First Financial Bankshares (FFIN) 10-K 16 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Feb. 16, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	FFIN
Entity Registrant Name	FIRST FINANCIAL BANKSHARES INC
Entity Central Index Key	36,029
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		67,574,990
Entity Public Float			$ 2,780

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
ASSETS
CASH AND DUE FROM BANKS	$ 209,583	$ 204,782
FEDERAL FUNDS SOLD		3,130
INTEREST-BEARING DEPOSITS IN BANKS	162,764	48,574
Total cash and cash equivalents	372,347	256,486
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,458	1,707
SECURITIES AVAILABLE-FOR-SALE, at fair value	3,087,473	2,860,837
SECURITIES HELD-TO-MATURITY (fair value of $- and $124 at December 31, 2017 and 2016, respectively)	0	121
LOANS:
Held for investment	3,485,569	3,357,307
Less - allowance for loan losses	(48,156)	(45,779)
Net loans held for investment	3,437,413	3,311,528
Held for sale	15,130	26,898
Net loans	3,452,543	3,338,426
BANK PREMISES AND EQUIPMENT, net	124,026	122,685
INTANGIBLE ASSETS	141,143	143,603
OTHER ASSETS	75,725	86,066
Total assets	7,254,715	6,809,931
LIABILITIES AND SHAREHOLDERS' EQUITY
NONINTEREST-BEARING DEPOSITS	2,041,650	1,717,722
INTEREST-BEARING DEPOSITS	3,921,311	3,760,817
Total deposits	5,962,961	5,478,539
DIVIDENDS PAYABLE	12,589	11,897
BORROWINGS	331,000	445,770
OTHER LIABILITIES	25,397	35,840
Total liabilities	6,331,947	5,972,046
COMMITMENTS AND CONTINGENCIES
SHAREHOLDERS' EQUITY:
Common stock - $0.01 par value; authorized 120,000,000 shares; 66,260,444 and 66,094,695 shares issued at December 31, 2017 and 2016, respectively	663	661
Capital surplus	378,062	372,245
Retained earnings	517,257	446,534
Treasury stock (shares at cost: 495,964 and 507,409 at December 31, 2017 and 2016, respectively)	(7,148)	(6,671)
Deferred Compensation	7,148	6,671
Accumulated other comprehensive earnings	26,786	18,445
Total shareholders' equity	922,768	837,885
Total liabilities and shareholders' equity	$ 7,254,715	$ 6,809,931

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
SECURITIES HELD-TO-MATURITY, fair value		$ 124
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares issued	66,260,444	66,094,695
Treasury stock, shares	495,964	507,409

Consolidated Statement of Earni

Consolidated Statement of Earnings - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
INTEREST INCOME:
Interest and fees on loans	$ 166,807	$ 161,018	$ 151,662
Interest on investment securities:
Taxable	32,825	27,626	29,673
Exempt from federal income tax	44,659	43,302	40,080
Interest on federal funds sold and interest-bearing deposits in banks	1,684	342	208
Total interest income	245,975	232,288	221,623
INTEREST EXPENSE:
Interest on deposits	8,213	4,503	3,642
Other	1,075	948	446
Total interest expense	9,288	5,451	4,088
Net interest income	236,687	226,837	217,535
PROVISION FOR LOAN LOSSES	6,530	10,212	9,685
Net interest income after provision for loan losses	230,157	216,625	207,850
NONINTEREST INCOME:
Trust fees	23,694	19,636	19,252
Service charges on deposit accounts	19,416	18,386	17,171
ATM, interchange and credit card fees	25,686	23,910	21,860
Real estate mortgage operations	15,109	16,086	10,409
Net gain on sale of available-for-sale securities (includes $1,828, $1,270 and $432 for the years ended December 31, 2017, 2016 and 2015, respectively, related to accumulated comprehensive earnings reclassifications)	1,828	1,270	432
Net gain (loss) on sale of foreclosed assets	(50)	456	538
Net gain (loss) on sale of assets	(396)	168	(820)
Interest on loan recoveries	1,128	2,112	1,050
Other	4,602	3,108	3,540
Total noninterest income	91,017	85,132	73,432
NONINTEREST EXPENSE:
Salaries and employee benefits	95,287	90,739	80,999
Loss from partial settlement of pension plan		267
Net occupancy expense	10,521	10,420	10,314
Equipment expense	13,765	13,479	12,222
FDIC insurance premiums	2,217	2,680	3,153
ATM, interchange and credit card expenses	7,452	7,231	6,384
Professional and service fees	8,063	6,877	4,831
Printing, stationery and supplies	1,989	2,093	2,278
Operational and other losses	3,192	2,170	1,889
Amortization of intangible assets	613	738	561
Other	30,887	29,136	26,833
Total noninterest expense	173,986	165,830	149,464
EARNINGS BEFORE INCOME TAXES	147,188	135,927	131,818
INCOME TAX EXPENSE (includes $640, $445 and $151 for the years ended December 31, 2017, 2016 and 2015, respectively, related to income tax expense from reclassification items)	26,817	31,153	31,437
NET EARNINGS	$ 120,371	$ 104,774	$ 100,381
NET EARNINGS PER SHARE, BASIC	$ 1.82	$ 1.59	$ 1.55
NET EARNINGS PER SHARE, ASSUMING DILUTION	$ 1.81	$ 1.59	$ 1.54

Consolidated Statement of Earn5

Consolidated Statement of Earnings (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Reclassifications adjustment for realized gains on investment securities included in net earnings (loss), before income tax	$ 1,828	$ 1,270	$ 432
Income tax expense from reclassification items	$ 640	$ 445	$ 151

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Earnings - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
NET EARNINGS	$ 120,371	$ 104,774	$ 100,381
OTHER ITEMS OF COMPREHENSIVE EARNINGS (LOSS):
Change in unrealized gain (loss) on investment securities available-for-sale, before income tax	23,266	(44,679)	2,273
Reclassification adjustment for realized losses (gains) on investment securities included in net earnings, before income tax	(1,828)	(1,270)	(432)
Minimum liability pension adjustment, before income tax	257	1,410	(1,986)
Total other items of comprehensive earnings (losses)	21,695	(44,539)	(145)
Income tax benefit (expense) related to:
Investment securities	(13,774)	16,082	(644)
Minimum liability pension adjustment	420	(493)	695
Total income tax benefit (expense)	(13,354)	15,589	51
COMPREHENSIVE EARNINGS	$ 128,712	$ 75,824	$ 100,287

Consolidated Statements of Shar

Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Deferred Compensation [Member]	Accumulated Other Comprehensive Earnings (Losses) [Member]
Beginning Balance at Dec. 31, 2014	$ 681,537	$ 641	$ 305,429	$ 327,978	$ (5,878)	$ 5,878	$ 47,489
Beginning Balance, Shares at Dec. 31, 2014		64,089,921
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2014					(529,563)
NET EARNINGS	100,381			100,381
Stock issued in acquisition of FBC Bancshares, Inc.	59,648	$ 18	59,630
Stock issued in acquisition of FBC Bancshare, shares		1,755,374
Stock option exercises	1,545	$ 1	1,544
Stock option exercises, Shares		105,121
Restricted Stock grant	1,350		1,350
Restricted Stock grant, Shares		39,818
Cash dividends declared	(40,353)			(40,353)
Minimum liability pension adjustment, net of related income taxes	(1,291)						(1,291)
Change in unrealized gain (loss) in investment securities available-for-sale, net of related income taxes	1,197						1,197
Additional tax benefit related to directors' deferred compensation plan	328		328
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net					$ (418)	418
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					8,912
Stock option expense	644		644
Ending Balance at Dec. 31, 2015	804,986	$ 660	368,925	388,006	$ (6,296)	6,296	47,395
Ending Balance, Shares at Dec. 31, 2015		65,990,234
Treasury Stock, Ending Balance, Shares at Dec. 31, 2015					(520,651)
NET EARNINGS	104,774			104,774
Stock option exercises	1,260	$ 1	1,259
Stock option exercises, Shares		82,871
Restricted Stock grant	809		809
Restricted Stock grant, Shares		21,590
Cash dividends declared	(46,246)			(46,246)
Minimum liability pension adjustment, net of related income taxes	917						917
Change in unrealized gain (loss) in investment securities available-for-sale, net of related income taxes	(29,867)						(29,867)
Additional tax benefit related to directors' deferred compensation plan	370		370
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net					$ (375)	375
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					13,242
Stock option expense	882		882
Ending Balance at Dec. 31, 2016	$ 837,885	$ 661	372,245	446,534	$ (6,671)	6,671	18,445
Ending Balance, Shares at Dec. 31, 2016		66,094,695
Treasury Stock, Ending Balance, Shares at Dec. 31, 2016	(507,409)				(507,409)
NET EARNINGS	$ 120,371			120,371
Stock option exercises	2,935	$ 2	2,933
Stock option exercises, Shares		140,250
Restricted Stock grant	1,139		1,139
Restricted Stock grant, Shares		25,499
Cash dividends declared	(49,648)			(49,648)
Minimum liability pension adjustment, net of related income taxes	677						677
Change in unrealized gain (loss) in investment securities available-for-sale, net of related income taxes	7,664						7,664
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net					$ (477)	477
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					11,445
Stock option expense	1,745		1,745
Ending Balance at Dec. 31, 2017	$ 922,768	$ 663	$ 378,062	$ 517,257	$ (7,148)	$ 7,148	$ 26,786
Ending Balance, Shares at Dec. 31, 2017		66,260,444
Treasury Stock, Ending Balance, Shares at Dec. 31, 2017	(495,964)				(495,964)

Consolidated Statements of Sha8

Consolidated Statements of Shareholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash dividend per share	$ 0.75	$ 0.70	$ 0.62
Retained Earnings [Member]
Cash dividend per share	$ 0.75	$ 0.70	$ 0.62

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
NET EARNINGS	$ 120,371	$ 104,774	$ 100,381
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	12,916	11,573	11,145
PROVISION FOR LOAN LOSSES	6,530	10,212	9,685
Securities premium amortization, net	30,310	29,005	27,705
Gain on sale of assets, net	(1,167)	(1,894)	(150)
Deferred federal income tax expense (benefit)	(53)	673	320
Change in loans held for sale	11,769	6,645	(24,739)
Change in other assets	9,313	2,397	(16,919)
Change in other liabilities	285	(2,643)	1,664
Total adjustments	69,903	55,968	8,711
Net cash provided by operating activities	190,274	160,742	109,092
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash paid for asset acquisition of 4Trust Mortgage, Inc., net			(1,931)
Cash received in acquisition of FBC Bancshares, Inc., net			65,197
Net decrease in interest-bearing time deposits in banks	249	1,788	13,507
Activity in available-for-sale securities:
Sales	120,576	40,510	35,580
Maturities	4,392,131	3,509,113	2,717,724
Purchases	(4,768,420)	(3,737,865)	(3,055,117)
Activity in held-to-maturity securities - maturities	124	157	163
Net increase in loans	(134,627)	(48,836)	(144,320)
Purchases of bank premises and equipment and other assets	(14,162)	(20,399)	(17,433)
Proceeds from sale of bank premises and equipment and other assets	6,085	3,572	2,405
Net cash used in (provided by) investing activities	(398,044)	(251,960)	(384,225)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net increase (decrease) in noninterest-bearing deposits	323,928	(28,230)	23,473
Net increase in interest-bearing deposits	160,494	316,600	72,857
Net increase (decrease) in borrowings	(114,770)	(169,905)	235,440
Common stock transactions:
Proceeds from stock issuances	2,934	1,260	1,545
Dividends paid	(48,955)	(44,907)	(38,767)
Net cash provided by financing activities	323,631	74,818	294,548
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	115,861	(16,400)	19,415
CASH AND CASH EQUIVALENTS, beginning of year	256,486	272,886	253,471
CASH AND CASH EQUIVALENTS, end of year	$ 372,347	$ 256,486	$ 272,886

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	1. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES: Nature of Operations First Financial Bankshares, Inc. (a Texas corporation) (“Bankshares”, “Company”, “we” or “us”) is a financial holding company which owns all of the capital stock of one bank with 69 locations located in Texas as of December 31, 2017. The subsidiary bank is First Financial Bank, National Association, Abilene. The bank’s primary source of revenue is providing loans and banking services to consumers and commercial customers in the market area in which the subsidiary is located. In addition, the Company also owns First Financial Trust & Asset Management Company, National Association, First Financial Insurance Agency, Inc., and First Technology Services, Inc. A summary of significant accounting policies of Bankshares and its subsidiaries applied in the preparation of the accompanying consolidated financial statements follows. The accounting principles followed by the Company and the methods of applying them are in conformity with both U.S. GAAP and prevailing practices of the banking industry. The Company evaluated subsequent events for potential recognition through the date the consolidated financial statements were issued. Use of Estimates in Preparation of Financial Statements The preparation of financial statements in conformity with U.S. GAAP 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 and reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. The Company’s significant estimates include its allowance for loan losses and its valuation of financial instruments. Consolidation The accompanying consolidated financial statements include the accounts of Bankshares and its subsidiaries, all of which are wholly-owned. All significant intercompany accounts and transactions have been eliminated. Stock Repurchase On June 25, 2017, the Company’s Board of Directors authorized the repurchase of up to 2,000,000 common shares through September 30, 2020. Previously, the Board of Directors had authorized the repurchase of up to 1,500,000 common shares through September 30, 2017. The stock buyback plan authorizes management to repurchase the stock at such time as repurchases are considered beneficial to stockholders. Any repurchase of stock will be made through the open market, block trades or in privately negotiated transactions in accordance with applicable laws and regulations. Under the repurchase plan, there is no minimum number of shares that the Company is required to repurchase. For the years ended December 31, 2017, 2016 and 2015, no shares were repurchased under this or the prior authorization that expired September 30, 2017. Acquisitions and Asset Purchase On July 31, 2015, the Company acquired 100% of the outstanding capital stock of FBC Bancshares, Inc. through the merger of a wholly-owned subsidiary with and into FBC Bancshares, Inc. Following such merger, FBC Bancshares, Inc. and its wholly-owned subsidiary, First Bank, N.A. were merged into the Company and First Financial Bank, National Association, respectively. The results of operations of FBC Bancshares, Inc. subsequent to the acquisition date, are included in the consolidated earnings of the Company. See Note 18 for additional information. On June 1, 2015, the Company completed the asset purchase of 4Trust Mortgage, Inc. The results of operation of 4Trust Mortgage Inc. subsequent to the asset purchase date, are included in the consolidated earnings of the Company. See Note 18 for additional information. Increase in Authorized Shares On April 28, 2015, the Company’s shareholders approved an amendment to the Company’s Amended and Restated Certificate of Formation to increase the number of authorized common shares to 120,000,000. Investment Securities Management classifies debt and equity securities as held-to-maturity, available-for-sale, held-to-maturity held-to-maturity available-for-sale Available-for-sale available-for-sale The Company records its available-for-sale When the fair value of a security is below its amortized cost, and depending on the length of time the condition exists and the extent the fair value is below amortized cost, additional analysis is performed to determine whether an other-than-temporary impairment condition exists. Available-for-sale held-to-maturity The Company’s investment portfolio consists of U.S. Treasury securities, obligations of U.S. government sponsored enterprises and agencies, obligations of state and political subdivisions, mortgage pass-through securities, corporate bonds and general obligation or revenue based municipal bonds. Pricing for such securities is generally readily available and transparent in the market. The Company utilizes independent third party pricing services to value its investment securities, which the Company reviews as well as the underlying pricing methodologies for reasonableness and to ensure such prices are aligned with pricing matrices. The Company validates quarterly, on a sample basis, prices supplied by the independent pricing services by comparison to prices obtained from other third party sources. Loans and Allowance for Loan Losses Loans held for investment are stated at the amount of unpaid principal, reduced by unearned income and an allowance for loan losses. Interest on loans is calculated by using the simple interest method on daily balances of the principal amounts outstanding. The Company defers and amortizes net loan origination fees and costs as an adjustment to yield. The allowance for loan losses is established through a provision for loan losses charged to expense. Loans are charged against the allowance for loan losses when management believes the collectability of the principal is unlikely. The allowance for loan losses is an amount which represents management’s best estimate of probable losses that are inherent in the Company’s loan portfolio as of the balance sheet date. The allowance for loan losses is comprised of three elements: (i) specific reserves determined based on probable losses on specific classified loans; (ii) a historical valuation reserve component that considers historical loss rates and estimated loss emergence periods; and (iii) qualitative reserves based upon general economic conditions and other qualitative risk factors both internal and external to the Company. The allowance for loan losses is increased by charges to income and decreased by charge-offs Although we believe we use the best information available to make loan loss allowance determinations, future adjustments could be necessary if circumstances or economic conditions differ substantially from the assumptions used in making our initial determinations. A decline in the economy and employment could result in increased levels of non-performing Accrual of interest is discontinued on a loan and payments are applied to principal when management believes, after considering economic and business conditions and collection efforts, the borrower’s financial condition is such that collection of interest is doubtful. Except consumer loans, generally all loans past due greater than 90 days, based on contractual terms, are placed on non-accrual. charged-off charge-off Loans are considered impaired when, based on current information and events, management determines that it is probable we will be unable to collect all amounts due in accordance with the loan agreement, including scheduled principal and interest payments. If a loan is impaired, a specific valuation allowance is allocated, if necessary. Interest payments on impaired loans are typically applied to principal unless collectability of the principal amount is reasonably assured, in which case interest is recognized on a cash basis. Impaired loans, or portions thereof, are charged off when deemed uncollectable. The Company’s policy requires measurement of the allowance for an impaired, collateral dependent loan based on the fair value of the collateral less cost to sell. Other loan impairments for non-collateral From time to time, the Company modifies its loan agreement with a borrower. A modified loan is considered a troubled debt restructuring when two conditions are met: (i) the borrower is experiencing financial difficulty and (ii) concessions are made by the Company that would not otherwise be considered for a borrower with similar credit risk characteristics. Modifications to loan terms may include a lower interest rate, a reduction of principal, or a longer term to maturity. For all impaired loans, including the Company’s troubled debt restructurings, the Company performs a periodic, well-documented credit evaluation of the borrower’s financial condition and prospects for repayment to assess the likelihood that all principal and interest payments required under the terms of the agreement will be collected in full. When doubt exists about the ultimate collectability of principal and interest, the troubled debt restructuring remains on non-accrual non-accrual The Company originates certain mortgage loans for sale in the secondary market. Accordingly, these loans are classified as held-for-sale Loans acquired, including loans acquired in a business combination, are initially recorded at fair value with no valuation allowance. Acquired loans are segregated between those considered to be credit impaired and those deemed performing. To make this determination, management considers such factors as past due status, non-accrual Purchased credit impaired loans are those loans that showed evidence of deterioration of credit quality since origination and for which it is probable, at acquisition, that the Company will be unable to collect all amounts contractually owed. Their acquisition fair value, which includes a credit component at the acquisition date, was based on the estimate of cash flows, both principal and interest, expected to be collected or estimated collateral values if cash flows are not estimable, discounted at prevailing market rates of interest. The difference between the discounted cash flows expected at acquisition and the investment in the loan is recognized as interest income on a level-yield method over the life of the loan, unless management was unable to reasonably forecast cash flows in which case the loans were placed on nonaccrual. Subsequent to the acquisition date, increases in expected cash flows will generally result in a recovery of any previously recorded allowance for loan loss, to the extent applicable, and/or a reclassification from the nonaccretable difference to accretable yield, which will be recognized prospectively. Decreases in expected cash flows subsequent to acquisition are recognized as impairment. Valuation allowances on these impaired loans reflect only losses incurred after the acquisition. The carrying amount of purchased credit impaired loans at December 31, 2017 and 2016 were $618,000 and $1,256,000, respectively, compared to a contractual balance of $755,000 and $1,865,000, respectively. Other purchased credit impaired loan disclosures were omitted due to immateriality. Other Real Estate Other real estate owned is foreclosed property held pending disposition and is initially recorded at fair value, less estimated costs to sell. At foreclosure, if the fair value of the real estate, less estimated costs to sell, is less than the Company’s recorded investment in the related loan, a write-down is recognized through a charge to the allowance for loan losses. Any subsequent reduction in value is recognized by a charge to income. Operating and holding expenses of such properties, net of related income, and gains and losses on their disposition are included in net gain (loss) on sale of foreclosed assets as incurred. Bank Premises and Equipment Bank premises and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization are computed principally on a straight-line basis over the estimated useful lives of the related assets. Leasehold improvements are amortized over the life of the respective lease or the estimated useful lives of the improvements, whichever is shorter. Business Combinations, Goodwill and Other Intangible Assets The Company accounts for all business combinations under the purchase method of accounting. Tangible and intangible assets and liabilities of the acquired entity are recorded at fair value. Intangible assets with finite useful lives represent the future benefit associated with the acquisition of the core deposits and are amortized over seven years, utilizing a method that approximates the expected attrition of the deposits. Goodwill with an indefinite life is not amortized, but rather tested annually for impairment as of June 30 each year and totaled $139,971,000 at both December 31, 2017 and 2016. There was no impairment recorded for the years ended December 31, 2017, 2016 and 2015. The carrying amount of goodwill arising from acquisitions that qualify as an asset purchase for federal income tax purposes was $74,376,000 both at December 31, 2017 and 2016, and is deductible for federal income tax purposes. Also included in other intangible assets are mortgage servicing rights totaling $1,795,000 at December 31, 2016. These servicing rights were sold to an unrelated third party in 2017 resulting in a loss on sale of approximately $215,000. Securities Sold Under Agreements To Repurchase Securities sold under agreements to repurchase, which are classified as borrowings, generally mature within one to four days from the transaction date. Securities sold under agreements to repurchase are reflected at the amount of the cash received in connection with the transaction. The Company may be required to provide additional collateral based on the estimated fair value of the underlying securities. Segment Reporting The Company has determined that its banking regions meet the aggregation criteria of the current authoritative accounting guidance since each of its banking regions offer similar products and services, operate in a similar manner, have similar customers and report to the same regulatory authority, and therefore operate one line of business (community banking) located in a single geographic area (Texas). Statements of Cash Flows For purposes of reporting cash flows, cash and cash equivalents includes cash on hand, amounts due from banks, including interest-bearing deposits in banks with original maturity of 90 days or less, and federal funds sold. Accumulated Other Comprehensive Income (Loss) Unrealized net gains on the Company’s available-for-sale Income Taxes The Company’s provision for income taxes is based on income before income taxes adjusted for permanent differences between financial reporting and taxable income. Deferred tax assets and liabilities are determined using the liability (or balance sheet) method. Under this method, the net deferred tax asset or liability is determined based on the tax effects of the temporary differences between the book and tax bases of the various balance sheet assets and liabilities and gives current recognition to changes in tax rates and laws. Stock Based Compensation The Company grants stock options for a fixed number of shares to employees with an exercise price equal to the fair value of the shares at the grant date. The Company recorded stock option expense totaling $1,745,000, $882,000, and $644,000 for the years ended December 31, 2017, 2016 and 2015, respectively. The Company also grants restricted stock for a fixed number of shares. The Company recorded expenses associated with its director and officer restricted stock grants totaling $483,000 and $562,000, respectively, for the year ended December 31, 2017, $278,000 and $381,000, respectively, for the year ended December 31, 2016, and $139,000 and $62,000, respectively, for the year ended December 31, 2015. See Note 15 for further information. Advertising Costs Advertising costs are expensed as incurred. Per Share Data Net earnings per share (“EPS”) are computed by dividing net earnings by the weighted average number of common stock shares outstanding during the period. The Company calculates dilutive EPS assuming all outstanding stock options to purchase common stock have been exercised at the beginning of the year (or the time of issuance, if later.) The dilutive effect of the outstanding options and restricted stock is reflected by application of the treasury stock method, whereby the proceeds from the exercised options and restricted stock are assumed to be used to purchase common stock at the average market price during the respective year. For the year ended December 31, 2015, 442,368 weighted average outstanding stock options were anti-dilutive and have been excluded from the EPS calculations. There were no such anti-dilutive stock options for the years ended December 31, 2017 and 2016. The following table reconciles the computation of basic EPS to dilutive EPS: Net Earnings Weighted Per Share For the year ended December 31, 2017: Net earnings per share, basic $ 120,371 66,126,863 $ 1.82 Effect of stock options and stock grants — 197,467 (0.01 ) Net earnings per share, assuming dilution $ 120,371 66,324,330 $ 1.81 For the year ended December 31, 2016: Net earnings per share, basic $ 104,774 66,013,004 $ 1.59 Effect of stock options and stock grants — 89,882 — Net earnings per share, assuming dilution $ 104,774 66,102,886 $ 1.59 For the year ended December 31, 2015: Net earnings per share, basic $ 100,381 64,892,934 $ 1.55 Effect of stock options and stock grants — 175,096 (0.01 ) Net earnings per share, assuming dilution $ 100,381 65,068,030 $ 1.54 Recently Issued Authoritative Accounting Guidance Accounting Standards Update (“ASU”) 2014-09, 2014-09 2015-4 2014-09 2014-09, non-interest ASU 2014-15, 2014-15 2014-15 ASU 2015-01, 2015-01 ASU 2015-05, Internal-Use 2015-05 2015-05 2015-05 ASU 2015-16, 2015-16 ASU 2016-1, 2016-1, available-for-sale 2016-1 ASU 2016-02, 2016-02 right-of-use 2016-02 ASU 2016-09, 2016-09 2016-09 2016-09 ASU 2016-13, 2016-13 held-to-maturity 2016-13 2016-13 ASU 2017-04, 2017-04 2017-04 ASU 2017-07, 2017-17 2017-17 ASU 2017-08, 2017-08 2017-08 2017-08 ASU 2018-02, “Income Statement — Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income.”

Interest-bearing Time Deposits

Interest-bearing Time Deposits in Banks and Securities	12 Months Ended
	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Interest-bearing Time Deposits in Banks and Securities	2. INTEREST-BEARING TIME DEPOSITS IN BANKS AND SECURITIES: Interest-bearing time deposits in banks totaled $1,458,000 and $1,707,000 at December 31, 2017 and 2016, respectively, and have original maturities generally ranging from one to three years. A summary of the Company’s available-for-sale December 31, 2017 Amortized Gross Gross Estimated Securities available-for-sale: Obligations of U.S. government sponsored enterprises and agencies $ 60,516 $ — $ (186 ) $ 60,330 Obligations of state and political subdivisions 1,369,295 52,491 (936 ) 1,420,850 Corporate bonds and other 11,421 43 (5 ) 11,459 Residential mortgage-backed securities 1,223,452 4,561 (8,916 ) 1,219,097 Commercial mortgage-backed securities 377,934 263 (2,460 ) 375,737 Total securities available-for-sale $ 3,042,618 $ 57,358 $ (12,503 ) $ 3,087,473 December 31, 2016 Amortized Gross Gross Estimated Securities available-for-sale: U.S. Treasury securities $ 10,649 $ 19 $ — $ 10,668 Obligations of U.S. government sponsored enterprises and agencies 113,450 253 — 113,703 Obligations of state and political subdivisions 1,534,095 40,194 (10,013 ) 1,564,276 Corporate bonds and other 51,920 476 (3 ) 52,393 Residential mortgage-backed securities 848,614 8,260 (5,513 ) 851,361 Commercial mortgage-backed securities 269,044 622 (1,230 ) 268,436 Total securities available-for-sale $ 2,827,772 $ 49,824 $ (16,759 ) $ 2,860,837 Disclosures related to the Company’s held-to-maturity held-to-maturity The Company invests in mortgage-backed securities that have expected maturities that differ from their contractual maturities. These differences arise because borrowers may have the right to call or prepay obligations with or without a prepayment penalty. These securities include collateralized mortgage obligations (CMOs) and other asset backed securities. The expected maturities of these securities at December 31, 2017, were computed by using scheduled amortization of balances and historical prepayment rates. At December 31, 2017 and 2016, the Company did not hold any CMOs that entail higher risks than standard mortgage-backed securities. The amortized cost and estimated fair value of available-for-sale Amortized Estimated Due within one year $ 184,538 $ 185,547 Due after one year through five years 631,376 654,092 Due after five years through ten years 623,628 650,923 Due after ten years 1,690 2,077 Mortgage-backed securities 1,601,386 1,594,834 Total $ 3,042,618 $ 3,087,473 The following tables disclose, as of December 31, 2017 and 2016, the Company’s investment securities that have been in a continuous unrealized-loss Less than 12 Months 12 Months or Longer Total December 31, 2017 Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized Obligations of U.S. government sponsored enterprises and agencies $ 60,329 $ 186 $ — $ — $ 60,329 $ 186 Obligations of state and political subdivisions 66,361 219 44,938 717 111,299 936 Corporate bonds and other 224 2 237 3 461 5 Residential mortgage-backed securities 701,252 3,988 239,641 4,928 940,893 8,916 Commercial mortgage-backed securities 239,548 1,500 92,549 960 332,097 2,460 Total $ 1,067,714 $ 5,895 $ 377,365 $ 6,608 $ 1,445,079 $ 12,503 Less than 12 Months 12 Months or Longer Total December 31, 2016 Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized Obligations of state and political subdivisions $ 446,052 $ 9,997 $ 1,209 $ 16 $ 447,261 $ 10,013 Corporate bonds and other 244 3 — — 244 3 Residential mortgage-backed securities 372,331 4,532 33,227 981 405,558 5,513 Commercial mortgage-backed securities 193,495 1,180 13,263 50 206,758 1,230 Total $ 1,012,122 $ 15,712 $ 47,699 $ 1,047 $ 1,059,821 $ 16,759 The number of investments in an unrealized loss position totaled 338 at December 31, 2017. We do not believe these unrealized losses are “other-than-temporary”. In estimating other-than-temporary impairment losses, management considers, among other things, the length of time and the extent to which the fair value has been less than cost and the financial condition and near-term prospects of the issuer. Additionally management does not (i) have the intent to sell our securities prior to recovery and/or maturity and, (ii) it is more likely than not that we will not have to sell our securities prior to recovery and/or maturity and (iii) that the length of time and extent that fair value has been less than cost is not indicative of recoverability. The unrealized losses noted are interest rate related due to the level of interest rates at December 31, 2017 compared to the time of purchase. We have reviewed the ratings of the issuers and have not identified any issues related to the ultimate repayment of principal as a result of credit concerns on these securities. Our mortgage related securities are backed by GNMA, FNMA and FHLMC or are collateralized by securities backed by these agencies. At December 31, 2017, 82.98% of our available-for-sale Securities, carried at approximately $2,018,420,000 and $1,940,460,000 at December 31, 2017 and 2016, respectively, were pledged as collateral for public or trust fund deposits, repurchase agreements and for other purposes required or permitted by law. During 2017, 2016 and 2015, sales of investment securities that were classified as available-for-sale

Loans and Allowance for Loan Lo

Loans and Allowance for Loan Losses	12 Months Ended
	Dec. 31, 2017
Receivables [Abstract]
Loans and Allowance for Loan Losses	3. LOANS AND ALLOWANCE FOR LOAN LOSSES: Loans held-for-investment December 31, 2017 2016 Commercial $ 684,099 $ 674,410 Agricultural 94,543 84,021 Real estate 2,302,998 2,189,844 Consumer 403,929 409,032 Total loans held-for-investment $ 3,485,569 $ 3,357,307 Loans held-for-sale The Company’s non-accrual December 31, 2017 2016 Non-accrual $ 17,670 $ 27,371 Loans still accruing and past due 90 days or more 288 284 Troubled debt restructured loans** 627 701 Total $ 18,585 $ 28,356 * Includes $618,000 and $1,256,000, respectively, of purchased credit impaired loans as of December 31, 2017 and 2016. ** Our troubled debt restructured loans of $4,629,000 and $6,863,000, whose interest collection, after considering economic and business conditions and collection efforts, is doubtful are included in non-accrual The Company’s recorded investment in impaired loans and the related valuation allowance are as follows (in thousands): December 31, 2017 December 31, 2016 Recorded Investment Valuation Allowance Recorded Investment Valuation Allowance $17,670 $ 3,996 $ 27,371 $ 5,012 The Company had $20,117,000 and $29,000,000 in non-accrual, Non-accrual December 31, 2017 2016 Commercial $ 3,612 $ 7,284 Agricultural 134 99 Real Estate 12,838 18,754 Consumer 1,086 1,234 Total $ 17,670 $ 27,371 No significant additional funds are committed to be advanced in connection with impaired loans as of December 31, 2017. The Company’s impaired loans and related allowance as of December 31, 2017 and 2016 are summarized in the following tables by class of financing receivables (in thousands). No interest income was recognized on impaired loans subsequent to their classification as impaired. December 31, 2017 Unpaid Recorded Recorded Total Related 12 Month Commercial $ 5,597 $ 518 $ 3,094 $ 3,612 $ 1,194 $ 4,849 Agricultural 147 — 134 134 31 120 Real Estate 16,823 2,348 10,490 12,838 2,316 13,835 Consumer 1,284 143 943 1,086 455 1,258 Total $ 23,851 $ 3,009 $ 14,661 $ 17,670 $ 3,996 $ 20,062 * Includes $618,000 of purchased credit impaired loans. December 31, 2016 Unpaid Recorded Recorded Total Related 12 Month Commercial $ 13,389 $ 1,148 $ 6,136 $ 7,284 $ 2,128 $ 4,921 Agricultural 103 — 99 99 25 50 Real Estate 23,466 6,229 12,525 18,754 2,428 16,170 Consumer 1,421 280 954 1,234 431 914 Total $ 38,379 $ 7,657 $ 19,714 $ 27,371 $ 5,012 $ 22,055 * Includes $1,256,000 of purchased credit impaired loans. The Company recognized interest income on impaired loans prior to being recognized as impaired of approximately $624,000, $829,000 and $922,000 during the years ended December 31, 2017, 2016 and 2015, respectively. From a credit risk standpoint, the Company rates its loans in one of four categories: (i) pass, (ii) special mention, (iii) substandard or (iv) doubtful. Loans rated as loss are charged-off. The ratings of loans reflect a judgment about the risks of default and loss associated with the loan. The Company reviews the ratings on our credits as part of our on-going Credits rated special mention show clear signs of financial weaknesses or deterioration in credit worthiness, however, such concerns are not so pronounced that the Company generally expects to experience significant loss within the short-term. Such credits typically maintain the ability to perform within standard credit terms and credit exposure is not as prominent as credits rated more harshly. Credits rated substandard are those in which the normal repayment of principal and interest may be, or has been, jeopardized by reason of adverse trends or developments of a financial, managerial, economic or political nature, or important weaknesses exist in collateral. A protracted workout on these credits is a distinct possibility. Prompt corrective action is therefore required to strengthen the Company’s position, and/or to reduce exposure and to assure that adequate remedial measures are taken by the borrower. Credit exposure becomes more likely in such credits and a serious evaluation of the secondary support to the credit is performed. Credits rated doubtful are those in which full collection of principal appears highly questionable, and which some degree of loss is anticipated, even though the ultimate amount of loss may not yet be certain and/or other factors exist which could affect collection of debt. Based upon available information, positive action by the Company is required to avert or minimize loss. Credits rated doubtful are generally also placed on non-accrual. The following summarizes the Company’s internal ratings of its loans held-for-investment December 31, 2017 Pass Special Substandard Doubtful Total Commercial $ 649,166 $ 6,282 $ 28,651 $ — $ 684,099 Agricultural 90,457 1,527 2,559 — 94,543 Real Estate 2,227,302 29,089 46,607 — 2,302,998 Consumer 401,434 181 2,314 — 403,929 Total $ 3,368,359 $ 37,079 $ 80,131 $ — $ 3,485,569 December 31, 2016 Pass Special Substandard Doubtful Total Commercial $ 629,756 $ 5,769 $ 38,885 $ — $ 674,410 Agricultural 81,620 715 1,686 — 84,021 Real Estate 2,111,947 18,091 59,806 — 2,189,844 Consumer 406,182 212 2,638 — 409,032 Total $ 3,229,505 $ 24,787 $ 103,015 $ — $ 3,357,307 At December 31, 2017 and 2016, the Company’s past due loans are as follows (in thousands): December 31, 2017 15-59 60-89 Greater Total Total Total Loans Total 90 Commercial $ 2,039 $ 1,104 $ 1,081 $ 4,224 $ 679,875 $ 684,099 $ 7 Agricultural 640 — — 640 93,903 94,543 — Real Estate 12,308 511 1,198 14,017 2,288,981 2,302,998 216 Consumer 1,360 361 135 1,856 402,073 403,929 65 Total $ 16,347 $ 1,976 $ 2,414 $ 20,737 $ 3,464,832 $ 3,485,569 $ 288 December 31, 2016 15-59 60-89 Greater Total Total Total Loans Total 90 Commercial $ 3,908 $ 1,122 $ 2,220 $ 7,250 $ 667,160 $ 674,410 $ 10 Agricultural 185 — — 185 83,836 84,021 — Real Estate 13,172 1,301 5,268 19,741 2,170,103 2,189,844 272 Consumer 1,845 368 122 2,335 406,697 409,032 2 Total $ 19,110 $ 2,791 $ 7,610 $ 29,511 $ 3,327,796 $ 3,357,307 $ 284 * The Company monitors commercial, agricultural and real estate loans after such loans are 15 days past due. Consumer loans are monitored after such loans are 30 days past due. The following table details the allowance for loan losses at December 31, 2017 and 2016 by portfolio segment (in thousands). There were no allowances for purchased credit impaired loans at December 31, 2017 or 2016. Allocation of a portion of the allowance to one category of loans does not preclude its availability to absorb losses in other categories. December 31, 2017 Commercial Agricultural Real Consumer Total Loans individually evaluated for impairment $ 1,194 $ 31 $ 2,316 $ 455 $ 3,996 Loan collectively evaluated for impairment 9,671 1,274 27,580 5,635 44,160 Total $ 10,865 $ 1,305 $ 29,896 $ 6,090 $ 48,156 December 31, 2016 Commercial Agricultural Real Consumer Total Loans individually evaluated for impairment $ 2,128 $ 25 $ 2,428 $ 431 $ 5,012 Loan collectively evaluated for impairment 9,579 1,076 24,436 5,676 40,767 Total $ 11,707 $ 1,101 $ 26,864 $ 6,107 $ 45,779 Changes in the allowance for loan losses for the years ended December 31, 2017 and 2016 are summarized as follows (in thousands): December 31, 2017 Commercial Agricultural Real Consumer Total Beginning balance $ 11,707 $ 1,101 $ 26,864 $ 6,107 $ 45,779 Provision for loan losses 1,233 243 4,055 999 6,530 Recoveries 943 32 192 501 1,668 Charge-offs (3,018 ) (71 ) (1,215 ) (1,517 ) (5,821 ) Ending balance $ 10,865 $ 1,305 $ 29,896 $ 6,090 $ 48,156 December 31, 2016 Commercial Agricultural Real Consumer Total Beginning balance $ 12,644 $ 1,191 $ 24,375 $ 3,667 $ 41,877 Provision for loan losses 5,101 104 1,150 3,857 10,212 Recoveries 952 25 2,021 508 3,506 Charge-offs (6,990 ) (219 ) (682 ) (1,925 ) (9,816 ) Ending balance $ 11,707 $ 1,101 $ 26,864 $ 6,107 $ 45,779 The Company’s recorded investment in loans as of December 31, 2017 and 2016 related to the balance in the allowance for loan losses on the basis of the Company’s impairment methodology was as follows (in thousands). Purchased credit impaired loans of $618,000 and $1,256,000, respectively, at December 31, 2017 and 2016 are included in loans individually evaluated for impairment. December 31, 2017 Commercial Agricultural Real Estate Consumer Total Loans individually evaluated for impairment $ 3,612 $ 134 $ 12,838 $ 1,086 $ 17,670 Loan collectively evaluated for impairment 680,487 94,409 2,290,160 402,843 3,467,899 Total $ 684,099 $ 94,543 $ 2,302,998 $ 403,929 $ 3,485,569 December 31, 2016 Commercial Agricultural Real Estate Consumer Total Loans individually evaluated for impairment $ 7,284 $ 99 $ 18,754 $ 1,234 $ 27,371 Loan collectively evaluated for impairment 667,126 83,922 2,171,090 407,798 3,329,936 Total $ 674,410 $ 84,021 $ 2,189,844 $ 409,032 $ 3,357,307 The Company’s loans that were modified in the years ended December 31, 2017 and 2016, and considered troubled debt restructurings are as follows (in thousands): Year Ended December 31, 2017 Year Ended December 31, 2016 Number Pre-Modification Post- Number Pre-Modification Post- Commercial 11 $ 895 $ 895 15 $ 3,208 $ 3,208 Agricultural — — — — — — Real Estate 5 625 625 6 1,460 1,460 Consumer 1 25 25 7 189 189 Total 17 $ 1,545 $ 1,545 28 $ 4,857 $ 4,857 The balances below provide information as to how the loans were modified as troubled debt restructured loans during the years ended December 31, 2017 and 2016 (in thousands): Year Ended December 31, 2017 Year Ended December 31, 2016 Adjusted Extended Combined Adjusted Extended Combined Commercial $ — $ 195 $ 700 $ — $ 2,560 $ 648 Agricultural — — — — — — Real Estate — 312 313 — 298 1,162 Consumer — 25 — — 70 119 Total $ — $ 532 $ 1,013 $ — $ 2,928 $ 1,929 During the years ended December 31, 2017 and 2016, certain loans were modified as a troubled debt restructured loans within the previous 12 months and for which there was a payment default. A default for purposes of this disclosure is a troubled debt restructured loan in which the borrower is 90 days past or more due or results in the foreclosure and repossession of the applicable collateral. The loans with payment default are as follows (dollars in thousands): Year Ended Year Ended Number Balance Number Balance Commercial 2 $ 88 4 $ 1,690 Agriculture — — — — Real Estate — — 3 921 Consumer — — — — Total 2 $ 88 7 $ 2,611 As of December 31, 2017, the Company has no commitments to lend additional funds to loan customers whose terms have been modified in troubled debt restructurings. An analysis of the changes in loans to officers, directors, principal shareholders, or associates of such persons for the year ended December 31, 2017 (determined as of each respective year-end) Beginning Additional Payments Ending Year ended December 31, 2017 $44,429 $58,420 $46,945 $55,904 In the opinion of management, those loans are on substantially the same terms, including interest rates and collateral requirements, as those prevailing at the time for comparable transactions with unaffiliated persons. Our subsidiary bank has established a line of credit with the Federal Home Loan Bank of Dallas (FHLB) to provide liquidity and meet pledging requirements for those customers eligible to have securities pledged to secure certain uninsured deposits. At December 31, 2017, $2,163,425,000 in loans held by our bank subsidiary were subject to blanket liens as security for this line of credit. At December 31, 2017, there was no balance outstanding under this line of credit.

Bank Premises and Equipment

Bank Premises and Equipment	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Bank Premises and Equipment	4. BANK PREMISES AND EQUIPMENT The following is a summary of bank premises and equipment (in thousands): Useful Life December 31, 2017 2016 Land — $ 29,508 $ 28,266 Buildings 20 to 40 years 119,728 115,566 Furniture and equipment 3 to 10 years 58,672 58,145 Leasehold improvements Lesser of lease term or 5 to 15 years 4,118 4,783 212,026 206,760 Less-accumulated depreciation and amortization (88,000 ) (84,075 ) Total Bank Premises and Equipment $ 124,026 $ 122,685 Depreciation expense for the years ended December 31, 2017, 2016 and 2015 amounted to $9,810,000, $9,390,000, and $9,125,000, respectively, and is included in the captions net occupancy expense and equipment expense in the accompanying consolidated statements of earnings. The Company is lessor for portions of its banking premises. Total rental income for all leases included in net occupancy expense is approximately $2,367,000, $2,139,000 and $1,949,000, for the years ended December 31, 2017, 2016 and 2015, respectively. During the years ended December 31, 2017, 2016 and 2015, the Company recorded gains (losses) on sale of the bank premises and equipment totaling ($396,000), $168,000 and ($820,000). In 2017, the Company sold its San Angelo main region branch building for $1,586,000 and recorded a gain of $210,000 and cancelled its San Angelo grocery store branch lease and recorded a write off of leasehold improvements of $360,000. In 2016, the Company sold its Weatherford and Orange main region branch building for $1,385,000 and $2,000,000 and recorded a gain of $560,000 and a loss of $31,000, respectively. The Company recorded a write down of $1,000,000 in 2015 in anticipation of the Orange branch building sale.

Deposits and Borrowings

Deposits and Borrowings	12 Months Ended
	Dec. 31, 2017
Banking and Thrift [Abstract]
Deposits and Borrowings	5. DEPOSITS AND BORROWINGS: Time deposits of $250,000 or more totaled approximately $115,203,000 and $130,385,000 at December 31, 2017 and 2016, respectively. At December 31, 2017, the scheduled maturities of time deposits (in thousands) were, as follows: Year ending December 31, 2018 $ 384,668 2019 38,031 2020 14,607 2021 6,645 2022 7,304 Thereafter — $ 451,255 Deposits received from related parties at December 31, 2017 and 2016 totaled $74,270,000 and $114,513,000, respectively. Borrowings at December 31, 2017 and 2016 consisted of the following (dollars in thousands): December 31, 2017 2016 Securities sold under agreements with customers to repurchase $ 320,450 $ 360,820 Federal funds purchased 10,550 9,950 Advances from Federal Home Loan Bank of Dallas — 75,000 Total $ 331,000 $ 445,770 Securities sold under repurchase agreements are generally with significant customers of the Company that require short-term liquidity for their funds for which the Company pledges certain securities that have a fair value equal to at least the amount of the borrowings. The agreements mature daily and therefore the risk arising from a decline in the fair value of the collateral pledged is minimal. The securities pledged are mortgage-backed securities. These agreements do not include “right of set-off” At December 31, 2016, the Company had advances from the Federal Home Loan Bank of Dallas of $75,000,000 that were repaid in 2017. The interest rate on this advance was 0.46% at December 31, 2016. There were no such advances outstanding at December 31, 2017.

Line of Credit

Line of Credit	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Line of Credit	6. LINE OF CREDIT: The Company renewed its loan agreement, effective June 30, 2017, with Frost Bank. Under the loan agreement, as renewed and amended, we are permitted to draw up to $25,000,000 on a revolving line of credit. Prior to June 30, 2019, interest is paid quarterly at The Wall Street Journal The Wall Street Journal non-performing

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	7. INCOME TAXES: On December 22, 2017 the Tax Cuts and Jobs Act was signed into law with sweeping modifications to the Internal Revenue Code. The primary change for the Company was to lower the corporate income tax rate to 21% from 35%. The Company’s deferred tax assets and liabilities were re-measured re-measurement The Company files a consolidated federal income tax return. Income tax expense is comprised of the following (dollars in thousands): Year Ended December 31, 2017 2016 2015 Current federal income tax $ 34,421 $ 30,381 $ 31,014 Current state income tax 99 99 103 Deferred federal income tax expense (benefit) (53 ) 673 320 Restatement of net deferred tax liability due to change in income tax rate (7,650 ) — — Income tax expense $ 26,817 $ 31,153 $ 31,437 Income tax expense, as a percentage of pretax earnings, differs from the statutory federal income tax rate as follows: As a Percent of Pretax 2017 2016 2015 Statutory federal income tax rate 35.0 % 35.0 % 35.0 % Restatement of net deferred tax liability due to change in income tax rate (5.3 ) — — Reductions in tax rate resulting from interest income exempt from federal income tax (11.5 ) (12.1 ) (11.4 ) Effect of state income tax 0.1 0.1 0.1 ESOP tax deduction (0.2 ) (0.2 ) (0.2 ) Other 0.1 0.1 0.3 Effective income tax rate 18.2 % 22.9 % 23.8 % The approximate effects of each type of difference that gave rise to the Company’s deferred tax assets and liabilities at December 31, 2017 and 2016 are as follows (dollars in thousands): 2017 2016 Deferred tax assets: Tax basis of loans in excess of financial statement basis $ 10,550 $ 17,006 Minimum liability in defined benefit plan 766 1,641 Recognized for financial reporting purposes but not yet for tax purposes: Deferred compensation 1,818 2,807 Write-downs and adjustments to other real estate owned and repossessed assets 11 9 Other deferred tax assets 79 226 Total deferred tax assets $ 13,224 $ 21,689 Deferred tax liabilities: Financial statement basis of fixed assets in excess of tax basis $ 3,343 5,870 Intangible asset amortization deductible for tax purposes, but not for financial reporting purposes 9,926 15,191 Recognized for financial reporting purposes but not yet for tax purposes: Accretion on investment securities 1,039 1,788 Pension plan contributions 1,086 1,799 Net unrealized gain on investment securities available-for-sale 9,420 11,573 Other deferred tax liabilities 31 83 Total deferred tax liabilities $ 24,845 $ 36,304 Net deferred tax asset (liability) $ (11,621 ) $ (14,615 ) At December 31, 2017 and 2016, management believes that it is more likely than not that all of the deferred tax amounts shown above will be realized and therefore no valuation allowance was recorded. Current authoritative accounting guidance prescribes a more-likely-than-not more-likely-than-not more-likely-than-not

Fair Value Disclosures

Fair Value Disclosures	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Disclosures	8. FAIR VALUE DISCLOSURES: The authoritative accounting guidance for fair value measurements defines fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. A fair value measurement assumes that the transaction to sell the asset or transfer the liability occurs in the principal market for the asset or liability or, in the absence of a principal market, the most advantageous market for the asset or liability. The price in the principal (or most advantageous) market used to measure the fair value of the asset or liability shall not be adjusted for transaction costs. An orderly transaction is a transaction that assumes exposure to the market for a period prior to the measurement date to allow for marketing activities that are usual and customary for transactions involving such assets and liabilities; it is not a forced transaction. Market participants are buyers and sellers in the principal market that are (i) independent, (ii) knowledgeable, (iii) able to transact, and (iv) willing to transact. The authoritative accounting guidance requires the use of valuation techniques that are consistent with the market approach, the income approach and/or the cost approach. The market approach uses prices and other relevant information generated by market transactions involving identical or comparable assets and liabilities. The income approach uses valuation techniques to convert future amounts, such as cash flows or earnings, to a single present amount on a discounted basis. The cost approach is based on the amount that currently would be required to replace the service capacity of an asset (replacement costs). Valuation techniques should be consistently applied. Inputs to valuation techniques refer to the assumptions that market participants would use in pricing the asset or liability. Inputs may be observable, meaning those that reflect the assumptions market participants would use in pricing the asset or liability developed based on market data obtained from independent sources, or unobservable, meaning those that reflect the reporting entity’s own assumptions about the assumptions market participants would use in pricing the asset or liability developed based on the best information available in the circumstances. In that regard, the authoritative guidance establishes a 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. The fair value hierarchy is as follows: • Level 1 Inputs — Unadjusted quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. • Level 2 Inputs — Inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. These include quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, inputs other than quoted prices that are observable for the asset or liability (for example, interest rates, volatilities, prepayment speeds, loss severities, credit risks and default rates) or inputs that are derived principally from or corroborated by observable market data by correlation or other means. • Level 3 Inputs — Significant unobservable inputs that reflect an entity’s own assumptions that market participants would use in pricing the assets or liabilities. A description of the valuation methodologies used for assets and liabilities measured at fair value, as well as the general classification of such instruments pursuant to the valuation hierarchy, is set forth below. In general, fair value is based upon quoted market prices, where available. If such quoted market prices are not available, fair value is based upon internally developed models that primarily use, as inputs, observable market-based parameters. Valuation adjustments may be made to ensure that financial instruments are recorded at fair value. While management believes the Company’s valuation methodologies are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different estimate of fair value at the reporting date. Securities classified as available-for-sale There were no transfers between Level 2 and Level 3 during the year ended December 31, 2017, 2016 and 2015. The following table summarizes financial assets and financial liabilities measured at fair value on a recurring basis as of December 31, 2017 and 2016 segregated by the level of the valuation inputs within the fair value hierarchy utilized to measure fair value (in thousands): December 31, 2017 Level 1 Level 2 Level 3 Total Fair Available-for-sale Obligations of U.S. government sponsored enterprises and agencies $ — $ 60,330 $ — $ 60,330 Obligations of state and political subdivisions — 1,420,850 — 1,420,850 Corporate bonds — 7,031 — 7,031 Residential mortgage-backed securities — 1,219,097 — 1,219,097 Commercial mortgage-backed securities — 375,737 — 375,737 Other securities 4,428 — — 4,428 Total $ 4,428 $ 3,083,045 $ — $ 3,087,473 December 31, 2016 Level 1 Level 2 Level 3 Total Fair Available-for-sale U.S. Treasury securities $ 10,668 $ — $ — $ 10,668 Obligations of U.S. government sponsored enterprises and agencies — 113,703 — 113,703 Obligations of state and political subdivisions — 1,564,276 — 1,564,276 Corporate bonds — 47,965 — 47,965 Residential mortgage-backed securities — 851,361 — 851,361 Commercial mortgage-backed securities — 268,436 — 268,436 Other securities 4,428 — — 4,428 Total $ 15,096 $ 2,845,741 $ — $ 2,860,837 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 and financial liabilities measured at fair value on a non-recurring Impaired Loans — Impaired loans are reported at the fair value of the underlying collateral less selling costs if repayment is expected solely from the collateral. Collateral values are estimated using Level 2 inputs based on observable market data. At December 31, 2017, impaired loans with a carrying value of $17,670,000 were reduced by specific valuation reserves totaling $3,996,000 resulting in a net fair value of $13,674,000. Loans Held-for-Sale held-for-sale held-for-sale held-for-sale Certain non-financial non-financial non-recurring non-financial Non-financial non-recurring re-measured re-measured Year Ended 2017 2016 Carrying value of other real estate owned prior to re-measurement $ 1,067 $ — Write-downs included in gain (loss) on sale of other real estate owned (306 ) — Fair value $ 761 $ — At December 31, 2017 and 2016, other real estate owned totaled $1,347,000 and $413,000, respectively. The Company is required under current authoritative accounting guidance to disclose the estimated fair value of their financial instrument assets and liabilities including those subject to the requirements discussed above. For the Company, as for most financial institutions, substantially all of its assets and liabilities are considered financial instruments. Many of the Company’s financial instruments, however, lack an available trading market as characterized by a willing buyer and willing seller engaging in an exchange transaction. The estimated fair value amounts of financial instruments have been determined by the Company using available market information and appropriate valuation methodologies. However, considerable judgment is required to interpret data to develop the estimates of fair value. Accordingly, the estimates presented herein are not necessarily indicative of the amounts the Company could realize in a current market exchange. The use of different market assumptions and/or estimation methodologies may have a material effect on the estimated fair value amounts. In addition, reasonable comparability between financial institutions may not be likely due to the wide range of permitted valuation techniques and numerous estimates that must be made given the absence of active secondary markets for many of the financial instruments. This lack of uniform valuation methodologies also introduces a greater degree of subjectivity to these estimated fair values. Cash and due from banks, federal funds sold, interest-bearing deposits and time deposits in banks and accrued interest receivable and payable are liquid in nature and considered Levels 1 or 2 of the fair value hierarchy. Financial instruments with stated maturities have been valued using a present value discounted cash flow with a discount rate approximating current market for similar assets and liabilities and are considered Levels 2 and 3 of the fair value hierarchy. Financial instrument liabilities with no stated maturities have an estimated fair value equal to both the amount payable on demand and the carrying value and are considered Level 1 of the fair value hierarchy. The carrying value and the estimated fair value of the Company’s contractual off-balance-sheet The estimated fair values and carrying values of all financial instruments under current authoritative guidance at December 31, 2017 and 2016, were as follows (dollars in thousands): 2017 2016 Fair Value Carrying Estimated Carrying Estimated Cash and due from banks $ 209,583 $ 209,583 $ 204,782 $ 204,782 Level 1 Federal funds sold — — 3,130 3,130 Level 1 Interest-bearing deposits in banks 162,764 162,764 48,574 48,574 Level 1 Interest-bearing time deposits in banks 1,458 1,458 1,707 1,709 Level 2 Available-for-sale 3,087,473 3,087,473 2,860,837 2,860,837 Levels 1 and 2 Held-to-maturity — — 121 124 Level 2 Loans 3,452,543 3,470,133 3,338,426 3,361,735 Level 3 Accrued interest receivable 36,081 36,081 36,469 36,469 Level 2 Deposits with stated maturities 451,255 452,000 508,996 510,304 Level 2 Deposits with no stated maturities 5,511,706 5,511,706 4,969,543 4,969,543 Level 1 Borrowings 331,000 331,000 445,770 445,770 Level 2 Accrued interest payable 197 197 225 225 Level 2

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	9. COMMITMENTS AND CONTINGENCIES: The Company is engaged in legal actions arising from the normal course of business. In management’s opinion, the Company has adequate legal defenses with respect to these actions, and as of December 31, 2017 the resolution of these matters is not expected to have material adverse effects upon the results of operations or financial condition of the Company. The Company leases a portion of its bank premises and equipment under operating leases. At December 31, 2017, future minimum lease commitments were: 2018 — $385,000, 2019 — $303,000, 2020 — $173,000, 2021 — $41,000 and 2022 — $7,000.

Financial Instruments with Off-

Financial Instruments with Off-Balance-Sheet Risk	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Financial Instruments with Off-Balance-Sheet Risk	10. FINANCIAL INSTRUMENTS WITH OFF-BALANCE-SHEET We are a party to financial instruments with off-balance-sheet Our exposure to credit loss in the event of nonperformance by the counterparty to the financial instrument for unfunded lines of credit, commitments to extend credit and standby letters of credit is represented by the contractual notional amount of these instruments. We generally use the same credit policies in making commitments and conditional obligations as we do for on-balance-sheet Total Notional Financial instruments whose contract amounts represent credit risk: Unfunded lines of credit $ 534,468 Unfunded commitments to extend credit 244,658 Standby letters of credit 28,858 Total commercial commitments $ 807,984 Unfunded lines of credit and commitments to extend credit are agreements to lend to a customer as long as there is no violation of any condition established in the contract. These commitments generally have fixed expiration dates or other termination clauses and may require payment of a fee. Since many of the commitments are expected to expire without being drawn upon, the total commitment amounts do not necessarily represent future cash requirements. We evaluate each customer’s creditworthiness on a case-by-case Standby letters of credit are conditional commitments we issue to guarantee the performance of a customer to a third party. The credit risk involved in issuing letters of credit is essentially the same as that involved in extending loan facilities to customers. The average collateral value held on letters of credit usually exceeds the contract amount. We believe we have no other off-balance

Concentration of Credit Risk

Concentration of Credit Risk	12 Months Ended
	Dec. 31, 2017
Risks and Uncertainties [Abstract]
Concentration of Credit Risk	11. CONCENTRATION OF CREDIT RISK: The Company grants commercial, retail, agriculture and residential real estate loans to customers primarily in North Central, Southeastern and West Texas. Although the Company has a diversified loan portfolio, a substantial portion of its borrowers’ ability to honor their commitments is dependent upon each local economic sector. In addition, the Company holds mortgage related securities which are guaranteed by GNMA, FNMA or FHLMC or are collateralized by loans backed by these agencies.

Pension and Profit Sharing Plan

Pension and Profit Sharing Plans	12 Months Ended
	Dec. 31, 2017
Retirement Benefits [Abstract]
Pension and Profit Sharing Plans	12. PENSION AND PROFIT SHARING PLANS: The Company’s defined benefit pension plan was frozen effective January 1, 2004, whereby no new participants will be added to the Plan and no additional years of service will accrue to participants, unless the pension plan is reinstated at a future date. The pension plan covered substantially all of the Company’s employees at the time. The benefits for each employee were based on years of service and a percentage of the employee’s qualifying compensation during the final years of employment. The Company’s funding policy was and is to contribute annually the amount necessary to satisfy the Internal Revenue Service’s funding standards. Contributions to the pension plan, prior to freezing the plan, were intended to provide not only for benefits attributed to service to date but also for those expected to be earned in the future. As a result of the Pension Protection Act of 2006 (the “Protection Act”), the Company will be required to contribute amounts in future years to fund any shortfalls. The Company has evaluated the provisions of the Protection Act as well as the Internal Revenue Service’s funding standards to develop a plan for funding in future years. As a result, the Company made no contribution in 2017 and made a contribution totaling $500,000 in 2016, and is continuing to evaluate future funding amounts. Using an actuarial measurement date of December 31, 2017 and 2016, benefit obligation activity and fair value of plan assets for the years ended December 31, 2017 and 2016, and a statement of the funded status as of December 31, 2017 and 2016, are as follows (dollars in thousands): 2017 2016 Reconciliation of benefit obligations: Benefit obligation at January 1 $ 15,453 $ 16,002 Interest cost on projected benefit obligation 635 665 Actuarial loss 486 139 Benefits paid, including partial settlement of certain participant balances (1,043 ) (1,353 ) Benefit obligation at December 31 $ 15,531 $ 15,453 Reconciliation of fair value of plan assets: Fair value of plan assets at January 1 $ 15,787 $ 14,820 Actual return on plan assets 2,302 1,820 Employer contributions — 500 Benefits paid, including partial settlement of certain participant balances (1,043 ) (1,353 ) Fair value of plan assets at December 31 17,046 15,787 Funded status $ 1,515 $ 334 Amounts recognized as a component of accumulated other comprehensive earnings as of year-end 2017 2016 Net actuarial loss $ (3,597 ) $ (4,688 ) Deferred tax benefit 1,227 1,641 Amounts included in accumulated other comprehensive earnings, net of tax $ (2,370 ) $ (3,047 ) Net periodic benefit cost for the years ended December 31, 2017, 2016 and 2015, are as follows (dollars in thousands): Year Ended December 31, 2017 2016 2015 Service cost — benefits earned during the period $ — $ — $ — Interest cost on projected benefit obligation 635 665 622 Expected return on plan assets (974 ) (912 ) (948 ) Amortization of unrecognized net loss 249 375 222 Recognized loss on partial settlement of certain participant balances — 267 — Net periodic pension benefit expense (benefit) $ (90 ) $ 395 $ (104 ) The following table sets forth the rates used in the actuarial calculations of the present value of benefit obligations and net periodic pension cost and the rate of return on plan assets: 2017 2016 2015 Weighted average discount rate 3.50 % 4.25 % 4.25 % Expected long-term rate of return on assets 6.25 % 6.25 % 6.25 % The weighted average discount rate is estimated based on setting a discount rate to establish an obligation for pension benefits equivalent to an amount that, if invested in high quality fixed income securities, would produce a return that matches the expected benefit payment stream. The expected long-term rate of return on plan assets is based on historical returns and expectations of future returns based on asset mix, after consultation with our investment advisors and actuaries. The major type of plan assets in the pension plan and the targeted allocation percentage as of December 31, 2017 and 2016 is as follows: December 31, December 31, Targeted Equity securities 75 % 77 % 75 % Debt securities 24 % 22 % 25 % Cash and equivalents 1 % 1 % — The range and weighted average final maturities of debt securities held in the pension plan as of December 31, 2017 are 1.52 to 19.76 years and approximately 5.64 years, respectively. Assets held in the pension plan are considered either Level 1 consisting of the money market funds, publicly traded common stocks and publicly traded mutual funds or Level 2 consisting of obligations of state and political subdivisions, corporate bonds and mortgage-backed securities. There were no Level 3 securities. See note 8 for a discussion of the fair value hierarchy. The breakdown by level is as follows (dollars in thousands): Level 1 Level 2 Level 3 Total Fair Money market fund $ 274 $ — $ — $ 274 U.S. Treasury notes — 149 — 149 Obligations of state and political subdivisions — 634 — 634 Corporate bonds — 868 — 868 Mortgage-backed securities — 1,130 — 1,130 Corporate stocks and mutual funds 13,991 — — 13,991 Total $ 14,265 $ 2,781 — $ 17,046 First Financial Trust & Asset Management Company, National Association, a wholly owned subsidiary of the Company, manages the pension plan assets as well as the profit sharing plan assets (see below). The investment strategy and targeted allocations are based on similar strategies First Financial Trust & Asset Management Company, National Association employs for most of its managed accounts whereby appropriate diversification is achieved. First Financial Trust & Asset Management Company, National Association is prohibited from holding investments deemed to be high risk by the Office of the Comptroller of the Currency. An estimate of the undiscounted projected future payments to eligible participants for the next five years and the following five years in the aggregate is as follows (in thousands): Year Ending December 31, 2018 $ 1,064 2019 $ 1,018 2020 $ 1,036 2021 $ 1,077 2022 $ 1,063 2023 forward $ 5,309 As of December 31, 2017 and 2016, the pension plan’s total assets included First Financial Bankshares, Inc. common stock valued at approximately $2,776,000 and $2,786,000, respectively. The Company also provides a profit sharing plan, which covers substantially all full-time employees. The profit sharing plan is a defined contribution plan and allows employees to contribute a percentage of their base annual salary. Employees are fully vested to the extent of their contributions and become fully vested in the Company’s contributions over a six-year In 2004, after freezing our pension plan, we added a safe harbor match to the 401(k) plan. We match a maximum of 4% on employee deferrals of 5% of their employee compensation. Total expense for this matching in 2017, 2016 and 2015 was $2,392,000, $2,331,000 and $2,043,000, respectively, and is included in salaries and employee benefits in the statements of earnings. The Company has a directors’ deferred compensation plan whereby the directors may elect to defer up to 100% of their directors’ fees. All deferred compensation is invested in the Company’s common stock held in a rabbi trust. The stock is held in nominee name of the trustee, and the principal and earnings of the trust are held separate and apart from other funds of the Company, and are used exclusively for the uses and purposes of the deferred compensation agreement. The accounts of the trust have been consolidated in the financial statements of the Company.

Dividends from Subsidiaries

Dividends from Subsidiaries	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Dividends from Subsidiaries	13. DIVIDENDS FROM SUBSIDIARIES: At December 31, 2017, approximately $205,751,000 was available for the declaration of dividends by the Company’s subsidiaries without the prior approval of regulatory agencies.

Regulatory Matters

Regulatory Matters	12 Months Ended
	Dec. 31, 2017
Banking and Thrift [Abstract]
Regulatory Matters	14. REGULATORY MATTERS: Banking regulators measure capital adequacy by means of the risk-based capital ratios and the leverage ratio under the Basel III regulatory capital framework and prompt corrective action regulations. The risk-based capital rules provide for the weighting of assets and off-balance-sheet quarter-to-date Beginning in January 2016, under the Basel III regulatory capital framework, the implementation of the capital conservation buffer was effective for the Company starting at the 0.625% level and increasing 0.625% each year thereafter, until it reaches 2.5% on January 1, 2019. The capital conservation buffer is designed to absorb losses during periods of economic stress and requires increased capital levels for the purpose of capital distributions and other payments. Failure to meet the amount of the buffer will result in restrictions on the Company’s ability to make capital distributions, including dividend payments and stock repurchases, and to pay discretionary bonuses to executive officers. As of December 31, 2017 and 2016, we had a total risk-based capital ratio of 19.85% and 18.45%, a Tier 1 capital to risk-weighted assets ratio of 18.66% and 17.30%; a common equity Tier 1 capital to risk-weighted assets ratio of 18.66% and 17.30%, and a Tier 1 leverage ratio of 11.09% and 10.71%, respectively. The regulatory capital ratios as of December 31, 2017 and 2016 were calculated under Basel III rules. There is no threshold for well-capitalized status for bank holding companies. As of December 31, 2017 and 2016, the regulatory capital ratios of the Company and Bank under the Basel III regulatory capital framework are as follows: Actual Minimum Capital Phase-In Minimum Capital Phased-In Required to be As of December 31, 2017: Amount Ratio Amount Ratio Amount Ratio Amount Ratio Total Capital to Risk-Weighted Assets: Consolidated $ 814,634 19.85 % $ 379,578 9.250 % $ 430,872 10.50 % — N/A First Financial Bank, N.A $ 723,563 17.68 % $ 378,614 9.250 % $ 429,777 10.50 % $ 409,312 10.00 % Tier 1 Capital to Risk-Weighted Assets: Consolidated $ 765,882 18.66 % $ 297,507 7.250 % $ 348,801 8.50 % — N/A First Financial Bank, N.A $ 674,811 16.49 % $ 296,751 7.250 % $ 347,915 8.50 % $ 327,450 8.00 % Common Equity Tier 1 Capital to Risk-Weighted Assets: Consolidated $ 765,882 18.66 % $ 235,954 5.750 % $ 287,248 7.00 % — N/A First Financial Bank, N.A $ 674,811 16.49 % $ 235,354 5.750 % $ 286,518 7.00 % $ 266,053 6.50 % Leverage Ratio: Consolidated $ 765,882 11.09 % $ 276,296 4.000 % $ 276,296 4.00 % — N/A First Financial Bank, N.A $ 674,811 9.80 % $ 275,320 4.000 % $ 275,320 4.00 % $ 344,151 5.00 % Actual Minimum Capital Phase-In Minimum Capital Phased-In Required to be As of December 31, 2016: Amount Ratio Amount Ratio Amount Ratio Amount Ratio Total Capital to Risk-Weighted Assets: Consolidated $ 739,959 18.45 % $ 345,827 8.625 % $ 421,007 10.50 % — N/A First Financial Bank, N.A $ 633,403 15.84 % $ 344,930 8.625 % $ 419,915 10.50 % $ 399,919 10.00 % Tier 1 Capital to Risk-Weighted Assets: Consolidated $ 693,584 17.30 % $ 265,635 6.625 % $ 340,815 8.50 % — N/A First Financial Bank, N.A $ 587,028 14.68 % $ 264,946 6.625 % $ 339,931 8.50 % $ 319,935 8.00 % Common Equity Tier 1 Capital to Risk-Weighted Assets: Consolidated $ 693,584 17.30 % $ 205,491 5.125 % $ 280,671 7.00 % — N/A First Financial Bank, N.A $ 587,028 14.68 % $ 204,959 5.125 % $ 279,943 7.00 % $ 259,947 6.50 % Leverage Ratio: Consolidated $ 693,584 10.71 % $ 258,978 4.00 % $ 258,978 4.00 % — N/A First Financial Bank, N.A $ 587,028 9.10 % $ 257,941 4.00 % $ 257,941 4.00 % $ 322,426 5.00 % We have performed a preliminary assessment using the regulatory capital estimation tool made available by the OCC and believe the Company and Bank are prepared to meet the new requirements upon full adoption of Basel III that will be effective December 31, 2019. In connection with the adoption of the Basel III regulatory capital framework, our subsidiary bank made the election to continue to exclude most accumulated other comprehensive income (“AOCI”) from capital in connection with its March 31, 2015 quarterly financial filing and, in effect, to retain the AOCI treatment under the prior capital rules. In connection with the First Financial Trust & Asset Management Company, National Association’s (the “Trust Company”) application to obtain our trust charter, the Trust Company is required to maintain tangible net assets of $2,000,000 at all times. As of December 31, 2017, our Trust Company had tangible net assets totaling $16,602,000. Our subsidiary bank may be required at times to maintain reserve balances with the Federal Reserve Bank. At December 31, 2017 and 2016, the subsidiary bank’s reserve balances were $11,504,000 and $4,340,000, respectively.

Stock Option Plan and Restricte

Stock Option Plan and Restricted Stock Plan	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock Option Plan and Restricted Stock Plan	15. STOCK OPTION PLAN AND RESTRICTED STOCK PLAN: The Company has an incentive stock plan to provide for the granting of options to employees of the Company at prices not less than market at the date of grant. At December 31, 2017, the Company had allocated 3,180,000 shares of stock for issuance under the plan. The plan provides that options granted are exercisable after two years from date of grant at a rate of 20% each year cumulatively during the 10-year Shares Weighted- Weighted- Aggregate Outstanding, beginning of year 1,094,035 $ 27.40 Granted 452,450 42.35 Exercised (140,250 ) 20.92 Cancelled (80,270 ) 30.33 Outstanding, end of year 1,325,965 33.01 7.12 $ 15,966 Exercisable at end of year 458,715 $ 23.94 4.66 $ 9,684 The options outstanding at December 31, 2017 had exercise prices ranging between $16.78 and $42.35. Stock options have been adjusted retroactively for the effects of stock dividends and splits. The following table summarizes information concerning outstanding and vested stock options as of December 31, 2017: Exercise Price Number Outstanding Remaining Contracted Number Vested $16.78 87,640 1.4 87,640 15.73 142,620 3.8 142,620 30.85 270,505 5.8 156,225 33.89 382,750 7.8 72,230 $42.35 442,450 9.5 — The fair value of the options granted during 2017 and 2015 were estimated using the Black-Scholes options pricing model with the following weighted-average assumptions: risk-free interest rates of 1.89% and 1.89%; expected dividend yields of 1.79% and 1.89%; expected lives of 6.24 years and 5.78 years; and expected volatilities of 26.51% and 23.36%, respectively. The weighted-average grant-date fair value of options granted during 2017 and 2015 was $9.90 and $6.72, respectively. There were no grants during 2016. The total intrinsic value of options exercised during the years ended December 31, 2017, 2016 and 2015, was $3,082,000, $1,226,000 and $1,539,000, respectively. As of December 31, 2017, there was $5,031,000 of total unrecognized compensation cost related to nonvested share-based compensation arrangements granted under the Plan. That cost is expected to be recognized over a weighted-average period of 2.21 years. The total fair value of shares vested during the years ended December 31, 2017, 2016 and 2015 was $1,246,000, $592,000 and $810,000. The aggregate intrinsic value of vested stock options at December 31, 2017 totaled $10,981,000. On April 28, 2015, shareholders of the Company approved a restricted stock plan for selected employees, officers, non-employee On July 21, 2015, 7,070 shares were granted to the ten non-employee re-election non-employee re-election non-employee On October 27, 2015, the Company granted 31,273 shares with a total value of $1,060,000 to certain officers that is being expensed over the vesting period of three years. On October 25, 2016, the Company granted 15,405 shares with a total value of $560,000 to certain officers that is being expensed over the vesting period of three years. On October 24, 2017, the Company granted 14,191 restricted shares with a total value of $655,000 to certain officers that is being expensed over the vesting period of one to three years. The Company recorded restricted stock expense for officers of $562,000, $381,000 and $62,000, respectively, for the year ended December 31, 2017, 2016 and 2015.

Condensed Financial Information

Condensed Financial Information - Parent Company	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed Financial Information - Parent Company	16. CONDENSED FINANCIAL INFORMATION—PARENT COMPANY: Condensed Balance Sheets-December 31, 2017 and 2016 2017 2016 ASSETS Cash in subsidiary bank $ 14,272 $ 15,070 Cash in unaffiliated banks 2 2 Interest-bearing deposits in subsidiary bank 64,195 78,179 Total cash and cash equivalents 78,469 93,251 Securities available-for-sale, 8,515 11,593 Investment in and advances to subsidiaries, at equity 847,445 744,971 Intangible assets 723 723 Other assets 2,654 2,668 Total assets $ 937,806 $ 853,206 LIABILITIES AND SHAREHOLDERS’ EQUITY Total liabilities $ 15,038 $ 15,321 Shareholders’ equity: Common stock 663 661 Capital surplus 378,062 372,245 Retained earnings 517,257 446,534 Treasury stock (7,148 ) (6,671 ) Deferred compensation 7,148 6,671 Accumulated other comprehensive earnings 26,786 18,445 Total shareholders’ equity 922,768 837,885 Total liabilities and shareholders’ equity $ 937,806 $ 853,206 Condensed Statements of Earnings- For the Years Ended December 31, 2017, 2016 and 2015 2017 2016 2015 Income: Cash dividends from subsidiaries $ 30,800 $ 48,800 $ 51,200 Excess of earnings over dividends of subsidiaries 92,929 58,809 52,911 Other 6,590 4,184 4,185 Total Income 130,319 111,793 108,296 Expenses: Salaries and employee benefits 8,606 5,655 6,067 Other operating expenses 3,871 3,531 4,439 Total Expense 12,477 9,186 10,506 Earnings before income taxes 117,842 102,607 97,790 Income tax benefit 2,529 2,167 2,591 Net earnings $ 120,371 $ 104,774 $ 100,381 Condensed Statements of Cash Flows- For the Years Ended December 31, 2017, 2016 and 2015 2017 2016 2015 Cash flows from operating activities: Net earnings $ 120,371 $ 104,774 $ 100,381 Adjustments to reconcile net earnings to net cash provided by operating activities: Excess of earnings over dividends of subsidiary bank (92,929 ) (58,809 ) (52,911 ) Depreciation and amortization, net 207 208 197 Decrease (increase) in other assets 438 1,702 507 Increase (decrease) in other liabilities 183 (1,374 ) 3,743 Other 2 8 — Net cash provided by operating activities 28,272 46,509 51,917 Cash flows from investing activities: Cash received in connection with acquisition of banks — — 13,125 Maturity of available-for-sale 2,997 — — Purchases of bank premises and equipment (30 ) (94 ) (107 ) Repayment from investment in and advances to subsidiaries, net — — 5,800 Other — 10 — Net cash used in (provided by) investing activities 2,967 (84 ) 18,818 Cash flows from financing activities: Repayment of subordinated debt — — (13,125 ) Proceeds of stock issuances 2,934 1,260 1,545 Cash dividends paid (48,955 ) (44,907 ) (38,767 ) Net cash used in financing activities (46,021 ) (43,647 ) (50,347 ) Net increase (decrease) in cash and cash equivalents (14,782 ) 2,778 20,388 Cash and cash equivalents, beginning of year 93,251 90,473 70,085 Cash and cash equivalents, end of year $ 78,469 $ 93,251 $ 90,473

Cash Flow Information

Cash Flow Information	12 Months Ended
	Dec. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Cash Flow Information	17. CASH FLOW INFORMATION: Supplemental information on cash flows and noncash transactions is as follows (dollars in thousands): Year Ended December 31, 2017 2016 2015 Supplemental cash flow information: Interest paid $ 9,316 $ 5,465 $ 4,085 Federal income taxes paid 29,695 28,348 29,674 Schedule of noncash investing and financing activities: Assets acquired through foreclosure 2,211 2,269 203 Investment securities purchased but not settled — 12,381 — Restricted stock grant to officers and directors 1,139 810 1,310

Acquisitions and Asset Purchase

Acquisitions and Asset Purchase	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Acquisitions and Asset Purchase	18. ACQUISITIONS AND ASSET PURCHASE: On April 1, 2015, we entered into an agreement and plan of reorganization to acquire FBC Bancshares, Inc. and its wholly owned bank subsidiary, First Bank, N.A., Conroe, Texas. On July 31, 2015, the transaction was completed. Pursuant to the agreement, we issued 1,755,374 shares of the Company’s common stock in exchange for all of the outstanding shares of FBC Bancshares, Inc. At closing, FBC Bancshares, Inc. was merged into the Company and First Bank, N.A., Conroe, Texas, was merged into First Financial Bank, National Association, Abilene, Texas, a wholly owned subsidiary of the Company. The primary purpose of the acquisition was to expand the Company’s market share along Interstate Highway 45 in southern Texas, north of Houston. Factors that contributed to a purchase price resulting in goodwill include First Bank, N.A.’s historic record of earnings, strong local economic environment and opportunity for growth. The results of operations from this acquisition are included in the consolidated earnings of the Company commencing August 1, 2015. The assets acquired and liabilities assumed were recorded on the consolidated balance sheet at estimated fair value on the acquisition date. The acquisition was not considered to be a significant business combination. The following table presents the amounts recorded on the consolidated balance sheet on the acquisition date (dollars in thousands): Fair value of consideration paid: Common stock issued (1,755,374 shares) $ 59,648 Fair value of identifiable assets acquired: Cash and cash equivalents 65,197 Securities available-for-sale 42,903 Loans 248,380 Identifiable intangible assets 2,343 Other assets 15,262 Total identifiable assets acquired 374,085 Fair value of liabilities assumed: Deposits 343,583 Subordinated debt 13,125 Other liabilities 1,651 Total liabilities assumed 358,359 Fair value of net identifiable assets acquired 15,726 Goodwill resulting from acquisition $ 43,922 Goodwill recorded in the acquisition was accounted for in accordance with the authoritative business combination guidance. Accordingly, goodwill will not be amortized, but will be tested for impairment annually. The goodwill recorded is not deductible for federal income tax purposes. The subordinated debt of $13,125,000 was paid off August 3, 2015, subsequent to closing. The fair value of total loans acquired was $248,380,000 at acquisition compared to contractual amounts of $252,458,000. The fair value of purchased credit impaired loans at acquisition was $1,398,000 compared to contractual amounts of $1,704,000. Additional purchased credit impaired loan disclosures were omitted due to immateriality. All other acquired loans were considered performing loans. First Bank, N.A. had branches in Conroe, Magnolia, Tomball, Willis, Cut and Shoot and Huntsville, all located north of Houston, Texas. In February 2016, the Company closed First Bank’s Huntsville location and consolidated the branch with the Company’s existing Huntsville location. On April 8, 2015, the Company announced that it had entered into an asset purchase agreement with 4Trust Mortgage, Inc. for a cash purchase price of $1,900,000. The asset purchase was finalized on June 1, 2015. The total asset purchase price exceeded the estimated fair value of assets purchased by approximately $1,750,000 and the Company recorded such excess as goodwill.

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Event	19. SUBSEQUENT EVENT On January 1, 2018, the Company acquired 100% of the outstanding capital stock of Commercial Bancshares, Inc. through the merger of a wholly owned subsidiary with and into Commercial Bancshares, Inc. Following such merger, Commercial Bancshares, Inc. and its wholly owned subsidiary, Commercial State Bank, Kingwood, Texas, were merged into the Company and First Financial Bank, National Association, respectively. Considerations paid to the shareholders of Commercial Bancshares, Inc. totaled 1,289,371 shares of the company’s common stock with an aggregate value of $59,400,000 at January 1, 2018. In addition, Commercial Bancshares, Inc. made a $22,075,000 special dividend to its shareholders prior to closing of the transaction, which was increased for the amount by which Commercial Bancshares, Inc.’s consolidated shareholders’ equity as of January 1, 2018 exceeded $42,402,000, after certain adjustments per the merger agreement. At December 31, 2017, Commercial State Bank, Kingwood, Texas had gross loans totaling $271,533,000, total deposits of $345,908,000 and total assets of $390,112,000.

Summary of Significant Accoun29

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Nature of Operations	Nature of Operations First Financial Bankshares, Inc. (a Texas corporation) (“Bankshares”, “Company”, “we” or “us”) is a financial holding company which owns all of the capital stock of one bank with 69 locations located in Texas as of December 31, 2017. The subsidiary bank is First Financial Bank, National Association, Abilene. The bank’s primary source of revenue is providing loans and banking services to consumers and commercial customers in the market area in which the subsidiary is located. In addition, the Company also owns First Financial Trust & Asset Management Company, National Association, First Financial Insurance Agency, Inc., and First Technology Services, Inc. A summary of significant accounting policies of Bankshares and its subsidiaries applied in the preparation of the accompanying consolidated financial statements follows. The accounting principles followed by the Company and the methods of applying them are in conformity with both U.S. GAAP and prevailing practices of the banking industry. The Company evaluated subsequent events for potential recognition through the date the consolidated financial statements were issued.
Use of Estimates in Preparation of Financial Statements	Use of Estimates in Preparation of Financial Statements The preparation of financial statements in conformity with U.S. GAAP 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 and reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. The Company’s significant estimates include its allowance for loan losses and its valuation of financial instruments.
Consolidation	Consolidation The accompanying consolidated financial statements include the accounts of Bankshares and its subsidiaries, all of which are wholly-owned. All significant intercompany accounts and transactions have been eliminated.
Stock Repurchase	Stock Repurchase On June 25, 2017, the Company’s Board of Directors authorized the repurchase of up to 2,000,000 common shares through September 30, 2020. Previously, the Board of Directors had authorized the repurchase of up to 1,500,000 common shares through September 30, 2017. The stock buyback plan authorizes management to repurchase the stock at such time as repurchases are considered beneficial to stockholders. Any repurchase of stock will be made through the open market, block trades or in privately negotiated transactions in accordance with applicable laws and regulations. Under the repurchase plan, there is no minimum number of shares that the Company is required to repurchase. For the years ended December 31, 2017, 2016 and 2015, no shares were repurchased under this or the prior authorization that expired September 30, 2017.
Acquisitions and Asset Purchase	Acquisitions and Asset Purchase On July 31, 2015, the Company acquired 100% of the outstanding capital stock of FBC Bancshares, Inc. through the merger of a wholly-owned subsidiary with and into FBC Bancshares, Inc. Following such merger, FBC Bancshares, Inc. and its wholly-owned subsidiary, First Bank, N.A. were merged into the Company and First Financial Bank, National Association, respectively. The results of operations of FBC Bancshares, Inc. subsequent to the acquisition date, are included in the consolidated earnings of the Company. See Note 18 for additional information. On June 1, 2015, the Company completed the asset purchase of 4Trust Mortgage, Inc. The results of operation of 4Trust Mortgage Inc. subsequent to the asset purchase date, are included in the consolidated earnings of the Company. See Note 18 for additional information.
Increase in Authorized Shares	Increase in Authorized Shares On April 28, 2015, the Company’s shareholders approved an amendment to the Company’s Amended and Restated Certificate of Formation to increase the number of authorized common shares to 120,000,000.
Investment Securities	Investment Securities Management classifies debt and equity securities as held-to-maturity, available-for-sale, held-to-maturity held-to-maturity available-for-sale Available-for-sale available-for-sale The Company records its available-for-sale When the fair value of a security is below its amortized cost, and depending on the length of time the condition exists and the extent the fair value is below amortized cost, additional analysis is performed to determine whether an other-than-temporary impairment condition exists. Available-for-sale held-to-maturity The Company’s investment portfolio consists of U.S. Treasury securities, obligations of U.S. government sponsored enterprises and agencies, obligations of state and political subdivisions, mortgage pass-through securities, corporate bonds and general obligation or revenue based municipal bonds. Pricing for such securities is generally readily available and transparent in the market. The Company utilizes independent third party pricing services to value its investment securities, which the Company reviews as well as the underlying pricing methodologies for reasonableness and to ensure such prices are aligned with pricing matrices. The Company validates quarterly, on a sample basis, prices supplied by the independent pricing services by comparison to prices obtained from other third party sources.
Loans and Allowance for Loan Losses	Loans and Allowance for Loan Losses Loans held for investment are stated at the amount of unpaid principal, reduced by unearned income and an allowance for loan losses. Interest on loans is calculated by using the simple interest method on daily balances of the principal amounts outstanding. The Company defers and amortizes net loan origination fees and costs as an adjustment to yield. The allowance for loan losses is established through a provision for loan losses charged to expense. Loans are charged against the allowance for loan losses when management believes the collectability of the principal is unlikely. The allowance for loan losses is an amount which represents management’s best estimate of probable losses that are inherent in the Company’s loan portfolio as of the balance sheet date. The allowance for loan losses is comprised of three elements: (i) specific reserves determined based on probable losses on specific classified loans; (ii) a historical valuation reserve component that considers historical loss rates and estimated loss emergence periods; and (iii) qualitative reserves based upon general economic conditions and other qualitative risk factors both internal and external to the Company. The allowance for loan losses is increased by charges to income and decreased by charge-offs Although we believe we use the best information available to make loan loss allowance determinations, future adjustments could be necessary if circumstances or economic conditions differ substantially from the assumptions used in making our initial determinations. A decline in the economy and employment could result in increased levels of non-performing Accrual of interest is discontinued on a loan and payments are applied to principal when management believes, after considering economic and business conditions and collection efforts, the borrower’s financial condition is such that collection of interest is doubtful. Except consumer loans, generally all loans past due greater than 90 days, based on contractual terms, are placed on non-accrual. charged-off charge-off Loans are considered impaired when, based on current information and events, management determines that it is probable we will be unable to collect all amounts due in accordance with the loan agreement, including scheduled principal and interest payments. If a loan is impaired, a specific valuation allowance is allocated, if necessary. Interest payments on impaired loans are typically applied to principal unless collectability of the principal amount is reasonably assured, in which case interest is recognized on a cash basis. Impaired loans, or portions thereof, are charged off when deemed uncollectable. The Company’s policy requires measurement of the allowance for an impaired, collateral dependent loan based on the fair value of the collateral less cost to sell. Other loan impairments for non-collateral From time to time, the Company modifies its loan agreement with a borrower. A modified loan is considered a troubled debt restructuring when two conditions are met: (i) the borrower is experiencing financial difficulty and (ii) concessions are made by the Company that would not otherwise be considered for a borrower with similar credit risk characteristics. Modifications to loan terms may include a lower interest rate, a reduction of principal, or a longer term to maturity. For all impaired loans, including the Company’s troubled debt restructurings, the Company performs a periodic, well-documented credit evaluation of the borrower’s financial condition and prospects for repayment to assess the likelihood that all principal and interest payments required under the terms of the agreement will be collected in full. When doubt exists about the ultimate collectability of principal and interest, the troubled debt restructuring remains on non-accrual non-accrual The Company originates certain mortgage loans for sale in the secondary market. Accordingly, these loans are classified as held-for-sale Loans acquired, including loans acquired in a business combination, are initially recorded at fair value with no valuation allowance. Acquired loans are segregated between those considered to be credit impaired and those deemed performing. To make this determination, management considers such factors as past due status, non-accrual Purchased credit impaired loans are those loans that showed evidence of deterioration of credit quality since origination and for which it is probable, at acquisition, that the Company will be unable to collect all amounts contractually owed. Their acquisition fair value, which includes a credit component at the acquisition date, was based on the estimate of cash flows, both principal and interest, expected to be collected or estimated collateral values if cash flows are not estimable, discounted at prevailing market rates of interest. The difference between the discounted cash flows expected at acquisition and the investment in the loan is recognized as interest income on a level-yield method over the life of the loan, unless management was unable to reasonably forecast cash flows in which case the loans were placed on nonaccrual. Subsequent to the acquisition date, increases in expected cash flows will generally result in a recovery of any previously recorded allowance for loan loss, to the extent applicable, and/or a reclassification from the nonaccretable difference to accretable yield, which will be recognized prospectively. Decreases in expected cash flows subsequent to acquisition are recognized as impairment. Valuation allowances on these impaired loans reflect only losses incurred after the acquisition. The carrying amount of purchased credit impaired loans at December 31, 2017 and 2016 were $618,000 and $1,256,000, respectively, compared to a contractual balance of $755,000 and $1,865,000, respectively. Other purchased credit impaired loan disclosures were omitted due to immateriality.
Other Real Estate	Other Real Estate Other real estate owned is foreclosed property held pending disposition and is initially recorded at fair value, less estimated costs to sell. At foreclosure, if the fair value of the real estate, less estimated costs to sell, is less than the Company’s recorded investment in the related loan, a write-down is recognized through a charge to the allowance for loan losses. Any subsequent reduction in value is recognized by a charge to income. Operating and holding expenses of such properties, net of related income, and gains and losses on their disposition are included in net gain (loss) on sale of foreclosed assets as incurred.
Bank Premises and Equipment	Bank Premises and Equipment Bank premises and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization are computed principally on a straight-line basis over the estimated useful lives of the related assets. Leasehold improvements are amortized over the life of the respective lease or the estimated useful lives of the improvements, whichever is shorter.
Business Combinations, Goodwill and Other Intangible Assets	Business Combinations, Goodwill and Other Intangible Assets The Company accounts for all business combinations under the purchase method of accounting. Tangible and intangible assets and liabilities of the acquired entity are recorded at fair value. Intangible assets with finite useful lives represent the future benefit associated with the acquisition of the core deposits and are amortized over seven years, utilizing a method that approximates the expected attrition of the deposits. Goodwill with an indefinite life is not amortized, but rather tested annually for impairment as of June 30 each year and totaled $139,971,000 at both December 31, 2017 and 2016. There was no impairment recorded for the years ended December 31, 2017, 2016 and 2015. The carrying amount of goodwill arising from acquisitions that qualify as an asset purchase for federal income tax purposes was $74,376,000 both at December 31, 2017 and 2016, and is deductible for federal income tax purposes. Also included in other intangible assets are mortgage servicing rights totaling $1,795,000 at December 31, 2016. These servicing rights were sold to an unrelated third party in 2017 resulting in a loss on sale of approximately $215,000
Securities Sold Under Agreements To Repurchase	Securities Sold Under Agreements To Repurchase Securities sold under agreements to repurchase, which are classified as borrowings, generally mature within one to four days from the transaction date. Securities sold under agreements to repurchase are reflected at the amount of the cash received in connection with the transaction. The Company may be required to provide additional collateral based on the estimated fair value of the underlying securities.
Segment Reporting	Segment Reporting The Company has determined that its banking regions meet the aggregation criteria of the current authoritative accounting guidance since each of its banking regions offer similar products and services, operate in a similar manner, have similar customers and report to the same regulatory authority, and therefore operate one line of business (community banking) located in a single geographic area (Texas).
Statements of Cash Flows	Statements of Cash Flows For purposes of reporting cash flows, cash and cash equivalents includes cash on hand, amounts due from banks, including interest-bearing deposits in banks with original maturity of 90 days or less, and federal funds sold.
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Unrealized net gains on the Company’s available-for-sale
Income Taxes	Income Taxes The Company’s provision for income taxes is based on income before income taxes adjusted for permanent differences between financial reporting and taxable income. Deferred tax assets and liabilities are determined using the liability (or balance sheet) method. Under this method, the net deferred tax asset or liability is determined based on the tax effects of the temporary differences between the book and tax bases of the various balance sheet assets and liabilities and gives current recognition to changes in tax rates and laws.
Stock Based Compensation	Stock Based Compensation The Company grants stock options for a fixed number of shares to employees with an exercise price equal to the fair value of the shares at the grant date. The Company recorded stock option expense totaling $1,745,000, $882,000, and $644,000 for the years ended December 31, 2017, 2016 and 2015, respectively. The Company also grants restricted stock for a fixed number of shares. The Company recorded expenses associated with its director and officer restricted stock grants totaling $483,000 and $562,000, respectively, for the year ended December 31, 2017, $278,000 and $381,000, respectively, for the year ended December 31, 2016, and $139,000 and $62,000, respectively, for the year ended December 31, 2015.
Advertising Costs	Advertising Costs Advertising costs are expensed as incurred.
Per Share Data	Per Share Data Net earnings per share (“EPS”) are computed by dividing net earnings by the weighted average number of common stock shares outstanding during the period. The Company calculates dilutive EPS assuming all outstanding stock options to purchase common stock have been exercised at the beginning of the year (or the time of issuance, if later.) The dilutive effect of the outstanding options and restricted stock is reflected by application of the treasury stock method, whereby the proceeds from the exercised options and restricted stock are assumed to be used to purchase common stock at the average market price during the respective year. For the year ended December 31, 2015, 442,368 weighted average outstanding stock options were anti-dilutive and have been excluded from the EPS calculations. There were no such anti-dilutive stock options for the years ended December 31, 2017 and 2016. The following table reconciles the computation of basic EPS to dilutive EPS: Net Earnings Weighted Per Share For the year ended December 31, 2017: Net earnings per share, basic $ 120,371 66,126,863 $ 1.82 Effect of stock options and stock grants — 197,467 (0.01 ) Net earnings per share, assuming dilution $ 120,371 66,324,330 $ 1.81 For the year ended December 31, 2016: Net earnings per share, basic $ 104,774 66,013,004 $ 1.59 Effect of stock options and stock grants — 89,882 — Net earnings per share, assuming dilution $ 104,774 66,102,886 $ 1.59 For the year ended December 31, 2015: Net earnings per share, basic $ 100,381 64,892,934 $ 1.55 Effect of stock options and stock grants — 175,096 (0.01 ) Net earnings per share, assuming dilution $ 100,381 65,068,030 $ 1.54
Recently Issued Authoritative Accounting Guidance	Recently Issued Authoritative Accounting Guidance Accounting Standards Update (“ASU”) 2014-09, 2014-09 2015-4 2014-09 2014-09, non-interest ASU 2014-15, 2014-15 2014-15 ASU 2015-01, 2015-01 ASU 2015-05, Internal-Use 2015-05 2015-05 2015-05 ASU 2015-16, 2015-16 ASU 2016-1, 2016-1, available-for-sale 2016-1 ASU 2016-02, 2016-02 right-of-use 2016-02 ASU 2016-09, 2016-09 2016-09 2016-09 ASU 2016-13, 2016-13 held-to-maturity 2016-13 2016-13 ASU 2017-04, 2017-04 2017-04 ASU 2017-07, 2017-17 2017-17 ASU 2017-08, 2017-08 2017-08 2017-08 ASU 2018-02, “Income Statement — Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income.”

Summary of Significant Accoun30

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Computation of Basic EPS to Dilutive EPS	The following table reconciles the computation of basic EPS to dilutive EPS: Net Earnings Weighted Per Share For the year ended December 31, 2017: Net earnings per share, basic $ 120,371 66,126,863 $ 1.82 Effect of stock options and stock grants — 197,467 (0.01 ) Net earnings per share, assuming dilution $ 120,371 66,324,330 $ 1.81 For the year ended December 31, 2016: Net earnings per share, basic $ 104,774 66,013,004 $ 1.59 Effect of stock options and stock grants — 89,882 — Net earnings per share, assuming dilution $ 104,774 66,102,886 $ 1.59 For the year ended December 31, 2015: Net earnings per share, basic $ 100,381 64,892,934 $ 1.55 Effect of stock options and stock grants — 175,096 (0.01 ) Net earnings per share, assuming dilution $ 100,381 65,068,030 $ 1.54

Interest-bearing Time Deposit31

Interest-bearing Time Deposits in Banks and Securities (Tables)	12 Months Ended
	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Summary of Available-for-Sale Securities	A summary of the Company’s available-for-sale December 31, 2017 Amortized Gross Gross Estimated Securities available-for-sale: Obligations of U.S. government sponsored enterprises and agencies $ 60,516 $ — $ (186 ) $ 60,330 Obligations of state and political subdivisions 1,369,295 52,491 (936 ) 1,420,850 Corporate bonds and other 11,421 43 (5 ) 11,459 Residential mortgage-backed securities 1,223,452 4,561 (8,916 ) 1,219,097 Commercial mortgage-backed securities 377,934 263 (2,460 ) 375,737 Total securities available-for-sale $ 3,042,618 $ 57,358 $ (12,503 ) $ 3,087,473 December 31, 2016 Amortized Gross Gross Estimated Securities available-for-sale: U.S. Treasury securities $ 10,649 $ 19 $ — $ 10,668 Obligations of U.S. government sponsored enterprises and agencies 113,450 253 — 113,703 Obligations of state and political subdivisions 1,534,095 40,194 (10,013 ) 1,564,276 Corporate bonds and other 51,920 476 (3 ) 52,393 Residential mortgage-backed securities 848,614 8,260 (5,513 ) 851,361 Commercial mortgage-backed securities 269,044 622 (1,230 ) 268,436 Total securities available-for-sale $ 2,827,772 $ 49,824 $ (16,759 ) $ 2,860,837
Amortized Cost and Estimated Fair Value of Available-for-Sale Securities	The amortized cost and estimated fair value of available-for-sale Amortized Estimated Due within one year $ 184,538 $ 185,547 Due after one year through five years 631,376 654,092 Due after five years through ten years 623,628 650,923 Due after ten years 1,690 2,077 Mortgage-backed securities 1,601,386 1,594,834 Total $ 3,042,618 $ 3,087,473
Continuous Unrealized-Loss Position of Available-for-Sale Securities	The following tables disclose, as of December 31, 2017 and 2016, the Company’s investment securities that have been in a continuous unrealized-loss Less than 12 Months 12 Months or Longer Total December 31, 2017 Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized Obligations of U.S. government sponsored enterprises and agencies $ 60,329 $ 186 $ — $ — $ 60,329 $ 186 Obligations of state and political subdivisions 66,361 219 44,938 717 111,299 936 Corporate bonds and other 224 2 237 3 461 5 Residential mortgage-backed securities 701,252 3,988 239,641 4,928 940,893 8,916 Commercial mortgage-backed securities 239,548 1,500 92,549 960 332,097 2,460 Total $ 1,067,714 $ 5,895 $ 377,365 $ 6,608 $ 1,445,079 $ 12,503 Less than 12 Months 12 Months or Longer Total December 31, 2016 Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized Obligations of state and political subdivisions $ 446,052 $ 9,997 $ 1,209 $ 16 $ 447,261 $ 10,013 Corporate bonds and other 244 3 — — 244 3 Residential mortgage-backed securities 372,331 4,532 33,227 981 405,558 5,513 Commercial mortgage-backed securities 193,495 1,180 13,263 50 206,758 1,230 Total $ 1,012,122 $ 15,712 $ 47,699 $ 1,047 $ 1,059,821 $ 16,759

Loans and Allowance for Loan 32

Loans and Allowance for Loan Losses (Tables)	12 Months Ended
	Dec. 31, 2017
Receivables [Abstract]
Loans Held-for-Investment by Class of Financing Receivables	Loans held-for-investment December 31, 2017 2016 Commercial $ 684,099 $ 674,410 Agricultural 94,543 84,021 Real estate 2,302,998 2,189,844 Consumer 403,929 409,032 Total loans held-for-investment $ 3,485,569 $ 3,357,307
Non-Accrual Loans, Loan Still Accruing and Past Due 90 Days or More and Restructured Loans	The Company’s non-accrual December 31, 2017 2016 Non-accrual $ 17,670 $ 27,371 Loans still accruing and past due 90 days or more 288 284 Troubled debt restructured loans** 627 701 Total $ 18,585 $ 28,356 * Includes $618,000 and $1,256,000, respectively, of purchased credit impaired loans as of December 31, 2017 and 2016. ** Our troubled debt restructured loans of $4,629,000 and $6,863,000, whose interest collection, after considering economic and business conditions and collection efforts, is doubtful are included in non-accrual
Recorded Investment in Impaired Loans and Related Valuation Allowance	The Company’s recorded investment in impaired loans and the related valuation allowance are as follows (in thousands): December 31, 2017 December 31, 2016 Recorded Investment Valuation Allowance Recorded Investment Valuation Allowance $17,670 $ 3,996 $ 27,371 $ 5,012
Schedule of Non-Accrual Loans	Non-accrual December 31, 2017 2016 Commercial $ 3,612 $ 7,284 Agricultural 134 99 Real Estate 12,838 18,754 Consumer 1,086 1,234 Total $ 17,670 $ 27,371
Schedule of Impaired Loans and Related Allowance	The Company’s impaired loans and related allowance as of December 31, 2017 and 2016 are summarized in the following tables by class of financing receivables (in thousands). No interest income was recognized on impaired loans subsequent to their classification as impaired. December 31, 2017 Unpaid Recorded Recorded Total Related 12 Month Commercial $ 5,597 $ 518 $ 3,094 $ 3,612 $ 1,194 $ 4,849 Agricultural 147 — 134 134 31 120 Real Estate 16,823 2,348 10,490 12,838 2,316 13,835 Consumer 1,284 143 943 1,086 455 1,258 Total $ 23,851 $ 3,009 $ 14,661 $ 17,670 $ 3,996 $ 20,062 * Includes $618,000 of purchased credit impaired loans. December 31, 2016 Unpaid Recorded Recorded Total Related 12 Month Commercial $ 13,389 $ 1,148 $ 6,136 $ 7,284 $ 2,128 $ 4,921 Agricultural 103 — 99 99 25 50 Real Estate 23,466 6,229 12,525 18,754 2,428 16,170 Consumer 1,421 280 954 1,234 431 914 Total $ 38,379 $ 7,657 $ 19,714 $ 27,371 $ 5,012 $ 22,055 * Includes $1,256,000 of purchased credit impaired loans.
Schedule of Internal Ratings of Loans	The following summarizes the Company’s internal ratings of its loans held-for-investment December 31, 2017 Pass Special Substandard Doubtful Total Commercial $ 649,166 $ 6,282 $ 28,651 $ — $ 684,099 Agricultural 90,457 1,527 2,559 — 94,543 Real Estate 2,227,302 29,089 46,607 — 2,302,998 Consumer 401,434 181 2,314 — 403,929 Total $ 3,368,359 $ 37,079 $ 80,131 $ — $ 3,485,569 December 31, 2016 Pass Special Substandard Doubtful Total Commercial $ 629,756 $ 5,769 $ 38,885 $ — $ 674,410 Agricultural 81,620 715 1,686 — 84,021 Real Estate 2,111,947 18,091 59,806 — 2,189,844 Consumer 406,182 212 2,638 — 409,032 Total $ 3,229,505 $ 24,787 $ 103,015 $ — $ 3,357,307
Schedule of Past Due Loans	At December 31, 2017 and 2016, the Company’s past due loans are as follows (in thousands): December 31, 2017 15-59 60-89 Greater Total Total Total Loans Total 90 Commercial $ 2,039 $ 1,104 $ 1,081 $ 4,224 $ 679,875 $ 684,099 $ 7 Agricultural 640 — — 640 93,903 94,543 — Real Estate 12,308 511 1,198 14,017 2,288,981 2,302,998 216 Consumer 1,360 361 135 1,856 402,073 403,929 65 Total $ 16,347 $ 1,976 $ 2,414 $ 20,737 $ 3,464,832 $ 3,485,569 $ 288 December 31, 2016 15-59 60-89 Greater Total Total Total Loans Total 90 Commercial $ 3,908 $ 1,122 $ 2,220 $ 7,250 $ 667,160 $ 674,410 $ 10 Agricultural 185 — — 185 83,836 84,021 — Real Estate 13,172 1,301 5,268 19,741 2,170,103 2,189,844 272 Consumer 1,845 368 122 2,335 406,697 409,032 2 Total $ 19,110 $ 2,791 $ 7,610 $ 29,511 $ 3,327,796 $ 3,357,307 $ 284 * The Company monitors commercial, agricultural and real estate loans after such loans are 15 days past due. Consumer loans are monitored after such loans are 30 days past due.
Schedule of Allowance for Loan Losses by Portfolio Segment	The following table details the allowance for loan losses at December 31, 2017 and 2016 by portfolio segment (in thousands). There were no allowances for purchased credit impaired loans at December 31, 2017 or 2016. Allocation of a portion of the allowance to one category of loans does not preclude its availability to absorb losses in other categories. December 31, 2017 Commercial Agricultural Real Consumer Total Loans individually evaluated for impairment $ 1,194 $ 31 $ 2,316 $ 455 $ 3,996 Loan collectively evaluated for impairment 9,671 1,274 27,580 5,635 44,160 Total $ 10,865 $ 1,305 $ 29,896 $ 6,090 $ 48,156 December 31, 2016 Commercial Agricultural Real Consumer Total Loans individually evaluated for impairment $ 2,128 $ 25 $ 2,428 $ 431 $ 5,012 Loan collectively evaluated for impairment 9,579 1,076 24,436 5,676 40,767 Total $ 11,707 $ 1,101 $ 26,864 $ 6,107 $ 45,779
Changes in Allowance for Loan Losses	Changes in the allowance for loan losses for the years ended December 31, 2017 and 2016 are summarized as follows (in thousands): December 31, 2017 Commercial Agricultural Real Consumer Total Beginning balance $ 11,707 $ 1,101 $ 26,864 $ 6,107 $ 45,779 Provision for loan losses 1,233 243 4,055 999 6,530 Recoveries 943 32 192 501 1,668 Charge-offs (3,018 ) (71 ) (1,215 ) (1,517 ) (5,821 ) Ending balance $ 10,865 $ 1,305 $ 29,896 $ 6,090 $ 48,156 December 31, 2016 Commercial Agricultural Real Consumer Total Beginning balance $ 12,644 $ 1,191 $ 24,375 $ 3,667 $ 41,877 Provision for loan losses 5,101 104 1,150 3,857 10,212 Recoveries 952 25 2,021 508 3,506 Charge-offs (6,990 ) (219 ) (682 ) (1,925 ) (9,816 ) Ending balance $ 11,707 $ 1,101 $ 26,864 $ 6,107 $ 45,779
Schedule of Investment in Loans Related to Balance in Allowance for Loan Losses on Basis of Company's Impairment Methodology	The Company’s recorded investment in loans as of December 31, 2017 and 2016 related to the balance in the allowance for loan losses on the basis of the Company’s impairment methodology was as follows (in thousands). Purchased credit impaired loans of $618,000 and $1,256,000, respectively, at December 31, 2017 and 2016 are included in loans individually evaluated for impairment. December 31, 2017 Commercial Agricultural Real Estate Consumer Total Loans individually evaluated for impairment $ 3,612 $ 134 $ 12,838 $ 1,086 $ 17,670 Loan collectively evaluated for impairment 680,487 94,409 2,290,160 402,843 3,467,899 Total $ 684,099 $ 94,543 $ 2,302,998 $ 403,929 $ 3,485,569 December 31, 2016 Commercial Agricultural Real Estate Consumer Total Loans individually evaluated for impairment $ 7,284 $ 99 $ 18,754 $ 1,234 $ 27,371 Loan collectively evaluated for impairment 667,126 83,922 2,171,090 407,798 3,329,936 Total $ 674,410 $ 84,021 $ 2,189,844 $ 409,032 $ 3,357,307
Schedule of Loans Modified and Considered Troubled Debt Restructurings	The Company’s loans that were modified in the years ended December 31, 2017 and 2016, and considered troubled debt restructurings are as follows (in thousands): Year Ended December 31, 2017 Year Ended December 31, 2016 Number Pre-Modification Post- Number Pre-Modification Post- Commercial 11 $ 895 $ 895 15 $ 3,208 $ 3,208 Agricultural — — — — — — Real Estate 5 625 625 6 1,460 1,460 Consumer 1 25 25 7 189 189 Total 17 $ 1,545 $ 1,545 28 $ 4,857 $ 4,857
Schedule of How Loans Were Modified as Troubled Debt Restructured Loans	The balances below provide information as to how the loans were modified as troubled debt restructured loans during the years ended December 31, 2017 and 2016 (in thousands): Year Ended December 31, 2017 Year Ended December 31, 2016 Adjusted Extended Combined Adjusted Extended Combined Commercial $ — $ 195 $ 700 $ — $ 2,560 $ 648 Agricultural — — — — — — Real Estate — 312 313 — 298 1,162 Consumer — 25 — — 70 119 Total $ — $ 532 $ 1,013 $ — $ 2,928 $ 1,929
Schedule of Troubled Debt Restructurings	The loans with payment default are as follows (dollars in thousands): Year Ended Year Ended Number Balance Number Balance Commercial 2 $ 88 4 $ 1,690 Agriculture — — — — Real Estate — — 3 921 Consumer — — — — Total 2 $ 88 7 $ 2,611
Analysis of Changes in Loans to Officers, Directors, Principal Shareholders, or Associates of Such Persons	An analysis of the changes in loans to officers, directors, principal shareholders, or associates of such persons for the year ended December 31, 2017 (determined as of each respective year-end) Beginning Additional Payments Ending Year ended December 31, 2017 $44,429 $58,420 $46,945 $55,904

Bank Premises and Equipment (Ta

Bank Premises and Equipment (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Summary of Bank Premises and Equipment	The following is a summary of bank premises and equipment (in thousands): Useful Life December 31, 2017 2016 Land — $ 29,508 $ 28,266 Buildings 20 to 40 years 119,728 115,566 Furniture and equipment 3 to 10 years 58,672 58,145 Leasehold improvements Lesser of lease term or 5 to 15 years 4,118 4,783 212,026 206,760 Less-accumulated depreciation and amortization (88,000 ) (84,075 ) Total Bank Premises and Equipment $ 124,026 $ 122,685

Deposits and Borrowings (Tables

Deposits and Borrowings (Tables)	12 Months Ended
	Dec. 31, 2017
Banking and Thrift [Abstract]
Scheduled Maturities of Time Deposits	At December 31, 2017, the scheduled maturities of time deposits (in thousands) were, as follows: Year ending December 31, 2018 $ 384,668 2019 38,031 2020 14,607 2021 6,645 2022 7,304 Thereafter — $ 451,255
Schedule of Borrowings	Borrowings at December 31, 2017 and 2016 consisted of the following (dollars in thousands): December 31, 2017 2016 Securities sold under agreements with customers to repurchase $ 320,450 $ 360,820 Federal funds purchased 10,550 9,950 Advances from Federal Home Loan Bank of Dallas — 75,000 Total $ 331,000 $ 445,770

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Tax Expense	The Company files a consolidated federal income tax return. Income tax expense is comprised of the following (dollars in thousands): Year Ended December 31, 2017 2016 2015 Current federal income tax $ 34,421 $ 30,381 $ 31,014 Current state income tax 99 99 103 Deferred federal income tax expense (benefit) (53 ) 673 320 Restatement of net deferred tax liability due to change in income tax rate (7,650 ) — — Income tax expense $ 26,817 $ 31,153 $ 31,437
Percentage of Pretax Earnings, Differs from Statutory Federal Income Tax Rate	Income tax expense, as a percentage of pretax earnings, differs from the statutory federal income tax rate as follows: As a Percent of Pretax 2017 2016 2015 Statutory federal income tax rate 35.0 % 35.0 % 35.0 % Restatement of net deferred tax liability due to change in income tax rate (5.3 ) — — Reductions in tax rate resulting from interest income exempt from federal income tax (11.5 ) (12.1 ) (11.4 ) Effect of state income tax 0.1 0.1 0.1 ESOP tax deduction (0.2 ) (0.2 ) (0.2 ) Other 0.1 0.1 0.3 Effective income tax rate 18.2 % 22.9 % 23.8 %
Schedule of Deferred Tax Assets and Liabilities	The approximate effects of each type of difference that gave rise to the Company’s deferred tax assets and liabilities at December 31, 2017 and 2016 are as follows (dollars in thousands): 2017 2016 Deferred tax assets: Tax basis of loans in excess of financial statement basis $ 10,550 $ 17,006 Minimum liability in defined benefit plan 766 1,641 Recognized for financial reporting purposes but not yet for tax purposes: Deferred compensation 1,818 2,807 Write-downs and adjustments to other real estate owned and repossessed assets 11 9 Other deferred tax assets 79 226 Total deferred tax assets $ 13,224 $ 21,689 Deferred tax liabilities: Financial statement basis of fixed assets in excess of tax basis $ 3,343 5,870 Intangible asset amortization deductible for tax purposes, but not for financial reporting purposes 9,926 15,191 Recognized for financial reporting purposes but not yet for tax purposes: Accretion on investment securities 1,039 1,788 Pension plan contributions 1,086 1,799 Net unrealized gain on investment securities available-for-sale 9,420 11,573 Other deferred tax liabilities 31 83 Total deferred tax liabilities $ 24,845 $ 36,304 Net deferred tax asset (liability) $ (11,621 ) $ (14,615 )

Fair Value Disclosures (Tables)

Fair Value Disclosures (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis	The following table summarizes financial assets and financial liabilities measured at fair value on a recurring basis as of December 31, 2017 and 2016 segregated by the level of the valuation inputs within the fair value hierarchy utilized to measure fair value (in thousands): December 31, 2017 Level 1 Level 2 Level 3 Total Fair Available-for-sale Obligations of U.S. government sponsored enterprises and agencies $ — $ 60,330 $ — $ 60,330 Obligations of state and political subdivisions — 1,420,850 — 1,420,850 Corporate bonds — 7,031 — 7,031 Residential mortgage-backed securities — 1,219,097 — 1,219,097 Commercial mortgage-backed securities — 375,737 — 375,737 Other securities 4,428 — — 4,428 Total $ 4,428 $ 3,083,045 $ — $ 3,087,473 December 31, 2016 Level 1 Level 2 Level 3 Total Fair Available-for-sale U.S. Treasury securities $ 10,668 $ — $ — $ 10,668 Obligations of U.S. government sponsored enterprises and agencies — 113,703 — 113,703 Obligations of state and political subdivisions — 1,564,276 — 1,564,276 Corporate bonds — 47,965 — 47,965 Residential mortgage-backed securities — 851,361 — 851,361 Commercial mortgage-backed securities — 268,436 — 268,436 Other securities 4,428 — — 4,428 Total $ 15,096 $ 2,845,741 $ — $ 2,860,837
Other Real Estate Owned	The following table presents other real estate owned that were re-measured Year Ended 2017 2016 Carrying value of other real estate owned prior to re-measurement $ 1,067 $ — Write-downs included in gain (loss) on sale of other real estate owned (306 ) — Fair value $ 761 $ —
Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments	The estimated fair values and carrying values of all financial instruments under current authoritative guidance at December 31, 2017 and 2016, were as follows (dollars in thousands): 2017 2016 Fair Value Carrying Estimated Carrying Estimated Cash and due from banks $ 209,583 $ 209,583 $ 204,782 $ 204,782 Level 1 Federal funds sold — — 3,130 3,130 Level 1 Interest-bearing deposits in banks 162,764 162,764 48,574 48,574 Level 1 Interest-bearing time deposits in banks 1,458 1,458 1,707 1,709 Level 2 Available-for-sale 3,087,473 3,087,473 2,860,837 2,860,837 Levels 1 and 2 Held-to-maturity — — 121 124 Level 2 Loans 3,452,543 3,470,133 3,338,426 3,361,735 Level 3 Accrued interest receivable 36,081 36,081 36,469 36,469 Level 2 Deposits with stated maturities 451,255 452,000 508,996 510,304 Level 2 Deposits with no stated maturities 5,511,706 5,511,706 4,969,543 4,969,543 Level 1 Borrowings 331,000 331,000 445,770 445,770 Level 2 Accrued interest payable 197 197 225 225 Level 2

Financial Instruments with Of37

Financial Instruments with Off-Balance-Sheet Risk (Tables)	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Schedule of Financial Instruments with Off-Balance-Sheet Risk	We generally use the same credit policies in making commitments and conditional obligations as we do for on-balance-sheet Total Notional Financial instruments whose contract amounts represent credit risk: Unfunded lines of credit $ 534,468 Unfunded commitments to extend credit 244,658 Standby letters of credit 28,858 Total commercial commitments $ 807,984

Pension and Profit Sharing Pl38

Pension and Profit Sharing Plans (Tables)	12 Months Ended
	Dec. 31, 2017
Retirement Benefits [Abstract]
Benefit Obligation Activity and Fair Value of Plan Assets and Statement of Funded Status	Using an actuarial measurement date of December 31, 2017 and 2016, benefit obligation activity and fair value of plan assets for the years ended December 31, 2017 and 2016, and a statement of the funded status as of December 31, 2017 and 2016, are as follows (dollars in thousands): 2017 2016 Reconciliation of benefit obligations: Benefit obligation at January 1 $ 15,453 $ 16,002 Interest cost on projected benefit obligation 635 665 Actuarial loss 486 139 Benefits paid, including partial settlement of certain participant balances (1,043 ) (1,353 ) Benefit obligation at December 31 $ 15,531 $ 15,453 Reconciliation of fair value of plan assets: Fair value of plan assets at January 1 $ 15,787 $ 14,820 Actual return on plan assets 2,302 1,820 Employer contributions — 500 Benefits paid, including partial settlement of certain participant balances (1,043 ) (1,353 ) Fair value of plan assets at December 31 17,046 15,787 Funded status $ 1,515 $ 334
Component of Accumulated Other Comprehensive Earnings	Amounts recognized as a component of accumulated other comprehensive earnings as of year-end 2017 2016 Net actuarial loss $ (3,597 ) $ (4,688 ) Deferred tax benefit 1,227 1,641 Amounts included in accumulated other comprehensive earnings, net of tax $ (2,370 ) $ (3,047 )
Net Periodic Pension Cost	Net periodic benefit cost for the years ended December 31, 2017, 2016 and 2015, are as follows (dollars in thousands): Year Ended December 31, 2017 2016 2015 Service cost — benefits earned during the period $ — $ — $ — Interest cost on projected benefit obligation 635 665 622 Expected return on plan assets (974 ) (912 ) (948 ) Amortization of unrecognized net loss 249 375 222 Recognized loss on partial settlement of certain participant balances — 267 — Net periodic pension benefit expense (benefit) $ (90 ) $ 395 $ (104 )
Benefit Obligations and Net Periodic Pension Cost and Rate of Return on Plan Assets	The following table sets forth the rates used in the actuarial calculations of the present value of benefit obligations and net periodic pension cost and the rate of return on plan assets: 2017 2016 2015 Weighted average discount rate 3.50 % 4.25 % 4.25 % Expected long-term rate of return on assets 6.25 % 6.25 % 6.25 %
Pension Plan and Targeted Allocation Percentage	The major type of plan assets in the pension plan and the targeted allocation percentage as of December 31, 2017 and 2016 is as follows: December 31, December 31, Targeted Equity securities 75 % 77 % 75 % Debt securities 24 % 22 % 25 % Cash and equivalents 1 % 1 % —
Breakdown by Level	The breakdown by level is as follows (dollars in thousands): Level 1 Level 2 Level 3 Total Fair Money market fund $ 274 $ — $ — $ 274 U.S. Treasury notes — 149 — 149 Obligations of state and political subdivisions — 634 — 634 Corporate bonds — 868 — 868 Mortgage-backed securities — 1,130 — 1,130 Corporate stocks and mutual funds 13,991 — — 13,991 Total $ 14,265 $ 2,781 — $ 17,046
Estimate of Undiscounted Projected Future Payments	An estimate of the undiscounted projected future payments to eligible participants for the next five years and the following five years in the aggregate is as follows (in thousands): Year Ending December 31, 2018 $ 1,064 2019 $ 1,018 2020 $ 1,036 2021 $ 1,077 2022 $ 1,063 2023 forward $ 5,309

Regulatory Matters (Tables)

Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2017
Banking and Thrift [Abstract]
Schedule of Regulatory Capital Ratios Under the Basel III Regulatory Capital Framework	As of December 31, 2017 and 2016, the regulatory capital ratios of the Company and Bank under the Basel III regulatory capital framework are as follows: Actual Minimum Capital Phase-In Minimum Capital Phased-In Required to be As of December 31, 2017: Amount Ratio Amount Ratio Amount Ratio Amount Ratio Total Capital to Risk-Weighted Assets: Consolidated $ 814,634 19.85 % $ 379,578 9.250 % $ 430,872 10.50 % — N/A First Financial Bank, N.A $ 723,563 17.68 % $ 378,614 9.250 % $ 429,777 10.50 % $ 409,312 10.00 % Tier 1 Capital to Risk-Weighted Assets: Consolidated $ 765,882 18.66 % $ 297,507 7.250 % $ 348,801 8.50 % — N/A First Financial Bank, N.A $ 674,811 16.49 % $ 296,751 7.250 % $ 347,915 8.50 % $ 327,450 8.00 % Common Equity Tier 1 Capital to Risk-Weighted Assets: Consolidated $ 765,882 18.66 % $ 235,954 5.750 % $ 287,248 7.00 % — N/A First Financial Bank, N.A $ 674,811 16.49 % $ 235,354 5.750 % $ 286,518 7.00 % $ 266,053 6.50 % Leverage Ratio: Consolidated $ 765,882 11.09 % $ 276,296 4.000 % $ 276,296 4.00 % — N/A First Financial Bank, N.A $ 674,811 9.80 % $ 275,320 4.000 % $ 275,320 4.00 % $ 344,151 5.00 % Actual Minimum Capital Phase-In Minimum Capital Phased-In Required to be As of December 31, 2016: Amount Ratio Amount Ratio Amount Ratio Amount Ratio Total Capital to Risk-Weighted Assets: Consolidated $ 739,959 18.45 % $ 345,827 8.625 % $ 421,007 10.50 % — N/A First Financial Bank, N.A $ 633,403 15.84 % $ 344,930 8.625 % $ 419,915 10.50 % $ 399,919 10.00 % Tier 1 Capital to Risk-Weighted Assets: Consolidated $ 693,584 17.30 % $ 265,635 6.625 % $ 340,815 8.50 % — N/A First Financial Bank, N.A $ 587,028 14.68 % $ 264,946 6.625 % $ 339,931 8.50 % $ 319,935 8.00 % Common Equity Tier 1 Capital to Risk-Weighted Assets: Consolidated $ 693,584 17.30 % $ 205,491 5.125 % $ 280,671 7.00 % — N/A First Financial Bank, N.A $ 587,028 14.68 % $ 204,959 5.125 % $ 279,943 7.00 % $ 259,947 6.50 % Leverage Ratio: Consolidated $ 693,584 10.71 % $ 258,978 4.00 % $ 258,978 4.00 % — N/A First Financial Bank, N.A $ 587,028 9.10 % $ 257,941 4.00 % $ 257,941 4.00 % $ 322,426 5.00 %

Stock Option Plan and Restric40

Stock Option Plan and Restricted Stock Plan (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Analysis of Stock Option Activity	An analysis of stock option activity for the year ended December 31, 2017 is presented in the table and narrative below: Shares Weighted- Weighted- Aggregate Outstanding, beginning of year 1,094,035 $ 27.40 Granted 452,450 42.35 Exercised (140,250 ) 20.92 Cancelled (80,270 ) 30.33 Outstanding, end of year 1,325,965 33.01 7.12 $ 15,966 Exercisable at end of year 458,715 $ 23.94 4.66 $ 9,684
Schedule of Information Concerning Outstanding and Vested Stock Options	The following table summarizes information concerning outstanding and vested stock options as of December 31, 2017: Exercise Price Number Outstanding Remaining Contracted Number Vested $16.78 87,640 1.4 87,640 15.73 142,620 3.8 142,620 30.85 270,505 5.8 156,225 33.89 382,750 7.8 72,230 $42.35 442,450 9.5 —

Condensed Financial Informati41

Condensed Financial Information - Parent Company (Tables)	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed Balance Sheets	Condensed Balance Sheets-December 31, 2017 and 2016 2017 2016 ASSETS Cash in subsidiary bank $ 14,272 $ 15,070 Cash in unaffiliated banks 2 2 Interest-bearing deposits in subsidiary bank 64,195 78,179 Total cash and cash equivalents 78,469 93,251 Securities available-for-sale, 8,515 11,593 Investment in and advances to subsidiaries, at equity 847,445 744,971 Intangible assets 723 723 Other assets 2,654 2,668 Total assets $ 937,806 $ 853,206 LIABILITIES AND SHAREHOLDERS’ EQUITY Total liabilities $ 15,038 $ 15,321 Shareholders’ equity: Common stock 663 661 Capital surplus 378,062 372,245 Retained earnings 517,257 446,534 Treasury stock (7,148 ) (6,671 ) Deferred compensation 7,148 6,671 Accumulated other comprehensive earnings 26,786 18,445 Total shareholders’ equity 922,768 837,885 Total liabilities and shareholders’ equity $ 937,806 $ 853,206
Condensed Statements of Earnings	Condensed Statements of Earnings- For the Years Ended December 31, 2017, 2016 and 2015 2017 2016 2015 Income: Cash dividends from subsidiaries $ 30,800 $ 48,800 $ 51,200 Excess of earnings over dividends of subsidiaries 92,929 58,809 52,911 Other 6,590 4,184 4,185 Total Income 130,319 111,793 108,296 Expenses: Salaries and employee benefits 8,606 5,655 6,067 Other operating expenses 3,871 3,531 4,439 Total Expense 12,477 9,186 10,506 Earnings before income taxes 117,842 102,607 97,790 Income tax benefit 2,529 2,167 2,591 Net earnings $ 120,371 $ 104,774 $ 100,381
Condensed Statements of Cash Flows	Condensed Statements of Cash Flows- For the Years Ended December 31, 2017, 2016 and 2015 2017 2016 2015 Cash flows from operating activities: Net earnings $ 120,371 $ 104,774 $ 100,381 Adjustments to reconcile net earnings to net cash provided by operating activities: Excess of earnings over dividends of subsidiary bank (92,929 ) (58,809 ) (52,911 ) Depreciation and amortization, net 207 208 197 Decrease (increase) in other assets 438 1,702 507 Increase (decrease) in other liabilities 183 (1,374 ) 3,743 Other 2 8 — Net cash provided by operating activities 28,272 46,509 51,917 Cash flows from investing activities: Cash received in connection with acquisition of banks — — 13,125 Maturity of available-for-sale 2,997 — — Purchases of bank premises and equipment (30 ) (94 ) (107 ) Repayment from investment in and advances to subsidiaries, net — — 5,800 Other — 10 — Net cash used in (provided by) investing activities 2,967 (84 ) 18,818 Cash flows from financing activities: Repayment of subordinated debt — — (13,125 ) Proceeds of stock issuances 2,934 1,260 1,545 Cash dividends paid (48,955 ) (44,907 ) (38,767 ) Net cash used in financing activities (46,021 ) (43,647 ) (50,347 ) Net increase (decrease) in cash and cash equivalents (14,782 ) 2,778 20,388 Cash and cash equivalents, beginning of year 93,251 90,473 70,085 Cash and cash equivalents, end of year $ 78,469 $ 93,251 $ 90,473

Cash Flow Information (Tables)

Cash Flow Information (Tables)	12 Months Ended
	Dec. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Supplemental Information on Cash Flows and Noncash Transactions	Supplemental information on cash flows and noncash transactions is as follows (dollars in thousands): Year Ended December 31, 2017 2016 2015 Supplemental cash flow information: Interest paid $ 9,316 $ 5,465 $ 4,085 Federal income taxes paid 29,695 28,348 29,674 Schedule of noncash investing and financing activities: Assets acquired through foreclosure 2,211 2,269 203 Investment securities purchased but not settled — 12,381 — Restricted stock grant to officers and directors 1,139 810 1,310

Acquisitions and Asset Purcha43

Acquisitions and Asset Purchase (Tables) - FBC Bancshares, Inc. and First Bank, N.A. [Member]	12 Months Ended
	Dec. 31, 2017
Schedule of Amounts Recorded on Consolidated Balance Sheet on Acquisition Date	The assets acquired and liabilities assumed were recorded on the consolidated balance sheet at estimated fair value on the acquisition date. The acquisition was not considered to be a significant business combination. The following table presents the amounts recorded on the consolidated balance sheet on the acquisition date (dollars in thousands): Fair value of consideration paid: Common stock issued (1,755,374 shares) $ 59,648
Schedule of Preliminary Estimated Fair Value Amounts Assigned to Major Asset and Liability Categories at Acquisition Date	Fair value of consideration paid: Common stock issued (1,755,374 shares) $ 59,648 Fair value of identifiable assets acquired: Cash and cash equivalents 65,197 Securities available-for-sale 42,903 Loans 248,380 Identifiable intangible assets 2,343 Other assets 15,262 Total identifiable assets acquired 374,085 Fair value of liabilities assumed: Deposits 343,583 Subordinated debt 13,125 Other liabilities 1,651 Total liabilities assumed 358,359 Fair value of net identifiable assets acquired 15,726 Goodwill resulting from acquisition $ 43,922

Summary of Significant Accoun44

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017USD ($)Locationshares	Dec. 21, 2017	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)shares	Sep. 30, 2017shares	Jun. 25, 2017shares	Jul. 31, 2015	Apr. 28, 2015shares
Summary Of Significant Accounting Policies [Line Items]
Number of locations | Location		69
Stock repurchase program expiration date		Sep. 30, 2020
Minimum number of shares that company is required to repurchase | shares		0
Stock repurchased under authorization | shares		0		0	0
Common stock, shares authorized | shares		120,000,000		120,000,000					120,000,000
Minimum number of days to consider the loans as non-accrual		90 days
Loans individually evaluated for impairment		$ 17,670,000		$ 27,371,000
Contractual balance		23,851,000		38,379,000
Impairment recorded		$ 0		0	$ 0
Other identifiable intangible assets, amortized period (years)		7 years
Goodwill and intangible assets with indefinite lives		$ 139,971,000		139,971,000
Carrying amount of goodwill arising from acquisitions, is deductible for federal income tax purposes		74,376,000		74,376,000
Other intangible assets mortgage servicing rights				1,795,000
Loss on sale of mortgage servicing rights		$ (215,000)
Securities sold under agreements to repurchase maturity range (in days)		One to four days
Interest-bearing time deposits, maturity period (in days)		90 days or less
Unrealized net gains in available-for-sale securities, included in accumulated other comprehensive income		$ 29,156,000		21,492,000
Minimum pension liability, included in accumulated other comprehensive income		(2,370,000)		(3,047,000)
Stock compensation expense		$ 1,745,000		$ 882,000	$ 644,000
Anti-dilutive securities excluded from the EPS calculations | shares		0		0	442,368
Federal statutory tax rate		35.00%	35.00%	35.00%	35.00%
Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Reclassification of standard tax debit within accumulated other comprehensive income to retained earnings		$ 5,761,000
Scenario, Forecast [Member]
Summary Of Significant Accounting Policies [Line Items]
Federal statutory tax rate	21.00%
Purchased Credit Impaired Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Loans individually evaluated for impairment		618,000		$ 1,256,000
Contractual balance		755,000		1,865,000
Director [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted stock expense		483,000		278,000	$ 139,000
Officers [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted stock expense		562,000		$ 381,000	$ 62,000
FBC Bancshares, Inc. and First Bank, N.A. [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of common stock acquired								100.00%
Goodwill and intangible assets with indefinite lives		43,922,000
FBC Bancshares, Inc. and First Bank, N.A. [Member] | Purchased Credit Impaired Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Contractual balance		$ 1,704,000
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Stock repurchase program, number of shares authorized to be repurchased | shares						1,500,000	2,000,000

Summary of Significant Accoun45

Summary of Significant Accounting Policies - Computation of Basic EPS to Dilutive EPS (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net earnings	$ 120,371	$ 104,774	$ 100,381
Effect of stock options and stock grants, Net Earnings	0	0	0
Net earnings per share, assuming dilution, Net Earnings	$ 120,371	$ 104,774	$ 100,381
Net earnings per share, basic, Weighted Average Shares	66,126,863	66,013,004	64,892,934
Effect of stock options and stock grants, Weighted Average Shares	197,467	89,882	175,096
Net earnings per share, assuming dilution, Weighted Average Shares	66,324,330	66,102,886	65,068,030
Net earnings per share, basic, Per Share Amount	$ 1.82	$ 1.59	$ 1.55
Effect of stock options and stock grants, Per Share Amount	(0.01)		(0.01)
Net earnings per share, assuming dilution, Per Share Amount	$ 1.81	$ 1.59	$ 1.54

Interest-bearing Time Deposit46

Interest-bearing Time Deposits in Banks and Securities - Additional Information (Detail) $ in Thousands	Dec. 31, 2017USD ($)Investment	Dec. 31, 2017USD ($)Investment	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Interest Bearing Time Deposits In Banks And Securities [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	$ 1,458	$ 1,458	$ 1,707
Held-to-maturity securities	$ 0	$ 0	121
Number of investment positions | Investment	338	338
Securities pledged as collateral	$ 2,018,420	$ 2,018,420	1,940,460
Sales of investment securities available-for-sale		120,576	40,510	$ 35,580
Gross realized gains from security sales		2,643	1,579	443
Gross realized losses from security sales or calls		$ 815	$ 309	$ 11
Obligations of State and Political Subdivisions [Member] | Texas [Member]
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Percentage of securities guaranteed by Texas Permanent School Fund	31.21%
Obligations of State and Political Subdivisions [Member] | Texas [Member] | Available-for-Sale Securities [Member] | Geographic Concentration Risk [Member]
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Concentration risk, percentage	82.98%
Minimum [Member]
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Interest-bearing time deposits, maturity period, years		1 year
Maximum [Member]
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Interest-bearing time deposits, maturity period, years		3 years

Interest-bearing Time Deposit47

Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 3,042,618	$ 2,827,772
Securities available-for-sale, Gross Unrealized Holding Gains	57,358	49,824
Securities available-for-sale, Gross Unrealized Holding Losses	(12,503)	(16,759)
Securities available-for-sale, Estimated Fair Value	3,087,473	2,860,837
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total		10,649
Securities available-for-sale, Gross Unrealized Holding Gains		19
Securities available-for-sale, Estimated Fair Value		10,668
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	60,516	113,450
Securities available-for-sale, Gross Unrealized Holding Gains		253
Securities available-for-sale, Gross Unrealized Holding Losses	(186)
Securities available-for-sale, Estimated Fair Value	60,330	113,703
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,369,295	1,534,095
Securities available-for-sale, Gross Unrealized Holding Gains	52,491	40,194
Securities available-for-sale, Gross Unrealized Holding Losses	(936)	(10,013)
Securities available-for-sale, Estimated Fair Value	1,420,850	1,564,276
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	11,421	51,920
Securities available-for-sale, Gross Unrealized Holding Gains	43	476
Securities available-for-sale, Gross Unrealized Holding Losses	(5)	(3)
Securities available-for-sale, Estimated Fair Value	11,459	52,393
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,223,452	848,614
Securities available-for-sale, Gross Unrealized Holding Gains	4,561	8,260
Securities available-for-sale, Gross Unrealized Holding Losses	(8,916)	(5,513)
Securities available-for-sale, Estimated Fair Value	1,219,097	851,361
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	377,934	269,044
Securities available-for-sale, Gross Unrealized Holding Gains	263	622
Securities available-for-sale, Gross Unrealized Holding Losses	(2,460)	(1,230)
Securities available-for-sale, Estimated Fair Value	$ 375,737	$ 268,436

Interest-bearing Time Deposit48

Interest-bearing Time Deposits in Banks and Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 184,538
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	631,376
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	623,628
Available-for-Sale, Amortized Cost Basis, Due after ten years	1,690
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,601,386
Available-for-Sale, Amortized Cost Basis, Total	3,042,618	$ 2,827,772
Available-for-Sale, Estimated Fair Value, Due within one year	185,547
Available-for-Sale, Estimated Fair Value, Due after one year through five years	654,092
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	650,923
Available-for-Sale, Estimated Fair Value, Due after ten years	2,077
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,594,834
Securities available-for-sale, Estimated Fair Value, Total	$ 3,087,473	$ 2,860,837

Interest-bearing Time Deposit49

Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 1,067,714	$ 1,012,122
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	5,895	15,712
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	377,365	47,699
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	6,608	1,047
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,445,079	1,059,821
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	12,503	16,759
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	60,329
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	186
Available-for-sale securities, continuous unrealized loss position, Fair Value	60,329
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	186
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	66,361	446,052
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	219	9,997
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	44,938	1,209
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	717	16
Available-for-sale securities, continuous unrealized loss position, Fair Value	111,299	447,261
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	936	10,013
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	224	244
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	2	3
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	237
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	3
Available-for-sale securities, continuous unrealized loss position, Fair Value	461	244
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	5	3
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	701,252	372,331
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	3,988	4,532
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	239,641	33,227
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	4,928	981
Available-for-sale securities, continuous unrealized loss position, Fair Value	940,893	405,558
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	8,916	5,513
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	239,548	193,495
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1,500	1,180
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	92,549	13,263
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	960	50
Available-for-sale securities, continuous unrealized loss position, Fair Value	332,097	206,758
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 2,460	$ 1,230

Loans and Allowance for Loan 50

Loans and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 3,485,569	$ 3,357,307
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	684,099	674,410
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	94,543	84,021
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	2,302,998	2,189,844
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 403,929	$ 409,032

Loans and Allowance for Loan 51

Loans and Allowance for Loan Losses - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	$ 15,130,000	$ 26,898,000	$ 15,130,000	$ 26,898,000
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	20,117,000	29,000,000	20,117,000	29,000,000
Non accrual loans	17,670,000	27,371,000	17,670,000	27,371,000
Additional funds advanced in connection with impaired loans	0		0
Interest income recognized on impaired loans	0	0
Interest income recognized on impaired loans			624,000	829,000	$ 922,000
Allowance for loan losses	48,156,000	45,779,000	48,156,000	45,779,000	$ 41,877,000
Loans individually evaluated for impairment	17,670,000	27,371,000	$ 17,670,000	$ 27,371,000
Default for purposes of this disclosure is a troubled debt restructured loan			90 days	90 days
Commitments to lend additional funds to borrowers with loan that have been modified as TDRs	0		$ 0
Loans held by subsidiaries subject to blanket liens	2,163,425,000		2,163,425,000
Advances from Federal Home Loan Bank of Dallas	0	75,000,000	0	$ 75,000,000
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non accrual loans	618,000	1,256,000	618,000	1,256,000
Allowance for loan losses	0	0	0	0
Loans individually evaluated for impairment	$ 618,000	$ 1,256,000	$ 618,000	$ 1,256,000

Loans and Allowance for Loan 52

Loans and Allowance for Loan Losses - Non-Accrual Loans, Loan Still Accruing and Past Due 90 Days or More and Restructured Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Receivables [Abstract]
Non-accrual loans	$ 17,670	$ 27,371
Loans still accruing and past due 90 days or more	288	284
Troubled debt restructured loans	627	701
Total	$ 18,585	$ 28,356

Loans and Allowance for Loan 53

Loans and Allowance for Loan Losses - Non-Accrual Loans, Loan Still Accruing and Past Due 90 Days or More and Restructured Loans (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	$ 17,670	$ 27,371
Troubled debt restructured loans	627	701
Purchased Credit Impaired Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	618	1,256
Doubtful [Member]
Loans and Leases Receivable Disclosure [Line Items]
Troubled debt restructured loans	$ 4,629	$ 6,863

Loans and Allowance for Loan 54

Loans and Allowance for Loan Losses - Recorded Investment in Impaired Loans and Related Valuation Allowance (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Receivables [Abstract]
Recorded Investment	$ 17,670	$ 27,371
Valuation Allowance	$ 3,996	$ 5,012

Loans and Allowance for Loan 55

Loans and Allowance for Loan Losses - Schedule of Non-Accrual Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	$ 17,670	$ 27,371
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	3,612	7,284
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	134	99
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	12,838	18,754
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	$ 1,086	$ 1,234

Loans and Allowance for Loan 56

Loans and Allowance for Loan Losses - Schedule of Impaired Loans and Related Allowance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	$ 23,851	$ 38,379
Recorded Investment With No Allowance	3,009	7,657
Recorded Investment With Allowance	14,661	19,714
Total Recorded Investment	17,670	27,371
Related Allowance	3,996	5,012
Average Recorded Investment	20,062	22,055
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	5,597	13,389
Recorded Investment With No Allowance	518	1,148
Recorded Investment With Allowance	3,094	6,136
Total Recorded Investment	3,612	7,284
Related Allowance	1,194	2,128
Average Recorded Investment	4,849	4,921
Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	147	103
Recorded Investment With Allowance	134	99
Total Recorded Investment	134	99
Related Allowance	31	25
Average Recorded Investment	120	50
Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	16,823	23,466
Recorded Investment With No Allowance	2,348	6,229
Recorded Investment With Allowance	10,490	12,525
Total Recorded Investment	12,838	18,754
Related Allowance	2,316	2,428
Average Recorded Investment	13,835	16,170
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	1,284	1,421
Recorded Investment With No Allowance	143	280
Recorded Investment With Allowance	943	954
Total Recorded Investment	1,086	1,234
Related Allowance	455	431
Average Recorded Investment	$ 1,258	$ 914

Loans and Allowance for Loan 57

Loans and Allowance for Loan Losses - Schedule of Impaired Loans and Related Allowance (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance	$ 3,009	$ 7,657
Purchased Credit Impaired Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance	$ 618	$ 1,256

Loans and Allowance for Loan 58

Loans and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 3,485,569	$ 3,357,307
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	684,099	674,410
Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	94,543	84,021
Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,302,998	2,189,844
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	403,929	409,032
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,368,359	3,229,505
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	649,166	629,756
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	90,457	81,620
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,227,302	2,111,947
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	401,434	406,182
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	37,079	24,787
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	6,282	5,769
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	1,527	715
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	29,089	18,091
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	181	212
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	80,131	103,015
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	28,651	38,885
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,559	1,686
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	46,607	59,806
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 2,314	$ 2,638

Loans and Allowance for Loan 59

Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 20,737	$ 29,511
Total Current	3,464,832	3,327,796
Total Loans	3,485,569	3,357,307
Total 90 Days Past Due Still Accruing	288	284
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,224	7,250
Total Current	679,875	667,160
Total Loans	684,099	674,410
Total 90 Days Past Due Still Accruing	7	10
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	640	185
Total Current	93,903	83,836
Total Loans	94,543	84,021
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	14,017	19,741
Total Current	2,288,981	2,170,103
Total Loans	2,302,998	2,189,844
Total 90 Days Past Due Still Accruing	216	272
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,856	2,335
Total Current	402,073	406,697
Total Loans	403,929	409,032
Total 90 Days Past Due Still Accruing	65	2
15-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	16,347	19,110
15-59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,039	3,908
15-59 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	640	185
15-59 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	12,308	13,172
15-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,360	1,845
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,976	2,791
60-89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,104	1,122
60-89 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	511	1,301
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	361	368
Greater than 90 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,414	7,610
Greater than 90 Days [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,081	2,220
Greater than 90 Days [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,198	5,268
Greater than 90 Days [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 135	$ 122

Loans and Allowance for Loan 60

Loans and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 3,996	$ 5,012
Loan collectively evaluated for impairment	44,160	40,767
Total allowance for loan losses	48,156	45,779	$ 41,877
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,194	2,128
Loan collectively evaluated for impairment	9,671	9,579
Total allowance for loan losses	10,865	11,707	12,644
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	31	25
Loan collectively evaluated for impairment	1,274	1,076
Total allowance for loan losses	1,305	1,101	1,191
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	2,316	2,428
Loan collectively evaluated for impairment	27,580	24,436
Total allowance for loan losses	29,896	26,864	24,375
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	455	431
Loan collectively evaluated for impairment	5,635	5,676
Total allowance for loan losses	$ 6,090	$ 6,107	$ 3,667

Loans and Allowance for Loan 61

Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 45,779	$ 41,877
Provision for loan losses	6,530	10,212
Recoveries	1,668	3,506
Charge-offs	(5,821)	(9,816)
Ending balance	48,156	45,779
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	11,707	12,644
Provision for loan losses	1,233	5,101
Recoveries	943	952
Charge-offs	(3,018)	(6,990)
Ending balance	10,865	11,707
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,101	1,191
Provision for loan losses	243	104
Recoveries	32	25
Charge-offs	(71)	(219)
Ending balance	1,305	1,101
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	26,864	24,375
Provision for loan losses	4,055	1,150
Recoveries	192	2,021
Charge-offs	(1,215)	(682)
Ending balance	29,896	26,864
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,107	3,667
Provision for loan losses	999	3,857
Recoveries	501	508
Charge-offs	(1,517)	(1,925)
Ending balance	$ 6,090	$ 6,107

Loans and Allowance for Loan 62

Loans and Allowance for Loan Losses - Schedule of Investment in Loans Related to Balance in Allowance for Loan Losses on Basis of Company's Impairment Methodology (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 17,670	$ 27,371
Loan collectively evaluated for impairment	3,467,899	3,329,936
Total	3,485,569	3,357,307
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	3,612	7,284
Loan collectively evaluated for impairment	680,487	667,126
Total	684,099	674,410
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	134	99
Loan collectively evaluated for impairment	94,409	83,922
Total	94,543	84,021
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	12,838	18,754
Loan collectively evaluated for impairment	2,290,160	2,171,090
Total	2,302,998	2,189,844
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,086	1,234
Loan collectively evaluated for impairment	402,843	407,798
Total	$ 403,929	$ 409,032

Loans and Allowance for Loan 63

Loans and Allowance for Loan Losses - Schedule of Loans Modified and Considered Troubled Debt Restructurings (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)SecurityLoan	Dec. 31, 2016USD ($)SecurityLoan
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	17	28
Pre-Modification Recorded Investment	$ 1,545	$ 4,857
Post-Modification Recorded Investment	$ 1,545	$ 4,857
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	11	15
Pre-Modification Recorded Investment	$ 895	$ 3,208
Post-Modification Recorded Investment	$ 895	$ 3,208
Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	5	6
Pre-Modification Recorded Investment	$ 625	$ 1,460
Post-Modification Recorded Investment	$ 625	$ 1,460
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	1	7
Pre-Modification Recorded Investment	$ 25	$ 189
Post-Modification Recorded Investment	$ 25	$ 189

Loans and Allowance for Loan 64

Loans and Allowance for Loan Losses - Schedule of How Loans Were Modified as Troubled Debt Restructured Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	$ 1,545	$ 4,857
Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	532	2,928
Combined Rate and Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	1,013	1,929
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	895	3,208
Commercial [Member] | Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	195	2,560
Commercial [Member] | Combined Rate and Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	700	648
Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	625	1,460
Real Estate [Member] | Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	312	298
Real Estate [Member] | Combined Rate and Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	313	1,162
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	25	189
Consumer [Member] | Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	$ 25	70
Consumer [Member] | Combined Rate and Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		$ 119

Loans and Allowance for Loan 65

Loans and Allowance for Loan Losses - Schedule of Troubled Debt Restructurings (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)Contract	Dec. 31, 2016USD ($)Contract
Financing Receivable, Modifications [Line Items]
Number | Contract	2	7
Balance | $	$ 88	$ 2,611
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number | Contract	2	4
Balance | $	$ 88	$ 1,690
Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number | Contract		3
Balance | $		$ 921

Loans and Allowance for Loan 66

Loans and Allowance for Loan Losses - Analysis of Changes in Loans to Officers, Directors, Principal Shareholders, or Associates of Such Persons (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Receivables [Abstract]
Beginning Balance	$ 44,429
Additional Loans	58,420
Payments	46,945
Ending Balance	$ 55,904

Bank Premises and Equipment - S

Bank Premises and Equipment - Summary of Bank Premises and Equipment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Land	$ 29,508	$ 28,266
Buildings	119,728	115,566
Furniture and equipment	58,672	58,145
Leasehold improvements	4,118	4,783
Gross	212,026	206,760
Less-accumulated depreciation and amortization	(88,000)	(84,075)
Total Bank Premises and Equipment	$ 124,026	$ 122,685
Minimum [Member] | Buildings [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years	20 years
Minimum [Member] | Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years	3 years
Minimum [Member] | Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years	5 years
Maximum [Member] | Buildings [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years		40 years
Maximum [Member] | Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years		10 years
Maximum [Member] | Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years		15 years

Bank Premises and Equipment - A

Bank Premises and Equipment - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 9,810,000	$ 9,390,000	$ 9,125,000
Rental income for leases included in net occupancy expense	2,367,000	2,139,000	1,949,000
Gain (loss) on sale the of bank premises and equipment	(396,000)	168,000	(820,000)
Gain (loss) on sale of building	1,167,000	1,894,000	150,000
Weatherford [Member]
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Building		1,385,000
Gain (loss) on sale of building		560,000
Orange [Member]
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Building		2,000,000
Gain (loss) on sale of building		$ (31,000)
Write down of anticipated building sale			$ 1,000,000
San Angelo [Member]
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Building	1,586,000
Gain (loss) on sale of building	210,000
Write down of anticipated building sale	$ 360,000

Deposits and Borrowings - Addit

Deposits and Borrowings - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Banking and Thrift [Abstract]
Time deposit threshold amount	$ 250,000
Time deposits of $250,000 or more totaled	115,203,000	$ 130,385,000
Deposits received from related parties	74,270,000	114,513,000
Advances from Federal Home Loan Bank of Dallas, maturing during 2017	$ 0	$ 75,000,000
Interest rate on advance		0.46%

Deposits and Borrowings - Sched

Deposits and Borrowings - Scheduled Maturities of Time Deposits (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Banking and Thrift [Abstract]
2,018	$ 384,668
2,019	38,031
2,020	14,607
2,021	6,645
2,022	7,304
Thereafter	0
Time deposits	$ 451,255

Deposits and Borrowings - Sch71

Deposits and Borrowings - Schedule of Borrowings (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Securities sold under agreements with customers to repurchase	$ 320,450,000	$ 360,820,000
Federal funds purchased	10,550,000	9,950,000
Advances from Federal Home Loan Bank of Dallas	0	75,000,000
Total	$ 331,000,000	$ 445,770,000

Line of Credit - Additional Inf

Line of Credit - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Revolving line of credit, permitted	$ 25,000,000
Line of credit, maturity date	Jun. 30, 2019
Term facility payable (in years)	Five
Interest paid on basis points	Prime Rate plus 50 basis points or LIBOR plus 250 basis points.
Percentage of restricted dividend payments	55.00%
Line of credit	$ 0	$ 0
Prime Rate [Member]
Line of Credit Facility [Line Items]
Interest rate spread above benchmark rate	0.50%
LIBOR [Member]
Line of Credit Facility [Line Items]
Interest rate spread above benchmark rate	2.50%
Minimum [Member]
Line of Credit Facility [Line Items]
Percentage of stock dividend declared	37.00%
Maximum [Member]
Line of Credit Facility [Line Items]
Percentage of stock dividend declared	53.00%

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 21, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Federal statutory tax rate		35.00%	35.00%	35.00%	35.00%
Restatement of deferred tax liability due to change in tax rate		$ 7,650,000
Deferred tax amounts, valuation allowance recorded		0		$ 0
Uncertain tax positions		$ 0		$ 0
Scenario, Forecast [Member]
Income Taxes [Line Items]
Federal statutory tax rate	21.00%
Minimum [Member]
Income Taxes [Line Items]
Tax position measured if more-likely-than-not threshold is exceeded, percentage		50.00%

Income Taxes - Income Tax Expen

Income Taxes - Income Tax Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Current federal income tax	$ 34,421	$ 30,381	$ 31,014
Current state income tax	99	99	103
Deferred federal income tax expense (benefit)	(53)	673	320
Restatement of net deferred tax liability due to change in income tax rate	(7,650)
Income tax expense	$ 26,817	$ 31,153	$ 31,437

Income Taxes - Percentage of Pr

Income Taxes - Percentage of Pretax Earnings, Differs from Statutory Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 21, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	35.00%	35.00%	35.00%	35.00%
Restatement of net deferred tax liability due to change in income tax rate	(5.30%)
Reductions in tax rate resulting from interest income exempt from federal income tax	(11.50%)		(12.10%)	(11.40%)
Effect of state income tax	0.10%		0.10%	0.10%
ESOP tax deduction	(0.20%)		(0.20%)	(0.20%)
Other	0.10%		0.10%	0.30%
Effective income tax rate	18.20%		22.90%	23.80%

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Tax basis of loans in excess of financial statement basis	$ 10,550	$ 17,006
Minimum liability in defined benefit plan	766	1,641
Deferred compensation	1,818	2,807
Write-downs and adjustments to other real estate owned and repossessed assets	11	9
Other deferred tax assets	79	226
Total deferred tax assets	13,224	21,689
Deferred tax liabilities:
Financial statement basis of fixed assets in excess of tax basis	3,343	5,870
Intangible asset amortization deductible for tax purposes, but not for financial reporting purposes	9,926	15,191
Accretion on investment securities	1,039	1,788
Pension plan contributions	1,086	1,799
Net unrealized gain on investment securities available-for-sale	9,420	11,573
Other deferred tax liabilities	31	83
Total deferred tax liabilities	24,845	36,304
Net deferred tax asset (liability)	$ (11,621)	$ (14,615)

Fair Value Disclosures - Additi

Fair Value Disclosures - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets transfer between Level 1 and Level 2		$ 0	$ 0	$ 0
Assets transfer between Level 2 and Level 3		0	0	$ 0
Impaired loans carrying value		17,670,000	27,371,000
Impaired loans valuation reserves		3,996,000	5,012,000
Impaired loans net fair value		13,674,000
Other real estate owned, total		$ 1,347,000	$ 413,000
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt discounts, percentage	5.00%
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt discounts, percentage	25.00%

Fair Value Disclosures - Financ

Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	$ 3,087,473	$ 2,860,837
U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		10,668
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	60,330	113,703
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,420,850	1,564,276
Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,219,097	851,361
Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	375,737	268,436
Fair Value, Measurements, Recurring [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	3,087,473	2,860,837
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	7,031	47,965
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		10,668
Fair Value, Measurements, Recurring [Member] | Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	60,330	113,703
Fair Value, Measurements, Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,420,850	1,564,276
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,219,097	851,361
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	375,737	268,436
Fair Value, Measurements, Recurring [Member] | Other Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,428	4,428
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,428	15,096
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		10,668
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Other Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,428	4,428
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	3,083,045	2,845,741
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	7,031	47,965
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	60,330	113,703
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,420,850	1,564,276
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,219,097	851,361
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	$ 375,737	$ 268,436

Fair Value Disclosures - Other

Fair Value Disclosures - Other Real Estate Owned (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Fair Value Disclosures [Abstract]
Carrying value of other real estate owned prior to re-measurement	$ 1,067
Write-downs included in gain (loss) on sale of other real estate owned	(306)
Other real estate owned, fair value	$ 761

Fair Value Disclosures - Schedu

Fair Value Disclosures - Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	$ 209,583	$ 204,782
FEDERAL FUNDS SOLD		3,130
INTEREST-BEARING DEPOSITS IN BANKS	162,764	48,574
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,458	1,707
Available-for-sale securities	3,087,473	2,860,837
Held-to-maturity securities	0	121
Loans	3,452,543	3,338,426
Deposits with stated maturities	451,255
BORROWINGS	331,000	445,770
Carrying Value [Member] | Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	209,583	204,782
FEDERAL FUNDS SOLD		3,130
INTEREST-BEARING DEPOSITS IN BANKS	162,764	48,574
Deposits with no stated maturities	5,511,706	4,969,543
Carrying Value [Member] | Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,458	1,707
Held-to-maturity securities		121
Accrued interest receivable	36,081	36,469
Deposits with stated maturities	451,255	508,996
BORROWINGS	331,000	445,770
Accrued interest payable	197	225
Carrying Value [Member] | Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	3,087,473	2,860,837
Carrying Value [Member] | Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,452,543	3,338,426
Estimated Fair Value [Member] | Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	209,583	204,782
FEDERAL FUNDS SOLD		3,130
INTEREST-BEARING DEPOSITS IN BANKS	162,764	48,574
Deposits with no stated maturities	5,511,706	4,969,543
Estimated Fair Value [Member] | Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,458	1,709
Held-to-maturity securities		124
Accrued interest receivable	36,081	36,469
Deposits with stated maturities	452,000	510,304
BORROWINGS	331,000	445,770
Accrued interest payable	197	225
Estimated Fair Value [Member] | Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	3,087,473	2,860,837
Estimated Fair Value [Member] | Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 3,470,133	$ 3,361,735

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Dec. 31, 2017USD ($)
Commitments and Contingencies Disclosure [Abstract]
Future minimum lease commitments, 2018	$ 385,000
Future minimum lease commitments, 2019	303,000
Future minimum lease commitments, 2020	173,000
Future minimum lease commitments, 2021	41,000
Future minimum lease commitments, 2022	$ 7,000

Financial Instruments with Of82

Financial Instruments with Off-Balance-Sheet Risk - Schedule of Financial Instruments with Off-Balance-sheet Risk (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Concentration Risk [Line Items]
Financial instruments whose contract amounts represent credit risk	$ 807,984
Unfunded Lines of Credit [Member]
Concentration Risk [Line Items]
Financial instruments whose contract amounts represent credit risk	534,468
Unfunded Commitments to Extend Credit [Member]
Concentration Risk [Line Items]
Financial instruments whose contract amounts represent credit risk	244,658
Standby Letters of Credit [Member]
Concentration Risk [Line Items]
Financial instruments whose contract amounts represent credit risk	$ 28,858

Pension and Profit Sharing Pl83

Pension and Profit Sharing Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Contributions to the plan	$ 0	$ 500,000
Weighted average final maturities of debt securities held in the pension plan, Years	5 years 7 months 21 days
Level 3 securities	0
Common stock value	$ 663,000	661,000
Vesting period	6 years
Defined contribution plan cost	$ 4,735,000	3,221,000	$ 5,455,000
Employee deferrals	4.00%
Employee compensation	5.00%
Directors' fees	100.00%
Profit Sharing Plan's Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Common stock value	$ 55,796,000	60,270,000
Pension Plan's Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Common stock value	2,776,000	2,786,000
Safe Harbor 401(k) Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan cost	$ 2,392,000	$ 2,331,000	$ 2,043,000
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted average final maturities of debt securities held in the pension plan, Years	1 year 6 months 7 days
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted average final maturities of debt securities held in the pension plan, Years	19 years 9 months 3 days

Pension and Profit Sharing Pl84

Pension and Profit Sharing Plans - Benefit Obligation Activity and Fair Value of Plan Assets and Statement of Funded Status (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	$ 635,000	$ 665,000	$ 622,000
Actuarial loss	(3,597,000)	(4,688,000)
Employer contributions	0	500,000
Fair value of plan assets at December 31	17,046,000
Reconciliation of Benefit Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at January 1	15,453,000	16,002,000
Interest cost on projected benefit obligation	635,000	665,000
Actuarial loss	486,000	139,000
Benefits paid, including partial settlement of certain participant balances	(1,043,000)	(1,353,000)
Benefit obligation at December 31	15,531,000	15,453,000	16,002,000
Reconciliation of Fair Value of Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at January 1	15,787,000	14,820,000
Actual return on plan assets	2,302,000	1,820,000
Employer contributions		500,000
Benefits paid, including partial settlement of certain participant balances	(1,043,000)	(1,353,000)
Fair value of plan assets at December 31	17,046,000	15,787,000	$ 14,820,000
Funded status	$ 1,515,000	$ 334,000

Pension and Profit Sharing Pl85

Pension and Profit Sharing Plans - Component of Accumulated other Comprehensive Earnings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Net actuarial loss	$ (3,597)	$ (4,688)
Deferred tax benefit	1,227	1,641
Amounts included in accumulated other comprehensive earnings, net of tax	$ (2,370)	$ (3,047)

Pension and Profit Sharing Pl86

Pension and Profit Sharing Plans - Net Periodic Pension Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Service cost - benefits earned during the period	$ 0	$ 0	$ 0
Interest cost on projected benefit obligation	635	665	622
Expected return on plan assets	(974)	(912)	(948)
Amortization of unrecognized net loss	249	375	222
Recognized loss on partial settlement of certain participant balances		267
Net periodic pension benefit expense (benefit)	$ (90)	$ 395	$ (104)

Pension and Profit Sharing Pl87

Pension and Profit Sharing Plans - Benefit Obligations and Net Periodic Pension Cost and Rate of Return on Plan Assets (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Weighted average discount rate	3.50%	4.25%	4.25%
Expected long-term rate of return on assets	6.25%	6.25%	6.25%

Pension and Profit Sharing Pl88

Pension and Profit Sharing Plans - Pension Plan and Targeted Allocation Percentage (Detail)	Dec. 31, 2017	Dec. 31, 2016
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation	75.00%	77.00%
Targeted Allocation	75.00%
Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation	24.00%	22.00%
Targeted Allocation	25.00%
Cash and Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation	1.00%	1.00%

Pension and Profit Sharing Pl89

Pension and Profit Sharing Plans - Breakdown by Level (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 17,046
Money Market Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	274
US Treasury Notes Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	149
Obligations of State and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	634
Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	868
Mortgage-backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	1,130
Corporate Stocks and Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	13,991
Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	14,265
Level 1 Inputs [Member] | Money Market Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	274
Level 1 Inputs [Member] | Corporate Stocks and Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	13,991
Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	2,781
Level 2 Inputs [Member] | US Treasury Notes Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	149
Level 2 Inputs [Member] | Obligations of State and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	634
Level 2 Inputs [Member] | Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	868
Level 2 Inputs [Member] | Mortgage-backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 1,130

Pension and Profit Sharing Pl90

Pension and Profit Sharing Plans - Estimate of Undiscounted Projected Future Payments (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Retirement Benefits [Abstract]
2,018	$ 1,064
2,019	1,018
2,020	1,036
2,021	1,077
2,022	1,063
2023 forward	$ 5,309

Dividends from Subsidiaries - A

Dividends from Subsidiaries - Additional Information (Detail)	Dec. 31, 2017USD ($)
Equity [Abstract]
Amount available for the declaration of dividends by subsidiary banks without prior approval of regulatory agencies	$ 205,751,000

Regulatory Matters - Additional

Regulatory Matters - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Periodic increase of capital conservation buffer	0.625%
Total risk-based capital ratio, Actual Ratio	19.85%	18.45%
Tier1 Capital (to Risk-Weighted Assets), Actual Ratio	18.66%	17.30%
Common Tier1 Capital (to Risk-Weighted Assets), Actual Ratio	18.66%	17.30%
Tier1 Leverage ratio, Actual Ratio	11.09%	10.71%
Required tangible net assets	$ 2,000,000
Tangible net assets, total	16,602,000
Subsidiary bank's reserve balance	$ 11,504,000	$ 4,340,000
Minimum [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Capital conservation buffer	0.625%
Maximum [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Capital conservation buffer	2.50%

Regulatory Matters - Schedule o

Regulatory Matters - Schedule of Regulatory Capital Ratios Under the Basel III Regulatory Capital Framework (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Actual Ratio	19.85%	18.45%
Tier1 Capital to Risk-Weighted Assets, Actual Ratio	18.66%	17.30%
Leverage Ratio, Actual Ratio	11.09%	10.71%
Consolidated [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Actual Amount	$ 814,634	$ 739,959
Tier1 Capital to Risk-Weighted Assets, Actual Amount	765,882	693,584
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Amount	765,882	693,584
Leverage Ratio, Actual Amount	$ 765,882	$ 693,584
Total Capital to Risk-Weighted Assets, Actual Ratio	19.85%	18.45%
Tier1 Capital to Risk-Weighted Assets, Actual Ratio	18.66%	17.30%
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Ratio	18.66%	17.30%
Leverage Ratio, Actual Ratio	11.09%	10.71%
Consolidated [Member] | Minimum [Member] | Basel III Fully Phased-In [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Minimum Capital Required	$ 430,872	$ 421,007
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	348,801	340,815
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	287,248	280,671
Leverage Ratio, Minimum Capital Required	$ 276,296	$ 258,978
Total Capital to Risk-Weighted Assets, Minimum Capital Required	10.50%	10.50%
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	8.50%	8.50%
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required, Ratio	7.00%	7.00%
Leverage Ratio, Minimum Capital Required, Ratio	4.00%	4.00%
Consolidated [Member] | Minimum [Member] | Basel III Phase-In [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Minimum Capital Required	$ 379,578	$ 345,827
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	297,507	265,635
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	235,954	205,491
Leverage Ratio, Minimum Capital Required	$ 276,296	$ 258,978
Total Capital to Risk-Weighted Assets, Minimum Capital Required	9.25%	8.625%
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	7.25%	6.625%
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required, Ratio	5.75%	5.125%
Leverage Ratio, Minimum Capital Required, Ratio	4.00%	4.00%
Parent Company [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Actual Amount	$ 723,563	$ 633,403
Tier1 Capital to Risk-Weighted Assets, Actual Amount	674,811	587,028
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Amount	674,811	587,028
Leverage Ratio, Actual Amount	$ 674,811	$ 587,028
Total Capital to Risk-Weighted Assets, Actual Ratio	17.68%	15.84%
Tier1 Capital to Risk-Weighted Assets, Actual Ratio	16.49%	14.68%
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Ratio	16.49%	14.68%
Leverage Ratio, Actual Ratio	9.80%	9.10%
Parent Company [Member] | Minimum [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized , Ratio	10.00%	10.00%
Total Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Amount	$ 409,312	$ 399,919
Tier1 Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized , Ratio	8.00%	8.00%
Tier1 Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Amount	$ 327,450	$ 319,935
Common Equity Tier1 Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized , Ratio	6.50%	6.50%
Common Equity Tier1 Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Amount	$ 266,053	$ 259,947
Leverage Ratio, Required To Be Considered Well Capitalized , Ratio	5.00%	5.00%
Leverage Ratio, Required To Be Considered Well Capitalized, Amount	$ 344,151	$ 322,426
Parent Company [Member] | Minimum [Member] | Basel III Fully Phased-In [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Minimum Capital Required	429,777	419,915
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	347,915	339,931
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	286,518	279,943
Leverage Ratio, Minimum Capital Required	$ 275,320	$ 257,941
Total Capital to Risk-Weighted Assets, Minimum Capital Required	10.50%	10.50%
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	8.50%	8.50%
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required, Ratio	7.00%	7.00%
Leverage Ratio, Minimum Capital Required, Ratio	4.00%	4.00%
Parent Company [Member] | Minimum [Member] | Basel III Phase-In [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Minimum Capital Required	$ 378,614	$ 344,930
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	296,751	264,946
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	235,354	204,959
Leverage Ratio, Minimum Capital Required	$ 275,320	$ 257,941
Total Capital to Risk-Weighted Assets, Minimum Capital Required	9.25%	8.625%
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	7.25%	6.625%
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required, Ratio	5.75%	5.125%
Leverage Ratio, Minimum Capital Required, Ratio	4.00%	4.00%

Stock Option Plan and Restric94

Stock Option Plan and Restricted Stock Plan - Additional Information (Detail)	Oct. 24, 2017USD ($)shares	Apr. 25, 2017USD ($)Non_Employee_Directorsshares	Oct. 25, 2016USD ($)shares	Apr. 26, 2016USD ($)Non_Employee_Directorsshares	Oct. 27, 2015USD ($)shares	Jul. 21, 2015USD ($)Non_Employee_Directorsshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)$ / shares	Dec. 31, 2015USD ($)$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of stock for issuance under the plan | shares							3,180,000
Restricted shares vesting period							6 years
Percentage of options granted							20.00%
Duration of options granted, years							10 years
Stock options outstanding, exercise price range, lower | $ / shares							$ 16.78
Stock options outstanding, exercise price range, upper | $ / shares							$ 42.35
Fair value assumptions method used							Black-Scholes options pricing model
Risk-free interest rate							1.89%		1.89%
Expected dividend yield							1.79%		1.89%
Expected life							6 years 2 months 27 days		5 years 9 months 11 days
Expected volatility							26.51%		23.36%
Weighted-average grant-date fair value of options granted | $ / shares							$ 9.90	$ 0	$ 6.72
Total intrinsic value of options exercised							$ 3,082,000	$ 1,226,000	$ 1,539,000
Total unrecognized compensation cost related to nonvested share-based compensation arrangements granted under the Plan							$ 5,031,000
Weighted-average period of unrecognized compensation cost recognition, years							2 years 2 months 16 days
Total fair value of shares vested							$ 1,246,000	592,000	810,000
Aggregate intrinsic value of vested stock options							10,981,000
Stock compensation expense							$ 1,745,000	882,000	644,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of stock for issuance under the plan | shares							413,000
Number of non employee directors | Non_Employee_Directors		10		10		10
Restricted Stock [Member] | Non-Employee Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted | shares		14,650		7,660		7,070
Restricted shares value		$ 600,000		$ 250,000		$ 250,000
Restricted Stock [Member] | Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense							$ 483,000	278,000	139,000
Restricted Stock [Member] | Officers [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares vesting period			3 years		3 years
Restricted shares granted | shares	14,191		15,405		31,273
Restricted shares value	$ 655,000		$ 560,000		$ 1,060,000
Stock compensation expense							$ 562,000	$ 381,000	$ 62,000
Restricted Stock [Member] | Officers [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares vesting period	1 year
Restricted Stock [Member] | Officers [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares vesting period	3 years
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares vesting period							2 years

Stock Option Plan and Restric95

Stock Option Plan and Restricted Stock Plan - Schedule of Analysis of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)$ / sharesshares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Outstanding, beginning of year, Shares | shares	1,094,035
Granted, Shares | shares	452,450
Exercised, Shares | shares	(140,250)
Cancelled, Shares | shares	(80,270)
Outstanding, end of year, Shares | shares	1,325,965
Exercisable at end of year, Shares | shares	458,715
Outstanding, beginning of year, Weighted-Average Ex. Price | $ / shares	$ 27.40
Granted, Weighted-Average Ex. Price | $ / shares	42.35
Exercised, Weighted-Average Ex. Price | $ / shares	20.92
Cancelled, Weighted-Average Ex. Price | $ / shares	30.33
Outstanding, end of year, Weighted-Average Ex. Price | $ / shares	33.01
Exercisable at end of year, Weighted-Average Ex. Price | $ / shares	$ 23.94
Outstanding, end of year, Weighted-Average Remaining Contractual Term (Years)	7 years 1 month 13 days
Exercisable at end of year, Weighted-Average Remaining Contractual Term (Years)	4 years 7 months 28 days
Outstanding, end of year, Aggregate Intrinsic Value | $	$ 15,966
Exercisable at end of year, Aggregate Intrinsic Value | $	$ 9,684

Stock Option Plan and Restric96

Stock Option Plan and Restricted Stock Plan - Schedule of Information Concerning Outstanding and Vested Stock Options (Detail)	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Exercise Price $16.78 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price | $ / shares	$ 16.78
Number Outstanding	87,640
Number Vested	87,640
Remaining Contracted Life (Years)	1 year 4 months 24 days
Exercise Price $15.73 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price | $ / shares	$ 15.73
Number Outstanding	142,620
Number Vested	142,620
Remaining Contracted Life (Years)	3 years 9 months 18 days
Exercise Price $30.85 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price | $ / shares	$ 30.85
Number Outstanding	270,505
Number Vested	156,225
Remaining Contracted Life (Years)	5 years 9 months 18 days
Exercise Price $33.89 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price | $ / shares	$ 33.89
Number Outstanding	382,750
Number Vested	72,230
Remaining Contracted Life (Years)	7 years 9 months 18 days
Exercise Price $42.35 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price | $ / shares	$ 42.35
Number Outstanding	442,450
Remaining Contracted Life (Years)	9 years 6 months

Condensed Financial Informati97

Condensed Financial Information - Parent Company - Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Interest-bearing deposits in subsidiary bank	$ 162,764	$ 48,574
Total cash and cash equivalents	372,347	256,486	$ 272,886	$ 253,471
OTHER ASSETS	75,725	86,066
Total assets	7,254,715	6,809,931
LIABILITIES AND SHAREHOLDERS' EQUITY
Total liabilities	6,331,947	5,972,046
SHAREHOLDERS' EQUITY:
Common stock	663	661
Capital surplus	378,062	372,245
Retained earnings	517,257	446,534
Accumulated other comprehensive earnings	26,786	18,445
Total shareholders' equity	922,768	837,885	804,986	681,537
Total liabilities and shareholders' equity	7,254,715	6,809,931
Subsidiary Bank [Member]
ASSETS
Cash in banks	14,272	15,070
Interest-bearing deposits in subsidiary bank	64,195	78,179
Unaffiliated Banks [Member]
ASSETS
Cash in banks	2	2
Parent Company [Member]
ASSETS
Total cash and cash equivalents	78,469	93,251	$ 90,473	$ 70,085
SECURITIES AVAILABLE-FOR-SALE, at fair value	8,515	11,593
Investment in and advances to subsidiaries, at equity	847,445	744,971
INTANGIBLE ASSETS	723	723
OTHER ASSETS	2,654	2,668
Total assets	937,806	853,206
LIABILITIES AND SHAREHOLDERS' EQUITY
Total liabilities	15,038	15,321
SHAREHOLDERS' EQUITY:
Common stock	663	661
Capital surplus	378,062	372,245
Retained earnings	517,257	446,534
Treasury stock	(7,148)	(6,671)
Deferred compensation	7,148	6,671
Accumulated other comprehensive earnings	26,786	18,445
Total shareholders' equity	922,768	837,885
Total liabilities and shareholders' equity	$ 937,806	$ 853,206

Condensed Financial Informati98

Condensed Financial Information - Parent Company - Condensed Statements of Earnings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income:
Total interest income	$ 245,975	$ 232,288	$ 221,623
Expenses:
Salaries and employee benefits	95,287	90,739	80,999
EARNINGS BEFORE INCOME TAXES	147,188	135,927	131,818
Income tax benefit	(26,817)	(31,153)	(31,437)
NET EARNINGS	120,371	104,774	100,381
Parent Company [Member]
Income:
Cash dividends from subsidiaries	30,800	48,800	51,200
Excess of earnings over dividends of subsidiaries	92,929	58,809	52,911
Other	6,590	4,184	4,185
Total interest income	130,319	111,793	108,296
Expenses:
Salaries and employee benefits	8,606	5,655	6,067
Other operating expenses	3,871	3,531	4,439
Total Expense	12,477	9,186	10,506
EARNINGS BEFORE INCOME TAXES	117,842	102,607	97,790
Income tax benefit	2,529	2,167	2,591
NET EARNINGS	$ 120,371	$ 104,774	$ 100,381

Condensed Financial Informati99

Condensed Financial Information - Parent Company - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
NET EARNINGS	$ 120,371	$ 104,774	$ 100,381
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization, net	12,916	11,573	11,145
Decrease (increase) in other assets	9,313	2,397	(16,919)
Increase (decrease) in other liabilities	285	(2,643)	1,664
Net cash provided by operating activities	190,274	160,742	109,092
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash received in connection with acquisition of banks			65,197
Maturity of available-for-sale security	4,392,131	3,509,113	2,717,724
Purchases of bank premises and equipment	(14,162)	(20,399)	(17,433)
Net cash used in (provided by) investing activities	(398,044)	(251,960)	(384,225)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds of stock issuances	2,934	1,260	1,545
Cash dividends paid	(48,955)	(44,907)	(38,767)
Net cash provided by financing activities	323,631	74,818	294,548
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	115,861	(16,400)	19,415
CASH AND CASH EQUIVALENTS, beginning of year	256,486	272,886	253,471
CASH AND CASH EQUIVALENTS, end of year	372,347	256,486	272,886
Parent Company [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
NET EARNINGS	120,371	104,774	100,381
Adjustments to reconcile net earnings to net cash provided by operating activities:
Excess of earnings over dividends of subsidiary bank	(92,929)	(58,809)	(52,911)
Depreciation and amortization, net	207	208	197
Decrease (increase) in other assets	438	1,702	507
Increase (decrease) in other liabilities	183	(1,374)	3,743
Other	2	8
Net cash provided by operating activities	28,272	46,509	51,917
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash received in connection with acquisition of banks			13,125
Maturity of available-for-sale security	2,997
Purchases of bank premises and equipment	(30)	(94)	(107)
Repayment from investment in and advances to subsidiaries, net			5,800
Other		10
Net cash used in (provided by) investing activities	2,967	(84)	18,818
CASH FLOWS FROM FINANCING ACTIVITIES:
Repayment of subordinated debt			(13,125)
Proceeds of stock issuances	2,934	1,260	1,545
Cash dividends paid	(48,955)	(44,907)	(38,767)
Net cash provided by financing activities	(46,021)	(43,647)	(50,347)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(14,782)	2,778	20,388
CASH AND CASH EQUIVALENTS, beginning of year	93,251	90,473	70,085
CASH AND CASH EQUIVALENTS, end of year	$ 78,469	$ 93,251	$ 90,473

Cash Flow Information - Supplem

Cash Flow Information - Supplemental Information on Cash Flows and Noncash Transactions (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental cash flow information:
Interest paid	$ 9,316	$ 5,465	$ 4,085
Federal income taxes paid	29,695	28,348	29,674
Schedule of noncash investing and financing activities:
Assets acquired through foreclosure	2,211	2,269	203
Investment securities purchased but not settled		12,381
Restricted stock grant to officers and directors	$ 1,139	$ 810	$ 1,310

Acquisitions and Asset Purch101

Acquisitions and Asset Purchase - Additional Information (Detail) - USD ($)	Aug. 03, 2015	Jul. 31, 2015	Apr. 08, 2015	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2016
Business Acquisition [Line Items]
Fair value of purchased credit impaired loans				$ 17,670,000		$ 27,371,000
Contractual amounts				23,851,000		38,379,000
Business acquisition, cash purchase price					$ 1,931,000
Goodwill resulting from acquisition				139,971,000		139,971,000
Purchased Credit Impaired Loans [Member]
Business Acquisition [Line Items]
Contractual amounts				$ 755,000		$ 1,865,000
FBC Bancshares, Inc. and First Bank, N.A. [Member]
Business Acquisition [Line Items]
Business acquisition, agreement date				Apr. 1, 2015
Payment for all outstanding shares of acquired entity by shares		1,755,374
Subordinated debt, paid off	$ 13,125,000
Fair value of total loans				$ 248,380,000
Total loans of contractual amounts				252,458,000
Goodwill resulting from acquisition				43,922,000
FBC Bancshares, Inc. and First Bank, N.A. [Member] | Purchased Credit Impaired Loans [Member]
Business Acquisition [Line Items]
Fair value of purchased credit impaired loans				1,398,000
Contractual amounts				$ 1,704,000
4Trust Mortgage, Inc. [Member]
Business Acquisition [Line Items]
Business acquisition, agreement date				Apr. 8, 2015
Business acquisition, cash purchase price			$ 1,900,000
Goodwill resulting from acquisition				$ 1,750,000

Acquisitions and Asset Purch102

Acquisitions and Asset Purchase - Schedule of Amounts Recorded on Consolidated Balance Sheet on Acquisition Date (Detail) $ in Thousands	Jul. 31, 2015USD ($)
FBC Bancshares, Inc. and First Bank, N.A. [Member]
Business Acquisition [Line Items]
Common stock issued (1,755,374 shares)	$ 59,648

Acquisitions and Asset Purch103

Acquisitions and Asset Purchase - Schedule of Amounts Recorded on Consolidated Balance Sheet on Acquisition Date (Parenthetical) (Detail)	Jul. 31, 2015shares
FBC Bancshares, Inc. and First Bank, N.A. [Member]
Business Acquisition [Line Items]
Common stock issued shares	1,755,374

Acquisitions and Asset Purch104

Acquisitions and Asset Purchase - Schedule of Preliminary Estimated Fair Value Amounts Assigned to Major Asset and Liability Categories at Acquisition Date (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill resulting from acquisition	$ 139,971,000	$ 139,971,000
FBC Bancshares, Inc. and First Bank, N.A. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	65,197,000
Securities available-for-sale	42,903,000
Loans	248,380,000
Identifiable intangible assets	2,343,000
Other assets	15,262,000
Total identifiable assets acquired	374,085,000
Deposits	343,583,000
Subordinated debt	13,125,000
Other liabilities	1,651,000
Total liabilities assumed	358,359,000
Fair value of net identifiable assets acquired	15,726,000
Goodwill resulting from acquisition	$ 43,922,000

Subsequent Event - Additional I

Subsequent Event - Additional Information (Detail) - USD ($)	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Subsequent Event [Line Items]
Gross loans		$ 3,485,569,000	$ 3,357,307,000
Total deposits		5,962,961,000	5,478,539,000
Total assets		7,254,715,000	$ 6,809,931,000
Commercial State Bank [Member]
Subsequent Event [Line Items]
Gross loans		271,533,000
Total deposits		345,908,000
Total assets		$ 390,112,000
Commercial Bancshares, Inc [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Percentage of common stock acquired	100.00%
Common stock issued as consideration	1,289,371
Business acquisition, expected stock purchase price	$ 59,400,000
Business acquisition, special dividend	22,075,000
Increase decrease in business acquisition, special dividend	$ 42,402,000