (4,603 ) 5,365 3,751 489 2,950 7,952 Charge-offs (3,527 ) (4,278 ) — (11 ) (3,548 ) (11,364 ) Recoveries 798 53 45 107 2,420 3,423 Net charge-offs (2,729 ) (4,225 ) 45 96 (1,128 ) (7,941 ) Ending balance $ 45,583 $ 61,793 $ 34,009 $ 4,823 $ 6,848 $ 153,056

Goodwill and Other Intangible31

Goodwill and Other Intangible Assets (Tables)	3 Months Ended
	Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	Goodwill is presented in the table below. March 31, 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 $ 3,748 $ 4,044 Customer relationship 900 986 Non-compete agreements 37 43 $ 4,685 $ 5,073
Estimated Aggregate Future Amortization Expense for Intangible Assets	The estimated aggregate future amortization expense for intangible assets remaining as of March 31, 2018 is as follows: Remainder of 2018 $ 1,036 2019 1,167 2020 918 2021 697 2022 481 Thereafter 386 $ 4,685

Deposits (Tables)

Deposits (Tables)	3 Months Ended
	Mar. 31, 2018
Banking and Thrift [Abstract]
Schedule of Deposits	Deposits were as follows: March 31, Percentage of Total December 31, Percentage of Total Non-interest-bearing demand deposits: Commercial and individual $ 10,294,977 38.6 % $ 10,412,882 38.8 % Correspondent banks 212,199 0.8 222,648 0.8 Public funds 426,986 1.6 561,563 2.1 Total non-interest-bearing demand deposits 10,934,162 41.0 11,197,093 41.7 Interest-bearing deposits: Private accounts: Savings and interest checking 6,869,783 25.8 6,788,766 25.2 Money market accounts 7,706,531 28.9 7,624,471 28.4 Time accounts of $100,000 or more 458,103 1.7 453,668 1.7 Time accounts under $100,000 322,105 1.2 324,636 1.2 Total private accounts 15,356,522 57.6 15,191,541 56.5 Public funds: Savings and interest checking 308,609 1.1 410,140 1.5 Money market accounts 63,155 0.2 59,008 0.2 Time accounts of $100,000 or more 14,892 0.1 14,301 0.1 Time accounts under $100,000 438 — 306 — Total public funds 387,094 1.4 483,755 1.8 Total interest-bearing deposits 15,743,616 59.0 15,675,296 58.3 Total deposits $ 26,677,778 100.0 % $ 26,872,389 100.0 %
Additional Information About Corporation's Deposits	The following table presents additional information about our deposits: March 31, December 31, Deposits from foreign sources (primarily Mexico) $ 736,786 $ 716,339 Deposits not covered by deposit insurance 13,104,781 13,281,040

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Financial Instruments with Off-Balance-Sheet Risk	Financial instruments with off-balance-sheet risk were as follows: March 31, December 31, Commitments to extend credit $ 7,939,929 $ 7,949,400 Standby letters of credit 220,294 236,595 Deferred standby letter of credit fees 1,586 1,843

Capital and Regulatory Matters

Capital and Regulatory Matters (Tables)	3 Months Ended
	Mar. 31, 2018
Banking and Thrift [Abstract]
Actual and Required Capital Ratios	The following tables present actual and required capital ratios as of March 31, 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 March 31, 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 March 31, 2018 Common Equity Tier 1 to Risk-Weighted Assets Cullen/Frost $ 2,504,319 12.69 % $ 1,258,491 6.375 % $ 1,381,873 7.00 % $ 1,283,167 6.50 % Frost Bank 2,560,828 13.00 1,255,352 6.375 1,378,426 7.00 1,279,967 6.50 Tier 1 Capital to Risk-Weighted Assets Cullen/Frost 2,648,805 13.42 1,554,607 7.875 1,677,988 8.50 1,579,283 8.00 Frost Bank 2,560,828 13.00 1,550,729 7.875 1,673,803 8.50 1,575,344 8.00 Total Capital to Risk-Weighted Assets Cullen/Frost 3,032,190 15.36 1,949,427 9.875 2,072,809 10.50 1,974,104 10.00 Frost Bank 2,711,213 13.77 1,944,565 9.875 2,067,639 10.50 1,969,180 10.00 Leverage Ratio Cullen/Frost 2,648,805 8.62 1,228,567 4.00 1,228,567 4.00 1,535,709 5.00 Frost Bank 2,560,828 8.34 1,227,539 4.00 1,227,539 4.00 1,534,424 5.00 Actual Minimum Capital Required - Basel III Phase-In Schedule Minimum Capital Required - Basel III Fully Phased-In Required to be Capital Ratio Capital Ratio Capital Ratio Capital Ratio December 31, 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 Instrume35

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2018 and December 31, 2017 . March 31, 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 $ 11,904 $ 286 $ 13,679 $ 242 Loan/lease interest rate swaps – liabilities 10,141 (445 ) 11,147 (593 ) Non-hedging interest rate derivatives: Financial institution counterparties: Loan/lease interest rate swaps – assets 615,433 3,416 430,449 1,418 Loan/lease interest rate swaps – liabilities 520,466 (8,461 ) 541,496 (12,820 ) Loan/lease interest rate caps – assets 121,976 906 114,619 480 Customer counterparties: Loan/lease interest rate swaps – assets 491,786 12,204 541,496 17,882 Loan/lease interest rate swaps – liabilities 596,473 (12,846 ) 430,449 (4,861 ) Loan/lease interest rate caps – liabilities 121,976 (906 ) 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 March 31, 2018 were as follows: Weighted-Average Interest Rate Paid Interest Rate Received Interest rate swaps: Fair value hedge loan/lease interest rate swaps 3.27 % 1.77 % Non-hedging interest rate swaps – financial institution counterparties 3.96 % 3.28 % Non-hedging interest rate swaps – customer counterparties 3.35 % 4.06 %
Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions	The notional amounts and estimated fair values of non-hedging commodity swap and option derivative positions outstanding are presented in the following table. We obtain dealer quotations and use internal valuation models with observable market data inputs to value our commodity derivative positions. March 31, 2018 December 31, 2017 Notional Units Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Oil – assets Barrels 462 $ 1,676 253 $ 193 Oil – liabilities Barrels 2,663 (19,122 ) 2,731 (13,448 ) Natural gas – assets MMBTUs 6,963 984 5,927 1,399 Natural gas – liabilities MMBTUs 6,230 (591 ) 3,917 (326 ) Customer counterparties: Oil – assets Barrels 2,699 19,301 2,731 13,709 Oil – liabilities Barrels 426 (1,668 ) 253 (187 ) Natural gas – assets MMBTUs 6,133 602 3,917 340 Natural gas – liabilities MMBTUs 7,060 (975 ) 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: March 31, 2018 December 31, 2017 Notional Currency Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Financial institution counterparties: Forward contracts – assets EUR 5,110 $ 167 4,014 $ 77 Forward contracts – assets GBP 832 8 127 1 Forward contracts – assets AUD 59 1 — — Forward contracts – liabilities EUR — — 4,846 (37 ) Forward contracts – liabilities CAD 23,228 (68 ) 25,413 (142 ) Forward contracts – liabilities GBP 453 (5 ) 1,178 (9 ) Forward contracts – liabilities AUD 58 — — — Customer counterparties: Forward contracts – assets EUR — — 3,867 58 Forward contracts – assets CAD 23,170 136 25,282 279 Forward contracts – assets GBP 88 1 — — Forward contracts – liabilities EUR 4,041 (135 ) 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 2018 2017 Commercial loan/lease interest rate swaps: Amount of gain (loss) included in interest income on loans $ (42 ) $ (245 ) Amount of (gain) loss included in other non-interest expense — (1 )
Schedule of Amounts Related to Non-Hedging Interest Rate and Commodity Derivatives	Amounts included in the consolidated statements of income related to non-hedging interest rate, commodity and foreign currency derivative instruments are presented in the table below. Three Months Ended 2018 2017 Non-hedging interest rate derivatives: Other non-interest income $ 1,488 $ 370 Other non-interest expense (21 ) (1 ) Non-hedging commodity derivatives: Other non-interest income 90 52 Non-hedging foreign currency derivatives: Other non-interest income 59 9

Balance Sheet Offsetting (Table

Balance Sheet Offsetting (Tables)	3 Months Ended
	Mar. 31, 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 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 Information about financial instruments that are eligible for offset in the consolidated balance sheet as of March 31, 2018 is presented in the following tables. Gross Amount Recognized Gross Amount Offset Net Amount Recognized March 31, 2018 Financial assets: Derivatives: Loan/lease interest rate swaps and caps $ 4,608 $ — $ 4,608 Commodity swaps and options 2,660 — 2,660 Foreign currency forward contracts 176 — 176 Total derivatives 7,444 — 7,444 Resell agreements 9,642 — 9,642 Total $ 17,086 $ — $ 17,086 Financial liabilities: Derivatives: Loan/lease interest rate swaps $ 8,906 $ — $ 8,906 Commodity swaps and options 19,713 — 19,713 Foreign currency forward contracts 73 — 73 Total derivatives 28,692 — 28,692 Repurchase agreements 1,015,021 — 1,015,021 Total $ 1,043,713 $ — $ 1,043,713
Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset	Gross Amounts Not Offset Net Amount Recognized Financial Instruments Collateral Net Amount December 31, 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 Gross Amounts Not Offset Net Amount Recognized Financial Instruments Collateral Net Amount March 31, 2018 Financial assets: Derivatives: Counterparty A $ 1,012 $ (1,012 ) $ — $ — Counterparty B 2,152 (2,152 ) — — Counterparty C 84 (84 ) — — Other counterparties 4,196 (1,217 ) (2,751 ) 228 Total derivatives 7,444 (4,465 ) (2,751 ) 228 Resell agreements 9,642 — (9,642 ) — Total $ 17,086 $ (4,465 ) $ (12,393 ) $ 228 Financial liabilities: Derivatives: Counterparty A $ 5,706 $ (1,012 ) $ (4,694 ) $ — Counterparty B 4,103 (2,152 ) (1,258 ) 693 Counterparty C 1,467 (84 ) (1,383 ) — Other counterparties 17,416 (1,217 ) (16,099 ) 100 Total derivatives 28,692 (4,465 ) (23,434 ) 793 Repurchase agreements 1,015,021 — (1,015,021 ) — Total $ 1,043,713 $ (4,465 ) $ (1,038,455 ) $ 793
Remaining Contractual Maturity of the Securities Sold Under Agreement [Table Text Block]	The remaining contractual maturity of repurchase agreements in the consolidated balance sheets as of March 31, 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 March 31, 2018 Repurchase agreements: U.S. Treasury $ 999,320 $ — $ — $ — $ 999,320 Residential mortgage-backed securities 15,701 — — — 15,701 Total borrowings $ 1,015,021 $ — $ — $ — $ 1,015,021 Gross amount of recognized liabilities for repurchase agreements $ 1,015,021 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)	3 Months Ended
	Mar. 31, 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 — — — — — — — — Exercised/vested — — — — — — (318,110 ) 60.25 Forfeited/expired — — (3,527 ) 86.54 — — (29,125 ) 69.67 Balance, March 31, 2018 53,008 $ 64.87 309,129 $ 81.66 80,103 $ 79.91 2,569,907 $ 63.65
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 2018 2017 New shares issued from available authorized shares — 283,342 Issued from available treasury stock 318,110 158,712 Total 318,110 442,054 Proceeds from stock option exercises $ 19,165 $ 24,747
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 2018 2017 Stock options $ 1,085 $ 1,787 Non-vested stock awards/stock units 1,468 1,033 Director deferred stock units — — Performance stock units 622 283 Total $ 3,175 $ 3,103 Income tax benefit $ 667 $ 1,086
Unrecognized Stock-Based Compensation Expense	Unrecognized stock-based compensation expense at March 31, 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 $ 4,148 Non-vested stock awards/stock units 13,218 Performance stock units 4,507 Total $ 21,873

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	3 Months Ended
	Mar. 31, 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 2018 2017 Net income $ 106,480 $ 84,941 Less: Preferred stock dividends 2,016 2,016 Net income available to common shareholders 104,464 82,925 Less: Earnings allocated to participating securities 702 435 Net earnings allocated to common stock $ 103,762 $ 82,490 Distributed earnings allocated to common stock $ 36,306 $ 34,475 Undistributed earnings allocated to common stock 67,456 48,015 Net earnings allocated to common stock $ 103,762 $ 82,490 Weighted-average shares outstanding for basic earnings per common share 63,649,198 63,738,191 Dilutive effect of stock compensation 1,012,997 999,194 Weighted-average shares outstanding for diluted earnings per common share 64,662,195 64,737,385

Defined Benefit Plans (Tables)

Defined Benefit Plans (Tables)	3 Months Ended
	Mar. 31, 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 2018 2017 Expected return on plan assets, net of expenses $ (2,979 ) $ (2,779 ) Interest cost on projected benefit obligation 1,475 1,547 Net amortization and deferral 1,250 1,357 Net periodic expense (benefit) $ (254 ) $ 125

Income Taxes (Tables)

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Tax Expense	Income tax expense was as follows: Three Months Ended 2018 2017 Current income tax expense $ 746 $ 15,702 Deferred income tax expense (benefit) 10,411 (4,301 ) Income tax expense, as reported $ 11,157 $ 11,401 Effective tax rate 9.5 % 11.8 %

Other Comprehensive Income (L41

Other Comprehensive Income (Loss) (Tables)	3 Months Ended
	Mar. 31, 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 $ (178,904 ) $ (37,570 ) $ (141,334 ) $ 33,811 $ 11,834 $ 21,977 Change in net unrealized gain on securities transferred to held to maturity (2,619 ) (550 ) (2,069 ) (6,286 ) (2,200 ) (4,086 ) Reclassification adjustment for net (gains) losses included in net income 19 4 15 — — — Total securities available for sale and transferred securities (181,504 ) (38,116 ) (143,388 ) 27,525 9,634 17,891 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) 1,250 263 987 1,357 475 882 Total defined-benefit post-retirement benefit plans 1,250 263 987 1,357 475 882 Total other comprehensive income (loss) $ (180,254 ) $ (37,853 ) $ (142,401 ) $ 28,882 $ 10,109 $ 18,773
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 (143,403 ) — (143,403 ) Reclassification of amounts included in net income 15 987 1,002 Net other comprehensive income (loss) during period (143,388 ) 987 (142,401 ) 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 March 31, 2018 $ (8,601 ) $ (44,753 ) $ (53,354 ) Balance January 1, 2017 $ 16,153 $ (40,776 ) $ (24,623 ) Other comprehensive income (loss) before reclassifications 17,891 — 17,891 Reclassification of amounts included in net income — 882 882 Net other comprehensive income (loss) during period 17,891 882 18,773 Balance at March 31, 2017 $ 34,044 $ (39,894 ) $ (5,850 )

Operating Segments (Tables)

Operating Segments (Tables)	3 Months Ended
	Mar. 31, 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: March 31, 2018 $ 288,561 $ 35,081 $ (2,449 ) $ 321,193 March 31, 2017 258,911 34,588 (1,290 ) 292,209 Net income (loss): Three months ended: March 31, 2018 $ 103,641 $ 5,634 $ (2,795 ) $ 106,480 March 31, 2017 80,869 5,294 (1,222 ) 84,941

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 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 March 31, 2018 Securities available for sale: U.S. Treasury $ 3,418,147 $ — $ — $ 3,418,147 Residential mortgage-backed securities — 645,695 — 645,695 States and political subdivisions — 6,430,105 — 6,430,105 Other — 42,585 — 42,585 Trading account securities: U.S. Treasury 19,772 — — 19,772 Derivative assets: Interest rate swaps, caps and floors — 16,812 — 16,812 Commodity swaps and options — 20,844 1,719 22,563 Foreign currency forward contracts 313 — — 313 Derivative liabilities: Interest rate swaps, caps and floors — 22,658 — 22,658 Commodity swaps and options — 22,356 — 22,356 Foreign currency forward contracts 208 — — 208 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. Three Months Ended Three Months Ended Level 2 Level 3 Level 2 Level 3 Carrying value of impaired loans before allocations $ 365 $ 56,788 $ — $ 24,171 Specific valuation allowance (allocations) reversals of prior allocations (64 ) (1,520 ) — (1,340 ) Fair value $ 301 $ 55,268 $ — $ 22,831
Foreclosed Assets Remeasured and Reported at Fair Value	The following table presents foreclosed assets that were remeasured and reported at fair value during the reported periods: Three Months Ended 2018 2017 Foreclosed assets remeasured at initial recognition: Carrying value of foreclosed assets prior to remeasurement $ 7 $ — Charge-offs recognized in the allowance for loan losses — — Fair value $ 7 $ — 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: March 31, 2018 December 31, 2017 Carrying Amount Estimated Fair Value Carrying Amount Estimated Fair Value Financial assets: Level 2 inputs: Cash and cash equivalents $ 4,680,948 $ 4,680,948 $ 5,053,047 $ 5,053,047 Securities held to maturity 1,247,154 1,257,886 1,432,098 1,455,791 Cash surrender value of life insurance policies 181,297 181,297 180,477 180,477 Accrued interest receivable 125,485 125,485 167,508 167,508 Level 3 inputs: Loans, net 13,214,144 13,184,663 12,990,301 12,981,165 Financial liabilities: Level 2 inputs: Deposits 26,677,778 26,671,005 26,872,389 26,866,676 Federal funds purchased and repurchase agreements 1,032,221 1,032,221 1,147,824 1,147,824 Junior subordinated deferrable interest debentures 136,198 137,115 136,184 137,115 Subordinated notes payable and other borrowings 98,591 103,409 98,552 105,311 Accrued interest payable 3,196 3,196 3,358 3,358

Significant Accounting Polici44

Significant Accounting Policies - Additional Cash Flow Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accounting Policies [Abstract]
Cash paid for interest	$ 13,740	$ 3,257
Cash paid for income taxes	0	0
Unsettled purchases of securities	47,723	33,466
Loans foreclosed and transferred to other real estate owned and foreclosed assets	$ 7	$ 0

Significant Accounting Polici45

Significant Accounting Policies Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of certain income tax effects related to U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act	$ 9,535
Cumulative Effect of New Accounting Principle in Period of Adoption			$ (2,285)	$ 0
Gross interchange and debit card transaction fees	6,071
Gross cost related to interchange and debit card transactions	2,913	$ 3,223
Interchange and debit card transaction fees	$ 3,158	$ 5,574

Securities - Additional Informa

Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 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	98.20%
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund	67.70%
Securities Pledged as Collateral	$ 3,500,000	$ 3,800,000
Available for Sale, Estimated Fair Value	10,536,532	$ 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	8,900
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	$ 7,100
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	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,247,154	$ 1,432,098
Held-to-maturity Securities, Gross Unrealized Gains	13,922	26,477
Held-to-maturity Securities, Gross Unrealized Losses	3,190	2,784
Held to Maturity, Estimated Fair Value	1,257,886	1,455,791
Available for Sale, Amortized Cost	10,556,359	10,329,951
Available for Sale Securities, Gross Unrealized Gains	87,745	193,835
Available for Sale Securities, Gross Unrealized Losses	107,572	34,777
Available for Sale, Estimated Fair Value	10,536,532	10,489,009
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Amortized Cost	3,453,887	3,453,391
Available for Sale Securities, Gross Unrealized Gains	373	7,494
Available for Sale Securities, Gross Unrealized Losses	36,113	15,732
Available for Sale, Estimated Fair Value	3,418,147	3,445,153
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	3,503	3,610
Held-to-maturity Securities, Gross Unrealized Gains	12	15
Held-to-maturity Securities, Gross Unrealized Losses	57	38
Held to Maturity, Estimated Fair Value	3,458	3,587
Available for Sale, Amortized Cost	636,498	648,288
Available for Sale Securities, Gross Unrealized Gains	16,012	19,048
Available for Sale Securities, Gross Unrealized Losses	6,815	2,250
Available for Sale, Estimated Fair Value	645,695	665,086
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,242,151	1,428,488
Held-to-maturity Securities, Gross Unrealized Gains	13,910	26,462
Held-to-maturity Securities, Gross Unrealized Losses	3,126	2,746
Held to Maturity, Estimated Fair Value	1,252,935	1,452,204
Available for Sale, Amortized Cost	6,423,389	6,185,711
Available for Sale Securities, Gross Unrealized Gains	71,360	167,293
Available for Sale Securities, Gross Unrealized Losses	64,644	16,795
Available for Sale, Estimated Fair Value	6,430,105	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	7	0
Held to Maturity, Estimated Fair Value	1,493	0
Available for Sale, Amortized Cost	42,585	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	$ 42,585	$ 42,561

Securities - Securities, with U

Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 234,519
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	930
Held to Maturity, More than 12 Months, Estimated Fair Value	46,385
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,260
Held to Maturity, Total Estimated Fair Value	280,904
Held-to-maturity Securities, Gross Unrealized Losses	3,190	$ 2,784
Available for Sale, Less than 12 Months, Estimated Fair Value	4,839,766
Available for Sale, Less than 12 Months, Unrealized Losses	58,771
Available for Sale, More than 12 Months, Estimated Fair Value	1,375,871
Available for Sale, More than 12 Months, Unrealized Losses	48,801
Available for Sale, Total Estimated Fair Value	6,215,637
Available-for-sale Securities, Gross Unrealized Loss, before Tax	107,572	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	1,120
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	23
Held to Maturity, More than 12 Months, Estimated Fair Value	1,516
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	34
Held to Maturity, Total Estimated Fair Value	2,636
Held-to-maturity Securities, Gross Unrealized Losses	57	38
Available for Sale, Less than 12 Months, Estimated Fair Value	219,091
Available for Sale, Less than 12 Months, Unrealized Losses	4,508
Available for Sale, More than 12 Months, Estimated Fair Value	45,424
Available for Sale, More than 12 Months, Unrealized Losses	2,307
Available for Sale, Total Estimated Fair Value	264,515
Available-for-sale Securities, Gross Unrealized Loss, before Tax	6,815	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	231,906
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	900
Held to Maturity, More than 12 Months, Estimated Fair Value	44,869
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,226
Held to Maturity, Total Estimated Fair Value	276,775
Held-to-maturity Securities, Gross Unrealized Losses	3,126	2,746
Available for Sale, Less than 12 Months, Estimated Fair Value	2,004,045
Available for Sale, Less than 12 Months, Unrealized Losses	26,466
Available for Sale, More than 12 Months, Estimated Fair Value	815,756
Available for Sale, More than 12 Months, Unrealized Losses	38,178
Available for Sale, Total Estimated Fair Value	2,819,801
Available-for-sale Securities, Gross Unrealized Loss, before Tax	64,644	16,795
Other Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	1,493
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7
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	1,493
Held-to-maturity Securities, Gross Unrealized Losses	7	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	2,616,630
Available for Sale, Less than 12 Months, Unrealized Losses	27,797
Available for Sale, More than 12 Months, Estimated Fair Value	514,691
Available for Sale, More than 12 Months, Unrealized Losses	8,316
Available for Sale, Total Estimated Fair Value	3,131,321
Available-for-sale Securities, Gross Unrealized Loss, before Tax	$ 36,113	$ 15,732

Securities - Amortized Cost a49

Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Held to Maturity, Due in one year or less, Amortized Cost	$ 105,054
Held to Maturity, Due after one year through five years, Amortized Cost	165,355
Held to Maturity, Due after five years through ten years, Amortized Cost	432,654
Held to Maturity, Due after ten years, Amortized Cost	540,588
Held to Maturity, Residential mortgage-backed securities, Amortized Cost	3,503
Held-to-maturity Securities, Equity Securities, Amortized cost	0
Held to Maturity, Amortized Cost	1,247,154	$ 1,432,098
Held to Maturity, Due in one year or less, Estimated Fair Value	106,904
Held to Maturity, Due after one year through five years, Estimated Fair Value	168,916
Held to Maturity, Due after five years through ten years, Estimated Fair Value	433,756
Held to Maturity, Due after ten years, Estimated Fair Value	544,852
Held to Maturity, Residential mortgage-backed securities, Estimated Fair Value	3,458
Held-to-maturity Securities, Equity Securities, Fair Value	0
Held to Maturity, Estimated Fair Value	1,257,886	1,455,791
Available-for-sale Securities, Due in one year or less, Amortized Cost	118,442
Available for Sale, Due after one year through five years, Amortized Cost	4,010,195
Available for Sale, Due after five years through ten years, Amortized Cost	486,543
Available for Sale, Due after ten years, Amortized Cost	5,262,096
Available for Sale, Residential mortgage-backed securities, Amortized Cost	636,498
Available-for-sale Securities, Equity Securities, Amortized cost	42,585
Available for Sale, Amortized Cost	10,556,359	10,329,951
Available for Sale, Due in one year or less, Estimated Fair Value	119,061
Available for Sale, Due after one year through five years, Estimated Fair Value	3,980,391
Available for Sale, Due after five years through ten years, Estimated Fair Value	491,153
Available for Sale, Due after ten years, Estimated Fair Value	5,257,647
Available for Sale, Residential mortgage-backed securities, Estimated Fair Value	645,695
Available For Sale Securities Equity Securities Fair Value	42,585
Available for Sale, Estimated Fair Value	$ 10,536,532	$ 10,489,009

Securities - Sales of Securitie

Securities - Sales of Securities Available for Sale (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 2,984,867	$ 0
Gross realized gains	0	0
Gross realized losses	(19)	0
Tax (expense)benefit of securities gains/losses	$ 4	$ 0

Securities Securities - Premium

Securities Securities - Premium Amortization and Discount Accretion Included in Income on Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Premium amortization	$ (26,036)	$ (24,028)
Discount accretion	1,776	2,390
Net (premium amortization) discount accretion	$ (24,260)	$ (21,638)

Securities - Trading Account Se

Securities - Trading Account Securities, at Estimated Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading account securities	$ 19,772	$ 21,098
U.S. Treasury [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading account securities	19,772	19,210
States and political subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading account securities	$ 0	$ 1,888

Securities - Net Gains and Loss

Securities - Net Gains and Losses on Trading Account Securities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Net gain on sales transactions	$ 505	$ 311
Net mark-to-market gains (losses)	(36)	13
Net gain (loss) on trading account securities	$ 469	$ 324

Loans - Additional Information

Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2017USD ($)	Feb. 28, 2018
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%
Maximum percentage loan related to single industry	10.00%
Percentage of total energy loans to total loans	10.70%		11.40%
Loans and Leases Receivable, Related Parties	$ 180,700		$ 166,400
Restructured loans on non-accrual status at period end	14,229	$ 11,212
Texas Leading Index economic condition indicator			129.6	131.5
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end	12,058	11,212
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end	2,171	11,212
Additional interest that would have been recognized on non-accrual loans, net of tax	1,500	$ 851
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,200,000
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 44,800

Loans - Loans (Detail)

Loans - Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 4,876,523	$ 4,792,388
Energy:	1,446,988	1,499,093
Commercial mortgages	4,060,946	3,887,742
Construction	1,076,785	1,066,696
Land	317,189	331,986
Total commercial real estate	5,454,920	5,286,424
Home equity loans	355,715	355,342
Home equity lines of credit	295,677	291,950
Other	388,271	376,002
Total consumer real estate	1,039,663	1,023,294
Total real estate	6,494,583	6,309,718
Consumer and other	545,935	544,466
Total loans	$ 13,364,029	$ 13,145,665
Total commercial and industrial percentage of total loans	36.50%	36.40%
Percentage of total energy loans to total loans	10.70%	11.40%
Commercial mortgages percentage of total loans	30.40%	29.60%
Construction percentage of total loans	8.10%	8.10%
Land percentage of total loans	2.40%	2.50%
Total commercial real estate percentage of total loans	40.90%	40.20%
Home equity loans percentage of total loans	2.70%	2.70%
Home equity lines of credit percentage of total loans	2.20%	2.20%
Other percentage of total loans	2.90%	2.90%
Total consumer real estate percentage of total loans	7.80%	7.80%
Total real estate percentage of total loans	48.70%	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:	$ 1,125,321	$ 1,182,326
Percentage of energy production loans to total loans	8.40%	9.00%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 192,115	$ 171,795
Percentage of energy service loans to total loans	1.40%	1.30%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 129,552	$ 144,972
Percentage of other energy loans to total loans	0.90%	1.10%

Loans - Non-Accrual Loans, Segr

Loans - Non-Accrual Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 123,152	$ 150,314
Commercial Portfolio Segment [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	17,314	46,186
Commercial and industrial, Energy [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	93,097	94,302
Commercial real estate, Buildings, land and other [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	10,858	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	1,878	2,109
Consumer and Other [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 5	$ 128

Loans - Age Analysis of Past Du

Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 148,448
Current Loans	13,215,581
Total loans	13,364,029	$ 13,145,665
Accruing Loans 90 or More Days Past Due	19,276
Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	49,647
Current Loans	4,826,876
Total loans	4,876,523
Accruing Loans 90 or More Days Past Due	10,357
Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	28,611
Current Loans	1,418,377
Total loans	1,446,988
Accruing Loans 90 or More Days Past Due	3,137
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	41,600
Current Loans	4,336,535
Total loans	4,378,135
Accruing Loans 90 or More Days Past Due	3,889
Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	9,497
Current Loans	1,067,288
Total loans	1,076,785
Accruing Loans 90 or More Days Past Due	0
Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	11,389
Current Loans	1,028,274
Total loans	1,039,663
Accruing Loans 90 or More Days Past Due	1,680
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	7,704
Current Loans	538,231
Total loans	545,935
Accruing Loans 90 or More Days Past Due	213
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	111,443
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	25,701
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	24,404
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	36,130
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	9,497
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	8,225
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	7,486
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	37,005
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	23,946
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	4,207
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	5,470
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	0
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	3,164
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	$ 218

Loans - Impaired Loans (Detail)

Loans - Impaired Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	$ 175,992	$ 172,396
Recorded Investment With No Allowance	52,375	68,726
Recorded Investment With Allowance	78,979	76,884
Total Recorded Investment	131,354	145,610
Related Allowance	23,410	20,820
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	38,136	60,781
Recorded Investment With No Allowance	1,802	28,038
Recorded Investment With Allowance	13,430	15,722
Total Recorded Investment	15,232	43,760
Related Allowance	7,930	7,553
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	123,351	99,606
Recorded Investment With No Allowance	41,569	33,080
Recorded Investment With Allowance	63,502	61,162
Total Recorded Investment	105,071	94,242
Related Allowance	14,772	13,267
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	13,078	10,795
Recorded Investment With No Allowance	7,577	6,394
Recorded Investment With Allowance	2,047	0
Total Recorded Investment	9,624	6,394
Related Allowance	708	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	1,427	1,214
Recorded Investment With No Allowance	1,427	1,214
Recorded Investment With Allowance	0	0
Total Recorded Investment	1,427	1,214
Related Allowance	0	0
Consumer and Other [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

Loans - Average Recorded Invest

Loans - Average Recorded Investment In Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 138,483	$ 102,283
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	29,496	26,393
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	99,657	68,101
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	8,009	6,660
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	0	0
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	1,321	1,102
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 0	$ 27

Loans - Troubled Debt Restructu

Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	$ 15,911	$ 11,262
Balance at period end	14,229	11,212
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	2,203	0
Balance at period end	2,171	0
Commercial and industrial, Energy [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	13,708	11,262
Balance at period end	$ 12,058	$ 11,212

Loans Loans - Restructured Loan

Loans Loans - Restructured Loans Past Due in Excess of 90 Days on Financing Receivables (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018USD ($)Item	Mar. 31, 2017USD ($)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	$ 747
Restructured loans on non-accrual status at period end	14,229	11,212
Charge-offs of restructured loans recognized in connection with restructuring	0	0
Charge-offs of restructured loans recognized on previously restructured loans	1,650	2,000
Non Accrual Loans [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans on non-accrual status at period end	$ 2,171	$ 11,212

Loans - Weighted Average Risk G

Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018USD ($)Grade	Dec. 31, 2017USD ($)Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 4,876,523	$ 4,792,388
Loans	5,454,920	5,286,424
Loans	$ 1,076,785	$ 1,066,696
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.35	6.41
Loans	$ 4,876,523	$ 4,792,388
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.02	6.97
Loans	$ 1,446,988	$ 1,499,093
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7	7
Loans	$ 4,378,135	$ 4,219,728
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.25	7.23
Loans	$ 1,076,785	$ 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	6.05	6.06
Loans	$ 4,505,491	$ 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	6.12	6.01
Loans	$ 1,173,702	$ 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	$ 4,051,952	$ 3,868,659
Risk grades 1-8 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.16	7.11
Loans	$ 1,040,071	$ 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	$ 151,084	$ 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	$ 50,364	$ 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	$ 124,048	$ 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	$ 15,541	$ 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	$ 121,457	$ 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	$ 37,670	$ 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	$ 120,399	$ 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	$ 17,361	$ 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	$ 81,148	$ 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	$ 92,155	$ 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	$ 70,878	$ 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	$ 3,812	$ 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	$ 9,412	$ 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	$ 78,325	$ 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	$ 10,150	$ 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	$ 7,931	$ 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	$ 14,772	$ 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	$ 708	$ 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
	Mar. 31, 2018	Mar. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	$ (12,424)	$ (7,941)
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	(7,675)	(2,729)
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	(2,849)	(4,225)
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	81	42
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	2	3
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	(526)	96
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	$ (1,457)	$ (1,128)

Loans - Unallocated Portion of

Loans - Unallocated Portion of Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 149,885	$ 155,364	$ 153,056	$ 153,045
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	23,410	20,820
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	126,475	134,544
Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	67,993	75,481
Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	23,410	20,820
General Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	21,599	23,283
Macroeconomic Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	36,883	35,780
Commercial Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	57,733	59,614	45,583	52,915
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	7,930	7,553
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	49,803	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	26,658	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	7,930	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	8,053	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	15,092	16,548
Commercial and industrial, Energy [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	39,039	51,528	61,793	60,653
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	14,772	13,267
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	24,267	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	12,266	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	14,772	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	8,015	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	3,986	8,224
Commercial Real Estate Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	38,474	30,948	34,009	30,213
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	708	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	37,766	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	19,870	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	708	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	4,025	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	13,871	7,852
Consumer Real Estate [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	6,349	5,657	4,823	4,238
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	6,349	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	2,470	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	1,601	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	2,278	1,051
Consumer and Other [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	8,290	7,617	$ 6,848	$ 5,026
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	8,290	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	6,729	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	0	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	(95)	(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	$ 1,656	$ 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	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 4,876,523	$ 4,792,388
Energy:	1,446,988	1,499,093
Total commercial real estate	5,454,920	5,286,424
Total consumer real estate	1,039,663	1,023,294
Consumer and other	545,935	544,466
Total loans	13,364,029	13,145,665
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	15,232	43,760
Energy:	105,071	94,242
Total commercial real estate	9,624	6,394
Total consumer real estate	1,427	1,214
Consumer and other	0	0
Total loans	131,354	145,610
Loans Collectively Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	4,861,291	4,748,628
Energy:	1,341,917	1,404,851
Total commercial real estate	5,445,296	5,280,030
Total consumer real estate	1,038,236	1,022,080
Consumer and other	545,935	544,466
Total loans	$ 13,232,675	$ 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
	Mar. 31, 2018	Mar. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 155,364	$ 153,045
Provision for loan losses	6,945	7,952
Charge-offs	(16,798)	(11,364)
Recoveries	4,374	3,423
Net charge-offs	(12,424)	(7,941)
Ending balance	149,885	153,056
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	59,614	52,915
Provision for loan losses	5,794	(4,603)
Charge-offs	(9,252)	(3,527)
Recoveries	1,577	798
Net charge-offs	(7,675)	(2,729)
Ending balance	57,733	45,583
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	51,528	60,653
Provision for loan losses	(9,640)	5,365
Charge-offs	(2,850)	(4,278)
Recoveries	1	53
Net charge-offs	(2,849)	(4,225)
Ending balance	39,039	61,793
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	30,948	30,213
Provision for loan losses	7,443	3,751
Charge-offs	(5)	0
Recoveries	88	45
Net charge-offs	83	45
Ending balance	38,474	34,009
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,657	4,238
Provision for loan losses	1,218	489
Charge-offs	(719)	(11)
Recoveries	193	107
Net charge-offs	(526)	96
Ending balance	6,349	4,823
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,617	5,026
Provision for loan losses	2,130	2,950
Charge-offs	(3,972)	(3,548)
Recoveries	2,515	2,420
Net charge-offs	(1,457)	(1,128)
Ending balance	$ 8,290	$ 6,848

Goodwill and Other Intangible67

Goodwill and Other Intangible Assets - Schedule of Goodwill (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	$ 654,952	$ 654,952

Goodwill and Other Intangible68

Goodwill and Other Intangible Assets - Schedule of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	$ 4,685	$ 5,073
Core deposits [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	3,748	4,044
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	900	986
Non-compete agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	$ 37	$ 43

Goodwill and Other Intangible69

Goodwill and Other Intangible Assets - Estimated Aggregate Future Amortization Expense for Intangible Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Remainder of 2018	$ 1,036
2,019	1,167
2,020	918
2,021	697
2,022	481
Thereafter	386
Future Amortization Expense, Total	$ 4,685	$ 5,073

Deposits - Schedule of Deposits

Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,934,162	$ 11,197,093
Total interest-bearing deposits	15,743,616	15,675,296
Total deposits	$ 26,677,778	$ 26,872,389
Total non-interest-bearing demand deposits, Percentage of Total	41.00%	41.70%
Total interest-bearing deposits, Percentage of Total	59.00%	58.30%
Total deposits, Percentage of Total	100.00%	100.00%
Commercial and Individual [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 10,294,977	$ 10,412,882
Total non-interest-bearing demand deposits, Percentage of Total	38.60%	38.80%
Correspondent Banks [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 212,199	$ 222,648
Total non-interest-bearing demand deposits, Percentage of Total	0.80%	0.80%
Private Accounts [Member]
Deposit [Line Items]
Savings and interest checking	$ 6,869,783	$ 6,788,766
Money market accounts	7,706,531	7,624,471
Time accounts of $100,000 or More	458,103	453,668
Time accounts under $100,000	322,105	324,636
Total private accounts	$ 15,356,522	$ 15,191,541
Savings and interest checking, Percentage of Total	25.80%	25.20%
Money market accounts, Percentage of Total	28.90%	28.40%
Time accounts of $100,000 or more, Percentage of Total	1.70%	1.70%
Time accounts under $100,000, Percentage of Total	1.20%	1.20%
Total private accounts, Percentage of Total	57.60%	56.50%
Public Funds [Member]
Deposit [Line Items]
Total non-interest-bearing demand deposits	$ 426,986	$ 561,563
Savings and interest checking	308,609	410,140
Money market accounts	63,155	59,008
Time accounts of $100,000 or More	14,892	14,301
Time accounts under $100,000	438	306
Total public funds	$ 387,094	$ 483,755
Total non-interest-bearing demand deposits, Percentage of Total	1.60%	2.10%
Savings and interest checking, Percentage of Total	1.10%	1.50%
Money market accounts, Percentage of Total	0.20%	0.20%
Time accounts of $100,000 or more, Percentage of Total	0.10%	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	Mar. 31, 2018	Dec. 31, 2017
Banking and Thrift [Abstract]
Deposits from foreign sources (primarily Mexico)	$ 736,786	$ 716,339
Deposits not covered by deposit insurance	$ 13,104,781	$ 13,281,040

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Rent expense for operating leases	$ 8.2	$ 7.7

Commitments and Contingencies73

Commitments and Contingencies - Financial Instruments with Off-Balance-Sheet Risk (Detail) - USD ($) $ in Thousands	Mar. 31, 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	$ 220,294	$ 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	1,586	1,843
Commitments to Extend Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 7,939,929	$ 7,949,400

Capital and Regulatory Matter74

Capital and Regulatory Matters - Additional Information (Detail) $ in Thousands	Oct. 24, 2017USD ($)	Mar. 31, 2018USD ($)quartershares	Sep. 30, 2017USD ($)shares	Mar. 31, 2017USD ($)shares	Dec. 31, 2017USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares		0		469
Treasury Stock, Value, Acquired, Cost Method		$ 0		$ 42
Preferred stock value issued include for regulatory tier one capital calculations		$ 144,500			$ 144,500
Preferred stock dividend rate percentage		5.375%			5.375%
Qualified subordinated debt		$ 98,591			$ 98,552
Maximum dividends available without prior regulatory approval		$ 446,700
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

Capital and Regulatory Matter75

Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands	Mar. 31, 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	$ 2,504,319	$ 2,426,048
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.69%	12.42%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,258,491	$ 1,123,430
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	6.375%	5.75%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,381,873	$ 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	$ 1,283,167	$ 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	$ 2,648,805	$ 2,570,534
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.42%	13.16%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,554,607	$ 1,416,499
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	7.875%	7.25%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,677,988	$ 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	$ 1,579,283	$ 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	$ 3,032,190	$ 2,959,326
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.36%	15.15%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,949,427	$ 1,807,257
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio	9.875%	9.25%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,072,809	$ 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	$ 1,974,104	$ 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	$ 2,648,805	$ 2,570,534
Leverage Ratio, Actual, Ratio	8.62%	8.46%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,228,567	$ 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	$ 1,228,567	$ 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	$ 1,535,709	$ 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	$ 2,560,828	$ 2,518,999
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	13.00%	12.92%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,255,352	$ 1,120,663
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	6.375%	5.75%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,378,426	$ 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	$ 1,279,967	$ 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	$ 2,560,828	$ 2,518,999
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.00%	12.92%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,550,729	$ 1,413,010
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	7.875%	7.25%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,673,803	$ 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	$ 1,575,344	$ 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	$ 2,711,213	$ 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	$ 1,944,565	$ 1,802,805
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio	9.875%	9.25%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,067,639	$ 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	$ 1,969,180	$ 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	$ 2,560,828	$ 2,518,999
Leverage Ratio, Actual, Ratio	8.34%	8.30%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,227,539	$ 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	$ 1,227,539	$ 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	$ 1,534,424	$ 1,517,869
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%

Derivative Financial Instrume76

Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Weighted-average strike rate for outstanding interest rate caps	3.47%
Approximate credit exposure related to swaps with bank customers	$ 29,300
Aggregate fair value of securities posted as collateral for derivative contracts	6,600
Cash collateral for borrowed securities deposited with other financial institutions	25,800
Interest rate swaps with upstream financial institution counterparties [Member]
Derivative [Line Items]
Approximate credit exposure related to swaps with bank customers	9,200
Not Designated as Hedging Instrument [Member] | Financial Institution Counterparties Loan Lease Interest Rate Swaps Liabilities [Member]
Derivative [Line Items]
Estimated fair value of derivative contracts cleared through Chicago Mercantile Exchange	$ 0	$ 0

Derivative Financial Instrume77

Derivative Financial Instruments - Schedule of Notional Amounts and Estimated Fair Values of Interest Rate Derivative Contracts Outstanding (Detail) - USD ($) $ in Thousands	Mar. 31, 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	$ 11,904	$ 13,679
Estimated Fair Value	286	242
Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	10,141	11,147
Estimated Fair Value	(445)	(593)
Non Designated as Hedging Instrument [Member] | Financial institution counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	615,433	430,449
Estimated Fair Value	3,416	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	520,466	541,496
Estimated Fair Value	(8,461)	(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	121,976	114,619
Estimated Fair Value	906	480
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate swaps assets [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	491,786	541,496
Estimated Fair Value	12,204	17,882
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate swaps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	596,473	430,449
Estimated Fair Value	(12,846)	(4,861)
Non Designated as Hedging Instrument [Member] | Customer counterparties Loan/lease interest rate caps liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	121,976	114,619
Estimated Fair Value	$ (906)	$ (480)

Derivative Financial Instrume78

Derivative Financial Instruments - Schedule of Weighted-Average Rates Paid and Received for Interest Rate Swaps Outstanding (Detail)	Mar. 31, 2018
Weighted Average Interest Rate Paid [Member]
Weighted Average Discount Rate [Line Items]
Fair value hedge loan/lease interest rate swaps	3.27%
Weighted-Average Interest Rate Received [Member]
Weighted Average Discount Rate [Line Items]
Fair value hedge loan/lease interest rate swaps	1.77%
Financial Institution Counterparties [Member] | Weighted Average Interest Rate Paid [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – financial institution counterparties	3.96%
Financial Institution Counterparties [Member] | Weighted-Average Interest Rate Received [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – financial institution counterparties	3.28%
Customer Counterparties [Member] | Weighted Average Interest Rate Paid [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – customer counterparties	3.35%
Customer Counterparties [Member] | Weighted-Average Interest Rate Received [Member]
Weighted Average Discount Rate [Line Items]
Non-hedging interest rate swaps – customer counterparties	4.06%

Derivative Financial Instrume79

Derivative Financial Instruments - Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions (Detail) bbl in Thousands, MMBTU in Thousands, $ in Thousands	Mar. 31, 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	462	253
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	2,663	2,731
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 1,676	$ 193
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (19,122)	$ (13,448)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	6,963	5,927
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	6,230	3,917
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 984	$ 1,399
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (591)	$ (326)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	2,699	2,731
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	426	253
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 19,301	$ 13,709
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (1,668)	$ (187)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	6,133	3,917
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	7,060	5,927
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 602	$ 340
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (975)	$ (1,366)

Derivative Financial Instrume80

Derivative Financial Instruments - Notional Amount and Fair Value of Foreign Currency Forward Contract (Detail) € in Thousands, £ in Thousands, $ in Thousands, $ in Thousands, $ in Thousands	Mar. 31, 2018USD ($)	Mar. 31, 2018CAD ($)	Mar. 31, 2018AUD ($)	Mar. 31, 2018EUR (€)	Mar. 31, 2018GBP (£)	Dec. 31, 2017USD ($)	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			$ 59	€ 5,110	£ 832			$ 0	€ 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	$ 58	$ 23,228		0	453		$ 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		$ 23,170		0	88		$ 25,282		3,867	0
Customer Counterparties [Member] | Other Liabilities [Member] | Non Designated as Hedging Instrument [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Notional Amount				€ 4,041	£ 0				€ 4,041	£ 127
Foreign Exchange Forward [Member] | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Liability Estimated Fair Value	(68)					$ (142)
Foreign Exchange Forward [Member] | Customer Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value	136					279
GBP Foreign Exchange Forward [Member] | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value	8					1
Forward Contracts – Liability Estimated Fair Value	(5)					(9)
GBP Foreign Exchange Forward [Member] | Customer Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value	1					0
Forward Contracts – Liability Estimated Fair Value	0					0
AUD Foreign Exchange Forward [Member] | Financial Institution Counterparties [Member]
Derivative Counter Party [Line Items]
Forward Contracts – Assets Estimated Fair Value	1					0
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	167					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	$ (135)					$ (51)

Derivative Financial Instrume81

Derivative Financial Instruments - Schedule of Amounts Related to Interest Rate Derivatives Designated as Hedges of Fair Value (Detail) - Designated as Hedging Instrument [Member] - Commercial Loan/Lease Interest Rate Swaps [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Interest Income on Loans [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ (42)	$ (245)
Other Non-Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) included in income	$ 0	$ (1)

Derivative Financial Instrume82

Derivative Financial Instruments - Schedule of Amounts Related to Non-Hedging Interest Rate and Commodity Derivatives (Detail) - Non Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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)	$ 1,488	$ 370
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)	(21)	(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)	90	52
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)	$ 59	$ 9

Balance Sheet Offsetting - Fina

Balance Sheet Offsetting - Financial Instruments Eligible for Offset Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	$ 7,444	$ 3,810
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	7,444	3,810
Gross Amount Recognized, Resell agreements, Financial assets	9,642	9,642
Gross Amount Offset, Resell agreements, Financial assets	0	0
Net Amount Recognized, Resell agreements, Financial assets	9,642	9,642
Gross Amount Recognized, Financial assets	17,086	13,452
Gross Amount Offset, Financial assets	0	0
Net Amount Recognized, Financial assets	17,086	13,452
Gross Amount Recognized, Derivatives, Financial Liabilities	28,692	27,375
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	28,692	27,375
Gross Amount Recognized, Repurchase agreements, Financial liabilities	1,015,021	1,117,199
Gross Amount Offset, Repurchase agreements, Financial liabilities	0	0
Net Amount Recognized, Repurchase agreements, Financial liabilities	1,015,021	1,117,199
Gross Amount Recognized, Financial liabilities	1,043,713	1,144,574
Gross Amount Offset, Financial liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,043,713	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	4,608	2,140
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	4,608	2,140
Commodity swaps and options [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	2,660	1,592
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	2,660	1,592
Gross Amount Recognized, Derivatives, Financial Liabilities	19,713	13,774
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	19,713	13,774
Foreign currency forward contracts [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Assets	176	78
Gross Amount Offset, Derivatives, Financial assets	0	0
Net Amount Recognized, Derivatives, Financial assets	176	78
Gross Amount Recognized, Derivatives, Financial Liabilities	73	188
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	73	188
Loan/lease interest rate swaps [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Gross Amount Recognized, Derivatives, Financial Liabilities	8,906	13,413
Gross Amount Offset, Derivatives, Financial liabilities	0	0
Net Amount Recognized, Derivatives, Financial liabilities	$ 8,906	$ 13,413

Balance Sheet Offsetting - Fi84

Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 7,444	$ 3,810
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(4,465)	(3,308)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(2,751)	(387)
Net Amount, Derivatives, Financial Assets	228	115
Net Amount Recognized, Resell agreements, Financial Assets	9,642	9,642
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(9,642)	(9,642)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	17,086	13,452
Gross Amounts Not Offset Collateral, Financial Assets	(12,393)	(10,029)
Net Amount, Financial Assets	228	115
Net Amount Recognized, Derivatives, Financial Liabilities	28,692	27,375
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(4,465)	(3,308)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(23,434)	(22,768)
Net Amount, Derivatives, Financial Liabilities	793	1,299
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	1,015,021	1,117,199
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(1,015,021)	(1,117,199)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,043,713	1,144,574
Gross Amounts Not Offset Collateral, Financial Liabilities	(1,038,455)	(1,139,967)
Net Amount, Financial Liabilities	793	1,299
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	1,012	395
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,012)	(395)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	5,706	7,397
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,012)	(395)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(4,694)	(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	2,152	1,028
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(2,152)	(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	4,103	4,466
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(2,152)	(1,028)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(1,258)	(3,101)
Net Amount, Derivatives, Financial Liabilities	693	337
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	84	55
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(84)	(55)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	1,467	1,520
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(84)	(55)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(1,383)	(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	4,196	2,332
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,217)	(1,830)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(2,751)	(387)
Net Amount, Derivatives, Financial Assets	228	115
Net Amount Recognized, Derivatives, Financial Liabilities	17,416	13,992
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,217)	(1,830)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(16,099)	(11,215)
Net Amount, Derivatives, Financial Liabilities	$ 100	$ 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	3 Months Ended	12 Months Ended
	Mar. 31, 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	$ 1,015,021	$ 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	1,015,021	1,117,199
Securities Sold under Agreements to Repurchase, Gross	1,015,021	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	999,320	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	999,320	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	15,701	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	$ 15,701	$ 80,308

Stock-Based Compensation Stock-

Stock-Based Compensation Stock-Based Compensation - Narrative (Details)	Mar. 31, 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	1,400,402

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Activity in Corporation's Active Stock Plans (Detail) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options exercised, Number of Shares	(318,110)	(442,054)
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	0
Stock Awards Vested and Expected to Vest, Exercisable, Number	0
Ending Balance, Number of Shares	53,008
Beginning Balance, Director Deferred, Weighted-Average Grant-Date Fair Value	$ 64.87
Granted, Directors Deferred, Weighted-Average Grant-Date Fair Value	0
Ending Balance, Director Deferred, Weighted-Average Grant-Date Fair Value	64.87
Stock Award Vested, Weighted Average Grant Date Fair Value	$ 0
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	0
Forfeited/expired, Number of Shares	(3,527)
Ending Balance, Number of Shares	309,129
Stock Award Vested, Weighted Average Grant Date Fair Value	$ 0
Beginning Balance, Weighted-Average Grant-Date Fair Value	81.71
Ending Balance, Weighted-Average Grant-Date Fair Value	81.66
Forfeited/expired, Weighted-Average Exercise Price	$ 86.54
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	(318,110)
Forfeited/expired, Number of Shares	(29,125)
Ending Balance, Number of Shares	2,569,907
Beginning Balance, Weighted-Average Exercise Price	$ 63.34
Stock options exercised, Weighted-Average Exercise Price	60.25
Forfeited/expired, Weighted-Average Exercise Price	69.67
Ending Balance, Weighted-Average Exercise Price	$ 63.65
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
Beginning Balance, Weighted-Average Grant-Date Fair Value	$ 79.91
Ending Balance, Weighted-Average Grant-Date Fair Value	$ 79.91

Stock-Based Compensation - Shar

Stock-Based Compensation - Shares Issued in Connection with Stock Compensation Awards (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
New shares issued from available authorized shares	0	283,342
Issued from available treasury stock	318,110	158,712
Total	318,110	442,054
Proceeds from stock option exercises	$ 19,165	$ 24,747

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock options	$ 1,085	$ 1,787
Non-vested stock awards/stock units	1,468	1,033
Director deferred stock units	0	0
Performance stock units	622	283
Total	3,175	3,103
Income tax benefit	$ 667	$ 1,086

Stock-Based Compensation - Unre

Stock-Based Compensation - Unrecognized Stock-Based Compensation Expense (Detail) $ in Thousands	Mar. 31, 2018USD ($)
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock options	$ 4,148
Non-vested stock awards/stock units	13,218
Performance stock units	4,507
Total	$ 21,873

Earnings Per Common Share - Bas

Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 106,480	$ 84,941
Less: Preferred stock dividends	2,016	2,016
Net income available to common shareholders	104,464	82,925
Less: Earnings allocated to participating securities	702	435
Distributed earnings allocated to common stock	36,306	34,475
Undistributed earnings allocated to common stock	67,456	48,015
Net earnings allocated to common stock	$ 103,762	$ 82,490
Weighted-average shares outstanding for basic earnings per common share	63,649,198	63,738,191
Dilutive effect of stock compensation	1,012,997	999,194
Weighted-average shares outstanding for diluted earnings per common share	64,662,195	64,737,385

Defined Benefit Plans - Net Per

Defined Benefit Plans - Net Periodic Cost (Benefit) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Retirement Benefits [Abstract]
Expected return on plan assets, net of expenses	$ (2,979)	$ (2,779)
Interest cost on projected benefit obligation	1,475	1,547
Net amortization and deferral	1,250	1,357
Net periodic expense (benefit)	$ (254)	$ 125

Defined Benefit Plans Additiona

Defined Benefit Plans Additional Information (Details)	Mar. 31, 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 Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax [Abstract]
Deferred Tax Assets, Net	$ 59.6		$ 31.7
Valuation allowance	$ 0
U.S. federal statutory income tax rate, percent	21.00%	35.00%
Unrecognized tax benefits	$ 0
Deferred tax adjustment related to reduction in U.S. federal statutory income tax rate.			$ 4

Income Taxes - Income Tax Expen

Income Taxes - Income Tax Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Current income tax expense	$ 746	$ 15,702
Deferred income tax expense (benefit)	10,411	(4,301)
Income tax expense, as reported	$ 11,157	$ 11,401
Effective tax rate	9.50%	11.80%

Other Comprehensive Income (L96

Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (178,904)	$ 33,811
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(2,619)	(6,286)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	19	0
Total securities available for sale and transferred securities, before tax amount	(181,504)	27,525
Change in the net actuarial gain/loss,Before Tax Amount	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
Total defined-benefit post-retirement benefit plans	1,250	1,357
Total other comprehensive income (loss), before tax	(180,254)	28,882
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(37,570)	11,834
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(550)	(2,200)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	4	0
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(38,116)	9,634
Change in the net actuarial gain/loss,Tax Expense (Benefit)	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)	263	475
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	263	475
Total other comprehensive income (loss), Tax Expense (Benefit)	(37,853)	10,109
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(141,334)	21,977
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(2,069)	(4,086)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	15	0
Total securities available for sale and transferred securities, Net of Tax	(143,388)	17,891
Change in the net actuarial gain/loss, Net of Tax	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	987	882
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	987	882
Other comprehensive income (loss), net of tax	$ (142,401)	$ 18,773

Other Comprehensive Income - Sc

Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 79,512	$ (24,623)
Other comprehensive income (loss) before reclassifications	(143,403)	17,891
Reclassification of amounts included in net income	1,002	882
Other comprehensive income (loss), net of tax	(142,401)	18,773
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	(53,354)	(5,850)
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	117,230	16,153
Other comprehensive income (loss) before reclassifications	(143,403)	17,891
Reclassification of amounts included in net income	15	0
Other comprehensive income (loss), net of tax	(143,388)	17,891
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	(8,601)	34,044
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	987	882
Other comprehensive income (loss), net of tax	987	882
Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings	(8,022)
Ending Balance	$ (44,753)	$ (39,894)

Operating Segments - Additional

Operating Segments - Additional Information (Detail)	3 Months Ended
	Mar. 31, 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
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	$ 321,193	$ 292,209
Net income	106,480	84,941
Operating Segments [Member] | Banking [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	288,561	258,911
Net income	103,641	80,869
Operating Segments [Member] | Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	35,081	34,588
Net income	5,634	5,294
Operating Segments [Member] | Non-Banks [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	(2,449)	(1,290)
Net income	$ (2,795)	$ (1,222)

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) $ in Thousands	Mar. 31, 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	15.40%	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	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	$ 10,536,532	$ 10,489,009
Trading account securities	19,772	21,098
Derivative assets	7,444	3,810
Derivative liabilities	28,692	27,375
Interest rate swaps, caps and floors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	16,812	20,022
Derivative liabilities	22,658	18,754
Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	22,563	15,641
Derivative liabilities	22,356	15,327
Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	313	415
Derivative liabilities	208	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	3,418,147	3,445,153
Trading account securities	19,772	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	645,695	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	6,430,105	6,336,209
Trading account securities		1,888
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, estimated fair value	42,585	42,561
Level 1 [Member] | Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	313	415
Derivative liabilities	208	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	3,418,147	3,445,153
Trading account securities	19,772	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	16,812	20,022
Derivative liabilities	22,658	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	20,844	14,408
Derivative liabilities	22,356	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	645,695	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	6,430,105	6,336,209
Trading account securities		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	42,585	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	$ 1,719	$ 1,233

Fair Value Measurements - Impai

Fair Value Measurements - Impaired Loans Remeasured and Reported at Fair Value of Underlying Collateral (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	$ 131,354	$ 145,610
Specific valuation allowance allocations	(23,410)	$ (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	365		$ 0
Specific valuation allowance allocations	(64)		0
Loans receivable, fair value disclosure	301		0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	56,788		24,171
Specific valuation allowance allocations	(1,520)		(1,340)
Loans receivable, fair value disclosure	$ 55,268		$ 22,831

Fair Value Measurements - Forec

Fair Value Measurements - Foreclosed Assets Remeasured and Reported at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 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	$ 7	$ 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	7	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	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 4,680,948	$ 5,053,047	$ 4,529,173	$ 4,141,445
Securities held to maturity	1,247,154	1,432,098
Loans, net	13,214,144	12,990,301
Deposits	26,677,778	26,872,389
Federal funds purchased and repurchase agreements	1,032,221	1,147,824
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,198	136,184
Subordinated notes payable and other borrowings	98,591	98,552
Carrying Amount [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4,680,948	5,053,047
Securities held to maturity	1,247,154	1,432,098
Cash surrender value of life insurance policies	181,297	180,477
Accrued interest receivable	125,485	167,508
Deposits	26,677,778	26,872,389
Federal funds purchased and repurchase agreements	1,032,221	1,147,824
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,198	136,184
Subordinated notes payable and other borrowings	98,591	98,552
Accrued interest payable	3,196	3,358
Carrying Amount [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	13,214,144	12,990,301
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4,680,948	5,053,047
Securities held to maturity	1,257,886	1,455,791
Cash surrender value of life insurance policies	181,297	180,477
Accrued interest receivable	125,485	167,508
Deposits	26,671,005	26,866,676
Federal funds purchased and repurchase agreements	1,032,221	1,147,824
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	137,115	137,115
Subordinated notes payable and other borrowings	103,409	105,311
Accrued interest payable	3,196	3,358
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 13,184,663	$ 12,981,165