S&T Bancorp (STBA) 10-K 1 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 28, 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
Entity Registrant Name	S&T BANCORP INC
Entity Central Index Key	719,220
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		34,969,422
Entity Public Float			$ 1,357,061,248

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Cash and due from banks, including interest-bearing deposits of $61,965 and $87,201 at December 31, 2017 and 2016	$ 117,152	$ 139,486
Securities available-for-sale, at fair value	698,291	693,487
Loans held for sale	4,485	3,793
Portfolio loans, net of unearned income	5,761,449	5,611,419
Allowance for loan losses	(56,390)	(52,775)
Portfolio loans, net	5,705,059	5,558,644
Bank owned life insurance	72,150	72,081
Premises and equipment, net	42,702	44,999
Federal Home Loan Bank and other restricted stock, at cost	29,270	31,817
Goodwill	291,670	291,670
Other intangible assets, net	3,677	4,910
Other assets	95,799	102,166
Total Assets	7,060,255	6,943,053
LIABILITIES
Noninterest-bearing demand	1,387,712	1,263,833
Interest-bearing demand	603,141	638,300
Money market	1,146,156	936,461
Savings	893,119	1,050,131
Certificates of deposit	1,397,763	1,383,652
Total Deposits	5,427,891	5,272,377
Securities sold under repurchase agreements	50,161	50,832
Short-term borrowings	540,000	660,000
Long-term borrowings	47,301	14,713
Junior subordinated debt securities	45,619	45,619
Other liabilities	65,252	57,556
Total Liabilities	6,176,224	6,101,097
SHAREHOLDERS’ EQUITY
Common stock ($2.50 par value) Authorized—50,000,000 shares Issued—36,130,480 shares at December 31, 2017 and December 31, 2016 Outstanding—34,971,929 shares at December 31, 2017 and 34,913,023 shares at December 31, 2016	90,326	90,326
Additional paid-in capital	216,106	213,098
Retained earnings	628,107	585,891
Accumulated other comprehensive income (loss)	(18,427)	(13,784)
Treasury stock (1,158,551 shares at December 31, 2017 and 1,217,457 shares at December 31, 2016, at cost)	(32,081)	(33,575)
Total Shareholders’ Equity	884,031	841,956
Total Liabilities and Shareholders’ Equity	$ 7,060,255	$ 6,943,053

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Cash and due from banks, interest-bearing amounts	$ 61,965	$ 87,201
Common stock, par value (in USD per share)	$ 2.50	$ 2.50
Common stock, shares authorized (in shares)	50,000,000	50,000,000
Common stock, shares issued (in shares)	36,130,480	36,130,480
Common stock, shares outstanding (in shares)	34,971,929	34,913,023
Treasury stock, shares (in shares)	1,158,551	1,217,457

CONSOLIDATED STATEMENTS OF NET

CONSOLIDATED STATEMENTS OF NET INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
INTEREST INCOME
Loans, including fees	$ 243,315	$ 212,301	$ 188,012
Investment Securities:
Taxable	11,947	10,340	9,792
Tax-exempt	3,615	3,658	3,954
Dividends	1,765	1,475	1,790
Total Interest Income	260,642	227,774	203,548
INTEREST EXPENSE
Deposits	25,330	19,692	12,944
Borrowings and junior subordinated debt securities	9,579	4,823	3,053
Total Interest Expense	34,909	24,515	15,997
NET INTEREST INCOME	225,733	203,259	187,551
Provision for loan losses	13,883	17,965	10,388
Net Interest Income After Provision for Loan Losses	211,850	185,294	177,163
NONINTEREST INCOME
Securities gains (losses), net	3,000	0	(34)
Service charges on deposit accounts	12,458	12,512	11,642
Debit and credit card	12,029	11,943	12,113
Wealth management	9,758	10,456	11,444
Insurance	5,418	5,253	5,500
Mortgage banking	2,915	2,879	2,554
Bank owned life insurance	2,756	2,122	2,221
Gain on sale of credit card portfolio	0	2,066	0
Other	7,128	7,404	5,593
Total Noninterest Income	55,462	54,635	51,033
NONINTEREST EXPENSE
Salaries and employee benefits	80,776	77,325	68,252
Net occupancy	10,994	11,057	10,652
Data processing	8,801	8,837	9,560
Furniture, equipment and software	7,946	7,290	6,093
FDIC insurance	4,543	3,984	3,416
Other taxes	4,509	4,050	3,616
Professional services and legal	4,096	3,466	3,006
Marketing	3,659	3,713	4,224
Merger-related expenses	0	0	3,167
Other	22,583	23,510	24,731
Total Noninterest Expense	147,907	143,232	136,717
Income Before Taxes	119,405	96,697	91,479
Provision for income taxes	46,437	25,305	24,398
Net Income	$ 72,968	$ 71,392	$ 67,081
Earnings per common share—basic (in USD per share)	$ 2.10	$ 2.06	$ 1.98
Earnings per common share—diluted (in USD per share)	2.09	2.05	1.98
Dividends declared per common share (in USD per share)	$ 0.82	$ 0.77	$ 0.73

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net Income	$ 72,968	$ 71,392	$ 67,081
Other Comprehensive (Loss) Income, Before Tax:
Net change in unrealized gains on securities available-for-sale	(1,275)	(2,899)	(663)
Net available-for-sale securities (gains) losses reclassified into earnings	(3,000)	0	34
Adjustment to funded status of employee benefit plans	(1,992)	6,974	(3,551)
Other Comprehensive (Loss) Income, Before Tax	(6,267)	4,075	(4,180)
Income tax benefit (expense) related to items of other comprehensive income	1,624	(1,402)	1,556
Other Comprehensive (Loss) Income, After Tax	(4,643)	2,673	(2,624)
Comprehensive Income	$ 68,325	$ 74,065	$ 64,457

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Beginning balance at Dec. 31, 2014	$ 608,389	$ 77,993	$ 78,818	$ 504,060	$ (13,833)	$ (38,649)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	67,081			67,081
Other comprehensive income (loss), net of tax	(2,624)				(2,624)
Cash dividends declared	(24,487)			(24,487)
Common stock issued in acquisition	142,469	12,333	130,136
Treasury stock issued	(182)			(2,426)		2,244
Recognition of restricted stock compensation expense	1,670		1,670
Tax benefit from stock-based compensation	53		53
Costs to issue equity securities	(132)		(132)
Ending balance at Dec. 31, 2015	792,237	90,326	210,545	544,228	(16,457)	(36,405)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	71,392			71,392
Other comprehensive income (loss), net of tax	2,673				2,673
Cash dividends declared	(26,784)			(26,784)
Treasury stock issued	(115)			(2,945)		2,830
Recognition of restricted stock compensation expense	2,544		2,544
Tax benefit from stock-based compensation	9		9
Ending balance at Dec. 31, 2016	841,956	90,326	213,098	585,891	(13,784)	(33,575)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	72,968			72,968
Other comprehensive income (loss), net of tax	(4,643)				(4,643)
Cash dividends declared	(28,569)			(28,569)
Treasury stock issued	(689)			(2,183)		1,494
Recognition of restricted stock compensation expense	3,008		3,008
Ending balance at Dec. 31, 2017	$ 884,031	$ 90,326	$ 216,106	$ 628,107	$ (18,427)	$ (32,081)

CONSOLIDATED STATEMENTS OF CHA7

CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLERS' EQUITY (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash dividends declared, per share (in USD per share)	$ 0.82	$ 0.77	$ 0.73
Retained Earnings
Cash dividends declared, per share (in USD per share)	$ 0.82	$ 0.77	$ 0.73
Treasury stock issued (in shares)	58,906	102,649	80,862
Common Stock
Common stock issued in acquisition (in shares)			4,933,115

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
OPERATING ACTIVITIES
Net Income	$ 72,968	$ 71,392	$ 67,081
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for loan losses	13,883	17,965	10,388
(Recovery) provision for unfunded loan commitments	(410)	65	258
Net depreciation, amortization and accretion	2,498	3,628	356
Net amortization of discounts and premiums on securities	4,003	3,829	3,600
Stock-based compensation expense	3,008	2,544	1,636
Securities (gains) losses, net	(3,000)	0	34
Gain on sale of bank branch	(1,042)	0	0
Net gain on sale of credit card portfolio	0	(2,066)	0
Pension plan curtailment gain	0	(1,017)	0
Tax benefit from stock-based compensation	0	(9)	(53)
Mortgage loans originated for sale	(93,382)	(106,020)	(107,489)
Proceeds from the sale of loans	93,991	108,209	99,458
Deferred income taxes	13,832	536	(427)
Loss (gain) on sale of fixed assets	128	0	(179)
Gain on the sale of loans, net	(1,551)	(1,621)	(1,044)
Net increase in interest receivable	(2,714)	(2,409)	(2,744)
Net increase (decrease) in interest payable	1,349	1,715	(193)
Net decrease (increase) in other assets	5,634	4,668	(11,396)
Net increase (decrease) in other liabilities	5,041	(4,613)	1,298
Net Cash Provided by Operating Activities	114,236	96,796	60,584
INVESTING ACTIVITIES
Proceeds from maturities, prepayments and calls of securities available-for-sale	80,956	74,110	50,142
Proceeds from sales of securities available-for-sale	65,801	0	11,119
Purchases of securities available-for-sale	(156,839)	(113,362)	(74,712)
Net sales (purchases) of Federal Home Loan Bank stock	2,547	(8,784)	(855)
Net increase in loans	(211,766)	(599,341)	(383,575)
Proceeds from the sale of loans not originated for resale	6,754	9,208	2,880
Purchases of premises and equipment	(4,694)	(3,560)	(5,133)
Proceeds from the sale of premises and equipment	422	57	467
Net cash paid in excess of cash acquired from bank merger	0	0	(16,347)
Proceeds from surrender of bank owned life insurance	0	0	10,277
Net Cash Used in Investing Activities	(212,415)	(616,653)	(405,737)
FINANCING ACTIVITIES
Net increase in core deposits	166,054	378,323	195,589
Net increase in certificates of deposit	27,132	18,095	51,209
Net (decrease) increase in short-term borrowings	(120,000)	304,000	(2,660)
Net (decrease) increase in securities sold under repurchase agreements	(671)	(11,254)	31,481
Proceeds from long-term borrowings	35,000	0	100,000
Repayments of long-term borrowings	(2,412)	(102,330)	(2,399)
Repayment of junior subordinated debt	0	0	(13,500)
Treasury shares issued-net	(689)	(115)	(182)
Common stock issuance costs	0	0	(132)
Cash dividends paid to common shareholders	(28,569)	(26,784)	(24,487)
Tax benefit from stock-based compensation	0	9	53
Net Cash Provided by Financing Activities	75,845	559,944	334,972
Net (decrease) increase in cash and cash equivalents	(22,334)	40,087	(10,181)
Cash and cash equivalents at beginning of year	139,486	99,399	109,580
Cash and Cash Equivalents at End of Year	117,152	139,486	99,399
Supplemental Disclosures
Transfers to other real estate owned and other repossessed assets	2,238	1,039	843
Interest paid	33,591	22,800	15,878
Income taxes paid, net of refunds	33,814	26,743	23,175
Loans transferred to held for sale	0	250	23,277
Loans transferred to portfolio from held for sale	250	7,933	0
Net assets from acquisitions, excluding cash and cash equivalents	0	0	43,433
Decrease in cash and cash equivalents from sale of bank branch	154	0	0
Investment commitment payable for an available for sale security	5,884	0	0
Branch
INVESTING ACTIVITIES
Proceeds from the divestiture of businesses	4,404	0	0
Credit Card Portfolio
INVESTING ACTIVITIES
Proceeds from the divestiture of businesses	$ 0	$ 25,019	$ 0

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Nature of Operations S&T Bancorp, Inc., or S&T, was incorporated on March 17, 1983 under the laws of the Commonwealth of Pennsylvania as a bank holding company and has three direct wholly owned subsidiaries, S&T Bank, 9th Street Holdings, Inc. and STBA Capital Trust I. We own a 50 percent interest in Commonwealth Trust Credit Life Insurance Company, or CTCLIC. We are presently engaged in nonbanking activities through the following five entities: 9th Street Holdings, Inc.; S&T Bancholdings, Inc.; CTCLIC; S&T Insurance Group, LLC; and Stewart Capital Advisors, LLC. 9th Street Holdings, Inc. and S&T Bancholdings, Inc. are investment holding companies. CTCLIC, which is a joint venture with another financial institution, acts as a reinsurer of credit life, accident and health insurance policies sold by S&T Bank and the other institution. S&T Insurance Group, LLC, through its subsidiaries, offers a variety of insurance products. Stewart Capital Advisors, LLC is a registered investment advisor that manages private investment accounts for individuals and institutions. On October 29, 2014, S&T and Integrity Bancshares, Inc., or Integrity, based in Camp Hill, Pennsylvania, entered into an agreement to acquire Integrity Bancshares, Inc. and the transaction was completed on March 4, 2015. Integrity Bank was subsequently merged into S&T Bank on May 8, 2015. Prior to 2017, we reported three operating segments: Community Banking, Wealth Management and Insurance. Effective January 1, 2017, we no longer report Wealth Management and Insurance segment information, as they do not meet the quantitative thresholds required for disclosure. Accounting Policies Our financial statements have been prepared in accordance with U.S. generally accepted accounting principles, or GAAP. In preparing the financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities as of the dates of the balance sheets and revenues and expenses for the periods then ended. Actual results could differ from those estimates. Our significant accounting policies are described below. Principles of Consolidation The Consolidated Financial Statements include the accounts of S&T and its wholly owned subsidiaries. All significant intercompany transactions have been eliminated in consolidation. Investments of 20 percent to 50 percent of the outstanding common stock of investees are accounted for using the equity method of accounting. Reclassification Amounts in prior years' financial statements and footnotes are reclassified whenever necessary to conform to the current year’s presentation. Reclassifications had no effect on our results of operations or financial condition. Business Combinations We account for business combinations using the acquisition method of accounting. Under this method of accounting, the acquired company’s net assets are recorded at fair value at the date of acquisition, and the results of operations of the acquired company are combined with our results from that date forward. Acquisition costs are expensed when incurred. The difference between the purchase price and the fair value of the net assets acquired (including identified intangibles) is recorded as goodwill. Fair Value Measurements We use fair value measurements when recording and disclosing certain financial assets and liabilities. Securities available-for-sale, trading assets and derivative financial instruments are recorded at fair value on a recurring basis. Additionally, from time to time, we may be required to record other assets at fair value on a nonrecurring basis, such as loans held for sale, impaired loans, other real estate owned, or OREO, and other repossessed assets, mortgage servicing rights, or MSRs, and certain other assets. Fair value is the price that would be received to sell an asset or paid to transfer a liability in the principal or most advantageous market in an orderly transaction between market participants at the measurement date. 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 or liabilities; it is not a forced transaction. In determining fair value, we use various valuation approaches, including market, income and cost approaches. The fair value standard establishes a hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing an asset or liability, which are developed based on market data we have obtained from independent sources. Unobservable inputs reflect our estimates of assumptions that market participants would use in pricing an asset or liability, which are developed based on the best information available in the circumstances. The fair value hierarchy gives the highest priority to unadjusted quoted market prices in active markets for identical assets or liabilities (Level 1 measurement) and the lowest priority to unobservable inputs (Level 3 measurement). The fair value hierarchy is broken down into three levels based on the reliability of inputs as follows: Level 1: valuation is based upon unadjusted quoted market prices for identical instruments traded in active markets. Level 2: valuation is based upon quoted market prices for similar instruments traded in active markets, quoted market prices for identical or similar instruments traded in markets that are not active and model-based valuation techniques for which all significant assumptions are observable in the market or can be corroborated by market data. Level 3: valuation is derived from other valuation methodologies, including discounted cash flow models and similar techniques that use significant assumptions not observable in the market. These unobservable assumptions reflect estimates of assumptions that market participants would use in determining fair value. A financial instrument’s level within the fair value hierarchy is based on the lowest level of input that is significant to the fair value measurement. Our policy is to recognize transfers between any of the fair value hierarchy levels at the end of the reporting period in which the transfer occurred. The following are descriptions of the valuation methodologies that we use for financial instruments recorded at fair value on either a recurring or nonrecurring basis. Recurring Basis Securities Available-for-Sale Securities available-for-sale include both debt and equity securities. We obtain fair values for debt securities from a third-party pricing service which utilizes several sources for valuing fixed-income securities. We validate prices received from our pricing service through comparison to a secondary pricing service and broker quotes. We review the methodologies of the pricing service which provides us with a sufficient understanding of the valuation models, assumptions, inputs and pricing to reasonably measure the fair value of our debt securities. The market evaluation sources for debt securities include observable inputs rather than significant unobservable inputs and are classified as Level 2. The service provider utilizes pricing models that vary by asset class and include available trade, bid and other market information. Generally, the methodologies include broker quotes, proprietary models, and vast descriptive terms and conditions databases, and extensive quality control programs. Marketable equity securities that have an active, quotable market are classified as Level 1. Marketable equity securities that are quotable, but are thinly traded or inactive, are classified as Level 2. Marketable equity securities that are not readily traded and do not have a quotable market are classified as Level 3. Trading Assets We use quoted market prices to determine the fair value of our trading assets. Our trading assets are held in a Rabbi Trust under a deferred compensation plan and are invested in readily quoted mutual funds. Accordingly, these assets are classified as Level 1. Derivative Financial Instruments We use derivative instruments, including interest rate swaps for commercial loans with our customers, interest rate lock commitments and the sale of mortgage loans in the secondary market. We calculate the fair value for derivatives using widely accepted valuation techniques, including discounted cash flow analysis on the expected cash flows of each derivative. Each valuation considers the contractual terms of the derivative, including the period to maturity, and uses observable market based inputs, such as interest rate curves and implied volatilities. Accordingly, derivatives are classified as Level 2. We incorporate credit valuation adjustments into the valuation models to appropriately reflect both our own nonperformance risk and the respective counterparties’ nonperformance risk in calculating fair value measurements. In adjusting the fair value of our derivative contracts for the effect of nonperformance risk, we have considered the impact of netting and any applicable credit enhancements and collateral postings. Nonrecurring Basis Loans Held for Sale Loans held for sale consist of 1-4 family residential loans originated for sale in the secondary market and, from time to time, certain loans transferred from the loan portfolio to loans held for sale, all of which are carried at the lower of cost or fair value. The fair value of 1-4 family residential loans is based on the principal or most advantageous market currently offered for similar loans using observable market data. The fair value of the loans transferred from the loan portfolio is based on the amounts offered for these loans in currently pending sales transactions. Loans held for sale carried at fair value are classified as Level 3. Impaired Loans Impaired loans are carried at the lower of carrying value or fair value. Fair value is determined as the recorded investment balance less any specific reserve. We establish specific reserves based on the following three impairment methods: 1) the present value of expected future cash flows discounted at the loan’s original effective interest rate; 2) the loan’s observable market price; or 3) the fair value of the collateral less estimated selling costs when the loan is collateral dependent and we expect to liquidate the collateral. However, if repayment is expected to come from the operation of the collateral, rather than liquidation, then we do not consider estimated selling costs in determining the fair value of the collateral. Collateral values are generally based upon appraisals by approved, independent state certified appraisers. Appraisals may be discounted based on our historical knowledge, changes in market conditions from the time of appraisal or our knowledge of the borrower and the borrower’s business. Impaired loans carried at fair value are classified as Level 3. OREO and Other Repossessed Assets OREO and other repossessed assets obtained in partial or total satisfaction of a loan are recorded at the lower of recorded investment in the loan or fair value less cost to sell. Subsequent to foreclosure, these assets are carried at the lower of the amount recorded at acquisition date or fair value less cost to sell. Accordingly, it may be necessary to record nonrecurring fair value adjustments. Fair value, when recorded, is generally based upon appraisals by approved, independent state certified appraisers. Like impaired loans, appraisals on OREO may be discounted based on our historical knowledge, changes in market conditions from the time of appraisal or other information available to us. OREO and other repossessed assets carried at fair value are classified as Level 3. Mortgage Servicing Rights The fair value of MSRs is determined by calculating the present value of estimated future net servicing cash flows, considering expected mortgage loan prepayment rates, discount rates, servicing costs and other economic factors, which are determined based on current market conditions. The expected rate of mortgage loan prepayments is the most significant factor driving the value of MSRs. MSRs are considered impaired if the carrying value exceeds fair value. The valuation model includes significant unobservable inputs; therefore, MSRs are classified as Level 3. Other Assets We measure certain other assets at fair value on a nonrecurring basis. Fair value is based on the application of lower of cost or fair value accounting, or write-downs of individual assets. Valuation methodologies used to measure fair value are consistent with overall principles of fair value accounting and consistent with those described above. Financial Instruments In addition to financial instruments recorded at fair value in our financial statements, fair value accounting guidance requires disclosure of the fair value of all of an entity’s assets and liabilities that are considered financial instruments. The majority of our assets and liabilities are considered financial instruments. Many of these instruments lack an available trading market as characterized by a willing buyer and willing seller engaged in an exchange transaction. Also, it is our general practice and intent to hold our financial instruments to maturity and to not engage in trading or sales activities with respect to such financial instruments. For fair value disclosure purposes, we substantially utilize the fair value measurement criteria as required and explained above. In cases where quoted fair values are not available, we use present value methods to determine the fair value of our financial instruments. Cash and Cash Equivalents The carrying amounts reported in the Consolidated Balance Sheets for cash and due from banks, including interest-bearing deposits, approximate fair value. Loans The fair value of variable rate performing loans that may reprice frequently at short-term market rates is based on carrying values adjusted for credit risk. The fair value of variable rate performing loans that reprice at intervals of one year or longer, such as adjustable rate mortgage products, is estimated using discounted cash flow analyses that utilize interest rates currently being offered for similar loans and adjusted for credit risk. The fair value of fixed rate performing loans is estimated using a discounted cash flow analysis that utilizes interest rates currently being offered for similar loans and adjusted for credit risk. The fair value of nonperforming loans is the carrying value less any specific reserve on the loan if it is impaired. The carrying amount of accrued interest approximates fair value. Bank Owned Life Insurance Fair value approximates net cash surrender value of bank owned life insurance, or BOLI. Federal Home Loan Bank, or FHLB, and Other Restricted Stock It is not practical to determine the fair value of our FHLB and other restricted stock due to the restrictions placed on the transferability of these stocks; it is presented at carrying value. Deposits The fair values disclosed for deposits without defined maturities (e.g., noninterest and interest-bearing demand, money market and savings accounts) are by definition equal to the amounts payable on demand. The carrying amounts for variable rate, fixed-term time deposits approximate their fair values. Estimated fair values for fixed rate and other time deposits are based on discounted cash flow analysis using interest rates currently offered for time deposits with similar terms. The carrying amount of accrued interest approximates fair value. Short-Term Borrowings The carrying amounts of securities sold under repurchase agreements, or REPOs, and other short-term borrowings approximate their fair values. Long-Term Borrowings The fair values disclosed for fixed rate long-term borrowings are determined by discounting their contractual cash flows using current interest rates for long-term borrowings of similar remaining maturities. The carrying amounts of variable rate long-term borrowings approximate their fair values. Junior Subordinated Debt Securities The interest rate on the variable rate junior subordinated debt securities is reset quarterly; therefore, the carrying values approximate their fair values. Loan Commitments and Standby Letters of Credit Off-balance sheet financial instruments consist of commitments to extend credit and letters of credit. Except for interest rate lock commitments, estimates of the fair value of these off-balance sheet items are not made because of the short-term nature of these arrangements and the credit standing of the counterparties. Other Estimates of fair value are not made for items that are not defined as financial instruments, including such items as our core deposit intangibles and the value of our trust operations. Cash and Cash Equivalents We consider cash and due from banks, interest-bearing deposits with banks and federal funds sold as cash and cash equivalents. Securities We determine the appropriate classification of securities at the time of purchase. All securities, including both debt and equity securities, are classified as available-for-sale. These are securities that we intend to hold for an indefinite period of time, but that may be sold in response to changes in interest rates, prepayment risk, liquidity needs or other factors. Such securities are carried at fair value with net unrealized gains and losses deemed to be temporary and reported as a component of other comprehensive income (loss), net of tax. Realized gains and losses on the sale of available-for-sale securities and other-than-temporary impairment, or OTTI, charges are recorded within noninterest income in the Consolidated Statements of Net Income. Realized gains and losses on the sale of securities are determined using the specific-identification method. Bond premiums are amortized to the call date and bond discounts are accreted to the maturity date, both on a level yield basis. An investment security is considered impaired if its fair value is less than its cost or amortized cost basis. We perform a quarterly review of our securities to identify those that may indicate an OTTI. Our policy for OTTI within the marketable equity securities portfolio generally requires an impairment charge when the security is in a loss position for 12 consecutive months, unless facts and circumstances would suggest the need for an OTTI prior to that time. Our policy for OTTI within the debt securities portfolio is based upon a number of factors, including but not limited to, the length of time and extent to which the estimated fair value has been less than cost, the financial condition of the underlying issuer, the ability of the issuer to meet contractual obligations, the best estimate of the impairment charge representing credit losses, the likelihood of the security’s ability to recover any decline in its estimated fair value and whether management intends to sell the security or if it is more likely than not that management will be required to sell the investment security prior to the security’s recovery of any decline in its estimated fair value. If the impairment is considered other-than-temporary based on management’s review, the impairment must be separated into credit and non-credit components. The credit component is recognized in the Consolidated Statements of Net Income and the non-credit component is recognized in other comprehensive income (loss), net of applicable taxes. Loans Held for Sale Loans held for sale consist of 1-4 family residential loans originated for sale in the secondary market and, from time to time, certain loans transferred from the loan portfolio to loans held for sale, all of which are carried at the lower of cost or fair value. If a loan is transferred from the loan portfolio to the held for sale category, any write-down in the carrying amount of the loan at the date of transfer is recorded as a charge-off against the allowance for loan losses, or ALL. Subsequent declines in fair value are recognized as a charge to noninterest income. When a loan is placed in the held for sale category, we stop amortizing the related deferred fees and costs. The remaining unamortized fees and costs are recognized as part of the cost basis of the loan at the time it is sold. Gains and losses on sales of loans held for sale are included in other noninterest income in the Consolidated Statements of Net Income. Loans Loans are reported at the principal amount outstanding net of unearned income, unamortized premiums or discounts and deferred origination fees and costs. We defer certain nonrefundable loan origination and commitment fees. Accretion of discounts and amortization of premiums on loans are included in interest income in the Consolidated Statements of Net Income. Loan origination fees and direct loan origination costs are deferred and amortized as an adjustment of loan yield over the respective lives of the loans without consideration of anticipated prepayments. If a loan is paid off, the remaining unaccreted or unamortized net origination fees and costs are immediately recognized into income or expense. Interest is accrued and interest income is recognized on loans as earned. Acquired loans are recorded at fair value on the date of acquisition with no carryover of the related ALL. Determining the fair value of the acquired loans involves estimating the principal and interest cash flows expected to be collected on the loans and discounting those cash flows at a market rate of interest. In estimating the fair value of our acquired loans, we consider a number of factors including the loan term, internal risk rating, delinquency status, prepayment rates, recovery periods, estimated value of the underlying collateral and the current interest rate environment. Closed-end installment loans, amortizing loans secured by real estate and any other loans with payments scheduled monthly are reported past due when the borrower is in arrears two or more monthly payments. Other multi-payment obligations with payments scheduled other than monthly are reported past due when one scheduled payment is due and unpaid for 30 days or more. Generally, consumer loans are charged off against the ALL upon the loan reaching 90 days past due. Commercial loans are charged off as management becomes aware of facts and circumstances that raise doubt as to the collectability of all or a portion of the principal and when we believe a confirmed loss exists. Nonaccrual or Nonperforming Loans We stop accruing interest on a loan when the borrower’s payment is 90 days past due. Loans are also placed on nonaccrual status when we have doubt about the borrower’s ability to comply with contractual repayment terms, even if payment is not past due. When the interest accrual is discontinued, all unpaid accrued interest is reversed against interest income. Interest income is recognized on nonaccrual loans on a cash basis if recovery of the remaining principal is reasonably assured. As a general rule, a nonaccrual loan may be restored to accrual status when its principal and interest is paid current and the bank expects repayment of the remaining contractual principal and interest, or when the loan otherwise becomes well secured and in the process of collection. Troubled Debt Restructurings Troubled debt restructurings, or TDRs, are loans where we, for economic or legal reasons related to a borrower’s financial difficulties, grant a concession to the borrower. We strive to identify borrowers with financial difficulty early and work with them to come to a mutual resolution to modify the terms of their loan before the loan reaches nonaccrual status. These modified terms generally include extensions of maturity dates at a stated interest rate lower than the current market rate for a new loan with similar risk characteristics, reductions in contractual interest rates or principal deferment. While unusual, there may be instances of principal forgiveness. These modifications are generally for longer term periods that would not be considered insignificant. Additionally, we classify loans where the debt obligation has been discharged through a Chapter 7 Bankruptcy and not reaffirmed as TDRs. We individually evaluate all substandard commercial loans that have experienced a forbearance or change in terms agreement, and all substandard consumer and residential mortgage loans that entered into an agreement to modify their existing loan, to determine if they should be designated as TDRs. All TDRs are considered to be impaired loans and will be reported as impaired loans for the remaining life of the loan, unless the restructuring agreement specifies an interest rate equal to or greater than the rate that would be accepted at the time of the restructuring for a new loan with comparable risk and it is fully expected that the remaining principal and interest will be collected according to the restructured agreement. Further, all impaired loans are reported as nonaccrual loans unless the loan is a TDR that has met the requirements to be returned to accruing status. TDRs can be returned to accruing status if the ultimate collectability of all contractual amounts due, according to the restructured agreement, is not in doubt and there is a period of a minimum of six months of satisfactory payment performance by the borrower either immediately before or after the restructuring. Allowance for Loan Losses The ALL reflects our estimates of probable losses inherent in the loan portfolio at the balance sheet date. The methodology for determining the ALL has two main components: evaluation and impairment tests of individual loans and evaluation and impairment tests of certain groups of homogeneous loans with similar risk characteristics. Loans are considered to be impaired when based upon current information and events it is probable that we will be unable to collect all principal and interest payments due according to the original contractual terms of the loan agreement. We individually evaluate all substandard and nonaccrual commercial loans greater than $0.5 million for impairment. All TDRs will be reported as an impaired loan for the remaining life of the loan, unless the restructuring agreement specifies an interest rate equal to or greater than the rate that would be accepted at the time of the restructuring for a new loan with comparable risk and it is fully expected that the remaining principal and interest will be fully collected according to the restructured agreement. For all TDRs, regardless of size, and all other impaired loans, we conduct further analysis to determine the probable loss and assign a specific reserve to the loan if deemed appropriate. Specific reserves are established based upon the following three impairment methods: 1) the present value of expected future cash flows discounted at the loan’s original effective interest rate, 2) the loan’s observable market price or 3) the estimated fair value of the collateral if the loan is collateral dependent. Our impairment evaluations consist primarily of the fair value of collateral method because most of our loans are collateral dependent. Collateral values are discounted to consider disposition costs when appropriate. A specific reserve is established or a charge-off is taken if the fair value of the impaired loan is less than the recorded investment in the loan balance. The ALL for homogeneous loans is calculated using a systematic methodology with both a quantitative and a qualitative analysis that is applied on a quarterly basis. The ALL model is comprised of five distinct portfolio segments: 1) Commercial Real Estate, or CRE, 2) Commercial and Industrial, or C&I, 3) Commercial Construction, 4) Consumer Real Estate and 5) Other Consumer. Each segment has a distinct set of risk characteristics monitored by management. We further assess and monitor risk and performance at a more disaggregated level which includes our internal risk rating system for the commercial segments and type of collateral, lien position and loan-to-value, or LTV, for the consumer segments. We first apply historical loss rates to pools of loans with similar risk characteristics. Loss rates are calculated by historical charge-offs that have occurred within each pool of loans over the loss emergence period, or LEP. The LEP is an estimate of the average amount of time from when an event happens that causes the borrower to be unable to pay on a loan until the loss is confirmed through a loan charge-off. In conjunction with our annual review of the ALL assumptions, we have updated our analysis of LEPs for our Commercial and Consumer loan portfolio segments using our loan charge-off history. No changes were made to our LEP assumptions in 2017. We estimate the LEP to be 3 years for CRE, 4 years for construction and 1.25 years for C&I. Our analysis resulted in an LEP for Consumer Real Estate of 2.75 years and Other Consumer of 1.25 years. Another key assumption is the look-back period, or LBP, which represents the historical data period utilized to calculate loss rates. We used 8.5 years for our LBP for all portfolio segments which encompasses our loss experience during the Financial Crisis, and our more recent improved loss experience. After consideration of the historic loss calculations, management applies additional qualitative adjustments so that the ALL is reflective of the inherent losses that exist in the loan portfolio at the balance sheet date. Qualitative adjustments are made based upon changes in lending policies and practices, economic conditions, changes in the loan portfolio, changes in lending management, results of internal loan reviews, asset quality trends, collateral values, concentrations of credit risk and other external factors. The evaluation of the various components of the ALL requires considerable judgment in order to estimate inherent loss exposures. Acquired loans are recorded at fair value on the date of acquisition with no carryover of the related ALL. Determining the fair value of acquired loans involves estimating the principal and interest cash flows expected to be collected on the loans and discounting those cash flows at a market rate of interest. In estimating the fair value of our acquired loans, we considered a number of factors including the loan term, internal risk rating, delinquency status, prepayment rates, recovery periods, estimated value of the underlying collateral and the current interest rate environment. Loans acquired with evidence of credit deterioration were evaluated and not considered to be significant. The premium or discount estimated through the loan fair value calculation is recognized into interest income on a level yield or straight-line basis over the remaining contractual life of the loans. Additional credit deterioration on acquired loans, in excess of the original credit discount embedded in the fair value determination on the date of acquisition, will be recognized in the ALL through the provision for loan losses. Our ALL Committee meets quarterly to verify the overall adequacy of the ALL. Additionally, on an annual basis, the ALL Committee meets to validate our ALL methodology. This validation includes reviewing the loan segmentation, LEP, LBP and the qualitative framework. As a result of this ongoing monitoring process, we may make changes to our ALL to be responsive to the economic environment. Although we believe our process for determining the ALL adequately considers all of the factors that would likely result in credit losses, the process includes subjective elements and may be susceptible to significant change. To the extent actual losses are higher than management estimates, additional provisions for loan losses could be required and could adversely affect our earnings or financial position in future periods. Bank Owned Life Insurance We have purchased life insurance policies on certain executive officers and employees. We receive the cash surrender value of each policy upon its termination or benefits are payable to us upon the death of the insured. Changes in net cash surrender value are recognized in noninterest income or expense in the Consolidated Statements of Net Income. Premises and Equipment Premises and equipment, including leasehold improvements, are stated at cost less accumulated depreciation. Maintenance and repair

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
EARNINGS PER SHARE	EARNINGS PER SHARE The following table reconciles the numerators and denominators of basic and diluted EPS: Years ended December 31, (dollars in thousands, except share and per share data) 2017 2016 2015 Numerator for Earnings per Common Share—Basic: Net income $ 72,968 $ 71,392 $ 67,081 Less: Income allocated to participating shares 242 225 280 Net Income Allocated to Common Shareholders $ 72,726 $ 71,167 $ 66,801 Numerator for Earnings per Common Share—Diluted: Net income $ 72,968 $ 71,392 $ 67,081 Denominators: Weighted Average Common Shares Outstanding—Basic 34,729,376 34,677,738 33,812,990 Add: Dilutive potential common shares 225,391 95,432 35,092 Denominator for Treasury Stock Method—Diluted 34,954,767 34,773,170 33,848,082 Weighted Average Common Shares Outstanding—Basic 34,729,376 34,677,738 33,812,990 Add: Average participating shares outstanding 115,418 109,755 141,558 Denominator for Two-Class Method—Diluted 34,844,794 34,787,493 33,954,548 Earnings per common share—basic $ 2.10 $ 2.06 $ 1.98 Earnings per common share—diluted $ 2.09 $ 2.05 $ 1.98 Warrants considered anti-dilutive excluded from dilutive potential common shares - exercise price $31.53 per share, expires January 2019 438,681 517,012 517,012 Restricted stock considered anti-dilutive excluded from dilutive potential common shares 88,578 116,749 106,466

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS The following tables present our assets and liabilities that are measured at fair value on a recurring basis by fair value hierarchy level at December 31, 2017 and 2016. December 31, 2017 (dollars in thousands) Level 1 Level 2 Level 3 Total ASSETS Securities available-for-sale: U.S. Treasury securities $ — $ 19,789 $ — $ 19,789 Obligations of U.S. government corporations and agencies — 162,193 — 162,193 Collateralized mortgage obligations of U.S. government corporations and agencies — 108,688 — 108,688 Residential mortgage-backed securities of U.S. government corporations and agencies — 32,854 — 32,854 Commercial mortgage-backed securities of U.S. government corporations and agencies — 242,221 — 242,221 Obligations of states and political subdivisions — 127,402 — 127,402 Marketable equity securities — 5,144 — 5,144 Total securities available-for-sale — 698,291 — 698,291 Trading securities held in a Rabbi Trust 5,080 — — 5,080 Total securities 5,080 698,291 — 703,371 Derivative financial assets: Interest rate swaps — 3,074 — 3,074 Interest rate lock commitments — 226 — 226 Total Assets $ 5,080 $ 701,591 $ — $ 706,671 LIABILITIES Derivative financial liabilities: Interest rate swaps $ — $ 3,055 $ — $ 3,055 Forward sale contracts — 5 — 5 Total Liabilities $ — $ 3,060 $ — $ 3,060 December 31, 2016 (dollars in thousands) Level 1 Level 2 Level 3 Total ASSETS Securities available-for-sale: U.S. Treasury securities $ — $ 24,811 $ — $ 24,811 Obligations of U.S. government corporations and agencies — 232,179 — 232,179 Collateralized mortgage obligations of U.S. government corporations and agencies — 129,777 — 129,777 Residential mortgage-backed securities of U.S. government corporations and agencies — 37,358 — 37,358 Commercial mortgage-backed securities of U.S. government corporations and agencies — 125,604 — 125,604 Obligations of states and political subdivisions — 132,509 — 132,509 Marketable equity securities — 11,249 — 11,249 Total securities available-for-sale — 693,487 — 693,487 Trading securities held in a Rabbi Trust 4,410 — — 4,410 Total securities 4,410 693,487 — 697,897 Derivative financial assets: Interest rate swaps — 6,960 — 6,960 Interest rate lock commitments — 236 — 236 Total Assets $ 4,410 $ 700,683 $ — $ 705,093 LIABILITIES Derivative financial liabilities: Interest rate swaps $ — $ 6,958 $ — $ 6,958 Forward sale contracts — 27 — 27 Total Liabilities $ — $ 6,985 $ — $ 6,985 We classify financial instruments as Level 3 when valuation models are used because significant inputs are not observable in the market. We may be required to measure certain assets and liabilities at fair value on a nonrecurring basis. Nonrecurring assets are recorded at the lower of cost or fair value in our financial statements. There were no liabilities measured at fair value on a nonrecurring basis at either December 31, 2017 or December 31, 2016. The following table presents our assets that are measured at fair value on a nonrecurring basis by the fair value hierarchy level as of the dates presented: December 31, 2017 December 31, 2016 (dollars in thousands) Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total ASSETS (1) Loans held for sale $ — $ — $ — $ — $ — $ — $ 1,802 $ 1,802 Impaired loans — — 6,759 6,759 — — 10,329 10,329 Other real estate owned — — 444 444 — — 396 396 Mortgage servicing rights — — 178 178 — — 538 538 Total Assets $ — $ — $ 7,381 $ 7,381 $ — $ — $ 13,065 $ 13,065 (1) This table presents only the nonrecurring items that are recorded at fair value in our financial statements. The carrying values and fair values of our financial instruments at December 31, 2017 and 2016 are presented in the following tables: Fair Value Measurements at December 31, 2017 (dollars in thousands) Carrying Value (1) Total Level 1 Level 2 Level 3 ASSETS Cash and due from banks, including interest-bearing deposits $ 117,152 $ 117,152 $ 117,152 $ — $ — Securities available-for-sale 698,291 698,291 — 698,291 — Loans held for sale 4,485 4,583 — — 4,583 Portfolio loans, net of unearned income 5,761,449 5,690,292 — — 5,690,292 Bank owned life insurance 72,150 72,150 — 72,150 — FHLB and other restricted stock 29,270 29,270 — — 29,270 Trading securities held in a Rabbi Trust 5,080 5,080 5,080 — — Mortgage servicing rights 4,133 4,571 — — 4,571 Interest rate swaps 3,074 3,074 — 3,074 — Interest rate lock commitments 226 226 — 226 — LIABILITIES Deposits $ 5,427,891 $ 5,426,928 $ — $ — $ 5,426,928 Securities sold under repurchase agreements 50,161 50,161 — — 50,161 Short-term borrowings 540,000 540,000 — — 540,000 Long-term borrowings 47,301 47,618 — — 47,618 Junior subordinated debt securities 45,619 45,619 — — 45,619 Interest rate swaps 3,055 3,055 — 3,055 — Forward sale contracts 5 5 — 5 — (1) As reported in the Consolidated Balance Sheets Fair Value Measurements at December 31, 2016 (dollars in thousands) Carrying Value (1) Total Level 1 Level 2 Level 3 ASSETS Cash and due from banks, including interest-bearing deposits $ 139,486 $ 139,486 $ 139,486 $ — $ — Securities available-for-sale 693,487 693,487 — 693,487 — Loans held for sale 3,793 3,815 — — 3,815 Portfolio loans, net of unearned income 5,611,419 5,551,266 — — 5,551,266 Bank owned life insurance 72,081 72,081 — 72,081 — FHLB and other restricted stock 31,817 31,817 — — 31,817 Trading securities held in a Rabbi Trust 4,410 4,410 4,410 — — Mortgage servicing rights 3,744 4,098 — — 4,098 Interest rate swaps 6,960 6,960 — 6,960 — Interest rate lock commitments 236 236 — 236 — LIABILITIES Deposits $ 5,272,377 $ 5,276,499 $ — $ — $ 5,276,499 Securities sold under repurchase agreements 50,832 50,832 — — 50,832 Short-term borrowings 660,000 660,000 — — 660,000 Long-term borrowings 14,713 15,267 — — 15,267 Junior subordinated debt securities 45,619 45,619 — — 45,619 Interest rate swaps 6,958 6,958 — 6,958 — Forward sale contracts 27 27 — 27 — (1) As reported in the Consolidated Balance Sheets

Restrictions on Cash and Due fr

Restrictions on Cash and Due from Bank Accounts	12 Months Ended
	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
RESTRICTIONS ON CASH AND DUE FROM BANK ACCOUNTS	RESTRICTIONS ON CASH AND DUE FROM BANK ACCOUNTS The Board of Governors of the Federal Reserve System, or the Federal Reserve, imposes certain reserve requirements on all depository institutions. These reserves are maintained in the form of vault cash or as an interest-bearing balance with the Federal Reserve. The required reserves averaged $36.2 million for 2017, $36.8 million for 2016 and $44.1 million for 2015.

Dividend and Loan Restrictions

Dividend and Loan Restrictions	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
DIVIDEND AND LOAN RESTRICTIONS	DIVIDEND AND LOAN RESTRICTIONS S&T is a legal entity separate and distinct from its banking and other subsidiaries. A substantial portion of our revenues consist of dividend payments we receive from S&T Bank. S&T Bank, in turn, is subject to state laws and regulations that limit the amount of dividends it can pay to us. In addition, both S&T and S&T Bank are subject to various general regulatory policies relating to the payment of dividends, including requirements to maintain adequate capital above regulatory minimums. The Federal Reserve has indicated that banking organizations should generally pay dividends only if (i) the organization’s net income available to common shareholders over the past year has been sufficient to fully fund the dividends and (ii) the prospective rate of earnings retention appears consistent with the organization’s capital needs, asset quality and overall financial condition. Thus, under certain circumstances based upon our financial condition, our ability to declare and pay quarterly dividends may require consultation with the Federal Reserve and may be prohibited by applicable Federal Reserve guidelines. Federal law prohibits us from borrowing from S&T Bank unless such loans are collateralized by specific obligations. Further, such loans are limited to 10 percent of S&T Bank’s capital stock and surplus.

Securities Available-for-Sale

Securities Available-for-Sale	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
SECURITIES AVAILABLE-FOR-SALE	SECURITIES AVAILABLE-FOR-SALE The following tables present the amortized cost and fair value of available-for-sale securities as of the dates presented: December 31, 2017 December 31, 2016 (dollars in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. Treasury securities $ 19,943 $ — $ (154 ) $ 19,789 $ 24,891 $ 47 $ (127 ) $ 24,811 Obligations of U.S. government corporations and agencies 162,045 341 (193 ) 162,193 230,989 1,573 (383 ) 232,179 Collateralized mortgage obligations of U.S. government corporations and agencies 109,916 93 (1,321 ) 108,688 130,046 465 (734 ) 129,777 Residential mortgage-backed securities of U.S. government corporations and agencies 32,388 679 (213 ) 32,854 36,606 984 (232 ) 37,358 Commercial mortgage-backed securities of U.S. government corporations and agencies (1) 244,018 247 (2,044 ) 242,221 127,311 243 (1,950 ) 125,604 Obligations of states and political subdivisions 123,159 4,285 (42 ) 127,402 128,783 3,772 (46 ) 132,509 Debt Securities 691,469 5,645 (3,967 ) 693,147 678,626 7,084 (3,472 ) 682,238 Marketable equity securities 3,815 1,330 (1 ) 5,144 7,579 3,670 — 11,249 Total $ 695,284 $ 6,975 $ (3,968 ) $ 698,291 $ 686,205 $ 10,754 $ (3,472 ) $ 693,487 (1) Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017. The following table shows the composition of gross and net realized gains and losses for the periods presented: Years ended December 31, (dollars in thousands) 2017 2016 2015 Gross realized gains $ 3,986 $ — $ — Gross realized losses (986 ) — (34 ) Net Realized (Losses) Gains $ 3,000 $ — $ (34 ) The following tables present the fair value and the age of gross unrealized losses by investment category as of the dates presented: December 31, 2017 Less Than 12 Months 12 Months or More Total (dollars in thousands) Number of Securities Fair Unrealized Losses Number of Securities Fair Unrealized Losses Number of Securities Fair Unrealized Losses U.S. Treasury securities 3 $ 19,789 $ (154 ) — $ — $ — 3 $ 19,789 $ (154 ) Obligations of U.S. government corporations and agencies 9 63,635 (144 ) 1 10,017 (49 ) 10 73,652 (193 ) Collateralized mortgage obligations of U.S. government corporations and agencies 7 47,465 (248 ) 7 45,809 (1,073 ) 14 93,274 (1,321 ) Residential mortgage-backed securities of U.S. government corporations and agencies 1 2,333 (10 ) 2 8,638 (203 ) 3 10,971 (213 ) Commercial mortgage-backed securities of U.S. government corporations and agencies 14 128,300 (775 ) 5 48,746 (1,269 ) 19 177,046 (2,044 ) Obligations of states and political subdivisions 2 10,330 (42 ) — — — 2 10,330 (42 ) Debt Securities 36 271,852 (1,373 ) 15 113,210 (2,594 ) 51 385,062 (3,967 ) Marketable equity securities 1 70 (1 ) — — — 1 70 (1 ) Total Temporarily Impaired Securities 37 $ 271,922 $ (1,374 ) 15 $ 113,210 $ (2,594 ) 52 $ 385,132 $ (3,968 ) December 31, 2016 Less Than 12 Months 12 Months or More Total (dollars in thousands) Number of Securities Fair Value Unrealized Losses Number of Securities Fair Value Unrealized Losses Number of Securities Fair Value Unrealized Losses U.S. Treasury securities 1 $ 9,811 $ (127 ) — $ — $ — 1 $ 9,811 $ (127 ) Obligations of U.S. government corporations and agencies 7 62,483 (383 ) — — — 7 62,483 (383 ) Collateralized mortgage obligations of U.S. government corporations and agencies 10 83,031 (734 ) — — — 10 83,031 (734 ) Residential mortgage-backed securities of U.S. government corporations and agencies 2 10,022 (232 ) — — — 2 10,022 (232 ) Commercial mortgage-backed securities of U.S. government corporations and agencies 10 96,576 (1,950 ) — — — 10 96,576 (1,950 ) Obligations of states and political subdivisions 1 5,577 (46 ) — — — 1 5,577 (46 ) Debt Securities 31 267,500 (3,472 ) — — — 31 267,500 (3,472 ) Marketable equity securities — — — — — — — — — Total Temporarily Impaired Securities 31 $ 267,500 $ (3,472 ) — $ — $ — 31 $ 267,500 $ (3,472 ) We do not believe any individual unrealized loss as of December 31, 2017 represents an other than temporary impairment, or OTTI. As of December 31, 2017, the unrealized losses on 51 debt securities were primarily attributable to changes in interest rates and not related to the credit quality of these securities. All debt securities are determined to be investment grade and are paying principal and interest according to the contractual terms of the security. There was one marketable equity security with an unrealized loss at December 31, 2017 and no marketable equity securities at an unrealized loss at December 31, 2016. We do not intend to sell and it is more likely than not that we will not be required to sell any of the securities in an unrealized loss position before recovery of their amortized cost. The following table displays net unrealized gains and losses, net of tax, on securities available-for-sale included in accumulated other comprehensive income/(loss), for the periods presented: December 31, 2017 December 31, 2016 (dollars in thousands) Gross Unrealized Gains Gross Unrealized Losses Net Unrealized Gains (Losses) Gross Unrealized Gains Gross Unrealized Losses Net Unrealized Gains (Losses) Total unrealized gains (losses) on securities available for sale $ 6,975 $ (3,968 ) $ 3,007 $ 10,754 $ (3,472 ) $ 7,282 Income tax (expense) benefit (2,450 ) 1,394 (1,056 ) (3,776 ) 1,219 (2,557 ) Net unrealized gains (losses), net of tax included in accumulated other comprehensive income(loss) $ 4,525 $ (2,574 ) $ 1,951 $ 6,978 $ (2,253 ) $ 4,725 The amortized cost and fair value of securities available-for-sale at December 31, 2017 by contractual maturity are included in the table below. Actual maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations with or without call or prepayment penalties. December 31, 2017 (dollars in thousands) Amortized Cost Fair Value Due in one year or less $ 58,501 $ 58,438 Due after one year through five years 141,646 142,859 Due after five years through ten years 70,584 72,132 Due after ten years 34,416 35,955 305,147 309,384 Collateralized mortgage obligations of U.S. government corporations and agencies 109,916 108,688 Residential mortgage-backed securities of U.S. government corporations and agencies 32,388 32,854 Commercial mortgage-backed securities of U.S. government corporations and agencies 244,018 242,221 Debt Securities 691,469 693,147 Marketable equity securities 3,815 5,144 Total $ 695,284 $ 698,291 At December 31, 2017 and 2016, securities with carrying values of $249 million and $342 million were pledged for various regulatory and legal requirements.

Loans and Loans Held for Sale

Loans and Loans Held for Sale	12 Months Ended
	Dec. 31, 2017
Receivables [Abstract]
LOANS AND LOANS HELD FOR SALE	LOANS AND LOANS HELD FOR SALE Loans are presented net of unearned income of $5.2 million at December 31, 2017 and 2016 and net of a discount related to purchase accounting fair value adjustments of $2.8 million and $7.1 million at December 31, 2017 and December 31, 2016. The following table indicates the composition of the acquired and originated loans as of the dates presented: December 31, (dollars in thousands) 2017 2016 Commercial Commercial real estate $ 2,685,994 $ 2,498,476 Commercial and industrial 1,433,266 1,401,035 Commercial construction 384,334 455,884 Total Commercial Loans 4,503,594 4,355,395 Consumer Residential mortgage 698,774 701,982 Home equity 487,326 482,284 Installment and other consumer 67,204 65,852 Consumer construction 4,551 5,906 Total Consumer Loans 1,257,855 1,256,024 Total Portfolio Loans 5,761,449 5,611,419 Loans held for sale 4,485 3,793 Total Loans $ 5,765,934 $ 5,615,212 As of December 31, 2017, our acquired loans from the Merger were $387 million including $209 million of CRE, $92.1 million of C&I, $11.1 million of commercial construction, $57.5 million of residential mortgage and $17.3 million of home equity, installment and other consumer construction. These acquired loans decreased from acquired loans at December 31, 2016 of $543 million , including $273 million of CRE, $141 million of C&I, $33.0 million of commercial construction, $74.0 million of residential mortgage, $22.0 million of home equity, installment and other consumer construction. We attempt to limit our exposure to credit risk by diversifying our loan portfolio by segment, geography, collateral and industry and actively managing concentrations. When concentrations exist in certain segments, we mitigate this risk by reviewing the relevant economic indicators and internal risk rating trends and through stress testing of the loans in these segments. Total commercial loans represented 78 percent of total portfolio loans at both December 31, 2017 and 2016. Within our commercial portfolio, the CRE and Commercial Construction portfolios combined comprised $3.1 billion or 68 percent of total commercial loans and 53 percent of total portfolio loans at December 31, 2017 and comprised of $3.0 billion or 68 percent of total commercial loans and 53 percent of total portfolio loans at December 31, 2016. Further segmentation of the CRE and Commercial Construction portfolios by collateral type reveals no concentration in excess of 14 percent of both total CRE and Commercial Construction loans at either December 31, 2017 or December 31, 2016. Our market area includes Pennsylvania and the contiguous states of Ohio, West Virginia, New York and Maryland. The majority of our commercial and consumer loans are made to businesses and individuals in this market area, resulting in a geographic concentration. We believe our knowledge and familiarity with customers and conditions locally outweighs this geographic concentration risk. The conditions of the local and regional economies are monitored closely through publicly available data and information supplied by our customers. Our CRE and Commercial Construction portfolios have out-of-market exposure of 5.2 percent of their combined portfolios and 2.8 percent of total portfolio loans at December 31, 2017 and 5.2 percent of their combined portfolios and 2.7 percent of total portfolio loans at December 31, 2016. The following table summarizes our restructured loans as of the dates presented: December 31, 2017 December 31, 2016 (dollars in thousands) Performing TDRs Nonperforming TDRs Total TDRs Performing TDRs Nonperforming TDRs Total TDRs Commercial real estate $ 2,579 $ 967 $ 3,546 $ 2,994 $ 646 $ 3,640 Commercial and industrial 3,946 3,197 7,143 1,387 4,493 5,880 Commercial construction 2,420 2,413 4,833 2,966 430 3,396 Residential mortgage 2,039 3,585 5,624 2,375 5,068 7,443 Home equity 3,885 979 4,864 3,683 954 4,637 Installment and other consumer 32 9 41 18 7 25 Total $ 14,901 $ 11,150 $ 26,051 $ 13,423 $ 11,598 $ 25,021 The following tables present the restructured loans by type of concession for the years ended December 31: 2017 2016 (dollars in thousands) Number of Loans Pre-Modification Outstanding Recorded Investment (1) Post-Modification Outstanding Recorded Investment (1) Total Difference in Recorded Investment Number of Loans Pre-Modification Outstanding Recorded Investment (1) Post-Modification Outstanding Recorded Investment (1) Total Difference in Recorded Investment Commercial real estate Chapter 7 bankruptcy (2) — $ — $ — $ — 1 $ 709 $ 646 $ (63 ) Interest Rate Reduction — — — — 1 250 242 (8 ) Maturity date extension 1 400 398 (2 ) — — — — Commercial and industrial Maturity date extension 1 274 777 503 4 4,756 3,334 (1,422 ) Maturity date extension and interest rate reduction 2 1,800 1,805 5 — — — — Principal deferral 2 113 113 — 5 985 986 1 Commercial construction Maturity date extension — — — — 3 1,251 1,151 (100 ) Principal forgiveness 2 1,996 1,996 — — — — — Residential mortgage Chapter 7 bankruptcy (2) 1 33 31 (2 ) 7 439 413 (26 ) Maturity date extension — — — — 1 483 414 (69 ) Maturity date extension and interest rate reduction — — — — 1 280 279 (1 ) Principal deferral — — — — 1 3,273 3,133 (140 ) Home equity Chapter 7 bankruptcy (2) 21 689 643 (46 ) 19 676 643 (33 ) Maturity date extension 1 231 231 — 5 305 298 (7 ) Maturity date extension and interest rate reduction 1 173 113 (60 ) 2 604 598 (6 ) Principal deferral — — — — 1 47 45 (2 ) Installment and other consumer Chapter 7 bankruptcy (2) 4 48 35 (13 ) 2 16 10 (6 ) Total by Concession Type Chapter 7 bankruptcy (2) 26 $ 770 $ 709 $ (61 ) 29 $ 1,840 $ 1,712 $ (128 ) Interest rate reduction — — — — 1 250 242 (8 ) Maturity date extension 3 905 1,406 501 13 6,795 5,197 (1,598 ) Maturity date extension and interest rate reduction 3 1,973 1,918 (55 ) 3 884 877 (7 ) Principal deferral 2 113 113 — 7 4,305 4,164 (141 ) Principal forgiveness 2 1,996 1,996 — — — — — Total 36 $ 5,757 $ 6,142 $ 385 53 $ 14,074 $ 12,192 $ (1,882 ) (1) Excludes loans that were fully paid off or fully charged-off by period end. The pre-modification balance represents the balance outstanding prior to modification. The post-modification balance represents the outstanding balance at period end. (2) Chapter 7 bankruptcy loans where the debt has been legally discharged through the bankruptcy court and not reaffirmed. During 2017, we modified 15 loans that were not considered to be TDRs, including 10 C&I loans for $10.8 million , and five CRE loans for $7.5 million . These modifications primarily represented insignificant delays in the timing of payments, concessions where we were adequately compensated through principal pay downs, fees or additional collateral or circumstances where we concluded that no concession was granted. As of December 31, 2017, we have two commitments totaling $0.2 million to lend additional funds on a TDR. We returned one TDR totaling $2.0 million to accruing status during 2017. We returned five TDRs to accruing status during 2016 totaling $0.9 million . Defaulted TDRs are defined as loans having a payment default of 90 days or more after the restructuring takes place. There were no TDRs that defaulted during the years ended December 31, 2017 and 2016 that were restructured within the last 12 months prior to defaulting. The following table is a summary of nonperforming assets as of the dates presented: December 31, (dollars in thousands) 2017 2016 Nonperforming Assets Nonaccrual loans $ 12,788 $ 31,037 Nonaccrual TDRs 11,150 11,598 Total nonaccrual loans 23,938 42,635 OREO 469 679 Total Nonperforming Assets $ 24,407 $ 43,314 NPAs decreased $18.9 million to $24.4 million during 2017 compared to $43.3 million for the year ended 2016. The decrease primarily related to two large commercial nonperforming, impaired loans that paid off during the year that totaled $10.5 million . The following table presents a summary of the aggregate amount of loans to certain officers, directors of S&T or any affiliates of such persons as of December 31: (dollars in thousands) 2017 2016 Balance at beginning of year $ 25,167 $ 24,517 New loans 25,203 22,740 Repayments or no longer considered a related party (40,300 ) (22,090 ) $ 10,070 $ 25,167

Allowance for Loan Losses

Allowance for Loan Losses	12 Months Ended
	Dec. 31, 2017
Receivables [Abstract]
ALLOWANCE FOR LOAN LOSSES	ALLOWANCE FOR LOAN LOSSES We maintain an ALL at a level determined to be adequate to absorb estimated probable credit losses inherent in the loan portfolio as of the balance sheet date. We develop and document a systematic ALL methodology based on the following portfolio segments: 1) CRE, 2) C&I, 3) Commercial Construction, 4) Consumer Real Estate and 5) Other Consumer. The following are key risks within each portfolio segment: CRE —Loans secured by commercial purpose real estate, including both owner-occupied properties and investment properties for various purposes such as hotels, strip malls and apartments. Operations of the individual projects and global cash flows of the debtors are the primary sources of repayment for these loans. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the collateral type and the business prospects of the lessee, if the project is not owner-occupied. C&I —Loans made to operating companies or manufacturers for the purpose of production, operating capacity, accounts receivable, inventory or equipment financing. Cash flow from the operations of the company is the primary source of repayment for these loans. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the industry of the company. Collateral for these types of loans often do not have sufficient value in a distressed or liquidation scenario to satisfy the outstanding debt. Commercial Construction —Loans made to finance construction of buildings or other structures, as well as to finance the acquisition and development of raw land for various purposes. While the risk of these loans is generally confined to the construction period, if there are problems, the project may not be completed, and as such, may not provide sufficient cash flow on its own to service the debt or have sufficient value in a liquidation to cover the outstanding principal. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the type of project and the experience and resources of the developer. Consumer Real Estate —Loans secured by first and second liens such as home equity loans, home equity lines of credit and 1-4 family residential mortgages, including purchase money mortgages. The primary source of repayment for these loans is the income and assets of the borrower. The condition of the local economy, in particular the unemployment rate, is an important indicator of risk for this segment. The state of the local housing market can also have a significant impact on this segment because low demand and/or declining home values can limit the ability of borrowers to sell a property and satisfy the debt. Other Consumer —Loans made to individuals that may be secured by assets other than 1-4 family residences, as well as unsecured loans. This segment includes auto loans, unsecured loans and lines and credit cards. The primary source of repayment for these loans is the income and assets of the borrower. The condition of the local economy, in particular the unemployment rate, is an important indicator of risk for this segment. The value of the collateral, if there is any, is less likely to be a source of repayment due to less certain collateral values. We further assess risk within each portfolio segment by pooling loans with similar risk characteristics. For the commercial loan classes, the most important indicator of risk is the internally assigned risk rating, including pass, special mention and substandard. Consumer loans are pooled by type of collateral, lien position and loan to value, or LTV, ratio for Consumer Real Estate loans. Historical loss rates are applied to these loan pools to determine the reserve for loans collectively evaluated for impairment. The ALL methodology for groups of loans collectively evaluated for impairment is comprised of both a quantitative and qualitative analysis. A key assumption in the quantitative component of the reserve is the loss emergence period, or LEP. The LEP is an estimate of the average amount of time from the point at which a loss is incurred on a loan to the point at which the loss is confirmed. Another key assumption is the look-back period, or LBP, which represents the historical data period utilized to calculate loss rates. Management monitors various credit quality indicators for both the commercial and consumer loan portfolios, including delinquency, nonperforming status and changes in risk ratings on a monthly basis. The following tables present the age analysis of past due loans segregated by class of loans as of the dates presented: December 31, 2017 (dollars in thousands) Current 30-59 Days Past Due 60-89 Days Past Due Non- performing Total Past Due Loans Total Loans Commercial real estate $ 2,681,395 $ 997 $ 134 $ 3,468 $ 4,599 $ 2,685,994 Commercial and industrial 1,426,754 420 446 5,646 6,512 1,433,266 Commercial construction 377,968 2,473 20 3,873 6,366 384,334 Residential mortgage 687,195 2,975 1,439 7,165 11,579 698,774 Home equity 480,956 2,065 590 3,715 6,370 487,326 Installment and other consumer 66,770 193 170 71 434 67,204 Consumer construction 4,551 — — — — 4,551 Loans held for sale 4,485 — — — — 4,485 Total $ 5,730,074 $ 9,123 $ 2,799 $ 23,938 $ 35,860 $ 5,765,934 December 31, 2016 (dollars in thousands) Current 30-59 Days Past Due 60-89 Days Past Due Non- performing Total Past Due Loans Total Loans Commercial real estate $ 2,479,513 $ 2,032 $ 759 $ 16,172 $ 18,963 $ 2,498,476 Commercial and industrial 1,391,475 1,061 428 8,071 9,560 1,401,035 Commercial construction 450,410 547 — 4,927 5,474 455,884 Residential mortgage 689,635 1,312 1,117 9,918 12,347 701,982 Home equity 476,866 1,470 509 3,439 5,418 482,284 Installment and other consumer 65,525 176 43 108 327 65,852 Consumer construction 5,906 — — — — 5,906 Loans held for sale 3,793 — — — — 3,793 Total $ 5,563,123 $ 6,598 $ 2,856 $ 42,635 $ 52,089 $ 5,615,212 We continually monitor the commercial loan portfolio through an internal risk rating system. Loan risk ratings are assigned based upon the creditworthiness of the borrower and are reviewed on an ongoing basis according to our internal policies. Loans within the pass rating generally have a lower risk of loss than loans risk rated as special mention and substandard. Our risk ratings are consistent with regulatory guidance and are as follows: Pass —The loan is currently performing and is of high quality. Special Mention —A special mention loan has potential weaknesses that warrant management’s close attention. If left uncorrected, these potential weaknesses may result in deterioration of the repayment prospects or in the strength of our credit position at some future date. Economic and market conditions, beyond the borrower’s control, may in the future necessitate this classification. Substandard —A substandard loan is not adequately protected by the net worth and/or paying capacity of the borrower or by the collateral pledged, if any. Substandard loans have a well-defined weakness or weaknesses that jeopardize the liquidation of the debt. These loans are characterized by the distinct possibility that we will sustain some loss if the deficiencies are not corrected. The following tables present the recorded investment in commercial loan classes by internally assigned risk ratings as of the dates presented: December 31, 2017 (dollars in thousands) Commercial Real Estate % of Total Commercial and Industrial % of Total Commercial Construction % of Total Total % of Total Pass $ 2,588,847 96.4 % $ 1,345,810 93.9 % $ 368,105 95.8 % $ 4,302,762 95.5 % Special mention 66,436 2.5 % 54,320 3.8 % 9,345 2.4 % 130,101 2.9 % Substandard 30,711 1.1 % 33,136 2.3 % 6,884 1.8 % 70,731 1.6 % Total $ 2,685,994 100.0 % $ 1,433,266 100.0 % $ 384,334 100.0 % $ 4,503,594 100.0 % December 31, 2016 (dollars in thousands) Commercial Real Estate % of Total Commercial and Industrial % of Total Commercial Construction % of Total Total % of Total Pass $ 2,423,742 97.0 % $ 1,315,507 93.9 % $ 430,472 94.4 % $ 4,169,721 95.7 % Special mention 33,098 1.3 % 40,409 2.9 % 14,691 3.2 % 88,198 2.0 % Substandard 41,636 1.7 % 45,119 3.2 % 10,721 2.4 % 97,476 2.3 % Total $ 2,498,476 100.0 % $ 1,401,035 100.0 % $ 455,884 100.0 % $ 4,355,395 100.0 % We monitor the delinquent status of the consumer portfolio on a monthly basis. Loans are considered nonperforming when interest and principal are 90 days or more past due or management has determined that a material deterioration in the borrower’s financial condition exists. The risk of loss is generally highest for nonperforming loans. The following tables present the recorded investment in consumer loan classes by performing and nonperforming status as of the dates presented: December 31, 2017 (dollars in thousands) Residential Mortgage % of Total Home Equity % of Total Installment and other consumer % of Total Consumer Construction % of Total Total % of Total Performing $ 691,609 99.0 % $ 483,611 99.2 % $ 67,133 99.9 % $ 4,551 100.0 % $ 1,246,904 99.1 % Nonperforming 7,165 1.0 % 3,715 0.8 % 71 0.1 % — — % 10,951 0.9 % Total $ 698,774 100.0 % $ 487,326 100.0 % $ 67,204 100.0 % $ 4,551 100.0 % $ 1,257,855 100.0 % December 31, 2016 (dollars in thousands) Residential Mortgage % of Total Home Equity % of Total Installment and other consumer % of Total Consumer Construction % of Total Total % of Total Performing $ 692,064 98.6 % $ 478,845 99.3 % $ 65,744 99.8 % $ 5,906 100.0 % $ 1,242,559 98.9 % Nonperforming 9,918 1.4 % 3,439 0.7 % 108 0.2 % — — % 13,465 1.1 % Total $ 701,982 100.0 % $ 482,284 100.0 % $ 65,852 100.0 % $ 5,906 100.0 % $ 1,256,024 100.0 % We individually evaluate all substandard and nonaccrual commercial loans greater than $0.5 million for impairment. Loans are considered to be impaired when based upon current information and events it is probable that we will be unable to collect all principal and interest payments due according to the original contractual terms of the loan agreement. All TDRs will be reported as an impaired loan for the remaining life of the loan, unless the restructuring agreement specifies an interest rate equal to or greater than the rate that would be accepted at the time of the restructuring for a new loan with comparable risk and it is expected that the remaining principal and interest will be fully collected according to the restructured agreement. For all TDRs, regardless of size, and all other impaired loans, we conduct further analysis to determine the probable loss and assign a specific reserve to the loan if deemed appropriate. The following tables summarize investments in loans considered to be impaired and related information on those impaired loans as of the dates presented: December 31, 2017 December 31, 2016 (dollars in thousands) Recorded Investment Unpaid Principal Balance Related Allowance Recorded Investment Unpaid Principal Balance Related Allowance With a related allowance recorded: Commercial real estate $ — $ — $ — $ — $ — $ — Commercial and industrial 1,735 1,787 29 964 2,433 771 Commercial construction — — — — — — Consumer real estate 21 21 21 26 26 26 Other consumer 27 27 27 1 1 1 Total with a Related Allowance Recorded 1,783 1,835 77 991 2,460 798 Without a related allowance recorded: Commercial real estate 3,546 3,811 — 16,352 17,654 — Commercial and industrial 5,549 7,980 — 5,902 7,699 — Commercial construction 5,464 8,132 — 6,613 10,306 — Consumer real estate 10,467 11,357 — 12,053 12,849 — Other consumer 14 22 — 24 31 — Total without a Related Allowance Recorded 25,040 31,302 — 40,944 48,539 — Total: Commercial real estate 3,546 3,811 — 16,352 17,654 — Commercial and industrial 7,284 9,767 29 6,866 10,132 771 Commercial construction 5,464 8,132 — 6,613 10,306 — Consumer real estate 10,488 11,378 21 12,079 12,875 26 Other consumer 41 49 27 25 32 1 Total $ 26,823 $ 33,137 $ 77 $ 41,935 $ 50,999 $ 798 As of December 31, 2017, we had $26.8 million of impaired loans which included $5.1 million of acquired loans that experienced credit deterioration since the acquisition date. The following table summarizes investments in loans considered to be impaired and related information on those impaired loans for the years presented: For the Year Ended December 31, 2017 December 31, 2016 (dollars in thousands) Average Recorded Investment Interest Income Recognized Average Recorded Investment Interest Income Recognized With a related allowance recorded: Commercial real estate $ — $ — $ — $ — Commercial and industrial 968 52 2,438 — Commercial construction — — — — Consumer real estate 23 2 28 2 Other consumer 34 2 2 — Total with a Related Allowance Recorded 1,025 56 2,468 2 Without a related allowance recorded: Commercial real estate 6,636 177 17,496 144 Commercial and industrial 9,897 257 6,141 160 Commercial construction 6,828 253 7,723 162 Consumer real estate 11,037 487 11,939 523 Other consumer 23 — 35 1 Total without a Related Allowance Recorded 34,421 1,174 43,334 990 Total: Commercial real estate 6,636 177 17,496 144 Commercial and industrial 10,865 309 8,579 160 Commercial construction 6,828 253 7,723 162 Consumer real estate 11,060 489 11,967 525 Other consumer 57 2 37 1 Total $ 35,446 $ 1,230 $ 45,802 $ 992 The following tables detail activity in the ALL for the periods presented: 2017 (dollars in thousands) Commercial Real Estate Commercial and Industrial Commercial Construction Consumer Real Estate Other Consumer Total Loans Balance at beginning of year $ 19,976 $ 10,810 $ 13,999 $ 6,095 $ 1,895 $ 52,775 Charge-offs (2,304 ) (4,709 ) (2,571 ) (2,274 ) (1,638 ) (13,496 ) Recoveries 810 654 851 342 571 3,228 Net (Charge-offs) Recoveries (1,494 ) (4,055 ) (1,720 ) (1,932 ) (1,067 ) (10,268 ) Provision for loan losses 8,753 2,211 888 1,316 715 13,883 Balance at End of Year $ 27,235 $ 8,966 $ 13,167 $ 5,479 $ 1,543 $ 56,390 2016 (dollars in thousands) Commercial Real Estate Commercial and Industrial Commercial Construction Consumer Real Estate Other Consumer Total Loans Balance at beginning of year $ 15,043 $ 10,853 $ 12,625 $ 8,400 $ 1,226 $ 48,147 Charge-offs (3,114 ) (6,810 ) (1,877 ) (1,657 ) (2,103 ) (15,561 ) Recoveries 692 722 21 433 356 2,224 Net Recoveries (Charge-offs) (2,422 ) (6,088 ) (1,856 ) (1,224 ) (1,747 ) (13,337 ) Provision for loan losses 7,355 6,045 3,230 (1,081 ) 2,416 17,965 Balance at End of Year $ 19,976 $ 10,810 $ 13,999 $ 6,095 $ 1,895 $ 52,775 Loans acquired in the Merger were recorded at fair value with no carryover of the related allowance for loan losses from Integrity. As of December 31, 2017, acquired loans from the Merger of $387 million were outstanding, which decreased from $543 million at December 31, 2016. Additional credit deterioration on acquired loans during 2017 in excess of the original credit discount embedded in the fair value determination on the date of acquisition was recognized in the ALL through the provision for loan losses. The following tables present the ALL and recorded investments in loans by category as of December 31: 2017 Allowance for Loan Losses Portfolio Loans (dollars in thousands) Individually Evaluated for Impairment Collectively Evaluated for Impairment Total Individually Evaluated for Impairment Collectively Evaluated for Impairment Total Commercial real estate $ — $ 27,235 $ 27,235 $ 3,546 $ 2,682,448 $ 2,685,994 Commercial and industrial 29 8,937 8,966 7,284 1,425,982 1,433,266 Commercial construction — 13,167 13,167 5,464 378,870 384,334 Consumer real estate 21 5,458 5,479 10,488 1,180,163 1,190,651 Other consumer 27 1,516 1,543 41 67,163 67,204 Total $ 77 $ 56,313 $ 56,390 $ 26,823 $ 5,734,626 $ 5,761,449 2016 Allowance for Loan Losses Portfolio Loans (dollars in thousands) Individually Evaluated for Impairment Collectively Evaluated for Impairment Total Individually Evaluated for Impairment Collectively Evaluated for Impairment Total Commercial real estate $ — $ 19,976 $ 19,976 $ 16,352 $ 2,482,124 $ 2,498,476 Commercial and industrial 771 10,039 10,810 6,866 1,394,169 1,401,035 Commercial construction — 13,999 13,999 6,613 449,271 455,884 Consumer real estate 26 6,069 6,095 12,079 1,178,093 1,190,172 Other consumer 1 1,894 1,895 25 65,827 65,852 Total $ 798 $ 51,977 $ 52,775 $ 41,935 $ 5,569,484 $ 5,611,419

Premises and Equipment

Premises and Equipment	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
PREMISES AND EQUIPMENT	PREMISES AND EQUIPMENT The following table is a summary of premises and equipment as of the dates presented: December 31, (dollars in thousands) 2017 2016 Land $ 6,266 $ 6,397 Premises 51,799 52,696 Furniture and equipment 34,836 32,328 Leasehold improvements 6,643 7,293 99,544 98,714 Accumulated depreciation (56,842 ) (53,715 ) Total $ 42,702 $ 44,999 Depreciation expense related to premises and equipment was $5.1 million in 2017, $5.0 million in 2016 and $4.7 million in 2015. Certain banking facilities are leased under arrangements expiring at various dates through the year 2054 . We account for these leases on a straight-line basis due to escalation clauses. All leases are accounted for as operating leases, except for one capital lease. Rental expense for premises amounted to $4.0 million , $4.1 million and $3.9 million in 2017, 2016 and 2015. Included in the rental expense for premises are leases entered into with two S&T directors, which totaled $0.2 million in 2017, and $0.3 million in 2016 and 2015. Minimum annual rental and renewal option payments for each of the following five years and thereafter are approximately: (dollars in thousands) Operating Capital Total 2018 $ 3,257 $ 76 $ 3,333 2019 3,277 77 3,354 2020 3,312 77 3,389 2021 3,351 76 3,427 2022 3,425 77 3,502 Thereafter 55,000 457 55,457 Total $ 71,622 $ 840 $ 72,462

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
GOODWILL AND OTHER INTANGIBLE ASSETS	GOODWILL AND OTHER INTANGIBLE ASSETS The following table presents goodwill as of the dates presented: December 31, (dollars in thousands) 2017 2016 Balance at beginning of year $ 291,670 $ 291,764 Additions — — Other adjustments — $ (94 ) Balance at End of Year $ 291,670 $ 291,670 Goodwill represents the excess of the purchase price over the fair value of net assets acquired. Purchase accounting guidance allows for a reasonable period of time following an acquisition for the acquirer to obtain the information necessary to complete the accounting for a business combination. This period is known as the measurement period. Additional measurement period purchase accounting adjustments, primarily related to taxes from the Merger, decreased goodwill by less than $0.1 million in 2016. Goodwill is reviewed for impairment annually or more frequently if it is determined that a triggering event has occurred. Based upon our qualitative assessment performed for our annual impairment analysis, we concluded that it is more likely than not that the fair value of the reporting units exceeds the carrying value. In general, the overall macroeconomic conditions and more specifically the economic conditions of the banking industry have been very good. Additionally, our overall performance has been good and we did not identify any other facts and circumstances causing us to conclude that it is more likely than not that the fair value of the reporting units would be less than the carrying value. The following table shows a summary of intangible assets as of the dates presented: December 31, (dollars in thousands) 2017 2016 Gross carrying amount at beginning of year $ 22,114 $ 22,114 Additions — — Accumulated amortization (18,437 ) (17,204 ) Balance at End of Year $ 3,677 $ 4,910 Intangible assets as of December 31, 2017 consisted of $3.4 million for core deposits and $0.3 million for insurance contract relationships resulting from acquisitions. We determined the amount of identifiable intangible assets based upon independent core deposit, wealth management and insurance contract valuations. Other intangible assets are evaluated for impairment whenever events or changes in circumstances indicate that their carrying amounts may not be recoverable. There were no triggering events in 2017 requiring an impairment analysis to be completed. Amortization expense on finite-lived intangible assets totaled $1.2 million , $1.6 million and $1.8 million for 2017, 2016 and 2015. The following is a summary of the expected amortization expense for finite-lived intangible assets, assuming no new additions, for each of the five years following December 31, 2017 and thereafter: (dollars in thousands) Amount 2018 $ 1,013 2019 655 2020 554 2021 477 2022 359 Thereafter 619 Total $ 3,677

Derivative Instruments and Hedg

Derivative Instruments and Hedging Activities	12 Months Ended
	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES	DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES The following table indicates the amounts representing the value of derivative assets and derivative liabilities at December 31: Derivatives (included in Other Assets) Derivatives (included in Other Liabilities) (dollars in thousands) 2017 2016 2017 2016 Derivatives not Designated as Hedging Instruments Interest Rate Swap Contracts—Commercial Loans Fair value $ 3,074 $ 6,960 $ 3,055 $ 6,958 Notional amount 263,841 282,930 263,841 282,930 Collateral posted — — 1,448 14,340 Interest Rate Lock Commitments—Mortgage Loans Fair value 226 236 — — Notional amount 6,860 8,490 — — Forward Sale Contracts—Mortgage Loans Fair value — — 5 27 Notional amount — — 6,580 8,216 Presenting offsetting derivatives that are subject to legally enforceable netting arrangements with the same party is permitted. For example, we may have a derivative asset and a derivative liability with the same counterparty to a swap transaction and are permitted to offset the asset position and the liability position resulting in a net presentation. The following table indicates the gross amounts of commercial loan swap derivative assets and derivative liabilities, the amounts offset and the carrying values in the Consolidated Balance Sheets at December 31: Derivatives (included in Other Assets) Derivatives (included in Other Liabilities) (dollars in thousands) 2017 2016 2017 2016 Derivatives not Designated as Hedging Instruments Gross amounts recognized $ 4,974 $ 8,590 $ 4,955 $ 8,588 Gross amounts offset (1,900 ) (1,630 ) (1,900 ) (1,630 ) Net amounts presented in the Consolidated Balance Sheets 3,074 6,960 3,055 6,958 Gross amounts not offset (1) — — (1,448 ) (14,340 ) Net Amount $ 3,074 $ 6,960 $ 1,607 $ (7,382 ) (1) Amounts represent posted collateral. NOTE 11. DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - continued The following table indicates the gain or loss recognized in income on derivatives for the years ended December 31: (dollars in thousands) 2017 2016 2015 Derivatives not Designated as Hedging Instruments Interest rate swap contracts—commercial loans $ 17 $ (16 ) $ (8 ) Interest rate lock commitments—mortgage loans (11 ) (25 ) 26 Forward sale contracts—mortgage loans 52 (22 ) 52 Total Derivative (Loss) Gain $ 58 $ (63 ) $ 70

Mortgage Servicing Rights

Mortgage Servicing Rights	12 Months Ended
	Dec. 31, 2017
Transfers and Servicing of Financial Assets [Abstract]
MORTGAGE SERVICING RIGHTS	MORTGAGE SERVICING RIGHTS For the years ended December 31, 2017, 2016 and 2015, the 1-4 family mortgage loans that were sold to Fannie Mae amounted to $78.8 million , $93.9 million and $76.8 million . At December 31, 2017, 2016 and 2015 our servicing portfolio totaled $441 million , $407 million and $361 million . The following table indicates MSRs and the net carrying values: (dollars in thousands) Servicing Rights Valuation Allowance Net Carrying Value Balance at December 31, 2015 $ 3,426 $ (189 ) $ 3,237 Additions 1,047 — 1,047 Amortization (615 ) — (615 ) Temporary recapture (impairment) — 75 75 Balance at December 31, 2016 $ 3,858 $ (114 ) $ 3,744 Additions 918 — 918 Amortization (584 ) — (584 ) Temporary recapture (impairment) — 55 55 Balance at December 31, 2017 $ 4,192 $ (59 ) $ 4,133

Qualified Affordable Housing

Qualified Affordable Housing	12 Months Ended
	Dec. 31, 2017
Investments in Affordable Housing Projects [Abstract]
QUALIFIED AFFORDABLE HOUSING	QUALIFIED AFFORDABLE HOUSING We invest in affordable housing projects primarily to satisfy our Community Reinvestment Act requirements. As a limited partner in these operating partnerships, we receive tax credits and tax deductions for losses incurred by the underlying properties. We use the cost method to account for these partnerships. Our total investment in qualified affordable housing projects was $9.0 million at December 31, 2017 and $11.7 million at December 31, 2016. We have one open commitment for $0.8 million to fund a new qualified affordable housing project at December 31, 2017. There were no open commitments to fund current or future investments in qualified affordable housing projects at December 31, 2016. Amortization expense, included in other noninterest expense in the Consolidated Statements of Net Income, was $3.0 million , $3.3 million and $3.6 million for December 31, 2017, 2016 and 2015. The amortization expense was offset by tax credits of $3.4 million , $3.7 million and $4.0 million for December 31, 2017, 2016 and 2015, as a reduction to our federal tax provision. We evaluated our investments in affordable housing projects for impairment at December 31, 2017 due to the enactment of the Tax Act in 2017; the results indicated that the tax benefits associated with each investment exceeded the carrying values.

Deposits

Deposits	12 Months Ended
	Dec. 31, 2017
Banking and Thrift [Abstract]
DEPOSITS	DEPOSITS The following table presents the composition of deposits at December 31 and interest expense for the years ended December 31: 2017 2016 2015 (dollars in thousands) Balance Interest Expense Balance Interest Expense Balance Interest Expense Noninterest-bearing demand $ 1,387,712 $ — $ 1,263,833 $ — $ 1,227,766 $ — Interest-bearing demand 603,141 67 638,300 111 616,188 818 Money market 1,146,156 9,204 936,461 4,199 605,184 1,299 Savings 893,119 2,081 1,050,131 2,002 1,061,265 1,712 Certificates of deposit 1,397,763 13,978 1,383,652 13,380 1,366,208 9,115 Total $ 5,427,891 $ 25,330 $ 5,272,377 $ 19,692 $ 4,876,611 $ 12,944 The aggregate of all certificates of deposits over $100,000, including brokered CDs, was $585 million and $672 million at December 31, 2017 and 2016. Certificates of deposits over $250,000, including brokered CDs, were $228 million and $303 million at December 31, 2017 and 2016. The following table indicates the scheduled maturities of certificates of deposit at December 31, 2017: (dollars in thousands) Amount 2018 $ 1,015,515 2019 233,932 2020 43,461 2021 65,563 2022 33,172 Thereafter 6,120 Total $ 1,397,763

Short-Term Borrowings

Short-Term Borrowings	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
SHORT-TERM BORROWINGS	SHORT-TERM BORROWINGS Short-term borrowings are for terms under one year and were comprised of REPOs and FHLB advances. REPOs are overnight short-term investments and are not insured by the Federal Deposit Insurance Corporation, or FDIC. Securities pledged as collateral under these REPO financing arrangements cannot be sold or repledged by the secured party and are therefore accounted for as a secured borrowing. Securities with amortized cost of $57.5 million and carrying value of $56.8 million at December 31, 2017 and amortized cost of $53.2 million and carrying value of $ 52.9 million at December 31, 2016 were pledged as collateral for these secured transactions. The pledged securities are held in safekeeping at the Federal Reserve. Due to the overnight short-term nature of REPOs, potential risk due to a decline in the value of the pledged collateral is low. Collateral pledging requirements with REPOs are monitored daily. The following table represents the composition of short-term borrowings, the weighted average interest rate as of December 31 and interest expense for the years ended December 31: 2017 2016 2015 (dollars in thousands) Balance Weighted Average Interest Rate Interest Expense Balance Weighted Average Interest Rate Interest Expense Balance Weighted Average Interest Rate Interest Expense REPOs $ 50,161 0.39 % $ 54 $ 50,832 0.01 % $ 5 $ 62,086 0.01 % $ 4 FHLB advances 540,000 1.47 % 7,399 660,000 0.76 % 2,713 356,000 0.52 % 932 Total Short-term Borrowings $ 590,161 1.38 % $ 7,453 $ 710,832 0.70 % $ 2,718 $ 418,086 0.44 % $ 936

Long-Term Borrowings and Subord

Long-Term Borrowings and Subordinated Debt	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
LONG-TERM BORROWINGS AND SUBORDINATED DEBT	LONG-TERM BORROWINGS AND SUBORDINATED DEBT Long-term borrowings are for original terms greater than or equal to one year and are comprised of FHLB advances and junior subordinated debt securities. Our long-term borrowings at the Pittsburgh FHLB were $47.2 million as of December 31, 2017 and $14.7 million as of December 31, 2016. FHLB borrowings are secured by a blanket lien on residential mortgages and other real estate secured loans. Total loans pledged as collateral at the FHLB were $3.5 billion at December 31, 2017. We were eligible to borrow up to an additional $1.8 billion based on qualifying collateral, to a maximum borrowing capacity of $2.5 billion at December 31, 2017. The following table represents the balance of long-term borrowings, the weighted average interest rate as of December 31 and interest expense for the years ended December 31: (dollars in thousand) 2017 2016 2015 Long-term borrowings $ 47,301 $ 14,713 $ 117,043 Weighted average interest rate 1.88 % 2.91 % 0.81 % Interest expense $ 463 $ 670 $ 790 Scheduled annual maturities and average interest rates for all of our long-term debt for each of the five years subsequent to December 31, 2017 and thereafter are as follows: (dollars in thousands) Balance Average Rate 2018 $ 2,496 3.60 % 2019 37,514 1.60 % 2020 2,004 3.22 % 2021 1,057 3.44 % 2022 529 4.50 % Thereafter 3,701 1.96 % Total $ 47,301 1.88 % Junior Subordinated Debt Securities The following table represents the composition of junior subordinated debt securities at December 31 and the interest expense for the years ended December 31: 2017 2016 2015 (dollars in thousands) Balance Interest Expense Balance Interest Expense Balance Interest Expense 2006 Junior subordinated debt $ 25,000 $ 708 $ 25,000 $ 580 $ 25,000 $ 554 2008 Junior subordinated debt—trust preferred securities 20,619 955 20,619 854 20,619 773 Total $ 45,619 $ 1,663 $ 45,619 $ 1,434 $ 45,619 $ 1,327 The following table summarizes the key terms of our junior subordinated debt securities: (dollars in thousands) 2006 Junior Subordinated Debt 2008 Trust Preferred Securities Junior Subordinated Debt $25,000 — Trust Preferred Securities — $20,619 Stated Maturity Date 12/15/2036 3/15/2038 Optional redemption date at par Any time after 9/15/2011 Any time after 3/15/2013 Regulatory Capital Tier 2 Tier 1 Interest Rate 3 month LIBOR plus 160 bps 3 month LIBOR plus 350 bps Interest Rate at December 31, 2017 2.92% 4.82% NOTE 16. LONG-TERM BORROWINGS AND SUBORDINATED DEBT - continued We completed a private placement of the trust preferred securities to a financial institution during the first quarter of 2008. As a result, we own 100 percent of the common equity of STBA Capital Trust I. The trust was formed to issue mandatorily redeemable capital securities to third-party investors. The proceeds from the sale of the securities and the issuance of the common equity by STBA Capital Trust I were invested in junior subordinated debt securities issued by us. The third party investors are considered the primary beneficiaries of STBA Capital Trust I; therefore, the trust qualifies as a VIE, but is not consolidated into our financial statements. STBA Capital Trust I pays dividends on the securities at the same rate as the interest paid by us on the junior subordinated debt held by STBA Capital Trust I.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES Commitments The following table sets forth our commitments and letters of credit as of the dates presented: December 31, (dollars in thousands) 2017 2016 Commitments to extend credit $ 1,420,428 $ 1,509,696 Standby letters of credit 80,918 84,534 Total $ 1,501,346 $ 1,594,230 Estimates of the fair value of these off-balance sheet items were not made because of the short-term nature of these arrangements and the credit standing of the counterparties. Our allowance for unfunded loan commitments totaled $2.2 million at December 31, 2017 and $2.6 million at December 31, 2016. We have future commitments with third party vendors for data processing and communication charges. Data processing and communication expense was consistent for 2017 and 2016 at $10.4 million and $11.7 million for 2015 due to the Merger. Included in the 2015 expense was $1.3 million of merger-related expenses. There were no data processing and communication merger- related expenses in 2017 or 2016. The following table sets forth the future estimated payments related to data processing and communication charges for each of the five years following December 31, 2017: (dollars in thousands) Total 2018 $ 12,237 2019 12,633 2020 13,047 2021 13,465 2022 13,926 Total $ 65,308 Litigation In the normal course of business, we are subject to various legal and administrative proceedings and claims. While any type of litigation contains a level of uncertainty, we believe that the outcome of such proceedings or claims pending will not have a material adverse effect on our consolidated financial position or results of operations.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES Income tax expense (benefit) for the years ended December 31 is comprised of: (dollars in thousands) 2017 2016 2015 Federal Current $ 32,282 $ 24,521 $ 24,825 Deferred 13,980 665 (427 ) Total Federal 46,262 25,186 24,398 State Current 323 248 — Deferred (148 ) (129 ) — Total State 175 119 — Total Federal and State $ 46,437 $ 25,305 $ 24,398 The Tax Act includes significant changes to the U.S. corporate tax system including: a federal corporate rate reduction from 35 percent to 21 percent. The Tax Act also establishes new tax laws that became effective January 1, 2018. U.S. GAAP requires a company to record the effects of a tax law change in the period of enactment. As a result, we re-measured our deferred tax assets and liabilities and recorded an adjustment of $13.4 million . The re-measurement adjustment was recognized as an increase to our income tax expense in the fourth quarter of 2017. This adjustment incorporates assumptions made based upon our current interpretation of the Tax Act and may change as we receive additional clarification and implementation guidance. The statutory to effective tax rate reconciliation for the years ended December 31 is as follows: 2017 2016 2015 Statutory tax rate 35.0 % 35.0 % 35.0 % Low income housing tax credits (2.9 )% (3.8 )% (4.4 )% Tax-exempt interest (4.0 )% (4.4 )% (4.1 )% Bank owned life insurance (0.8 )% (0.8 )% (0.8 )% Other 0.3 % 0.2 % 1.0 % Adjustment to net deferred tax assets for enacted changes in tax laws and rates 11.3 % — % — % Effective Tax Rate 38.9 % 26.2 % 26.7 % Significant components of our temporary differences were as follows at December 31: (dollars in thousands) 2017 2016 Deferred Tax Assets: Allowance for loan losses 12,440 19,446 Purchase accounting adjustments — 365 Other employee benefits 3,095 3,983 Low income housing partnerships 3,213 4,845 Net adjustment to funded status of pension 6,481 10,018 Impairment of securities 300 1,318 State net operating loss carryforwards 3,598 3,114 Other 2,355 4,984 Gross Deferred Tax Assets 31,482 48,073 Less: Valuation allowance (3,598 ) (3,114 ) Total Deferred Tax Assets 27,884 44,959 Deferred Tax Liabilities: Net unrealized holding gains on securities available-for-sale $ (638 ) $ (2,557 ) Prepaid pension (1,749 ) (2,770 ) Deferred loan income (2,937 ) (3,815 ) Purchase accounting adjustments (100 ) — Depreciation on premises and equipment (480 ) (1,239 ) Other (1,401 ) (1,766 ) Total Deferred Tax liabilities (7,305 ) (12,147 ) Net Deferred Tax Asset $ 20,579 $ 32,812 We establish a valuation allowance when it is more likely than not that we will not be able to realize the benefit of the deferred tax assets. Except for Pennsylvania net operating losses, or NOLs, we have determined that a valuation allowance is unnecessary for the deferred tax assets because it is more likely than not that these assets will be realized through future reversals of existing temporary differences and through future taxable income. The valuation allowance is reviewed quarterly and adjusted based on management’s assessments of realizable deferred tax assets. Gross deferred tax assets were reduced by a valuation allowance of $3.6 million in 2017 related to Pennsylvania income tax NOLs. The Pennsylvania NOL carryforwards total $36.0 million and will expire in the years 2020 - 2037 . Unrecognized Tax Benefits The following table reconciles the change in Federal and State gross unrecognized tax benefits, or UTB, for the years ended December 31: (dollars in thousands) 2017 2016 2015 Balance at beginning of year $ 804 $ 1,102 $ 284 Prior period tax positions Increase — — 818 Decrease (37 ) (449 ) — Current period tax positions 142 151 — Reductions for statute of limitations expirations — — — Balance at End of Year $ 909 $ 804 $ 1,102 Amount That Would Impact the Effective Tax Rate if Recognized $ 770 $ 610 $ 542 We classify interest and penalties as an element of tax expense. We monitor changes in tax statutes and regulations to determine if significant changes will occur over the next 12 months. As of December 31, 2017, no significant changes to UTB are projected, however, tax audit examinations are possible. As of December 31, 2017, all income tax returns filed for the tax years 2014 through 2016 remain subject to examination by the IRS. Currently, our income tax return for the 2015 tax year is under examination by the IRS. We do not expect that the results of this examination will have a material effect on our financial condition or results of operations.

Tax Effects on Other Comprehens

Tax Effects on Other Comprehensive Income (Loss)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
TAX EFFECTS ON OTHER COMPREHENSIVE INCOME (LOSS)	TAX EFFECTS ON OTHER COMPREHENSIVE INCOME (LOSS) The following tables present the tax effects of the components of other comprehensive income (loss) for the years ended December 31: (dollars in thousands) Pre-Tax Amount Tax (Expense) Benefit Net of Tax Amount 2017 Net change in unrealized gains on securities available-for-sale $ (1,275 ) $ 448 $ (827 ) Net available-for-sale securities losses (gains) reclassified into earnings (3,000 ) 1,054 (1,946 ) Adjustment to funded status of employee benefit plans (1,992 ) 122 (1,870 ) Other Comprehensive Income (Loss) $ (6,267 ) $ 1,624 $ (4,643 ) 2016 Net change in unrealized gains on securities available-for-sale $ (2,899 ) $ 1,006 $ (1,893 ) Net available-for-sale securities losses reclassified into earnings — — — Adjustment to funded status of employee benefit plans 6,974 (2,408 ) 4,566 Other Comprehensive Income (Loss) $ 4,075 $ (1,402 ) $ 2,673 2015 Net change in unrealized gains on securities available-for-sale $ (663 ) $ 232 $ (431 ) Net available-for-sale securities gains reclassified into earnings 34 (12 ) 22 Adjustment to funded status of employee benefit plans (3,551 ) 1,336 (2,215 ) Other Comprehensive Income (Loss) $ (4,180 ) $ 1,556 $ (2,624 )

Employee Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2017
Retirement Benefits [Abstract]
EMPLOYEE BENEFITS	EMPLOYEE BENEFITS We maintain a qualified defined benefit pension plan, or Plan, covering substantially all employees hired prior to January 1, 2008. The benefits are based on years of service and the employee’s compensation for the highest five consecutive years in the last ten years . Contributions are intended to provide for benefits attributed to employee service to date and for those benefits expected to be earned in the future. On January 25, 2016, the Board of Directors approved an amendment to freeze benefit accruals under the qualified and nonqualified defined benefit pension plans effective March 31, 2016. This change will result in no additional benefits being earned by participants in those plans based on service or pay after March 31, 2016. The Plan was previously closed to new participants effective December 31, 2007. The following table summarizes the activity in the benefit obligation and Plan assets deriving the funded status, which is recorded in other liabilities in the Consolidated Balance Sheets: (dollars in thousands) 2017 2016 Change in Projected Benefit Obligation Projected benefit obligation at beginning of year $ 105,834 $ 109,747 Service cost — 463 Interest cost 4,100 4,296 Actuarial loss 4,974 3,575 Curtailments — (6,997 ) Benefits paid (8,244 ) (5,250 ) Projected Benefit Obligation at End of Year $ 106,664 $ 105,834 Change in Plan Assets Fair value of plan assets at beginning of year $ 87,711 $ 84,585 Actual return on plan assets 7,687 8,376 Benefits paid (8,244 ) (5,250 ) Fair Value of Plan Assets at End of Year $ 87,154 $ 87,711 Funded Status $ (19,510 ) $ (18,123 ) The following table sets forth the amounts recognized in accumulated other comprehensive (loss) income at December 31: (dollars in thousands) 2017 2016 Prior service credit $ — $ — Net actuarial loss (27,825 ) (26,013 ) Total (Before Tax Effects) $ (27,825 ) $ (26,013 ) Below are the actuarial weighted average assumptions used in determining the benefit obligation: 2017 2016 Discount rate 3.75 % 4.00 % Rate of compensation increase (1) — % — % (1) Rate of compensation increase is not applicable for 2017 and 2016 due to the amendment to freeze benefit accruals under the qualified and nonqualified defined benefit pension plans effective March 31, 2016. The following table summarizes the components of net periodic pension cost and other changes in Plan assets and benefit obligations recognized in other comprehensive income (loss) for the years ended December 31: (dollars in thousands) 2017 2016 2015 Components of Net Periodic Pension Cost Service cost—benefits earned during the period $ — $ 463 $ 2,601 Interest cost on projected benefit obligation 4,100 4,296 4,425 Expected return on plan assets (6,313 ) (5,780 ) (7,180 ) Amortization of prior service credit — (11 ) (138 ) Recognized net actuarial loss 1,866 2,345 2,028 Curtailment gain — (1,017 ) — Net Periodic Pension Expense $ (347 ) $ 296 $ 1,736 Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income (Loss) Net actuarial (gain) loss $ 3,678 $ (6,018 ) $ 5,678 Recognized net actuarial loss (1,866 ) (2,345 ) (2,028 ) Recognized prior service credit — 1,029 138 Total (Before Tax Effects) $ 1,812 $ (7,334 ) $ 3,788 Total Recognized in Net Benefit Cost and Other Comprehensive (Loss)/Income (Before Tax Effects) $ 1,465 $ (7,038 ) $ 5,524 The following table summarizes the actuarial weighted average assumptions used in determining net periodic pension cost: 2017 2016 2015 Discount rate 4.00 % 4.25 % 4.00 % Rate of compensation increase (1) — % 3.00 % 3.00 % Expected return on assets 7.50 % 7.50 % 8.00 % 1) Rate of compensation increase is not applicable for 2017 and 2016 due to the amendment to freeze benefit accruals under the qualified and nonqualified defined benefit pension plans effective March 31, 2016. The net actuarial loss included in accumulated other comprehensive income (loss) expected to be recognized in net periodic pension cost during the year ended December 31, 2018 is $2.2 million . There will be no prior service credit recognized due to the amendment to freeze benefit accruals under the qualified and nonqualified defined benefit pension plans. The accumulated benefit obligation for the Plan was $106.7 million at December 31, 2017 and $105.8 million at December 31, 2016. We consider many factors when setting the assumed rate of return on Plan assets. As a general guideline the assumed rate of return is equal to the weighted average of the expected returns for each asset category and is estimated based on historical returns as well as expected future returns. The weighted average discount rate is derived from corporate yield curves. S&T Bank’s Retirement Plan Committee determines the investment policy for the Plan. In general, the targeted asset allocation is 50 percent to 70 percent equities and 30 percent to 50 percent fixed income. A strategic allocation within each asset class is employed based on the Plan’s time horizon, risk tolerances, performance expectations and asset class preferences. Investment managers have discretion to invest in any equity or fixed-income asset class, subject to the securities guidelines of the Plan’s Investment Policy Statement. On December 19, 2017, S&T Bank, as Plan Sponsor, entered into an agreement with an insurance company to purchase a single premium annuity contract for 124 retired Plan participants and their beneficiaries. Of these participants, 30 are receiving a $2,000 death benefit only. The total premium of $1.5 million was paid out of the Plan's assets, and the effective date of the annuity payments was January 1, 2018. The annuity purchase resulted in a reduction in the associated pension liability. At this time, S&T Bank is not required to make a cash contribution to the Plan in 2018. No contributions were made during 2017. The following table provides information regarding estimated future benefit payments to be paid in each of the next five years and in the aggregate for the five years thereafter: (dollars in thousands) Amount 2018 $ 7,175 2019 7,090 2020 6,956 2021 7,157 2022 7,106 2023 - 2027 32,658 We also have nonqualified supplemental executive pension plans, or SERPs, for certain key employees. The SERPs are unfunded. The projected benefit obligations related to the SERPs were $5.3 million and $4.7 million at December 31, 2017 and 2016. These amounts also represent the net amount recognized in the statement of financial position for the SERPs. Net periodic benefit costs for the SERPs were $0.5 million , $0.5 million and $0.6 million for each of the years ended December 31, 2017, 2016 and 2015. Additionally, $2.7 million , $2.5 million and $2.1 million before tax was reflected in accumulated other comprehensive income (loss) at December 31, 2017, 2016 and 2015, in relation to the SERPs. The actuarial assumptions used for the SERPs are the same as those used for the Plan. We maintain a Thrift Plan, a qualified defined contribution plan, in which substantially all employees are eligible to participate. We make matching contributions to the Thrift Plan up to 3.5 percent of participants’ eligible compensation and may make additional profit-sharing contributions as provided by the Thrift Plan. Expense related to these contributions amounted to $1.8 million in 2017, $1.7 million in 2016 and $1.5 million in 2015. Fair Value Measurements The following tables present our Plan assets measured at fair value on a recurring basis by fair value hierarchy level at December 31, 2017 and 2016. There were no transfers between Level 1 and Level 2 for items of a recurring basis during the periods presented. There were no purchases or transfers of Level 3 plan assets in 2017. December 31, 2017 Fair Value Asset Classes (1) (dollars in thousands) Level 1 Level 2 Level 3 Total Cash and cash equivalents (2) $ — $ 1,780 $ — $ 1,780 Fixed income (3) 27,738 — — 27,738 Equities: Equity index mutual funds—international (4) 4,016 — — 4,016 Domestic individual equities (5) 53,540 — — 53,540 Total Assets at Fair Value $ 85,294 $ 1,780 $ — $ 87,074 (1) Refer to Note 1 Summary of Significant Accounting Policies, Fair Value Measurements for a description of levels within the fair value hierarchy. (2) This asset class includes FDIC insured money market instruments. (3) This asset class includes a variety of fixed income mutual funds which primarily invest in investment grade rated securities. Investment managers have discretion to invest in fixed income related securities including futures, options and other derivatives. Investments may be made in currencies other than the U.S. dollar. (4) The sole investment within this asset class is the Harbor International Institutional Fund. (5) This asset class includes individual domestic equities invested in an active all-cap strategy. It may also include convertible bonds. December 31, 2016 Fair Value Asset Classes (1) (dollars in thousands) Level 1 Level 2 Level 3 Total Cash and cash equivalents (2) $ — $ 3,336 $ — $ 3,336 Fixed income (3) 27,279 — — 27,279 Equities: Equity index mutual funds—international (4) 3,362 — — 3,362 Domestic individual equities (5) 53,636 — — 53,636 Total Assets at Fair Value $ 84,277 $ 3,336 $ — $ 87,613 (1) Refer to Note 1 Summary of Significant Accounting Policies, Fair Value Measurements for a description of levels within the fair value hierarchy. (2) This asset class includes FDIC insured money market instruments. (3) This asset class includes a variety of fixed income mutual funds which primarily invest in investment grade rated securities. Investment managers have discretion to invest in fixed income related securities including futures, options and other derivatives. Investments may be made in currencies other than the U.S. dollar. (4) The sole investment within this asset class is MSCI EAFE Index iShares. (5) This asset class includes individual domestic equities invested in an active all-cap strategy. It may also include convertible bonds.

Incentive and Restricted Stock

Incentive and Restricted Stock Plan and Dividend Reinvestment Plan	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
INCENTIVE AND RESTRICTED STOCK PLAN AND DIVIDEND REINVESTMENT PLAN	INCENTIVE AND RESTRICTED STOCK PLAN AND DIVIDEND REINVESTMENT PLAN We adopted an Incentive Stock Plan in 2014 that provides for cash performance awards and for granting incentive stock options, nonstatutory stock options, restricted stock, restricted stock units and appreciation rights. A maximum of 750,000 shares of our common stock are available for awards granted under the 2014 Incentive Plan and the plan expires ten years from the date of board approval. Stock Options As of December 31, 2017, no nonstatutory stock options were outstanding under the 2014 Stock Plan. The fair value of nonstatutory stock option awards under the 2003 Stock Plan were estimated on the date of grant using the Black-Scholes valuation model, which is dependent upon certain assumptions. We used the simplified method in developing the estimated life of the option, whereby the expected life is presumed to be the midpoint between the vesting date and the end of the contractual term. There have been no nonstatutory stock options granted since 2006. There were 155,500 outstanding shares that expired December 19, 2015 at a weighted average exercise price of $37.86 . Restricted Stock We periodically issue restricted stock to employees and directors, pursuant to our 2014 Stock Plan. As of December 31, 2017, 366,570 restricted shares have been granted under the 2014 Stock Plan. During 2017, 2016, and 2015, we granted 12,728 , 15,613 and 16,142 restricted shares of common stock to outside directors under the 2014 Stock Plan. The grants are part of the compensation arrangement approved by the Compensation and Benefits Committee whereby the directors receive compensation in the form of both cash and restricted shares of common stock. These shares fully vest one year after the date of grant. The closing price of our stock is used to determine the fair value on the date of grant. During 2017, 2016, and 2015, we granted 77,387 , 95,030 and 71,699 restricted shares of common stock to senior management under our Long Term Incentive Plan, or LTIP, within the 2014 Stock Plan. The restricted shares granted under the LTIP consist of both time and performance-based awards. The awards were granted in accordance with performance levels set by the Compensation and Benefits Committee. Vesting for the time-based awards is 50 percent after two years and the remaining 50 percent at the end of the third year. The performance-based awards vest at the end of the three -year period. During the vesting period, if the recipient leaves S&T before the end of the vesting period, shares will be forfeited except in the case of retirement, disability or death where accelerated vesting provisions are defined within the awards agreement. The average of the high and low prices of the stock is used to determine the fair value on the date of grant. Compensation expense for time-based restricted stock is recognized ratably over the period of service, generally the entire vesting period, based on fair value on the grant date. Compensation expense for performance-based restricted stock is recognized ratably over the remaining vesting period once the likelihood of meeting the performance measure is probable. During 2017, 2016 and 2015, we recognized compensation expense of $3.0 million , $2.5 million and $1.7 million and realized a tax benefit of $1.1 million , $0.9 million and $0.6 million related to restricted stock grants. The following table provides information about restricted stock granted under the 2014 Stock Plan for the years ended December 31: Restricted Stock Weighted Average Grant Date Fair Value Non-vested at December 31, 2015 150,356 $ 26.34 Granted 110,643 25.58 Vested 32,164 25.03 Forfeited 3,335 26.04 Non-vested at December 31, 2016 225,500 $ 26.16 Granted 90,115 35.19 Vested 83,958 24.82 Forfeited 11,089 29.56 Non-vested at December 31, 2017 220,568 $ 30.19 As of December 31, 2017, there was $2.7 million of total unrecognized compensation cost related to restricted stock that will be recognized as compensation expense over a weighted average period of 1.65 years. Dividend Reinvestment Plan We also sponsor a Dividend Reinvestment and Stock Purchase Plan, or Dividend Plan, where shareholders may purchase shares of S&T common stock at the average fair value with reinvested dividends and voluntary cash contributions. The plan administrator and transfer agent may purchase shares directly from us from shares held in treasury or purchase shares in the open market to fulfill the Dividend Plan’s needs.

Parent Company Condensed Financ

Parent Company Condensed Financial Information	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
PARENT COMPANY CONDENSED FINANCIAL INFORMATION	PARENT COMPANY CONDENSED FINANCIAL INFORMATION The following condensed financial statements summarize the financial position of S&T Bancorp, Inc. as of December 31, 2017 and 2016 and the results of its operations and cash flows for each of the three years ended December 31, 2017, 2016 and 2015. BALANCE SHEETS December 31, (dollars in thousands) 2017 2016 ASSETS Cash $ 21,310 $ 17,057 Investments in: Bank subsidiary 857,293 819,531 Nonbank subsidiaries 19,569 21,980 Other assets 7,272 4,694 Total Assets $ 905,444 $ 863,262 LIABILITIES Long-term debt $ 20,619 $ 20,619 Other liabilities 794 687 Total Liabilities 21,413 21,306 Total Shareholders’ Equity 884,031 841,956 Total Liabilities and Shareholders’ Equity $ 905,444 $ 863,262 STATEMENTS OF NET INCOME Years ended December 31, (dollars in thousands) 2017 2016 2015 Dividends from subsidiaries $ 36,169 $ 34,134 $ 75,413 Investment income 22 17 19 Total Income 36,191 34,151 75,432 Interest expense on long-term debt 955 854 773 Other expenses 3,801 4,012 3,687 Total Expense 4,756 4,866 4,460 Income before income tax and undistributed net income of subsidiaries 31,435 29,285 70,972 Income tax benefit (1,596 ) (1,697 ) (1,549 ) Income before undistributed net income of subsidiaries 33,031 30,982 72,521 Equity in undistributed net income (distribution in excess of net income) of: Bank subsidiary 40,877 40,051 (5,064 ) Nonbank subsidiaries (940 ) 359 (376 ) Net Income $ 72,968 $ 71,392 $ 67,081 STATEMENTS OF CASH FLOWS Years ended December 31, (dollars in thousands) 2017 2016 2015 OPERATING ACTIVITIES Net Income $ 72,968 $ 71,392 $ 67,081 Equity in undistributed (earnings) losses of subsidiaries (39,937 ) (40,410 ) 5,440 Tax benefit from stock-based compensation — (9 ) (53 ) Other 480 379 3,129 Net Cash Provided by Operating Activities 33,511 31,352 75,597 INVESTING ACTIVITIES Net investments in subsidiaries — — (38,404 ) Acquisitions — — (29,510 ) Net Cash Used in Investing Activities — — (67,914 ) FINANCING ACTIVITIES Repayment of junior subordinated debt — — (8,500 ) (Purchase) Sale of treasury shares, net (689 ) (115 ) (182 ) Cash dividends paid to common shareholders (28,569 ) (26,784 ) (24,487 ) Tax benefit from stock-based compensation — 9 53 Net Cash Used in Financing Activities (29,258 ) (26,890 ) (33,116 ) Net increase (decrease) in cash 4,253 4,462 (25,433 ) Cash at beginning of year 17,057 12,595 38,028 Cash at End of Year $ 21,310 $ 17,057 $ 12,595

Regulatory Matters

Regulatory Matters	12 Months Ended
	Dec. 31, 2017
Banking and Thrift [Abstract]
REGULATORY MATTERS	REGULATORY MATTERS We are subject to various regulatory capital requirements administered by the federal banking agencies. Failure to meet the minimum capital requirements can initiate certain mandatory and possibly additional discretionary actions by regulators that, if undertaken, could have a direct material effect on our consolidated financial statements. Under capital guidelines and the regulatory framework for prompt corrective action, we must meet specific capital guidelines that involve quantitative measures of our assets, liabilities and certain off-balance sheet items as calculated under regulatory accounting practices. Our capital amounts and classification are also subject to qualitative judgments by the regulators about risk weightings and other factors. The most recent notifications from the Federal Reserve and the FDIC categorized S&T and S&T Bank as well capitalized under the regulatory framework for corrective action. There have been no conditions or events that we believe have changed S&T's or S&T Bank’s status during 2017 and 2016. Common equity tier 1 capital includes common stock and related surplus plus retained earnings, less goodwill and intangible assets subject to a limitation and certain deferred tax assets subject to a limitation. In addition, we made a one-time permanent election to exclude accumulated other comprehensive income from capital. For regulatory purposes, trust preferred securities totaling $20.0 million , issued by an unconsolidated trust subsidiary of S&T underlying junior subordinated debt, are included in Tier 1 capital for S&T. Total capital consists of Tier 1 capital plus junior subordinated debt and the ALL subject to limitation. We currently have $25.0 million in junior subordinated debt which is included in Tier 2 capital for S&T in accordance with current regulatory reporting requirements. Quantitative measures established by regulation to ensure capital adequacy require us to maintain minimum amounts and ratios of Total, Tier 1 and Common Equity Tier 1 capital to risk-weighted assets and Tier 1 capital to average assets. As of December 31, 2017 and 2016, we met all capital adequacy requirements to which we are subject. The following table summarizes risk-based capital amounts and ratios for S&T and S&T Bank: Actual Minimum Regulatory Capital Requirements To be Well Capitalized Under Prompt Corrective Action Provisions (dollars in thousands) Amount Ratio Amount Ratio Amount Ratio As of December 31, 2017 Leverage Ratio S&T $ 628,876 9.17 % $ 274,254 4.00 % $ 342,818 5.00 % S&T Bank 582,929 8.52 % 273,538 4.00 % 341,922 5.00 % Common Equity Tier 1 (to Risk-Weighted Assets) S&T 608,876 10.71 % 255,778 4.50 % 369,457 6.50 % S&T Bank 582,929 10.29 % 255,024 4.50 % 368,368 6.50 % Tier 1 Capital (to Risk-Weighted Assets) S&T 628,876 11.06 % 341,037 6.00 % 454,717 8.00 % S&T Bank 582,929 10.29 % 340,032 6.00 % 453,375 8.00 % Total Capital (to Risk-Weighted Assets) S&T 713,056 12.55 % 454,717 8.00 % 568,396 10.00 % S&T Bank 666,560 11.76 % 453,375 8.00 % 566,719 10.00 % As of December 31, 2016 Leverage Ratio S&T $ 582,155 8.98 % $ 259,170 4.00 % $ 323,963 5.00 % S&T Bank 542,048 8.39 % 258,460 4.00 % 323,075 5.00 % Common Equity Tier 1 (to Risk-Weighted Assets) S&T 562,155 10.04 % 252,079 4.50 % 364,114 6.50 % S&T Bank 542,048 9.71 % 251,213 4.50 % 362,864 6.50 % Tier 1 Capital (to Risk-Weighted Assets) S&T 582,155 10.39 % 336,105 6.00 % 448,140 8.00 % S&T Bank 542,048 9.71 % 334,951 6.00 % 446,601 8.00 % Total Capital (to Risk-Weighted Assets) S&T 664,184 11.86 % 448,140 8.00 % 560,175 10.00 % S&T Bank 622,469 11.15 % 446,602 8.00 % 558,252 10.00 %

Selected Financial Data

Selected Financial Data	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
SELECTED FINANCIAL DATA	SELECTED FINANCIAL DATA The following table presents selected financial data for the most recent eight quarters. 2017 2016 (dollars in thousands, except per share data) (unaudited) Fourth Quarter Third Quarter Second Quarter First Quarter Fourth Quarter Third Quarter Second Quarter First Quarter SUMMARY OF OPERATIONS Interest income $ 67,855 $ 66,723 $ 64,914 $ 61,150 $ 59,096 $ 57,808 $ 55,850 $ 55,019 Interest expense 10,027 9,267 8,344 7,272 6,638 6,353 6,142 5,382 Provision for loan losses 982 2,850 4,869 5,183 5,586 2,516 4,848 5,014 Net Interest Income After Provision For Loan Losses 56,846 54,606 51,701 48,695 46,872 48,939 44,860 44,623 Security (losses) gains, net (986 ) — 3,617 370 — — — — Noninterest income 13,636 13,551 12,648 12,626 12,922 13,448 12,448 15,817 Noninterest expense 37,947 36,553 36,597 36,808 35,625 34,439 34,753 38,416 Income Before Taxes 31,549 31,604 31,369 24,883 24,169 27,948 22,555 22,024 Provision for income taxes 22,255 8,883 8,604 6,695 6,510 7,367 5,496 5,931 Net Income Available to Common Shareholders $ 9,294 $ 22,721 $ 22,765 $ 18,188 $ 17,659 $ 20,581 $ 17,059 $ 16,093 Per Share Data Common earnings per share—diluted $ 0.27 $ 0.65 $ 0.65 $ 0.52 $ 0.51 $ 0.59 $ 0.49 $ 0.46 Dividends declared per common share 0.22 0.20 0.20 0.20 0.20 0.19 0.19 0.19 Common book value 25.28 25.37 24.90 24.45 24.12 24.02 23.63 23.23

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
SUBSEQUENT EVENT	SUBSEQUENT EVENT On January 1, 2018, we sold a majority interest in S&T Evergreen Insurance, LLC, a subsidiary of S&T Insurance Group. We received $5.0 million of consideration and a 30 percent equity ownership interest in the new limited liability corporation. As a result of this sale, we expect to recognize a gain of approximately $1.0 million in the first quarter of 2018.

Summary of Significant Accoun34

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Nature of Operations	Nature of Operations S&T Bancorp, Inc., or S&T, was incorporated on March 17, 1983 under the laws of the Commonwealth of Pennsylvania as a bank holding company and has three direct wholly owned subsidiaries, S&T Bank, 9th Street Holdings, Inc. and STBA Capital Trust I. We own a 50 percent interest in Commonwealth Trust Credit Life Insurance Company, or CTCLIC. We are presently engaged in nonbanking activities through the following five entities: 9th Street Holdings, Inc.; S&T Bancholdings, Inc.; CTCLIC; S&T Insurance Group, LLC; and Stewart Capital Advisors, LLC. 9th Street Holdings, Inc. and S&T Bancholdings, Inc. are investment holding companies. CTCLIC, which is a joint venture with another financial institution, acts as a reinsurer of credit life, accident and health insurance policies sold by S&T Bank and the other institution. S&T Insurance Group, LLC, through its subsidiaries, offers a variety of insurance products. Stewart Capital Advisors, LLC is a registered investment advisor that manages private investment accounts for individuals and institutions. On October 29, 2014, S&T and Integrity Bancshares, Inc., or Integrity, based in Camp Hill, Pennsylvania, entered into an agreement to acquire Integrity Bancshares, Inc. and the transaction was completed on March 4, 2015. Integrity Bank was subsequently merged into S&T Bank on May 8, 2015.
Segment Reporting	Prior to 2017, we reported three operating segments: Community Banking, Wealth Management and Insurance. Effective January 1, 2017, we no longer report Wealth Management and Insurance segment information, as they do not meet the quantitative thresholds required for disclosure.
Accounting Policies	Accounting Policies Our financial statements have been prepared in accordance with U.S. generally accepted accounting principles, or GAAP. In preparing the financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities as of the dates of the balance sheets and revenues and expenses for the periods then ended. Actual results could differ from those estimates.
Principles of Consolidation	Principles of Consolidation The Consolidated Financial Statements include the accounts of S&T and its wholly owned subsidiaries. All significant intercompany transactions have been eliminated in consolidation. Investments of 20 percent to 50 percent of the outstanding common stock of investees are accounted for using the equity method of accounting.
Reclassification	Reclassification Amounts in prior years' financial statements and footnotes are reclassified whenever necessary to conform to the current year’s presentation. Reclassifications had no effect on our results of operations or financial condition.
Business Combinations	Business Combinations We account for business combinations using the acquisition method of accounting. Under this method of accounting, the acquired company’s net assets are recorded at fair value at the date of acquisition, and the results of operations of the acquired company are combined with our results from that date forward. Acquisition costs are expensed when incurred. The difference between the purchase price and the fair value of the net assets acquired (including identified intangibles) is recorded as goodwill.
Fair Value Measurements	Fair Value Measurements We use fair value measurements when recording and disclosing certain financial assets and liabilities. Securities available-for-sale, trading assets and derivative financial instruments are recorded at fair value on a recurring basis. Additionally, from time to time, we may be required to record other assets at fair value on a nonrecurring basis, such as loans held for sale, impaired loans, other real estate owned, or OREO, and other repossessed assets, mortgage servicing rights, or MSRs, and certain other assets. Fair value is the price that would be received to sell an asset or paid to transfer a liability in the principal or most advantageous market in an orderly transaction between market participants at the measurement date. 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 or liabilities; it is not a forced transaction. In determining fair value, we use various valuation approaches, including market, income and cost approaches. The fair value standard establishes a hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing an asset or liability, which are developed based on market data we have obtained from independent sources. Unobservable inputs reflect our estimates of assumptions that market participants would use in pricing an asset or liability, which are developed based on the best information available in the circumstances. The fair value hierarchy gives the highest priority to unadjusted quoted market prices in active markets for identical assets or liabilities (Level 1 measurement) and the lowest priority to unobservable inputs (Level 3 measurement). The fair value hierarchy is broken down into three levels based on the reliability of inputs as follows: Level 1: valuation is based upon unadjusted quoted market prices for identical instruments traded in active markets. Level 2: valuation is based upon quoted market prices for similar instruments traded in active markets, quoted market prices for identical or similar instruments traded in markets that are not active and model-based valuation techniques for which all significant assumptions are observable in the market or can be corroborated by market data. Level 3: valuation is derived from other valuation methodologies, including discounted cash flow models and similar techniques that use significant assumptions not observable in the market. These unobservable assumptions reflect estimates of assumptions that market participants would use in determining fair value. A financial instrument’s level within the fair value hierarchy is based on the lowest level of input that is significant to the fair value measurement. Our policy is to recognize transfers between any of the fair value hierarchy levels at the end of the reporting period in which the transfer occurred. The following are descriptions of the valuation methodologies that we use for financial instruments recorded at fair value on either a recurring or nonrecurring basis. Recurring Basis Securities Available-for-Sale Securities available-for-sale include both debt and equity securities. We obtain fair values for debt securities from a third-party pricing service which utilizes several sources for valuing fixed-income securities. We validate prices received from our pricing service through comparison to a secondary pricing service and broker quotes. We review the methodologies of the pricing service which provides us with a sufficient understanding of the valuation models, assumptions, inputs and pricing to reasonably measure the fair value of our debt securities. The market evaluation sources for debt securities include observable inputs rather than significant unobservable inputs and are classified as Level 2. The service provider utilizes pricing models that vary by asset class and include available trade, bid and other market information. Generally, the methodologies include broker quotes, proprietary models, and vast descriptive terms and conditions databases, and extensive quality control programs. Marketable equity securities that have an active, quotable market are classified as Level 1. Marketable equity securities that are quotable, but are thinly traded or inactive, are classified as Level 2. Marketable equity securities that are not readily traded and do not have a quotable market are classified as Level 3. Trading Assets We use quoted market prices to determine the fair value of our trading assets. Our trading assets are held in a Rabbi Trust under a deferred compensation plan and are invested in readily quoted mutual funds. Accordingly, these assets are classified as Level 1. Derivative Financial Instruments We use derivative instruments, including interest rate swaps for commercial loans with our customers, interest rate lock commitments and the sale of mortgage loans in the secondary market. We calculate the fair value for derivatives using widely accepted valuation techniques, including discounted cash flow analysis on the expected cash flows of each derivative. Each valuation considers the contractual terms of the derivative, including the period to maturity, and uses observable market based inputs, such as interest rate curves and implied volatilities. Accordingly, derivatives are classified as Level 2. We incorporate credit valuation adjustments into the valuation models to appropriately reflect both our own nonperformance risk and the respective counterparties’ nonperformance risk in calculating fair value measurements. In adjusting the fair value of our derivative contracts for the effect of nonperformance risk, we have considered the impact of netting and any applicable credit enhancements and collateral postings. Nonrecurring Basis Loans Held for Sale Loans held for sale consist of 1-4 family residential loans originated for sale in the secondary market and, from time to time, certain loans transferred from the loan portfolio to loans held for sale, all of which are carried at the lower of cost or fair value. The fair value of 1-4 family residential loans is based on the principal or most advantageous market currently offered for similar loans using observable market data. The fair value of the loans transferred from the loan portfolio is based on the amounts offered for these loans in currently pending sales transactions. Loans held for sale carried at fair value are classified as Level 3. Impaired Loans Impaired loans are carried at the lower of carrying value or fair value. Fair value is determined as the recorded investment balance less any specific reserve. We establish specific reserves based on the following three impairment methods: 1) the present value of expected future cash flows discounted at the loan’s original effective interest rate; 2) the loan’s observable market price; or 3) the fair value of the collateral less estimated selling costs when the loan is collateral dependent and we expect to liquidate the collateral. However, if repayment is expected to come from the operation of the collateral, rather than liquidation, then we do not consider estimated selling costs in determining the fair value of the collateral. Collateral values are generally based upon appraisals by approved, independent state certified appraisers. Appraisals may be discounted based on our historical knowledge, changes in market conditions from the time of appraisal or our knowledge of the borrower and the borrower’s business. Impaired loans carried at fair value are classified as Level 3. OREO and Other Repossessed Assets OREO and other repossessed assets obtained in partial or total satisfaction of a loan are recorded at the lower of recorded investment in the loan or fair value less cost to sell. Subsequent to foreclosure, these assets are carried at the lower of the amount recorded at acquisition date or fair value less cost to sell. Accordingly, it may be necessary to record nonrecurring fair value adjustments. Fair value, when recorded, is generally based upon appraisals by approved, independent state certified appraisers. Like impaired loans, appraisals on OREO may be discounted based on our historical knowledge, changes in market conditions from the time of appraisal or other information available to us. OREO and other repossessed assets carried at fair value are classified as Level 3. Mortgage Servicing Rights The fair value of MSRs is determined by calculating the present value of estimated future net servicing cash flows, considering expected mortgage loan prepayment rates, discount rates, servicing costs and other economic factors, which are determined based on current market conditions. The expected rate of mortgage loan prepayments is the most significant factor driving the value of MSRs. MSRs are considered impaired if the carrying value exceeds fair value. The valuation model includes significant unobservable inputs; therefore, MSRs are classified as Level 3. Other Assets We measure certain other assets at fair value on a nonrecurring basis. Fair value is based on the application of lower of cost or fair value accounting, or write-downs of individual assets. Valuation methodologies used to measure fair value are consistent with overall principles of fair value accounting and consistent with those described above. Financial Instruments In addition to financial instruments recorded at fair value in our financial statements, fair value accounting guidance requires disclosure of the fair value of all of an entity’s assets and liabilities that are considered financial instruments. The majority of our assets and liabilities are considered financial instruments. Many of these instruments lack an available trading market as characterized by a willing buyer and willing seller engaged in an exchange transaction. Also, it is our general practice and intent to hold our financial instruments to maturity and to not engage in trading or sales activities with respect to such financial instruments. For fair value disclosure purposes, we substantially utilize the fair value measurement criteria as required and explained above. In cases where quoted fair values are not available, we use present value methods to determine the fair value of our financial instruments. Cash and Cash Equivalents The carrying amounts reported in the Consolidated Balance Sheets for cash and due from banks, including interest-bearing deposits, approximate fair value. Loans The fair value of variable rate performing loans that may reprice frequently at short-term market rates is based on carrying values adjusted for credit risk. The fair value of variable rate performing loans that reprice at intervals of one year or longer, such as adjustable rate mortgage products, is estimated using discounted cash flow analyses that utilize interest rates currently being offered for similar loans and adjusted for credit risk. The fair value of fixed rate performing loans is estimated using a discounted cash flow analysis that utilizes interest rates currently being offered for similar loans and adjusted for credit risk. The fair value of nonperforming loans is the carrying value less any specific reserve on the loan if it is impaired. The carrying amount of accrued interest approximates fair value. Bank Owned Life Insurance Fair value approximates net cash surrender value of bank owned life insurance, or BOLI. Federal Home Loan Bank, or FHLB, and Other Restricted Stock It is not practical to determine the fair value of our FHLB and other restricted stock due to the restrictions placed on the transferability of these stocks; it is presented at carrying value. Deposits The fair values disclosed for deposits without defined maturities (e.g., noninterest and interest-bearing demand, money market and savings accounts) are by definition equal to the amounts payable on demand. The carrying amounts for variable rate, fixed-term time deposits approximate their fair values. Estimated fair values for fixed rate and other time deposits are based on discounted cash flow analysis using interest rates currently offered for time deposits with similar terms. The carrying amount of accrued interest approximates fair value. Short-Term Borrowings The carrying amounts of securities sold under repurchase agreements, or REPOs, and other short-term borrowings approximate their fair values. Long-Term Borrowings The fair values disclosed for fixed rate long-term borrowings are determined by discounting their contractual cash flows using current interest rates for long-term borrowings of similar remaining maturities. The carrying amounts of variable rate long-term borrowings approximate their fair values. Junior Subordinated Debt Securities The interest rate on the variable rate junior subordinated debt securities is reset quarterly; therefore, the carrying values approximate their fair values. Loan Commitments and Standby Letters of Credit Off-balance sheet financial instruments consist of commitments to extend credit and letters of credit. Except for interest rate lock commitments, estimates of the fair value of these off-balance sheet items are not made because of the short-term nature of these arrangements and the credit standing of the counterparties. Other Estimates of fair value are not made for items that are not defined as financial instruments, including such items as our core deposit intangibles and the value of our trust operations.
Cash and Cash Equivalents	Cash and Cash Equivalents We consider cash and due from banks, interest-bearing deposits with banks and federal funds sold as cash and cash equivalents.
Securities	Securities We determine the appropriate classification of securities at the time of purchase. All securities, including both debt and equity securities, are classified as available-for-sale. These are securities that we intend to hold for an indefinite period of time, but that may be sold in response to changes in interest rates, prepayment risk, liquidity needs or other factors. Such securities are carried at fair value with net unrealized gains and losses deemed to be temporary and reported as a component of other comprehensive income (loss), net of tax. Realized gains and losses on the sale of available-for-sale securities and other-than-temporary impairment, or OTTI, charges are recorded within noninterest income in the Consolidated Statements of Net Income. Realized gains and losses on the sale of securities are determined using the specific-identification method. Bond premiums are amortized to the call date and bond discounts are accreted to the maturity date, both on a level yield basis. An investment security is considered impaired if its fair value is less than its cost or amortized cost basis. We perform a quarterly review of our securities to identify those that may indicate an OTTI. Our policy for OTTI within the marketable equity securities portfolio generally requires an impairment charge when the security is in a loss position for 12 consecutive months, unless facts and circumstances would suggest the need for an OTTI prior to that time. Our policy for OTTI within the debt securities portfolio is based upon a number of factors, including but not limited to, the length of time and extent to which the estimated fair value has been less than cost, the financial condition of the underlying issuer, the ability of the issuer to meet contractual obligations, the best estimate of the impairment charge representing credit losses, the likelihood of the security’s ability to recover any decline in its estimated fair value and whether management intends to sell the security or if it is more likely than not that management will be required to sell the investment security prior to the security’s recovery of any decline in its estimated fair value. If the impairment is considered other-than-temporary based on management’s review, the impairment must be separated into credit and non-credit components. The credit component is recognized in the Consolidated Statements of Net Income and the non-credit component is recognized in other comprehensive income (loss), net of applicable taxes.
Loans Held for Sale	Loans Held for Sale Loans held for sale consist of 1-4 family residential loans originated for sale in the secondary market and, from time to time, certain loans transferred from the loan portfolio to loans held for sale, all of which are carried at the lower of cost or fair value. If a loan is transferred from the loan portfolio to the held for sale category, any write-down in the carrying amount of the loan at the date of transfer is recorded as a charge-off against the allowance for loan losses, or ALL. Subsequent declines in fair value are recognized as a charge to noninterest income. When a loan is placed in the held for sale category, we stop amortizing the related deferred fees and costs. The remaining unamortized fees and costs are recognized as part of the cost basis of the loan at the time it is sold. Gains and losses on sales of loans held for sale are included in other noninterest income in the Consolidated Statements of Net Income.
Loans	Loans Loans are reported at the principal amount outstanding net of unearned income, unamortized premiums or discounts and deferred origination fees and costs. We defer certain nonrefundable loan origination and commitment fees. Accretion of discounts and amortization of premiums on loans are included in interest income in the Consolidated Statements of Net Income. Loan origination fees and direct loan origination costs are deferred and amortized as an adjustment of loan yield over the respective lives of the loans without consideration of anticipated prepayments. If a loan is paid off, the remaining unaccreted or unamortized net origination fees and costs are immediately recognized into income or expense. Interest is accrued and interest income is recognized on loans as earned. Acquired loans are recorded at fair value on the date of acquisition with no carryover of the related ALL. Determining the fair value of the acquired loans involves estimating the principal and interest cash flows expected to be collected on the loans and discounting those cash flows at a market rate of interest. In estimating the fair value of our acquired loans, we consider a number of factors including the loan term, internal risk rating, delinquency status, prepayment rates, recovery periods, estimated value of the underlying collateral and the current interest rate environment. Closed-end installment loans, amortizing loans secured by real estate and any other loans with payments scheduled monthly are reported past due when the borrower is in arrears two or more monthly payments. Other multi-payment obligations with payments scheduled other than monthly are reported past due when one scheduled payment is due and unpaid for 30 days or more. Generally, consumer loans are charged off against the ALL upon the loan reaching 90 days past due. Commercial loans are charged off as management becomes aware of facts and circumstances that raise doubt as to the collectability of all or a portion of the principal and when we believe a confirmed loss exists.
Nonaccrual or Nonperforming Loans	Nonaccrual or Nonperforming Loans We stop accruing interest on a loan when the borrower’s payment is 90 days past due. Loans are also placed on nonaccrual status when we have doubt about the borrower’s ability to comply with contractual repayment terms, even if payment is not past due. When the interest accrual is discontinued, all unpaid accrued interest is reversed against interest income. Interest income is recognized on nonaccrual loans on a cash basis if recovery of the remaining principal is reasonably assured. As a general rule, a nonaccrual loan may be restored to accrual status when its principal and interest is paid current and the bank expects repayment of the remaining contractual principal and interest, or when the loan otherwise becomes well secured and in the process of collection.
Troubled Debt Restructurings	Troubled Debt Restructurings Troubled debt restructurings, or TDRs, are loans where we, for economic or legal reasons related to a borrower’s financial difficulties, grant a concession to the borrower. We strive to identify borrowers with financial difficulty early and work with them to come to a mutual resolution to modify the terms of their loan before the loan reaches nonaccrual status. These modified terms generally include extensions of maturity dates at a stated interest rate lower than the current market rate for a new loan with similar risk characteristics, reductions in contractual interest rates or principal deferment. While unusual, there may be instances of principal forgiveness. These modifications are generally for longer term periods that would not be considered insignificant. Additionally, we classify loans where the debt obligation has been discharged through a Chapter 7 Bankruptcy and not reaffirmed as TDRs. We individually evaluate all substandard commercial loans that have experienced a forbearance or change in terms agreement, and all substandard consumer and residential mortgage loans that entered into an agreement to modify their existing loan, to determine if they should be designated as TDRs. All TDRs are considered to be impaired loans and will be reported as impaired loans for the remaining life of the loan, unless the restructuring agreement specifies an interest rate equal to or greater than the rate that would be accepted at the time of the restructuring for a new loan with comparable risk and it is fully expected that the remaining principal and interest will be collected according to the restructured agreement. Further, all impaired loans are reported as nonaccrual loans unless the loan is a TDR that has met the requirements to be returned to accruing status. TDRs can be returned to accruing status if the ultimate collectability of all contractual amounts due, according to the restructured agreement, is not in doubt and there is a period of a minimum of six months of satisfactory payment performance by the borrower either immediately before or after the restructuring.
Allowance for Loan Losses by Portfolio	Allowance for Loan Losses The ALL reflects our estimates of probable losses inherent in the loan portfolio at the balance sheet date. The methodology for determining the ALL has two main components: evaluation and impairment tests of individual loans and evaluation and impairment tests of certain groups of homogeneous loans with similar risk characteristics. Loans are considered to be impaired when based upon current information and events it is probable that we will be unable to collect all principal and interest payments due according to the original contractual terms of the loan agreement. We individually evaluate all substandard and nonaccrual commercial loans greater than $0.5 million for impairment. All TDRs will be reported as an impaired loan for the remaining life of the loan, unless the restructuring agreement specifies an interest rate equal to or greater than the rate that would be accepted at the time of the restructuring for a new loan with comparable risk and it is fully expected that the remaining principal and interest will be fully collected according to the restructured agreement. For all TDRs, regardless of size, and all other impaired loans, we conduct further analysis to determine the probable loss and assign a specific reserve to the loan if deemed appropriate. Specific reserves are established based upon the following three impairment methods: 1) the present value of expected future cash flows discounted at the loan’s original effective interest rate, 2) the loan’s observable market price or 3) the estimated fair value of the collateral if the loan is collateral dependent. Our impairment evaluations consist primarily of the fair value of collateral method because most of our loans are collateral dependent. Collateral values are discounted to consider disposition costs when appropriate. A specific reserve is established or a charge-off is taken if the fair value of the impaired loan is less than the recorded investment in the loan balance. The ALL for homogeneous loans is calculated using a systematic methodology with both a quantitative and a qualitative analysis that is applied on a quarterly basis. The ALL model is comprised of five distinct portfolio segments: 1) Commercial Real Estate, or CRE, 2) Commercial and Industrial, or C&I, 3) Commercial Construction, 4) Consumer Real Estate and 5) Other Consumer. Each segment has a distinct set of risk characteristics monitored by management. We further assess and monitor risk and performance at a more disaggregated level which includes our internal risk rating system for the commercial segments and type of collateral, lien position and loan-to-value, or LTV, for the consumer segments. We first apply historical loss rates to pools of loans with similar risk characteristics. Loss rates are calculated by historical charge-offs that have occurred within each pool of loans over the loss emergence period, or LEP. The LEP is an estimate of the average amount of time from when an event happens that causes the borrower to be unable to pay on a loan until the loss is confirmed through a loan charge-off. In conjunction with our annual review of the ALL assumptions, we have updated our analysis of LEPs for our Commercial and Consumer loan portfolio segments using our loan charge-off history. No changes were made to our LEP assumptions in 2017. We estimate the LEP to be 3 years for CRE, 4 years for construction and 1.25 years for C&I. Our analysis resulted in an LEP for Consumer Real Estate of 2.75 years and Other Consumer of 1.25 years. Another key assumption is the look-back period, or LBP, which represents the historical data period utilized to calculate loss rates. We used 8.5 years for our LBP for all portfolio segments which encompasses our loss experience during the Financial Crisis, and our more recent improved loss experience. After consideration of the historic loss calculations, management applies additional qualitative adjustments so that the ALL is reflective of the inherent losses that exist in the loan portfolio at the balance sheet date. Qualitative adjustments are made based upon changes in lending policies and practices, economic conditions, changes in the loan portfolio, changes in lending management, results of internal loan reviews, asset quality trends, collateral values, concentrations of credit risk and other external factors. The evaluation of the various components of the ALL requires considerable judgment in order to estimate inherent loss exposures. Acquired loans are recorded at fair value on the date of acquisition with no carryover of the related ALL. Determining the fair value of acquired loans involves estimating the principal and interest cash flows expected to be collected on the loans and discounting those cash flows at a market rate of interest. In estimating the fair value of our acquired loans, we considered a number of factors including the loan term, internal risk rating, delinquency status, prepayment rates, recovery periods, estimated value of the underlying collateral and the current interest rate environment. Loans acquired with evidence of credit deterioration were evaluated and not considered to be significant. The premium or discount estimated through the loan fair value calculation is recognized into interest income on a level yield or straight-line basis over the remaining contractual life of the loans. Additional credit deterioration on acquired loans, in excess of the original credit discount embedded in the fair value determination on the date of acquisition, will be recognized in the ALL through the provision for loan losses. Our ALL Committee meets quarterly to verify the overall adequacy of the ALL. Additionally, on an annual basis, the ALL Committee meets to validate our ALL methodology. This validation includes reviewing the loan segmentation, LEP, LBP and the qualitative framework. As a result of this ongoing monitoring process, we may make changes to our ALL to be responsive to the economic environment. Although we believe our process for determining the ALL adequately considers all of the factors that would likely result in credit losses, the process includes subjective elements and may be susceptible to significant change. To the extent actual losses are higher than management estimates, additional provisions for loan losses could be required and could adversely affect our earnings or financial position in future periods. We maintain an ALL at a level determined to be adequate to absorb estimated probable credit losses inherent in the loan portfolio as of the balance sheet date. We develop and document a systematic ALL methodology based on the following portfolio segments: 1) CRE, 2) C&I, 3) Commercial Construction, 4) Consumer Real Estate and 5) Other Consumer. The following are key risks within each portfolio segment: CRE —Loans secured by commercial purpose real estate, including both owner-occupied properties and investment properties for various purposes such as hotels, strip malls and apartments. Operations of the individual projects and global cash flows of the debtors are the primary sources of repayment for these loans. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the collateral type and the business prospects of the lessee, if the project is not owner-occupied. C&I —Loans made to operating companies or manufacturers for the purpose of production, operating capacity, accounts receivable, inventory or equipment financing. Cash flow from the operations of the company is the primary source of repayment for these loans. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the industry of the company. Collateral for these types of loans often do not have sufficient value in a distressed or liquidation scenario to satisfy the outstanding debt. Commercial Construction —Loans made to finance construction of buildings or other structures, as well as to finance the acquisition and development of raw land for various purposes. While the risk of these loans is generally confined to the construction period, if there are problems, the project may not be completed, and as such, may not provide sufficient cash flow on its own to service the debt or have sufficient value in a liquidation to cover the outstanding principal. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the type of project and the experience and resources of the developer. Consumer Real Estate —Loans secured by first and second liens such as home equity loans, home equity lines of credit and 1-4 family residential mortgages, including purchase money mortgages. The primary source of repayment for these loans is the income and assets of the borrower. The condition of the local economy, in particular the unemployment rate, is an important indicator of risk for this segment. The state of the local housing market can also have a significant impact on this segment because low demand and/or declining home values can limit the ability of borrowers to sell a property and satisfy the debt. Other Consumer —Loans made to individuals that may be secured by assets other than 1-4 family residences, as well as unsecured loans. This segment includes auto loans, unsecured loans and lines and credit cards. The primary source of repayment for these loans is the income and assets of the borrower. The condition of the local economy, in particular the unemployment rate, is an important indicator of risk for this segment. The value of the collateral, if there is any, is less likely to be a source of repayment due to less certain collateral values. We further assess risk within each portfolio segment by pooling loans with similar risk characteristics. For the commercial loan classes, the most important indicator of risk is the internally assigned risk rating, including pass, special mention and substandard. Consumer loans are pooled by type of collateral, lien position and loan to value, or LTV, ratio for Consumer Real Estate loans. Historical loss rates are applied to these loan pools to determine the reserve for loans collectively evaluated for impairment. The ALL methodology for groups of loans collectively evaluated for impairment is comprised of both a quantitative and qualitative analysis. A key assumption in the quantitative component of the reserve is the loss emergence period, or LEP. The LEP is an estimate of the average amount of time from the point at which a loss is incurred on a loan to the point at which the loss is confirmed. Another key assumption is the look-back period, or LBP, which represents the historical data period utilized to calculate loss rates. Management monitors various credit quality indicators for both the commercial and consumer loan portfolios, including delinquency, nonperforming status and changes in risk ratings on a monthly basis.
Bank Owned Life Insurance	Bank Owned Life Insurance We have purchased life insurance policies on certain executive officers and employees. We receive the cash surrender value of each policy upon its termination or benefits are payable to us upon the death of the insured. Changes in net cash surrender value are recognized in noninterest income or expense in the Consolidated Statements of Net Income.
Premises and Equipment	Premises and Equipment Premises and equipment, including leasehold improvements, are stated at cost less accumulated depreciation. Maintenance and repairs are charged to expense as incurred, while improvements that extend an asset’s useful life are capitalized and depreciated over the estimated remaining life of the asset. Depreciation expense is computed by the straight-line method for financial reporting purposes and accelerated methods for income tax purposes over the estimated useful lives of the particular assets. Management reviews long-lived assets using events and circumstances to determine if and when an asset is evaluated for recoverability. The estimated useful lives for the various asset categories are as follows: 1) Land and Land Improvements Non-depreciating assets 2) Buildings 25 years 3) Furniture and Fixtures 5 years 4) Computer Equipment and Software 5 years or term of license 5) Other Equipment 5 years 6) Vehicles 5 years 7) Leasehold Improvements Lesser of estimated useful life of the asset (generally 15 years unless established otherwise) or the remaining term of the lease, including renewal options in the lease that are reasonably assured of exercise
Restricted Investment in Bank Stock	Restricted Investment in Bank Stock Federal Home Loan Bank, or FHLB, stock is carried at cost and evaluated for impairment based on the ultimate recoverability of the par value. We hold FHLB stock because we are a member of the FHLB of Pittsburgh. The FHLB requires members to purchase and hold a specified level of FHLB stock based upon on the member's asset value, level of borrowings and participation in other programs offered. Stock in the FHLB is non-marketable and is redeemable at the discretion of the FHLB. Members do not purchase stock in the FHLB for the same reasons that traditional equity investors acquire stock in an investor-owned enterprise. Rather, members purchase stock to obtain access to the low-cost products and services offered by the FHLB. Unlike equity securities of traditional for-profit enterprises, the stock of the FHLB does not provide its holders with an opportunity for capital appreciation because, by regulation, FHLB stock can only be purchased, redeemed and transferred at par value. Both cash and stock dividends are reported as income in taxable investment securities in the Consolidated Statements of Net Income. FHLB stock is evaluated for OTTI on a quarterly basis. Atlantic Community Bankers’ Bank, or ACBB, stock is carried at cost and evaluated for impairment based on the ultimate recoverability of the carrying value. We do not currently use their membership products and services. We acquired ACBB stock through various mergers of banks that were ACBB members. ACBB stock is evaluated for OTTI on a quarterly basis.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets As a result of acquisitions, we have recorded goodwill and identifiable intangible assets in our Consolidated Balance Sheets. Goodwill represents the excess of the purchase price over the fair value of net assets acquired. We account for business combinations using the acquisition method of accounting. We have three reporting units: Community Bank, Insurance and Wealth Management. Existing goodwill relates to value inherent in the Community Banking and Insurance reporting units and that value is dependent upon our ability to provide quality, cost-effective services in the face of competition from other market participants. This ability relies upon continuing investments in processing systems, the development of value-added service features and the ease of use of our services. As such, goodwill value is supported ultimately by profitability that is driven by the volume of business transacted. A decline in earnings as a result of a lack of growth or the inability to deliver cost-effective services over sustained periods can lead to impairment of goodwill, which could adversely impact our earnings in future periods. The carrying value of goodwill is tested annually for impairment each October 1st or more frequently if it is determined that a triggering event has occurred. We first assess qualitatively whether it is more likely than not that the fair value of the a reporting unit is less than its carrying amount. Our qualitative assessment considers such factors as macroeconomic conditions, market conditions specifically related to the banking industry, our overall financial performance and various other factors. If we determine that it is more likely than not that the fair value is less than the carrying amount, we proceed to test for impairment. The evaluation for impairment involves comparing the current estimated fair value of each reporting unit to its carrying value, including goodwill. If the current estimated fair value of the reporting unit exceeds its carrying value, no additional testing is required and an impairment loss is not recorded. If the estimated fair value of an reporting unit is less than the carrying value, further valuation procedures are performed that could result in impairment of goodwill being recorded. Further valuation procedures would include allocating the estimated fair value to all assets and liabilities of the reporting unit to determine an implied goodwill value. If the implied value of goodwill of an reporting unit is less than the carrying amount of that goodwill, an impairment loss is recognized in an amount equal to that excess. We completed the annual goodwill impairment assessment as required in 2017, 2016 and 2015; the results indicated that the fair value each reporting unit exceeded the carrying value. Based upon the results of the qualitative assessment performed for our annual impairment analysis, we concluded that it is more likely than not that the fair value of the reporting units exceed the carrying value. Both the national economy and the local economies in our markets have shown improvement over the past couple of years. General economic activity and key indicators such as housing and unemployment continue to show improvement. While still challenging, the banking environment continues to improve with better asset quality, improved earnings and generally better stock prices. Activity in mergers and acquisitions demonstrated that there is premium value on banking franchises and a number of banks of our size have been able to access the capital markets over the past year. Our stock traded above book value for all of 2017. Additionally, our overall performance remains strong, and we have not identified any other facts or circumstances that would cause us to conclude that it is more likely than not that the fair value of each of the reporting units would be less than the carrying value of the reporting unit. We determine the amount of identifiable intangible assets based upon independent core deposit and insurance contract valuations at the time of acquisition. Intangible assets with finite useful lives, consisting primarily of core deposit and customer list intangibles, are amortized using straight-line or accelerated methods over their estimated weighted average useful lives, ranging from 10 to 20 years. Intangible assets with finite useful lives are evaluated for impairment whenever events or changes in circumstances indicate that their carrying amount may not be recoverable. No such events or changes in circumstances occurred during the years ended December 31, 2017, 2016 and 2015. The financial services industry and securities markets can be adversely affected by declining values. If economic conditions result in a prolonged period of economic weakness in the future, our operating segments, including the Community Banking segment, may be adversely affected. In the event that we determine that either our goodwill or finite lived intangible assets are impaired, recognition of an impairment charge could have a significant adverse impact on our financial position or results of operations in the period in which the impairment occurs.
Variable Interest Entities	Variable Interest Entities Variable interest entities, or VIEs, are legal entities that generally either do not have equity investors with voting rights or that have equity investors that do not provide sufficient financial resources for the entity to support its activities. When an enterprise has both the power to direct the economic activities of the VIE and the obligation to absorb losses of the VIE or the right to receive benefits of the VIE, the entity has a controlling financial interest in the VIE. A VIE often holds financial assets, including loans or receivables, or other property. The company with a controlling financial interest, the primary beneficiary, is required to consolidate the VIE into its consolidated balance sheets. S&T has one wholly-owned trust subsidiary, STBA Capital Trust I, or the Trust, for which it does not absorb a majority of expected losses or receive a majority of the expected residual returns. At its inception in 2008, the Trust issued floating rate trust preferred securities to the Trustee, another financial institution, and used the proceeds from the sale to invest in junior subordinated debt issued by us, which is the sole asset of the Trust. The Trust pays dividends on the trust preferred securities at the same rate as the interest we pay on our junior subordinated debt held by the Trust. Because the third-party investors are the primary beneficiaries, the Trust qualifies as a VIE. Accordingly, the Trust and its net assets are not included in our Consolidated Financial Statements. However, the junior subordinated debt issued by S&T is included in our Consolidated Balance Sheets.
Joint Ventures	Joint Ventures We have made investments directly in Low Income Housing Tax Credit, or LIHTC, partnerships formed with third parties. As a limited partner in these operating partnerships, we receive tax credits and tax deductions for losses incurred by the underlying properties. These investments are amortized over a maximum of 10 years , which represents the period over which the tax credits will be utilized. We have determined that we are not the primary beneficiary of these investments because the general partners have the power to direct the activities that most significantly impact the economic performance of the partnership and have both the obligation to absorb expected losses and the right to receive benefits.
OREO and Other Repossessed Assets	OREO and Other Repossessed Assets OREO and other repossessed assets are included in other assets in the Consolidated Balance Sheets and are comprised of properties acquired through foreclosure proceedings or acceptance of a deed in lieu of a foreclosure. At the time of foreclosure or acceptance of a deed in lieu of foreclosure, these properties are recorded at the lower of the recorded investment in the loan or fair value less cost to sell. Loan losses arising from the acquisition of any such property initially are charged against the ALL. Subsequently, these assets are carried at the lower of carrying value or current fair value less cost to sell. Gains or losses realized upon disposition of these assets are recorded in other expenses in the Consolidated Statements of Net Income.
Mortgage Servicing Rights	Mortgage Servicing Rights MSRs are recognized as separate assets when commitments to fund a loan to be sold are made. Upon commitment, the MSR is established, which represents the then current estimated fair value of future net cash flows expected to be realized for performing the servicing activities. The estimated fair value of the MSRs is estimated by calculating the present value of estimated future net servicing cash flows, considering expected mortgage loan prepayment rates, discount rates, servicing costs and other economic factors, which are determined based on current market conditions. The expected rate of mortgage loan prepayments is the most significant factor driving the value of MSRs. Increases in mortgage loan prepayments reduce estimated future net servicing cash flows because the life of the underlying loan is reduced. In determining the estimated fair value of MSRs, mortgage interest rates, which are used to determine prepayment rates, are held constant over the estimated life of the portfolio. MSRs are reported in other assets in the Consolidated Balance Sheets and are amortized into noninterest income in the Consolidated Statements of Net Income in proportion to, and over the period of, the estimated future net servicing income of the underlying mortgage loans. MSRs are regularly evaluated for impairment based on the estimated fair value of those rights. MSRs are stratified by certain risk characteristics, primarily loan term and note rate. If temporary impairment exists within a risk stratification tranche, a valuation allowance is established through a charge to income equal to the amount by which the carrying value exceeds the estimated fair value. If it is later determined that all or a portion of the temporary impairment no longer exists for a particular tranche, the valuation allowance is reduced. MSRs are also reviewed for OTTI. OTTI exists when the recoverability of a recorded valuation allowance is determined to be remote, taking into consideration historical and projected interest rates and loan pay-off activity. When this situation occurs, the unrecoverable portion of the valuation allowance is applied as a direct write-down to the carrying value of the MSR. Unlike a valuation allowance, a direct write-down permanently reduces the carrying value of the MSR and the valuation allowance, precluding subsequent recoveries.
Derivative Financial Instruments	Derivative Financial Instruments Interest Rate Swaps In accordance with applicable accounting guidance for derivatives and hedging, all derivatives are recognized as either assets or liabilities on the balance sheet at fair value. Interest rate swaps are contracts in which a series of interest rate flows (fixed and variable) are exchanged over a prescribed period. The notional amounts on which the interest payments are based are not exchanged. These derivative positions relate to transactions in which we enter into an interest rate swap with a commercial customer while at the same time entering into an offsetting interest rate swap with another financial institution. In connection with each transaction, we agree to pay interest to the customer on a notional amount at a variable interest rate and receive interest from the customer on a same notional amount at a fixed rate. At the same time, we agree to pay another financial institution the same fixed interest rate on the same notional amount and receive the same variable interest rate on the same notional amount. The transaction allows our customer to effectively convert a variable rate loan to a fixed rate loan with us receiving a variable rate. These agreements could have floors or caps on the contracted interest rates. Pursuant to our agreements with various financial institutions, we may receive collateral or may be required to post collateral based upon mark-to-market positions. Beyond unsecured threshold levels, collateral in the form of cash or securities may be made available to counterparties of interest rate swap transactions. Based upon our current positions and related future collateral requirements relating to them, we believe any effect on our cash flow or liquidity position to be immaterial. Derivatives contain an element of credit risk, the possibility that we will incur a loss because a counterparty, which may be a financial institution or a customer, fails to meet its contractual obligations. All derivative contracts with financial institutions may be executed only with counterparties approved by our Asset and Liability Committee, or ALCO, and derivatives with customers may only be executed with customers within credit exposure limits approved by our Senior Loan Committee. Interest rate swaps are considered derivatives, but are not accounted for using hedge accounting. As such, changes in the estimated fair value of the derivatives are recorded in current earnings and included in other noninterest income in the Consolidated Statements of Net Income. Interest Rate Lock Commitments and Forward Sale Contracts In the normal course of business, we sell originated mortgage loans into the secondary mortgage loan market. We also offer interest rate lock commitments to potential borrowers. The commitments are generally for a period of 60 days and guarantee a specified interest rate for a loan if underwriting standards are met, but the commitment does not obligate the potential borrower to close on the loan. Accordingly, some commitments expire prior to becoming loans. We can encounter pricing risks if interest rates increase significantly before the loan can be closed and sold. We may utilize forward sale contracts in order to mitigate this pricing risk. Whenever a customer desires these products, a mortgage originator quotes a secondary market rate guaranteed for that day by the investor. The rate lock is executed between the mortgagee and us and in turn a forward sale contract may be executed between us and the investor. Both the rate lock commitment and the corresponding forward sale contract for each customer are considered derivatives, but are not accounted for using hedge accounting. As such, changes in the estimated fair value of the derivatives during the commitment period are recorded in current earnings and included in mortgage banking in the Consolidated Statements of Net Income.
Allowance for Unfunded Commitments	Allowance for Unfunded Commitments In the normal course of business, we offer off-balance sheet credit arrangements to enable our customers to meet their financing objectives. These instruments involve, to varying degrees, elements of credit and interest rate risk in excess of the amount recognized in the financial statements. Our exposure to credit loss, in the event the customer does not satisfy the terms of the agreement, equals the contractual amount of the obligation less the value of any collateral. We apply the same credit policies in making commitments and standby letters of credit that are used for the underwriting of loans to customers. Commitments generally have fixed expiration dates, annual renewals or other termination clauses and may require payment of a fee. Because many of the commitments are expected to expire without being drawn upon, the total commitment amounts do not necessarily represent future cash requirements. The allowance for unfunded commitments is included in other liabilities in the Consolidated Balance Sheets. The allowance for unfunded commitments is determined using a similar methodology as our ALL methodology.
Treasury Stock	Treasury Stock The repurchase of our common stock is recorded at cost. At the time of reissuance, the treasury stock account is reduced using the average cost method. Gains and losses on the reissuance of common stock are recorded in additional paid-in capital, to the extent additional paid-in capital from previous treasury share transactions exists. Any deficiency is charged to retained earnings.
Revenue Recognition	Revenue Recognition We recognize revenues as they are earned based on contractual terms or as services are provided when collectability is reasonably assured. Our principal source of revenue is interest income, which is recognized on an accrual basis. Interest and dividend income, loan fees, trust fees, fees and charges on deposit accounts, insurance commissions and other ancillary income related to our deposits and lending activities are accrued as earned.
Wealth Management Fees	Wealth Management Fees Assets held in a fiduciary capacity by our subsidiary bank, S&T Bank, are not our assets and are therefore not included in our Consolidated Financial Statements. Wealth management fee income is reported in the Consolidated Statements of Net Income on an accrual basis.
Stock-Based Compensation	Stock-Based Compensation Stock-based compensation may include stock options and restricted stock which is measured using the fair value method of accounting. The grant date fair value is recognized over the period during which the recipient is required to provide service in exchange for the award. Stock option expense is determined utilizing the Black-Scholes model. Restricted stock expense is determined using the grant date fair value. We estimate expected forfeitures when stock-based awards are granted and record compensation expense only for awards that are expected to vest.
Pensions	Pensions The expense for S&T Bank’s qualified and nonqualified defined benefit pension plans is actuarially determined using the projected unit credit actuarial cost method. It requires us to make economic assumptions regarding future interest rates and asset returns and various demographic assumptions. We estimate the discount rate used to measure benefit obligations by applying the projected cash flow for future benefit payments to a yield curve of high-quality corporate bonds available in the marketplace and by employing a model that matches bonds to our pension cash flows. The expected return on plan assets is an estimate of the long-term rate of return on plan assets, which is determined based on the current asset mix and estimates of return by asset class. We recognize in the Consolidated Balance Sheets an asset for the plan’s overfunded status or a liability for the plan’s underfunded status. Gains or losses related to changes in benefit obligations or plan assets resulting from experience different from that assumed are recognized as other comprehensive income (loss) in the period in which they occur. To the extent that such gains or losses exceed ten percent of the greater of the projected benefit obligation or plan assets, they are recognized as a component of pension costs over the future service periods of actively employed plan participants. The funding policy for the qualified plan is to contribute an amount each year that is at least equal to the minimum required contribution as determined under the Pension Protection Act of 2006 and the Bipartisan Budget Act of 2015, but not more than the maximum amount permissible for taxable plan sponsors. Our nonqualified plans are unfunded.
Marketing Costs	Marketing Costs We expense all marketing-related costs, including advertising costs, as incurred.
Income Taxes	Income Taxes We estimate income tax expense based on amounts expected to be owed to the tax jurisdictions where we conduct business. On a quarterly basis, management assesses the reasonableness of our effective tax rate based upon our current estimate of the amount and components of net income, tax credits and the applicable statutory tax rates expected for the full year. We classify interest and penalties as an element of tax expense. Deferred income tax assets and liabilities are determined using the asset and liability method and are reported in other assets or other liabilities, as appropriate, in the Consolidated Balance Sheets. Under this method, the net deferred tax asset or liability is based on the tax effects of the differences between the book and tax basis of assets and liabilities and recognizes enacted changes in tax rate and laws. When deferred tax assets are recognized, they are subject to a valuation allowance based on management’s judgment as to whether realization is more likely than not. Accrued taxes represent the net estimated amount due to taxing jurisdictions and are reported in other assets or other liabilities, as appropriate, in the Consolidated Balance Sheets. We evaluate and assess the relative risks and appropriate tax treatment of transactions and filing positions after considering statutes, regulations, judicial precedent and other information and maintain tax accruals consistent with the evaluation of these relative risks and merits. Changes to the estimate of accrued taxes occur periodically due to changes in tax rates, interpretations of tax laws, the status of examinations being conducted by taxing authorities and changes to statutory, judicial and regulatory guidance. These changes, when they occur, can affect deferred taxes and accrued taxes, and the current period’s income tax expense and can be significant to our operating results. In the fourth quarter 2017, H.R.1, known as the Tax Cuts and Jobs Act, or Tax Act, was signed into law which requires the deferred tax assets and liabilities to be revalued using the 21 percent federal tax rate enacted. The effect was recorded in our fourth quarter tax provision. Tax positions are recognized as a benefit only if it is “more likely than not” that the tax position would be sustained in a tax examination, with a tax examination being presumed to occur. The amount recognized is the largest amount of tax benefit that is greater than 50 percent likely of being realized on examination. For tax positions not meeting the “more likely than not” test, no tax benefit is recorded.
Earnings Per Share	Earnings Per Share Basic earnings per share, or EPS, is calculated using the two-class method to determine income allocated to common shareholders. Unvested share-based payment awards that contain nonforfeitable rights to dividends are considered participating securities under the two-class method. Income allocated to common shareholders is then divided by the weighted average number of common shares outstanding during the period. Potentially dilutive securities are excluded from the basic EPS calculation. Diluted EPS is calculated under the more dilutive of either the treasury stock method or the two-class method. Under the treasury stock method, the weighted average number of common shares outstanding is increased by the potentially dilutive common shares. For the two-class method, diluted EPS is calculated for each class of shareholders using the weighted average number of shares attributed to each class. Potentially dilutive common shares are related to our outstanding warrants, stock options and restricted stock.
Recently Adopted Accounting Standards Updates, or ASU	Recently Adopted Accounting Standards Updates, or ASU Stock Compensation - Improvements to Employee Share-Based Payment Accounting On March 31, 2016 the Financial Accounting Standards Board, or FASB, issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting, which is intended to improve the accounting for share-based payment transactions as part of the FASB's simplification initiative. The ASU changes seven aspects of the accounting for share-based payment award transactions, including: 1. accounting for income taxes; 2. classification of excess tax benefits on the statement of cash flows; 3. forfeitures; 4. minimum statutory tax withholding requirements; 5. classification of employee taxes paid on the statement of cash flows when an employer withholds shares for tax-withholding purposes; 6. practical expedient - expected term (nonpublic only); and 7. intrinsic value (nonpublic only). This ASU is effective for fiscal years beginning after December 15, 2016 and interim periods within those years for public business entities. The adoption of this ASU on January 1, 2017, had no material impact on our results of operations or financial position. Equity Method and Joint Ventures - Simplifying the Transition to the Equity Method of Accounting In March 2016, the FASB issued ASU No. 2016-07, Simplifying the Transition to the Equity Method of Accounting, which eliminates the requirement for an investor to retroactively apply the equity method when its increase in ownership interest (or degree of influence) in an investee triggers equity method accounting. This ASU is effective for annual and interim periods in fiscal years beginning after December 15, 2016. The amendments will be applied prospectively upon their effective date to increases in the level of ownership interest or degree of influence that result in the adoption of the equity method. The adoption of this ASU on January 1, 2017, had no impact on our results of operations or financial position. Receivables - Nonrefundable Fees and Other Costs - Premium Amortization on Purchased Callable Debt Securities In March 2017, the FASB issued ASU No. 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities. The amendments in this ASU affect all entities that hold investments in callable debt securities that have an amortized cost basis in excess of the amount that is repayable by the issuer at the earliest call date. This ASU shortens the amortization period for certain callable debt securities held at a premium. Specifically, the amendments require the premium to be amortized to the earliest call date. The amendments do not require an accounting change for securities held at a discount, which continues to be amortized to maturity. This Update is effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted, including adoption in an interim period. We early adopted the provisions of this ASU on January 1, 2017, and it had no impact on our results of operations or financial position. Recently Issued Accounting Standards Updates not yet Adopted Income Statement -- Reporting Comprehensive Income - Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In February 2018, the FASB issued ASU No. 2018-02, Income Statement -- Reporting Comprehensive Income (Topic 220), Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. The amendments in this Update allow a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act, or Tax Act. The amendments eliminate the stranded tax effects resulting from the Tax Act and will improve the usefulness of information reported to financial statement users and will require certain disclosures about the stranded tax effects. This Update is effective for all entities for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. Early adoption is permitted, including adoption in any interim period, for public business entities for reporting periods for which financial statements have not been issued or made available for issuance. We intend to early adopt this Update and elect to reclassify the income tax effects of the Tax Act in our interim reporting period ending March 31, 2018. We estimate that the reclassification from accumulated other comprehensive income, or AOCI, to retained earnings will be $3.2 million for the release of stranded income tax effects relating to unrealized gains and losses on available for sale securities and our pension plan. The impact of this ASU will have no material impact on our results of operations or financial position upon adoption. Compensation - Retirement Benefits - Improving the Presentation of Net Periodic Pension Costs and Net Periodic Post Retirement Benefit Costs In March 2017, the FASB issued ASU No. 2017-07, Compensation Retirement Benefits - Improving the Presentation of Net Periodic Pension Costs and Net Periodic Post Retirement Benefit Costs (Topic 715). The main objective of this ASU is to provide financial statement users with clearer and disaggregated information related to the components of net periodic benefit cost and improve transparency of the presentation of net periodic benefit cost in the financial statements. This Update is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. Early adoption is permitted as of the beginning of an annual period for which financial statements have not been issued or made available for issuance. Effective March 31, 2016, our qualified and nonqualified defined benefit plans were amended to freeze benefit accruals for all persons entitled to benefits under the plan; as such, the provisions of this ASU will have no impact on our results of operations and financial position upon adoption. Other Income - Gains and Losses from the Derecognition of Nonfinancial Assets - Clarifying the Scope of Assets Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets In February 2017, the FASB issued ASU No. 2017-05, Other Income - Gains and Losses from the Derecognition of Nonfinancial Assets (Subtopic 610-20). The main objective in this ASU is intended to provide greater detail on what types of transactions should be accounted for as partial sales of nonfinancial assets. The scope of this ASU, as originally issued in ASU No. 2014-09 (described below), is intended to reduce the complexity of current GAAP requirements by clarifying which accounting guidance applies to various types of contracts that transfer assets or ownership interest to another entity. This Update is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017 and at the same time that ASU No. 2014-09 is effective. Early adoption is permitted, but only as of annual reporting periods beginning after December 15, 2016, including interim reporting periods within that reporting period. The provisions of this ASU will not impact our results of operations and financial position upon adoption. Intangibles - Goodwill and Other - Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued ASU No. 2017-04, Intangibles - Goodwill and Other - Simplifying the Test for Goodwill Impairment (Topic 350). The main objective in this ASU is intended to simplify the current requirements for testing goodwill for impairment by eliminating step two from the goodwill impairment test. The amendments are expected to reduce the complexity and costs associated with performing the goodwill impairment test, which could result in recording impairment charges sooner than under the current guidance. This Update is effective for any interim and annual impairment tests in reporting periods in fiscal years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. We are evaluating the provisions of this ASU; however, we do not anticipate that this ASU will materially impact our results of operations and financial position upon adoption. Business Combinations - Clarifying the Definition of a Business In January 2017, the FASB issued ASU No. 2017-01, Business Combinations - Clarifying the Definition of a Business (Topic 805). The main objective of this ASU is to help financial statement preparers evaluate whether a set of transferred assets and activities (either acquired or disposed of) is a business under Topic 805, Business Combinations by changing the definition of a business. The revised definition will result in fewer acquisitions being accounted for as business combinations than under existing guidance. The definition of a business is significant because it affects the accounting for acquisitions, the identification of reporting units, consolidation evaluations and the accounting for dispositions. This Update is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. Early adoption is permitted for transactions not yet reflected in financial statements that have been issued or made available for issuance. The provisions of this ASU will have no material impact on our results of operations and financial position. Income Taxes - Intra-Entity Transfers of Assets Other Than Inventory In October 2016, the FASB issued ASU No. 2016-16, Intra-Entity Transfers of Assets Other Than Inventory. The main objective of this ASU is to require companies to recognize the income tax effects of intercompany sales and transfers of assets other than inventory in the period in which the transfer occurs. This represents a change from existing guidance, which requires companies to defer the income tax effects of intercompany transfers of assets until the asset has been sold to an outside party or otherwise recognized. The new guidance will require companies to defer the income tax effects only of intercompany transfers of inventory. This Update is effective for annual periods beginning after December 15, 2017. Early adoption is permitted as of the beginning of an annual period. If an entity chooses to early adopt the amendments in the ASU, it must do so in the first interim period of its annual financial statements. That is, an entity cannot adopt the amendments in the ASU in a later interim period and apply them as if they were in effect as of the beginning of the year. The provisions of this ASU will have no impact on our results of operations and financial position. Statement of Cash Flows - Classification of Certain Cash Receipts and Cash Payments In August 2016, the FASB issued ASU No. 2016-15, Classification of Certain Cash Receipts and Cash Payments. The main objective of this ASU is to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. The amendments in this Update provide guidance on the following eight specific cash flow issues: debt prepayment or debt extinguishment costs, settlement of zero-coupon debt instruments, contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, proceeds from the settlement of bank-owned life insurance (BOLI) policies, distributions received from equity method investments, beneficial interests in securitization transactions, and separately identifiable cash flows and application of the predominance principle. This Update is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. Early adoption is permitted, provided that all of the amendments are adopted in the same period. The provisions of this ASU will not materially impact our results of operations and financial position. Financial Instruments - Credit Losses In June 2016, the FASB issued ASU No. 2016-13, Measurement of Credit Losses on Financial Instruments. The main objective of this ASU is to provide financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The amendments of this Update replace the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. The collective changes to the recognition and measurement accounting standards for financial instruments and their anticipated impact on the allowance for credit losses modeling have been universally referred to as the CECL, or current expected credit loss, model. This Update is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2019. Early adoption is permitted as of fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. We are evaluating the provisions of this ASU to determine the potential impact on our results of operations and financial position. Revenue from Contracts with Customers In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606). The new revenue pronouncement creates a single source of revenue guidance for all companies in all industries and is more principles-based than current revenue guidance. The pronouncement provides a five-step model for a company to recognize revenue when it transfers control of goods or services to customers at an amount that reflects the consideration to which it expects to be entitled in exchange for those goods or services. The five steps are: 1. identify the contract with the customer; 2. identify the separate performance obligations in the contract; 3. determine the transaction price; 4. allocate the transaction price to the separate performance obligations; and 5. recognize revenue when each performance obligation is satisfied. In August 2015, the FASB issued ASU No. 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date. This ASU defers the effective date of ASU No. 2014-09 for all entities by one year. In March 2016, the FASB issued ASU No. 2016-08, Principal versus Agent Considerations (Reporting Revenue Gross versus Net), as an amendment to ASU No. 2014-09 to improve Topic 606, Revenue from Contracts with Customers, by reducing: 1. The potential for diversity in practice arising from inconsistent application of the principal versus agent guidance, and 2. The cost and complexity of applying Topic 606 both at transition and on an ongoing basis. In April 2016, the FASB issued ASU No. 2016-10, Identifying Performance Obligations and Licensing, as an amendment to ASU No. 2014-09 to improve Topic 606, Revenue from Contracts with Customers, by reducing: 1. The potential for diversity in practice at initial application, and 2. The cost and complexity of applying Topic 606 both at transition and on an ongoing basis. In May 2016, the FASB issued ASU No. 2016-12, Narrow-scope Improvements and Practical Expedients. The amendments in this ASU do not change the core principles of Topic 606, Revenue from Contracts with Customers. These amendments affect only the narrow aspects of Topic 606: 1. Collectibility Criterion, 2. Presentation of Sales Taxes and Other Similar Taxes Collected from Customers, 3. Noncash Consideration, 4. Contract Modifications at Transition, and 5. Completed Contracts at Transition. ASU 2014-09, including transition requirements for all amendments, is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. Early adoption is permitted as of the original effective date for interim and annual reporting periods in fiscal years beginning after December 15, 2016. Our revenue is comprised of net interest income, which is excluded from the scope of ASU 2014-09, and noninterest income. We have completed our overall assessment of revenue streams and related contracts, including trust and asset management fees, deposit related fees, interchange fees, merchant income and annuity and insurance commissions. We have also completed our evaluation of certain costs related to these revenue streams to determine whether such costs should be presented as expenses or contra-revenue. We have evaluated the impact of this new standard and have concluded that our financial statements will not be materially impacted upon adoption; however, we will expand certain disclosures as required. We will adopt ASU No. 2014-09 on January 1, 2018 utilizing the modified retrospective approach with a cumulative effect adjustment to opening retained earnings. Leases - Section A-Amendments to the FASB Accounting Standards Codification, Section B-Conforming Amendments Related to Leases and Section C-Background Information and Basis for Conclusions In February 2016, the FASB issued ASU No. 2016-02, Leases, which requires lessees to recognize a right-to-use asset and a lease obligation for all leases on the balance sheet. Lessor accounting remains substantially similar to current GAAP. ASU 2016-02 supersedes Topic 840, Leases. This ASU is effective for annual and interim periods in fiscal years beginning after December 15, 2018. ASU 2016-02 mandates a modified retrospective transition method for all entities. Early adoption of this ASU is permitted. Adoption of ASU 2016-02 is not expected to have a material impact on our consolidated financial statements. We lease certain branch and limited purpose facilities, land and equipment under operating leases that will result in the recognition of right-to-use assets and lease obligations under the ASU. Approximately 55 percent of our facilities are owned, not leased. At December 31, 2017, we had contractual operating lease commitments of approximately $72 million including renewal options. We have developed an implementation plan for adoption of this ASU which includes specific identification of all lease contracts, evaluating our current processes and procedures, assessing internal controls, evaluating tax considerations and evaluating impact to regulatory capital requirements. We plan to adopt ASU No. 2016-02 January 1, 2019. Accounting for Financial Instruments - Overall: Classification and Measurement In January 2016, the FASB issued ASU No. 2016-01, Accounting for Financial Instruments - Overall: Classification and Measurement (Subtopic 825-10). The amendments in this ASU address the following: 1. require equity investments to be measured at fair value with changes in fair value recognized in net income; 2. simplify the impairment assessment of equity investments without readily-determinable fair values by requiring a qualitative assessment to identify impairment; 3. eliminate the requirement to disclose the method(s) and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost on the balance sheet; 4. require entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes; 5. require separate presentation in other comprehensive income for the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option for financial instruments; 6. require separate presentation of financial assets and financial liabilities by measurement category and form of financial asset (that is, securities or loans and receivables) on the balance sheet or in the accompanying notes to the financial statements; and 7. clarify that an entity should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale securities in combination with the entity's other deferred tax assets. This ASU is effective for annual and interim periods in fiscal years beginning after December 15, 2017. We adopted ASU No. 2016-01 January 1, 2018 and have concluded that the provisions of this ASU will not materially impact our results of operations and financial position. We have $5.1 million of equity securities at December 31, 2017 where changes in the fair value will be recognized in the income statement beginning January 1, 2018.
Loan Credit Risk Rating	We continually monitor the commercial loan portfolio through an internal risk rating system. Loan risk ratings are assigned based upon the creditworthiness of the borrower and are reviewed on an ongoing basis according to our internal policies. Loans within the pass rating generally have a lower risk of loss than loans risk rated as special mention and substandard. Our risk ratings are consistent with regulatory guidance and are as follows: Pass —The loan is currently performing and is of high quality. Special Mention —A special mention loan has potential weaknesses that warrant management’s close attention. If left uncorrected, these potential weaknesses may result in deterioration of the repayment prospects or in the strength of our credit position at some future date. Economic and market conditions, beyond the borrower’s control, may in the future necessitate this classification. Substandard —A substandard loan is not adequately protected by the net worth and/or paying capacity of the borrower or by the collateral pledged, if any. Substandard loans have a well-defined weakness or weaknesses that jeopardize the liquidation of the debt. These loans are characterized by the distinct possibility that we will sustain some loss if the deficiencies are not corrected.

Summary of Significant Accoun35

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Estimated Useful Lives for Various Asset	The estimated useful lives for the various asset categories are as follows: 1) Land and Land Improvements Non-depreciating assets 2) Buildings 25 years 3) Furniture and Fixtures 5 years 4) Computer Equipment and Software 5 years or term of license 5) Other Equipment 5 years 6) Vehicles 5 years 7) Leasehold Improvements Lesser of estimated useful life of the asset (generally 15 years unless established otherwise) or the remaining term of the lease, including renewal options in the lease that are reasonably assured of exercise

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Reconciles Numerators and Denominators of Basic Earnings Per Share and Diluted Earnings Per Share	The following table reconciles the numerators and denominators of basic and diluted EPS: Years ended December 31, (dollars in thousands, except share and per share data) 2017 2016 2015 Numerator for Earnings per Common Share—Basic: Net income $ 72,968 $ 71,392 $ 67,081 Less: Income allocated to participating shares 242 225 280 Net Income Allocated to Common Shareholders $ 72,726 $ 71,167 $ 66,801 Numerator for Earnings per Common Share—Diluted: Net income $ 72,968 $ 71,392 $ 67,081 Denominators: Weighted Average Common Shares Outstanding—Basic 34,729,376 34,677,738 33,812,990 Add: Dilutive potential common shares 225,391 95,432 35,092 Denominator for Treasury Stock Method—Diluted 34,954,767 34,773,170 33,848,082 Weighted Average Common Shares Outstanding—Basic 34,729,376 34,677,738 33,812,990 Add: Average participating shares outstanding 115,418 109,755 141,558 Denominator for Two-Class Method—Diluted 34,844,794 34,787,493 33,954,548 Earnings per common share—basic $ 2.10 $ 2.06 $ 1.98 Earnings per common share—diluted $ 2.09 $ 2.05 $ 1.98 Warrants considered anti-dilutive excluded from dilutive potential common shares - exercise price $31.53 per share, expires January 2019 438,681 517,012 517,012 Restricted stock considered anti-dilutive excluded from dilutive potential common shares 88,578 116,749 106,466

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present our assets and liabilities that are measured at fair value on a recurring basis by fair value hierarchy level at December 31, 2017 and 2016. December 31, 2017 (dollars in thousands) Level 1 Level 2 Level 3 Total ASSETS Securities available-for-sale: U.S. Treasury securities $ — $ 19,789 $ — $ 19,789 Obligations of U.S. government corporations and agencies — 162,193 — 162,193 Collateralized mortgage obligations of U.S. government corporations and agencies — 108,688 — 108,688 Residential mortgage-backed securities of U.S. government corporations and agencies — 32,854 — 32,854 Commercial mortgage-backed securities of U.S. government corporations and agencies — 242,221 — 242,221 Obligations of states and political subdivisions — 127,402 — 127,402 Marketable equity securities — 5,144 — 5,144 Total securities available-for-sale — 698,291 — 698,291 Trading securities held in a Rabbi Trust 5,080 — — 5,080 Total securities 5,080 698,291 — 703,371 Derivative financial assets: Interest rate swaps — 3,074 — 3,074 Interest rate lock commitments — 226 — 226 Total Assets $ 5,080 $ 701,591 $ — $ 706,671 LIABILITIES Derivative financial liabilities: Interest rate swaps $ — $ 3,055 $ — $ 3,055 Forward sale contracts — 5 — 5 Total Liabilities $ — $ 3,060 $ — $ 3,060 December 31, 2016 (dollars in thousands) Level 1 Level 2 Level 3 Total ASSETS Securities available-for-sale: U.S. Treasury securities $ — $ 24,811 $ — $ 24,811 Obligations of U.S. government corporations and agencies — 232,179 — 232,179 Collateralized mortgage obligations of U.S. government corporations and agencies — 129,777 — 129,777 Residential mortgage-backed securities of U.S. government corporations and agencies — 37,358 — 37,358 Commercial mortgage-backed securities of U.S. government corporations and agencies — 125,604 — 125,604 Obligations of states and political subdivisions — 132,509 — 132,509 Marketable equity securities — 11,249 — 11,249 Total securities available-for-sale — 693,487 — 693,487 Trading securities held in a Rabbi Trust 4,410 — — 4,410 Total securities 4,410 693,487 — 697,897 Derivative financial assets: Interest rate swaps — 6,960 — 6,960 Interest rate lock commitments — 236 — 236 Total Assets $ 4,410 $ 700,683 $ — $ 705,093 LIABILITIES Derivative financial liabilities: Interest rate swaps $ — $ 6,958 $ — $ 6,958 Forward sale contracts — 27 — 27 Total Liabilities $ — $ 6,985 $ — $ 6,985
Assets Measured at Fair Value on Nonrecurring Basis by Fair Value Hierarchy	The following table presents our assets that are measured at fair value on a nonrecurring basis by the fair value hierarchy level as of the dates presented: December 31, 2017 December 31, 2016 (dollars in thousands) Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total ASSETS (1) Loans held for sale $ — $ — $ — $ — $ — $ — $ 1,802 $ 1,802 Impaired loans — — 6,759 6,759 — — 10,329 10,329 Other real estate owned — — 444 444 — — 396 396 Mortgage servicing rights — — 178 178 — — 538 538 Total Assets $ — $ — $ 7,381 $ 7,381 $ — $ — $ 13,065 $ 13,065 (1) This table presents only the nonrecurring items that are recorded at fair value in our financial statements.
Carrying Values and Fair Values of Financial Instruments	The carrying values and fair values of our financial instruments at December 31, 2017 and 2016 are presented in the following tables: Fair Value Measurements at December 31, 2017 (dollars in thousands) Carrying Value (1) Total Level 1 Level 2 Level 3 ASSETS Cash and due from banks, including interest-bearing deposits $ 117,152 $ 117,152 $ 117,152 $ — $ — Securities available-for-sale 698,291 698,291 — 698,291 — Loans held for sale 4,485 4,583 — — 4,583 Portfolio loans, net of unearned income 5,761,449 5,690,292 — — 5,690,292 Bank owned life insurance 72,150 72,150 — 72,150 — FHLB and other restricted stock 29,270 29,270 — — 29,270 Trading securities held in a Rabbi Trust 5,080 5,080 5,080 — — Mortgage servicing rights 4,133 4,571 — — 4,571 Interest rate swaps 3,074 3,074 — 3,074 — Interest rate lock commitments 226 226 — 226 — LIABILITIES Deposits $ 5,427,891 $ 5,426,928 $ — $ — $ 5,426,928 Securities sold under repurchase agreements 50,161 50,161 — — 50,161 Short-term borrowings 540,000 540,000 — — 540,000 Long-term borrowings 47,301 47,618 — — 47,618 Junior subordinated debt securities 45,619 45,619 — — 45,619 Interest rate swaps 3,055 3,055 — 3,055 — Forward sale contracts 5 5 — 5 — (1) As reported in the Consolidated Balance Sheets Fair Value Measurements at December 31, 2016 (dollars in thousands) Carrying Value (1) Total Level 1 Level 2 Level 3 ASSETS Cash and due from banks, including interest-bearing deposits $ 139,486 $ 139,486 $ 139,486 $ — $ — Securities available-for-sale 693,487 693,487 — 693,487 — Loans held for sale 3,793 3,815 — — 3,815 Portfolio loans, net of unearned income 5,611,419 5,551,266 — — 5,551,266 Bank owned life insurance 72,081 72,081 — 72,081 — FHLB and other restricted stock 31,817 31,817 — — 31,817 Trading securities held in a Rabbi Trust 4,410 4,410 4,410 — — Mortgage servicing rights 3,744 4,098 — — 4,098 Interest rate swaps 6,960 6,960 — 6,960 — Interest rate lock commitments 236 236 — 236 — LIABILITIES Deposits $ 5,272,377 $ 5,276,499 $ — $ — $ 5,276,499 Securities sold under repurchase agreements 50,832 50,832 — — 50,832 Short-term borrowings 660,000 660,000 — — 660,000 Long-term borrowings 14,713 15,267 — — 15,267 Junior subordinated debt securities 45,619 45,619 — — 45,619 Interest rate swaps 6,958 6,958 — 6,958 — Forward sale contracts 27 27 — 27 — (1) As reported in the Consolidated Balance Sheets

Securities Available-for-Sale (

Securities Available-for-Sale (Tables)	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Composition of Securities Available-for-Sale	The following tables present the amortized cost and fair value of available-for-sale securities as of the dates presented: December 31, 2017 December 31, 2016 (dollars in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. Treasury securities $ 19,943 $ — $ (154 ) $ 19,789 $ 24,891 $ 47 $ (127 ) $ 24,811 Obligations of U.S. government corporations and agencies 162,045 341 (193 ) 162,193 230,989 1,573 (383 ) 232,179 Collateralized mortgage obligations of U.S. government corporations and agencies 109,916 93 (1,321 ) 108,688 130,046 465 (734 ) 129,777 Residential mortgage-backed securities of U.S. government corporations and agencies 32,388 679 (213 ) 32,854 36,606 984 (232 ) 37,358 Commercial mortgage-backed securities of U.S. government corporations and agencies (1) 244,018 247 (2,044 ) 242,221 127,311 243 (1,950 ) 125,604 Obligations of states and political subdivisions 123,159 4,285 (42 ) 127,402 128,783 3,772 (46 ) 132,509 Debt Securities 691,469 5,645 (3,967 ) 693,147 678,626 7,084 (3,472 ) 682,238 Marketable equity securities 3,815 1,330 (1 ) 5,144 7,579 3,670 — 11,249 Total $ 695,284 $ 6,975 $ (3,968 ) $ 698,291 $ 686,205 $ 10,754 $ (3,472 ) $ 693,487 (1) Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017.
Schedule of Gross and Net Realized Gains and Losses on Sale of Securities	The following table shows the composition of gross and net realized gains and losses for the periods presented: Years ended December 31, (dollars in thousands) 2017 2016 2015 Gross realized gains $ 3,986 $ — $ — Gross realized losses (986 ) — (34 ) Net Realized (Losses) Gains $ 3,000 $ — $ (34 )
Fair Value and Age of Gross Unrealized Losses by Investment Category	The following tables present the fair value and the age of gross unrealized losses by investment category as of the dates presented: December 31, 2017 Less Than 12 Months 12 Months or More Total (dollars in thousands) Number of Securities Fair Unrealized Losses Number of Securities Fair Unrealized Losses Number of Securities Fair Unrealized Losses U.S. Treasury securities 3 $ 19,789 $ (154 ) — $ — $ — 3 $ 19,789 $ (154 ) Obligations of U.S. government corporations and agencies 9 63,635 (144 ) 1 10,017 (49 ) 10 73,652 (193 ) Collateralized mortgage obligations of U.S. government corporations and agencies 7 47,465 (248 ) 7 45,809 (1,073 ) 14 93,274 (1,321 ) Residential mortgage-backed securities of U.S. government corporations and agencies 1 2,333 (10 ) 2 8,638 (203 ) 3 10,971 (213 ) Commercial mortgage-backed securities of U.S. government corporations and agencies 14 128,300 (775 ) 5 48,746 (1,269 ) 19 177,046 (2,044 ) Obligations of states and political subdivisions 2 10,330 (42 ) — — — 2 10,330 (42 ) Debt Securities 36 271,852 (1,373 ) 15 113,210 (2,594 ) 51 385,062 (3,967 ) Marketable equity securities 1 70 (1 ) — — — 1 70 (1 ) Total Temporarily Impaired Securities 37 $ 271,922 $ (1,374 ) 15 $ 113,210 $ (2,594 ) 52 $ 385,132 $ (3,968 ) December 31, 2016 Less Than 12 Months 12 Months or More Total (dollars in thousands) Number of Securities Fair Value Unrealized Losses Number of Securities Fair Value Unrealized Losses Number of Securities Fair Value Unrealized Losses U.S. Treasury securities 1 $ 9,811 $ (127 ) — $ — $ — 1 $ 9,811 $ (127 ) Obligations of U.S. government corporations and agencies 7 62,483 (383 ) — — — 7 62,483 (383 ) Collateralized mortgage obligations of U.S. government corporations and agencies 10 83,031 (734 ) — — — 10 83,031 (734 ) Residential mortgage-backed securities of U.S. government corporations and agencies 2 10,022 (232 ) — — — 2 10,022 (232 ) Commercial mortgage-backed securities of U.S. government corporations and agencies 10 96,576 (1,950 ) — — — 10 96,576 (1,950 ) Obligations of states and political subdivisions 1 5,577 (46 ) — — — 1 5,577 (46 ) Debt Securities 31 267,500 (3,472 ) — — — 31 267,500 (3,472 ) Marketable equity securities — — — — — — — — — Total Temporarily Impaired Securities 31 $ 267,500 $ (3,472 ) — $ — $ — 31 $ 267,500 $ (3,472 )
Unrealized Gain (Loss) on Investments	The following table displays net unrealized gains and losses, net of tax, on securities available-for-sale included in accumulated other comprehensive income/(loss), for the periods presented: December 31, 2017 December 31, 2016 (dollars in thousands) Gross Unrealized Gains Gross Unrealized Losses Net Unrealized Gains (Losses) Gross Unrealized Gains Gross Unrealized Losses Net Unrealized Gains (Losses) Total unrealized gains (losses) on securities available for sale $ 6,975 $ (3,968 ) $ 3,007 $ 10,754 $ (3,472 ) $ 7,282 Income tax (expense) benefit (2,450 ) 1,394 (1,056 ) (3,776 ) 1,219 (2,557 ) Net unrealized gains (losses), net of tax included in accumulated other comprehensive income(loss) $ 4,525 $ (2,574 ) $ 1,951 $ 6,978 $ (2,253 ) $ 4,725
Amortized Cost and Fair Value of Available-for-Sale Securities	The amortized cost and fair value of securities available-for-sale at December 31, 2017 by contractual maturity are included in the table below. Actual maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations with or without call or prepayment penalties. December 31, 2017 (dollars in thousands) Amortized Cost Fair Value Due in one year or less $ 58,501 $ 58,438 Due after one year through five years 141,646 142,859 Due after five years through ten years 70,584 72,132 Due after ten years 34,416 35,955 305,147 309,384 Collateralized mortgage obligations of U.S. government corporations and agencies 109,916 108,688 Residential mortgage-backed securities of U.S. government corporations and agencies 32,388 32,854 Commercial mortgage-backed securities of U.S. government corporations and agencies 244,018 242,221 Debt Securities 691,469 693,147 Marketable equity securities 3,815 5,144 Total $ 695,284 $ 698,291

Loans and Loans Held for Sale (

Loans and Loans Held for Sale (Tables)	12 Months Ended
	Dec. 31, 2017
Receivables [Abstract]
Composition of Loans	The following table indicates the composition of the acquired and originated loans as of the dates presented: December 31, (dollars in thousands) 2017 2016 Commercial Commercial real estate $ 2,685,994 $ 2,498,476 Commercial and industrial 1,433,266 1,401,035 Commercial construction 384,334 455,884 Total Commercial Loans 4,503,594 4,355,395 Consumer Residential mortgage 698,774 701,982 Home equity 487,326 482,284 Installment and other consumer 67,204 65,852 Consumer construction 4,551 5,906 Total Consumer Loans 1,257,855 1,256,024 Total Portfolio Loans 5,761,449 5,611,419 Loans held for sale 4,485 3,793 Total Loans $ 5,765,934 $ 5,615,212
Restructured Loans for Periods Presented and Type of Concession	The following tables present the restructured loans by type of concession for the years ended December 31: 2017 2016 (dollars in thousands) Number of Loans Pre-Modification Outstanding Recorded Investment (1) Post-Modification Outstanding Recorded Investment (1) Total Difference in Recorded Investment Number of Loans Pre-Modification Outstanding Recorded Investment (1) Post-Modification Outstanding Recorded Investment (1) Total Difference in Recorded Investment Commercial real estate Chapter 7 bankruptcy (2) — $ — $ — $ — 1 $ 709 $ 646 $ (63 ) Interest Rate Reduction — — — — 1 250 242 (8 ) Maturity date extension 1 400 398 (2 ) — — — — Commercial and industrial Maturity date extension 1 274 777 503 4 4,756 3,334 (1,422 ) Maturity date extension and interest rate reduction 2 1,800 1,805 5 — — — — Principal deferral 2 113 113 — 5 985 986 1 Commercial construction Maturity date extension — — — — 3 1,251 1,151 (100 ) Principal forgiveness 2 1,996 1,996 — — — — — Residential mortgage Chapter 7 bankruptcy (2) 1 33 31 (2 ) 7 439 413 (26 ) Maturity date extension — — — — 1 483 414 (69 ) Maturity date extension and interest rate reduction — — — — 1 280 279 (1 ) Principal deferral — — — — 1 3,273 3,133 (140 ) Home equity Chapter 7 bankruptcy (2) 21 689 643 (46 ) 19 676 643 (33 ) Maturity date extension 1 231 231 — 5 305 298 (7 ) Maturity date extension and interest rate reduction 1 173 113 (60 ) 2 604 598 (6 ) Principal deferral — — — — 1 47 45 (2 ) Installment and other consumer Chapter 7 bankruptcy (2) 4 48 35 (13 ) 2 16 10 (6 ) Total by Concession Type Chapter 7 bankruptcy (2) 26 $ 770 $ 709 $ (61 ) 29 $ 1,840 $ 1,712 $ (128 ) Interest rate reduction — — — — 1 250 242 (8 ) Maturity date extension 3 905 1,406 501 13 6,795 5,197 (1,598 ) Maturity date extension and interest rate reduction 3 1,973 1,918 (55 ) 3 884 877 (7 ) Principal deferral 2 113 113 — 7 4,305 4,164 (141 ) Principal forgiveness 2 1,996 1,996 — — — — — Total 36 $ 5,757 $ 6,142 $ 385 53 $ 14,074 $ 12,192 $ (1,882 ) (1) Excludes loans that were fully paid off or fully charged-off by period end. The pre-modification balance represents the balance outstanding prior to modification. The post-modification balance represents the outstanding balance at period end. (2) Chapter 7 bankruptcy loans where the debt has been legally discharged through the bankruptcy court and not reaffirmed. The following table summarizes our restructured loans as of the dates presented: December 31, 2017 December 31, 2016 (dollars in thousands) Performing TDRs Nonperforming TDRs Total TDRs Performing TDRs Nonperforming TDRs Total TDRs Commercial real estate $ 2,579 $ 967 $ 3,546 $ 2,994 $ 646 $ 3,640 Commercial and industrial 3,946 3,197 7,143 1,387 4,493 5,880 Commercial construction 2,420 2,413 4,833 2,966 430 3,396 Residential mortgage 2,039 3,585 5,624 2,375 5,068 7,443 Home equity 3,885 979 4,864 3,683 954 4,637 Installment and other consumer 32 9 41 18 7 25 Total $ 14,901 $ 11,150 $ 26,051 $ 13,423 $ 11,598 $ 25,021
Schedule of Nonperforming Assets	The following table is a summary of nonperforming assets as of the dates presented: December 31, (dollars in thousands) 2017 2016 Nonperforming Assets Nonaccrual loans $ 12,788 $ 31,037 Nonaccrual TDRs 11,150 11,598 Total nonaccrual loans 23,938 42,635 OREO 469 679 Total Nonperforming Assets $ 24,407 $ 43,314
Summary of Aggregate Amount of Loans to Officers and Directors	The following table presents a summary of the aggregate amount of loans to certain officers, directors of S&T or any affiliates of such persons as of December 31: (dollars in thousands) 2017 2016 Balance at beginning of year $ 25,167 $ 24,517 New loans 25,203 22,740 Repayments or no longer considered a related party (40,300 ) (22,090 ) $ 10,070 $ 25,167

Allowance for Loan Losses (Tabl

Allowance for Loan Losses (Tables)	12 Months Ended
	Dec. 31, 2017
Receivables [Abstract]
Age Analysis of Past Due Loans Segregated by Class of Loans	The following tables present the age analysis of past due loans segregated by class of loans as of the dates presented: December 31, 2017 (dollars in thousands) Current 30-59 Days Past Due 60-89 Days Past Due Non- performing Total Past Due Loans Total Loans Commercial real estate $ 2,681,395 $ 997 $ 134 $ 3,468 $ 4,599 $ 2,685,994 Commercial and industrial 1,426,754 420 446 5,646 6,512 1,433,266 Commercial construction 377,968 2,473 20 3,873 6,366 384,334 Residential mortgage 687,195 2,975 1,439 7,165 11,579 698,774 Home equity 480,956 2,065 590 3,715 6,370 487,326 Installment and other consumer 66,770 193 170 71 434 67,204 Consumer construction 4,551 — — — — 4,551 Loans held for sale 4,485 — — — — 4,485 Total $ 5,730,074 $ 9,123 $ 2,799 $ 23,938 $ 35,860 $ 5,765,934 December 31, 2016 (dollars in thousands) Current 30-59 Days Past Due 60-89 Days Past Due Non- performing Total Past Due Loans Total Loans Commercial real estate $ 2,479,513 $ 2,032 $ 759 $ 16,172 $ 18,963 $ 2,498,476 Commercial and industrial 1,391,475 1,061 428 8,071 9,560 1,401,035 Commercial construction 450,410 547 — 4,927 5,474 455,884 Residential mortgage 689,635 1,312 1,117 9,918 12,347 701,982 Home equity 476,866 1,470 509 3,439 5,418 482,284 Installment and other consumer 65,525 176 43 108 327 65,852 Consumer construction 5,906 — — — — 5,906 Loans held for sale 3,793 — — — — 3,793 Total $ 5,563,123 $ 6,598 $ 2,856 $ 42,635 $ 52,089 $ 5,615,212
Recorded Investment in Commercial Loan Classes by Internally Assigned Risk Ratings	The following tables present the recorded investment in commercial loan classes by internally assigned risk ratings as of the dates presented: December 31, 2017 (dollars in thousands) Commercial Real Estate % of Total Commercial and Industrial % of Total Commercial Construction % of Total Total % of Total Pass $ 2,588,847 96.4 % $ 1,345,810 93.9 % $ 368,105 95.8 % $ 4,302,762 95.5 % Special mention 66,436 2.5 % 54,320 3.8 % 9,345 2.4 % 130,101 2.9 % Substandard 30,711 1.1 % 33,136 2.3 % 6,884 1.8 % 70,731 1.6 % Total $ 2,685,994 100.0 % $ 1,433,266 100.0 % $ 384,334 100.0 % $ 4,503,594 100.0 % December 31, 2016 (dollars in thousands) Commercial Real Estate % of Total Commercial and Industrial % of Total Commercial Construction % of Total Total % of Total Pass $ 2,423,742 97.0 % $ 1,315,507 93.9 % $ 430,472 94.4 % $ 4,169,721 95.7 % Special mention 33,098 1.3 % 40,409 2.9 % 14,691 3.2 % 88,198 2.0 % Substandard 41,636 1.7 % 45,119 3.2 % 10,721 2.4 % 97,476 2.3 % Total $ 2,498,476 100.0 % $ 1,401,035 100.0 % $ 455,884 100.0 % $ 4,355,395 100.0 %
Recorded Investment in Consumer Loan Classes by Performing and Nonperforming Status	The following tables present the recorded investment in consumer loan classes by performing and nonperforming status as of the dates presented: December 31, 2017 (dollars in thousands) Residential Mortgage % of Total Home Equity % of Total Installment and other consumer % of Total Consumer Construction % of Total Total % of Total Performing $ 691,609 99.0 % $ 483,611 99.2 % $ 67,133 99.9 % $ 4,551 100.0 % $ 1,246,904 99.1 % Nonperforming 7,165 1.0 % 3,715 0.8 % 71 0.1 % — — % 10,951 0.9 % Total $ 698,774 100.0 % $ 487,326 100.0 % $ 67,204 100.0 % $ 4,551 100.0 % $ 1,257,855 100.0 % December 31, 2016 (dollars in thousands) Residential Mortgage % of Total Home Equity % of Total Installment and other consumer % of Total Consumer Construction % of Total Total % of Total Performing $ 692,064 98.6 % $ 478,845 99.3 % $ 65,744 99.8 % $ 5,906 100.0 % $ 1,242,559 98.9 % Nonperforming 9,918 1.4 % 3,439 0.7 % 108 0.2 % — — % 13,465 1.1 % Total $ 701,982 100.0 % $ 482,284 100.0 % $ 65,852 100.0 % $ 5,906 100.0 % $ 1,256,024 100.0 %
Investments in Loans Considered to be Impaired and Related Information on Impaired Loans	The following table summarizes investments in loans considered to be impaired and related information on those impaired loans for the years presented: For the Year Ended December 31, 2017 December 31, 2016 (dollars in thousands) Average Recorded Investment Interest Income Recognized Average Recorded Investment Interest Income Recognized With a related allowance recorded: Commercial real estate $ — $ — $ — $ — Commercial and industrial 968 52 2,438 — Commercial construction — — — — Consumer real estate 23 2 28 2 Other consumer 34 2 2 — Total with a Related Allowance Recorded 1,025 56 2,468 2 Without a related allowance recorded: Commercial real estate 6,636 177 17,496 144 Commercial and industrial 9,897 257 6,141 160 Commercial construction 6,828 253 7,723 162 Consumer real estate 11,037 487 11,939 523 Other consumer 23 — 35 1 Total without a Related Allowance Recorded 34,421 1,174 43,334 990 Total: Commercial real estate 6,636 177 17,496 144 Commercial and industrial 10,865 309 8,579 160 Commercial construction 6,828 253 7,723 162 Consumer real estate 11,060 489 11,967 525 Other consumer 57 2 37 1 Total $ 35,446 $ 1,230 $ 45,802 $ 992 The following tables summarize investments in loans considered to be impaired and related information on those impaired loans as of the dates presented: December 31, 2017 December 31, 2016 (dollars in thousands) Recorded Investment Unpaid Principal Balance Related Allowance Recorded Investment Unpaid Principal Balance Related Allowance With a related allowance recorded: Commercial real estate $ — $ — $ — $ — $ — $ — Commercial and industrial 1,735 1,787 29 964 2,433 771 Commercial construction — — — — — — Consumer real estate 21 21 21 26 26 26 Other consumer 27 27 27 1 1 1 Total with a Related Allowance Recorded 1,783 1,835 77 991 2,460 798 Without a related allowance recorded: Commercial real estate 3,546 3,811 — 16,352 17,654 — Commercial and industrial 5,549 7,980 — 5,902 7,699 — Commercial construction 5,464 8,132 — 6,613 10,306 — Consumer real estate 10,467 11,357 — 12,053 12,849 — Other consumer 14 22 — 24 31 — Total without a Related Allowance Recorded 25,040 31,302 — 40,944 48,539 — Total: Commercial real estate 3,546 3,811 — 16,352 17,654 — Commercial and industrial 7,284 9,767 29 6,866 10,132 771 Commercial construction 5,464 8,132 — 6,613 10,306 — Consumer real estate 10,488 11,378 21 12,079 12,875 26 Other consumer 41 49 27 25 32 1 Total $ 26,823 $ 33,137 $ 77 $ 41,935 $ 50,999 $ 798
Summary of Allowance for Loan Losses	The following tables detail activity in the ALL for the periods presented: 2017 (dollars in thousands) Commercial Real Estate Commercial and Industrial Commercial Construction Consumer Real Estate Other Consumer Total Loans Balance at beginning of year $ 19,976 $ 10,810 $ 13,999 $ 6,095 $ 1,895 $ 52,775 Charge-offs (2,304 ) (4,709 ) (2,571 ) (2,274 ) (1,638 ) (13,496 ) Recoveries 810 654 851 342 571 3,228 Net (Charge-offs) Recoveries (1,494 ) (4,055 ) (1,720 ) (1,932 ) (1,067 ) (10,268 ) Provision for loan losses 8,753 2,211 888 1,316 715 13,883 Balance at End of Year $ 27,235 $ 8,966 $ 13,167 $ 5,479 $ 1,543 $ 56,390 2016 (dollars in thousands) Commercial Real Estate Commercial and Industrial Commercial Construction Consumer Real Estate Other Consumer Total Loans Balance at beginning of year $ 15,043 $ 10,853 $ 12,625 $ 8,400 $ 1,226 $ 48,147 Charge-offs (3,114 ) (6,810 ) (1,877 ) (1,657 ) (2,103 ) (15,561 ) Recoveries 692 722 21 433 356 2,224 Net Recoveries (Charge-offs) (2,422 ) (6,088 ) (1,856 ) (1,224 ) (1,747 ) (13,337 ) Provision for loan losses 7,355 6,045 3,230 (1,081 ) 2,416 17,965 Balance at End of Year $ 19,976 $ 10,810 $ 13,999 $ 6,095 $ 1,895 $ 52,775
Summary of Allowance for Loan Losses and Recorded Investments	The following tables present the ALL and recorded investments in loans by category as of December 31: 2017 Allowance for Loan Losses Portfolio Loans (dollars in thousands) Individually Evaluated for Impairment Collectively Evaluated for Impairment Total Individually Evaluated for Impairment Collectively Evaluated for Impairment Total Commercial real estate $ — $ 27,235 $ 27,235 $ 3,546 $ 2,682,448 $ 2,685,994 Commercial and industrial 29 8,937 8,966 7,284 1,425,982 1,433,266 Commercial construction — 13,167 13,167 5,464 378,870 384,334 Consumer real estate 21 5,458 5,479 10,488 1,180,163 1,190,651 Other consumer 27 1,516 1,543 41 67,163 67,204 Total $ 77 $ 56,313 $ 56,390 $ 26,823 $ 5,734,626 $ 5,761,449 2016 Allowance for Loan Losses Portfolio Loans (dollars in thousands) Individually Evaluated for Impairment Collectively Evaluated for Impairment Total Individually Evaluated for Impairment Collectively Evaluated for Impairment Total Commercial real estate $ — $ 19,976 $ 19,976 $ 16,352 $ 2,482,124 $ 2,498,476 Commercial and industrial 771 10,039 10,810 6,866 1,394,169 1,401,035 Commercial construction — 13,999 13,999 6,613 449,271 455,884 Consumer real estate 26 6,069 6,095 12,079 1,178,093 1,190,172 Other consumer 1 1,894 1,895 25 65,827 65,852 Total $ 798 $ 51,977 $ 52,775 $ 41,935 $ 5,569,484 $ 5,611,419

Premises and Equipment (Tables)

Premises and Equipment (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Summary of Premises and Equipment	The following table is a summary of premises and equipment as of the dates presented: December 31, (dollars in thousands) 2017 2016 Land $ 6,266 $ 6,397 Premises 51,799 52,696 Furniture and equipment 34,836 32,328 Leasehold improvements 6,643 7,293 99,544 98,714 Accumulated depreciation (56,842 ) (53,715 ) Total $ 42,702 $ 44,999
Minimum Annual Rental and Renewal Option Payments	Minimum annual rental and renewal option payments for each of the following five years and thereafter are approximately: (dollars in thousands) Operating Capital Total 2018 $ 3,257 $ 76 $ 3,333 2019 3,277 77 3,354 2020 3,312 77 3,389 2021 3,351 76 3,427 2022 3,425 77 3,502 Thereafter 55,000 457 55,457 Total $ 71,622 $ 840 $ 72,462

Goodwill and Other Intangible42

Goodwill and Other Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	The following table presents goodwill as of the dates presented: December 31, (dollars in thousands) 2017 2016 Balance at beginning of year $ 291,670 $ 291,764 Additions — — Other adjustments — $ (94 ) Balance at End of Year $ 291,670 $ 291,670
Summary of Intangible Assets	The following table shows a summary of intangible assets as of the dates presented: December 31, (dollars in thousands) 2017 2016 Gross carrying amount at beginning of year $ 22,114 $ 22,114 Additions — — Accumulated amortization (18,437 ) (17,204 ) Balance at End of Year $ 3,677 $ 4,910
Summary of Expected Amortization Expense for Finite-Lived Intangibles Assets	The following is a summary of the expected amortization expense for finite-lived intangible assets, assuming no new additions, for each of the five years following December 31, 2017 and thereafter: (dollars in thousands) Amount 2018 $ 1,013 2019 655 2020 554 2021 477 2022 359 Thereafter 619 Total $ 3,677

Derivative Instruments and He43

Derivative Instruments and Hedging Activities (Tables)	12 Months Ended
	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Value of Derivative Assets and Derivative Liabilities	The following table indicates the amounts representing the value of derivative assets and derivative liabilities at December 31: Derivatives (included in Other Assets) Derivatives (included in Other Liabilities) (dollars in thousands) 2017 2016 2017 2016 Derivatives not Designated as Hedging Instruments Interest Rate Swap Contracts—Commercial Loans Fair value $ 3,074 $ 6,960 $ 3,055 $ 6,958 Notional amount 263,841 282,930 263,841 282,930 Collateral posted — — 1,448 14,340 Interest Rate Lock Commitments—Mortgage Loans Fair value 226 236 — — Notional amount 6,860 8,490 — — Forward Sale Contracts—Mortgage Loans Fair value — — 5 27 Notional amount — — 6,580 8,216
Schedule of Gross Amounts of Derivative Assets	The following table indicates the gross amounts of commercial loan swap derivative assets and derivative liabilities, the amounts offset and the carrying values in the Consolidated Balance Sheets at December 31: Derivatives (included in Other Assets) Derivatives (included in Other Liabilities) (dollars in thousands) 2017 2016 2017 2016 Derivatives not Designated as Hedging Instruments Gross amounts recognized $ 4,974 $ 8,590 $ 4,955 $ 8,588 Gross amounts offset (1,900 ) (1,630 ) (1,900 ) (1,630 ) Net amounts presented in the Consolidated Balance Sheets 3,074 6,960 3,055 6,958 Gross amounts not offset (1) — — (1,448 ) (14,340 ) Net Amount $ 3,074 $ 6,960 $ 1,607 $ (7,382 ) (1) Amounts represent posted collateral.
Schedule of Gross Amounts of Derivative Liabilities	The following table indicates the gross amounts of commercial loan swap derivative assets and derivative liabilities, the amounts offset and the carrying values in the Consolidated Balance Sheets at December 31: Derivatives (included in Other Assets) Derivatives (included in Other Liabilities) (dollars in thousands) 2017 2016 2017 2016 Derivatives not Designated as Hedging Instruments Gross amounts recognized $ 4,974 $ 8,590 $ 4,955 $ 8,588 Gross amounts offset (1,900 ) (1,630 ) (1,900 ) (1,630 ) Net amounts presented in the Consolidated Balance Sheets 3,074 6,960 3,055 6,958 Gross amounts not offset (1) — — (1,448 ) (14,340 ) Net Amount $ 3,074 $ 6,960 $ 1,607 $ (7,382 ) (1) Amounts represent posted collateral.
Amount of Gain or Loss Recognized in Income on Derivatives	The following table indicates the gain or loss recognized in income on derivatives for the years ended December 31: (dollars in thousands) 2017 2016 2015 Derivatives not Designated as Hedging Instruments Interest rate swap contracts—commercial loans $ 17 $ (16 ) $ (8 ) Interest rate lock commitments—mortgage loans (11 ) (25 ) 26 Forward sale contracts—mortgage loans 52 (22 ) 52 Total Derivative (Loss) Gain $ 58 $ (63 ) $ 70

Mortgage Servicing Rights (Tabl

Mortgage Servicing Rights (Tables)	12 Months Ended
	Dec. 31, 2017
Transfers and Servicing of Financial Assets [Abstract]
Schedule of Mortgage Servicing Rights at Net Carrying Value	The following table indicates MSRs and the net carrying values: (dollars in thousands) Servicing Rights Valuation Allowance Net Carrying Value Balance at December 31, 2015 $ 3,426 $ (189 ) $ 3,237 Additions 1,047 — 1,047 Amortization (615 ) — (615 ) Temporary recapture (impairment) — 75 75 Balance at December 31, 2016 $ 3,858 $ (114 ) $ 3,744 Additions 918 — 918 Amortization (584 ) — (584 ) Temporary recapture (impairment) — 55 55 Balance at December 31, 2017 $ 4,192 $ (59 ) $ 4,133

Deposits (Tables)

Deposits (Tables)	12 Months Ended
	Dec. 31, 2017
Banking and Thrift [Abstract]
Composition of Deposits and Interest Expenses	The following table presents the composition of deposits at December 31 and interest expense for the years ended December 31: 2017 2016 2015 (dollars in thousands) Balance Interest Expense Balance Interest Expense Balance Interest Expense Noninterest-bearing demand $ 1,387,712 $ — $ 1,263,833 $ — $ 1,227,766 $ — Interest-bearing demand 603,141 67 638,300 111 616,188 818 Money market 1,146,156 9,204 936,461 4,199 605,184 1,299 Savings 893,119 2,081 1,050,131 2,002 1,061,265 1,712 Certificates of deposit 1,397,763 13,978 1,383,652 13,380 1,366,208 9,115 Total $ 5,427,891 $ 25,330 $ 5,272,377 $ 19,692 $ 4,876,611 $ 12,944
Scheduled Maturities of Certificates of Deposit	The following table indicates the scheduled maturities of certificates of deposit at December 31, 2017: (dollars in thousands) Amount 2018 $ 1,015,515 2019 233,932 2020 43,461 2021 65,563 2022 33,172 Thereafter 6,120 Total $ 1,397,763

Short-Term Borrowings (Tables)

Short-Term Borrowings (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Composition of Short-Term Borrowings, Interest Expense and Weighted Average Interest Rate	The following table represents the composition of short-term borrowings, the weighted average interest rate as of December 31 and interest expense for the years ended December 31: 2017 2016 2015 (dollars in thousands) Balance Weighted Average Interest Rate Interest Expense Balance Weighted Average Interest Rate Interest Expense Balance Weighted Average Interest Rate Interest Expense REPOs $ 50,161 0.39 % $ 54 $ 50,832 0.01 % $ 5 $ 62,086 0.01 % $ 4 FHLB advances 540,000 1.47 % 7,399 660,000 0.76 % 2,713 356,000 0.52 % 932 Total Short-term Borrowings $ 590,161 1.38 % $ 7,453 $ 710,832 0.70 % $ 2,718 $ 418,086 0.44 % $ 936

Long-Term Borrowings and Subo47

Long-Term Borrowings and Subordinated Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of Long-Term Borrowings, Interest Expense and Weighted Average Interest Rate	The following table represents the balance of long-term borrowings, the weighted average interest rate as of December 31 and interest expense for the years ended December 31: (dollars in thousand) 2017 2016 2015 Long-term borrowings $ 47,301 $ 14,713 $ 117,043 Weighted average interest rate 1.88 % 2.91 % 0.81 % Interest expense $ 463 $ 670 $ 790
Schedule of Annual Maturities and Average Interest Rate of Long-Term Debt	Scheduled annual maturities and average interest rates for all of our long-term debt for each of the five years subsequent to December 31, 2017 and thereafter are as follows: (dollars in thousands) Balance Average Rate 2018 $ 2,496 3.60 % 2019 37,514 1.60 % 2020 2,004 3.22 % 2021 1,057 3.44 % 2022 529 4.50 % Thereafter 3,701 1.96 % Total $ 47,301 1.88 %
Schedule of Junior Subordinated Debt Securities and Interest Expense	The following table represents the composition of junior subordinated debt securities at December 31 and the interest expense for the years ended December 31: 2017 2016 2015 (dollars in thousands) Balance Interest Expense Balance Interest Expense Balance Interest Expense 2006 Junior subordinated debt $ 25,000 $ 708 $ 25,000 $ 580 $ 25,000 $ 554 2008 Junior subordinated debt—trust preferred securities 20,619 955 20,619 854 20,619 773 Total $ 45,619 $ 1,663 $ 45,619 $ 1,434 $ 45,619 $ 1,327
Schedule of Junior Subordinated Debt Securities	The following table summarizes the key terms of our junior subordinated debt securities: (dollars in thousands) 2006 Junior Subordinated Debt 2008 Trust Preferred Securities Junior Subordinated Debt $25,000 — Trust Preferred Securities — $20,619 Stated Maturity Date 12/15/2036 3/15/2038 Optional redemption date at par Any time after 9/15/2011 Any time after 3/15/2013 Regulatory Capital Tier 2 Tier 1 Interest Rate 3 month LIBOR plus 160 bps 3 month LIBOR plus 350 bps Interest Rate at December 31, 2017 2.92% 4.82%

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Letters of Credit	The following table sets forth our commitments and letters of credit as of the dates presented: December 31, (dollars in thousands) 2017 2016 Commitments to extend credit $ 1,420,428 $ 1,509,696 Standby letters of credit 80,918 84,534 Total $ 1,501,346 $ 1,594,230
Future Estimated Payments Related to Data Processing and Communication Charges	The following table sets forth the future estimated payments related to data processing and communication charges for each of the five years following December 31, 2017: (dollars in thousands) Total 2018 $ 12,237 2019 12,633 2020 13,047 2021 13,465 2022 13,926 Total $ 65,308

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of Income Tax Expense (Benefit)	Income tax expense (benefit) for the years ended December 31 is comprised of: (dollars in thousands) 2017 2016 2015 Federal Current $ 32,282 $ 24,521 $ 24,825 Deferred 13,980 665 (427 ) Total Federal 46,262 25,186 24,398 State Current 323 248 — Deferred (148 ) (129 ) — Total State 175 119 — Total Federal and State $ 46,437 $ 25,305 $ 24,398
Schedule of Statutory to Effective Tax Rate Reconciliation	The statutory to effective tax rate reconciliation for the years ended December 31 is as follows: 2017 2016 2015 Statutory tax rate 35.0 % 35.0 % 35.0 % Low income housing tax credits (2.9 )% (3.8 )% (4.4 )% Tax-exempt interest (4.0 )% (4.4 )% (4.1 )% Bank owned life insurance (0.8 )% (0.8 )% (0.8 )% Other 0.3 % 0.2 % 1.0 % Adjustment to net deferred tax assets for enacted changes in tax laws and rates 11.3 % — % — % Effective Tax Rate 38.9 % 26.2 % 26.7 %
Schedule of Significant Components of Temporary Differences	Significant components of our temporary differences were as follows at December 31: (dollars in thousands) 2017 2016 Deferred Tax Assets: Allowance for loan losses 12,440 19,446 Purchase accounting adjustments — 365 Other employee benefits 3,095 3,983 Low income housing partnerships 3,213 4,845 Net adjustment to funded status of pension 6,481 10,018 Impairment of securities 300 1,318 State net operating loss carryforwards 3,598 3,114 Other 2,355 4,984 Gross Deferred Tax Assets 31,482 48,073 Less: Valuation allowance (3,598 ) (3,114 ) Total Deferred Tax Assets 27,884 44,959 Deferred Tax Liabilities: Net unrealized holding gains on securities available-for-sale $ (638 ) $ (2,557 ) Prepaid pension (1,749 ) (2,770 ) Deferred loan income (2,937 ) (3,815 ) Purchase accounting adjustments (100 ) — Depreciation on premises and equipment (480 ) (1,239 ) Other (1,401 ) (1,766 ) Total Deferred Tax liabilities (7,305 ) (12,147 ) Net Deferred Tax Asset $ 20,579 $ 32,812
Schedule of Reconciliation of Change in Federal and State Gross Unrecognized Tax Benefits	The following table reconciles the change in Federal and State gross unrecognized tax benefits, or UTB, for the years ended December 31: (dollars in thousands) 2017 2016 2015 Balance at beginning of year $ 804 $ 1,102 $ 284 Prior period tax positions Increase — — 818 Decrease (37 ) (449 ) — Current period tax positions 142 151 — Reductions for statute of limitations expirations — — — Balance at End of Year $ 909 $ 804 $ 1,102 Amount That Would Impact the Effective Tax Rate if Recognized $ 770 $ 610 $ 542

Tax Effects on Other Comprehe50

Tax Effects on Other Comprehensive Income (Loss) (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Schedule of Tax Effects of Components of Other Comprehensive Income (Loss)	The following tables present the tax effects of the components of other comprehensive income (loss) for the years ended December 31: (dollars in thousands) Pre-Tax Amount Tax (Expense) Benefit Net of Tax Amount 2017 Net change in unrealized gains on securities available-for-sale $ (1,275 ) $ 448 $ (827 ) Net available-for-sale securities losses (gains) reclassified into earnings (3,000 ) 1,054 (1,946 ) Adjustment to funded status of employee benefit plans (1,992 ) 122 (1,870 ) Other Comprehensive Income (Loss) $ (6,267 ) $ 1,624 $ (4,643 ) 2016 Net change in unrealized gains on securities available-for-sale $ (2,899 ) $ 1,006 $ (1,893 ) Net available-for-sale securities losses reclassified into earnings — — — Adjustment to funded status of employee benefit plans 6,974 (2,408 ) 4,566 Other Comprehensive Income (Loss) $ 4,075 $ (1,402 ) $ 2,673 2015 Net change in unrealized gains on securities available-for-sale $ (663 ) $ 232 $ (431 ) Net available-for-sale securities gains reclassified into earnings 34 (12 ) 22 Adjustment to funded status of employee benefit plans (3,551 ) 1,336 (2,215 ) Other Comprehensive Income (Loss) $ (4,180 ) $ 1,556 $ (2,624 )

Employee Benefits (Tables)

Employee Benefits (Tables)	12 Months Ended
	Dec. 31, 2017
Retirement Benefits [Abstract]
Benefit Obligation and Plan Assets Deriving Funded Status, in Other Liabilities	The following table summarizes the activity in the benefit obligation and Plan assets deriving the funded status, which is recorded in other liabilities in the Consolidated Balance Sheets: (dollars in thousands) 2017 2016 Change in Projected Benefit Obligation Projected benefit obligation at beginning of year $ 105,834 $ 109,747 Service cost — 463 Interest cost 4,100 4,296 Actuarial loss 4,974 3,575 Curtailments — (6,997 ) Benefits paid (8,244 ) (5,250 ) Projected Benefit Obligation at End of Year $ 106,664 $ 105,834 Change in Plan Assets Fair value of plan assets at beginning of year $ 87,711 $ 84,585 Actual return on plan assets 7,687 8,376 Benefits paid (8,244 ) (5,250 ) Fair Value of Plan Assets at End of Year $ 87,154 $ 87,711 Funded Status $ (19,510 ) $ (18,123 )
Accumulated Other Comprehensive Income (Loss)	The following table sets forth the amounts recognized in accumulated other comprehensive (loss) income at December 31: (dollars in thousands) 2017 2016 Prior service credit $ — $ — Net actuarial loss (27,825 ) (26,013 ) Total (Before Tax Effects) $ (27,825 ) $ (26,013 )
Actuarial Weighted Average Assumptions Used in Determining Benefit Obligation	Below are the actuarial weighted average assumptions used in determining the benefit obligation: 2017 2016 Discount rate 3.75 % 4.00 % Rate of compensation increase (1) — % — % (1) Rate of compensation increase is not applicable for 2017 and 2016 due to the amendment to freeze benefit accruals under the qualified and nonqualified defined benefit pension plans effective March 31, 2016.
Components of Net Periodic Pension Cost and Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income (Loss)	The following table summarizes the components of net periodic pension cost and other changes in Plan assets and benefit obligations recognized in other comprehensive income (loss) for the years ended December 31: (dollars in thousands) 2017 2016 2015 Components of Net Periodic Pension Cost Service cost—benefits earned during the period $ — $ 463 $ 2,601 Interest cost on projected benefit obligation 4,100 4,296 4,425 Expected return on plan assets (6,313 ) (5,780 ) (7,180 ) Amortization of prior service credit — (11 ) (138 ) Recognized net actuarial loss 1,866 2,345 2,028 Curtailment gain — (1,017 ) — Net Periodic Pension Expense $ (347 ) $ 296 $ 1,736 Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income (Loss) Net actuarial (gain) loss $ 3,678 $ (6,018 ) $ 5,678 Recognized net actuarial loss (1,866 ) (2,345 ) (2,028 ) Recognized prior service credit — 1,029 138 Total (Before Tax Effects) $ 1,812 $ (7,334 ) $ 3,788 Total Recognized in Net Benefit Cost and Other Comprehensive (Loss)/Income (Before Tax Effects) $ 1,465 $ (7,038 ) $ 5,524
Actuarial Weighted Average Assumptions Used in Determining Net Periodic Pension Cost	The following table summarizes the actuarial weighted average assumptions used in determining net periodic pension cost: 2017 2016 2015 Discount rate 4.00 % 4.25 % 4.00 % Rate of compensation increase (1) — % 3.00 % 3.00 % Expected return on assets 7.50 % 7.50 % 8.00 % 1) Rate of compensation increase is not applicable for 2017 and 2016 due to the amendment to freeze benefit accruals under the qualified and nonqualified defined benefit pension plans effective March 31, 2016.
Estimated Future Benefit Payments	The following table provides information regarding estimated future benefit payments to be paid in each of the next five years and in the aggregate for the five years thereafter: (dollars in thousands) Amount 2018 $ 7,175 2019 7,090 2020 6,956 2021 7,157 2022 7,106 2023 - 2027 32,658
Pension Plan Assets Measured at Fair Value on Recurring Basis	The following tables present our Plan assets measured at fair value on a recurring basis by fair value hierarchy level at December 31, 2017 and 2016. There were no transfers between Level 1 and Level 2 for items of a recurring basis during the periods presented. There were no purchases or transfers of Level 3 plan assets in 2017. December 31, 2017 Fair Value Asset Classes (1) (dollars in thousands) Level 1 Level 2 Level 3 Total Cash and cash equivalents (2) $ — $ 1,780 $ — $ 1,780 Fixed income (3) 27,738 — — 27,738 Equities: Equity index mutual funds—international (4) 4,016 — — 4,016 Domestic individual equities (5) 53,540 — — 53,540 Total Assets at Fair Value $ 85,294 $ 1,780 $ — $ 87,074 (1) Refer to Note 1 Summary of Significant Accounting Policies, Fair Value Measurements for a description of levels within the fair value hierarchy. (2) This asset class includes FDIC insured money market instruments. (3) This asset class includes a variety of fixed income mutual funds which primarily invest in investment grade rated securities. Investment managers have discretion to invest in fixed income related securities including futures, options and other derivatives. Investments may be made in currencies other than the U.S. dollar. (4) The sole investment within this asset class is the Harbor International Institutional Fund. (5) This asset class includes individual domestic equities invested in an active all-cap strategy. It may also include convertible bonds. December 31, 2016 Fair Value Asset Classes (1) (dollars in thousands) Level 1 Level 2 Level 3 Total Cash and cash equivalents (2) $ — $ 3,336 $ — $ 3,336 Fixed income (3) 27,279 — — 27,279 Equities: Equity index mutual funds—international (4) 3,362 — — 3,362 Domestic individual equities (5) 53,636 — — 53,636 Total Assets at Fair Value $ 84,277 $ 3,336 $ — $ 87,613 (1) Refer to Note 1 Summary of Significant Accounting Policies, Fair Value Measurements for a description of levels within the fair value hierarchy. (2) This asset class includes FDIC insured money market instruments. (3) This asset class includes a variety of fixed income mutual funds which primarily invest in investment grade rated securities. Investment managers have discretion to invest in fixed income related securities including futures, options and other derivatives. Investments may be made in currencies other than the U.S. dollar. (4) The sole investment within this asset class is MSCI EAFE Index iShares. (5) This asset class includes individual domestic equities invested in an active all-cap strategy. It may also include convertible bonds.

Incentive and Restricted Stoc52

Incentive and Restricted Stock Plan and Dividend Reinvestment Plan (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Non-Vested Restricted Stock	The following table provides information about restricted stock granted under the 2014 Stock Plan for the years ended December 31: Restricted Stock Weighted Average Grant Date Fair Value Non-vested at December 31, 2015 150,356 $ 26.34 Granted 110,643 25.58 Vested 32,164 25.03 Forfeited 3,335 26.04 Non-vested at December 31, 2016 225,500 $ 26.16 Granted 90,115 35.19 Vested 83,958 24.82 Forfeited 11,089 29.56 Non-vested at December 31, 2017 220,568 $ 30.19

Parent Company Condensed Fina53

Parent Company Condensed Financial Information (Tables)	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Balance Sheets of S&T Bancorp, Inc.	BALANCE SHEETS December 31, (dollars in thousands) 2017 2016 ASSETS Cash $ 21,310 $ 17,057 Investments in: Bank subsidiary 857,293 819,531 Nonbank subsidiaries 19,569 21,980 Other assets 7,272 4,694 Total Assets $ 905,444 $ 863,262 LIABILITIES Long-term debt $ 20,619 $ 20,619 Other liabilities 794 687 Total Liabilities 21,413 21,306 Total Shareholders’ Equity 884,031 841,956 Total Liabilities and Shareholders’ Equity $ 905,444 $ 863,262
Statements of Net Income of S&T Bancorp, Inc.	STATEMENTS OF NET INCOME Years ended December 31, (dollars in thousands) 2017 2016 2015 Dividends from subsidiaries $ 36,169 $ 34,134 $ 75,413 Investment income 22 17 19 Total Income 36,191 34,151 75,432 Interest expense on long-term debt 955 854 773 Other expenses 3,801 4,012 3,687 Total Expense 4,756 4,866 4,460 Income before income tax and undistributed net income of subsidiaries 31,435 29,285 70,972 Income tax benefit (1,596 ) (1,697 ) (1,549 ) Income before undistributed net income of subsidiaries 33,031 30,982 72,521 Equity in undistributed net income (distribution in excess of net income) of: Bank subsidiary 40,877 40,051 (5,064 ) Nonbank subsidiaries (940 ) 359 (376 ) Net Income $ 72,968 $ 71,392 $ 67,081
Statements of Cash Flows of S&T Bancorp, Inc.	STATEMENTS OF CASH FLOWS Years ended December 31, (dollars in thousands) 2017 2016 2015 OPERATING ACTIVITIES Net Income $ 72,968 $ 71,392 $ 67,081 Equity in undistributed (earnings) losses of subsidiaries (39,937 ) (40,410 ) 5,440 Tax benefit from stock-based compensation — (9 ) (53 ) Other 480 379 3,129 Net Cash Provided by Operating Activities 33,511 31,352 75,597 INVESTING ACTIVITIES Net investments in subsidiaries — — (38,404 ) Acquisitions — — (29,510 ) Net Cash Used in Investing Activities — — (67,914 ) FINANCING ACTIVITIES Repayment of junior subordinated debt — — (8,500 ) (Purchase) Sale of treasury shares, net (689 ) (115 ) (182 ) Cash dividends paid to common shareholders (28,569 ) (26,784 ) (24,487 ) Tax benefit from stock-based compensation — 9 53 Net Cash Used in Financing Activities (29,258 ) (26,890 ) (33,116 ) Net increase (decrease) in cash 4,253 4,462 (25,433 ) Cash at beginning of year 17,057 12,595 38,028 Cash at End of Year $ 21,310 $ 17,057 $ 12,595

Regulatory Matters (Tables)

Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2017
Banking and Thrift [Abstract]
Summary of Risk-Based Capital Amounts and Ratios	The following table summarizes risk-based capital amounts and ratios for S&T and S&T Bank: Actual Minimum Regulatory Capital Requirements To be Well Capitalized Under Prompt Corrective Action Provisions (dollars in thousands) Amount Ratio Amount Ratio Amount Ratio As of December 31, 2017 Leverage Ratio S&T $ 628,876 9.17 % $ 274,254 4.00 % $ 342,818 5.00 % S&T Bank 582,929 8.52 % 273,538 4.00 % 341,922 5.00 % Common Equity Tier 1 (to Risk-Weighted Assets) S&T 608,876 10.71 % 255,778 4.50 % 369,457 6.50 % S&T Bank 582,929 10.29 % 255,024 4.50 % 368,368 6.50 % Tier 1 Capital (to Risk-Weighted Assets) S&T 628,876 11.06 % 341,037 6.00 % 454,717 8.00 % S&T Bank 582,929 10.29 % 340,032 6.00 % 453,375 8.00 % Total Capital (to Risk-Weighted Assets) S&T 713,056 12.55 % 454,717 8.00 % 568,396 10.00 % S&T Bank 666,560 11.76 % 453,375 8.00 % 566,719 10.00 % As of December 31, 2016 Leverage Ratio S&T $ 582,155 8.98 % $ 259,170 4.00 % $ 323,963 5.00 % S&T Bank 542,048 8.39 % 258,460 4.00 % 323,075 5.00 % Common Equity Tier 1 (to Risk-Weighted Assets) S&T 562,155 10.04 % 252,079 4.50 % 364,114 6.50 % S&T Bank 542,048 9.71 % 251,213 4.50 % 362,864 6.50 % Tier 1 Capital (to Risk-Weighted Assets) S&T 582,155 10.39 % 336,105 6.00 % 448,140 8.00 % S&T Bank 542,048 9.71 % 334,951 6.00 % 446,601 8.00 % Total Capital (to Risk-Weighted Assets) S&T 664,184 11.86 % 448,140 8.00 % 560,175 10.00 % S&T Bank 622,469 11.15 % 446,602 8.00 % 558,252 10.00 %

Selected Financial Data (Tables

Selected Financial Data (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Summary of Selected Financial Data	The following table presents selected financial data for the most recent eight quarters. 2017 2016 (dollars in thousands, except per share data) (unaudited) Fourth Quarter Third Quarter Second Quarter First Quarter Fourth Quarter Third Quarter Second Quarter First Quarter SUMMARY OF OPERATIONS Interest income $ 67,855 $ 66,723 $ 64,914 $ 61,150 $ 59,096 $ 57,808 $ 55,850 $ 55,019 Interest expense 10,027 9,267 8,344 7,272 6,638 6,353 6,142 5,382 Provision for loan losses 982 2,850 4,869 5,183 5,586 2,516 4,848 5,014 Net Interest Income After Provision For Loan Losses 56,846 54,606 51,701 48,695 46,872 48,939 44,860 44,623 Security (losses) gains, net (986 ) — 3,617 370 — — — — Noninterest income 13,636 13,551 12,648 12,626 12,922 13,448 12,448 15,817 Noninterest expense 37,947 36,553 36,597 36,808 35,625 34,439 34,753 38,416 Income Before Taxes 31,549 31,604 31,369 24,883 24,169 27,948 22,555 22,024 Provision for income taxes 22,255 8,883 8,604 6,695 6,510 7,367 5,496 5,931 Net Income Available to Common Shareholders $ 9,294 $ 22,721 $ 22,765 $ 18,188 $ 17,659 $ 20,581 $ 17,059 $ 16,093 Per Share Data Common earnings per share—diluted $ 0.27 $ 0.65 $ 0.65 $ 0.52 $ 0.51 $ 0.59 $ 0.49 $ 0.46 Dividends declared per common share 0.22 0.20 0.20 0.20 0.20 0.19 0.19 0.19 Common book value 25.28 25.37 24.90 24.45 24.12 24.02 23.63 23.23

Summary of Significant Accoun56

Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)entityunitsubsidiary	Dec. 31, 2016USD ($)segment
Summary of Significant Accounting Policies [Line Items]
Number of wholly owned subsidiaries | subsidiary		3
Percentage of outstanding common stock of investees accounted for using equity method of accounting		100.00%
Number of entities non-banking activities | entity		5
Number of operating segments | segment			3
Prior period reclassification adjustment		$ 0
Period for satisfactory payment of TDRs		6 months
Evaluation for impairment of substandard and nonaccrual commercial loans		$ 500,000
Look-back period used for assumptions of allowance for loans and lease losses		8 years 6 months
Number of reporting units | unit		3
Goodwill		$ 291,670,000	$ 291,670,000
Percentage of facilities owned		55.00%
Contractual operating lease commitments		$ 71,622,000
Securities available-for-sale, at fair value		698,291,000	693,487,000
Equity Securities
Summary of Significant Accounting Policies [Line Items]
Securities available-for-sale, at fair value		$ 5,144,000	$ 11,249,000
Scenario, Forecast
Summary of Significant Accounting Policies [Line Items]
Provisional estimate for reclassification of AOCI to retain earnings for other income tax effects of Tax Act	$ 3,200,000
Interest rate lock commitments
Summary of Significant Accounting Policies [Line Items]
Period for interest rate lock commitment		60 days
Commercial real estate
Summary of Significant Accounting Policies [Line Items]
Loss emergence period for used for assumptions of allowance for loans and lease losses		3 years
Commercial construction
Summary of Significant Accounting Policies [Line Items]
Loss emergence period for used for assumptions of allowance for loans and lease losses		4 years
Commercial and industrial
Summary of Significant Accounting Policies [Line Items]
Loss emergence period for used for assumptions of allowance for loans and lease losses		1 year 3 months
Consumer real estate
Summary of Significant Accounting Policies [Line Items]
Loss emergence period for used for assumptions of allowance for loans and lease losses		2 years 9 months
Other consumer
Summary of Significant Accounting Policies [Line Items]
Loss emergence period for used for assumptions of allowance for loans and lease losses		1 year 3 months
Minimum | Core Deposits And Customers Lists
Summary of Significant Accounting Policies [Line Items]
Weighted average estimated useful of acquired intangibles		10 years
Maximum
Summary of Significant Accounting Policies [Line Items]
Amortization period of investments in joint ventures		10 years
Maximum | Core Deposits And Customers Lists
Summary of Significant Accounting Policies [Line Items]
Weighted average estimated useful of acquired intangibles		20 years
Corporate Joint Venture | Common Wealth Trust Life Insurance Company
Summary of Significant Accounting Policies [Line Items]
Percentage of outstanding common stock of investees accounted for using equity method of accounting		50.00%

Summary of Significant Accoun57

Summary of Significant Accounting Policies - Estimated Useful Lives for Various Asset (Details)	12 Months Ended
	Dec. 31, 2017
Buildings
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	25 years
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	5 years
Computer Equipment and Software
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	5 years
Other Equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	5 years
Vehicles
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	5 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	15 years

Earnings Per Share - Reconcilia

Earnings Per Share - Reconciliation of Numerators and Denominators of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator for Earnings per Common Share-Basic:
Net Income									$ 72,968	$ 71,392	$ 67,081
Less: Income allocated to participating shares									242	225	280
Net Income Allocated to Common Shareholders	$ 9,294	$ 22,721	$ 22,765	$ 18,188	$ 17,659	$ 20,581	$ 17,059	$ 16,093	72,726	71,167	66,801
Numerator for Earnings per Common Share—Diluted:
Net Income									$ 72,968	$ 71,392	$ 67,081
Denominators:
Weighted Average Common Shares Outstanding-Basic (in shares)									34,729,376	34,677,738	33,812,990
Add: Dilutive potential common shares (in shares)									225,391	95,432	35,092
Denominator for Treasury Stock Method-Diluted (in shares)									34,954,767	34,773,170	33,848,082
Weighted Average Common Shares Outstanding-Basic (in shares)									34,729,376	34,677,738	33,812,990
Add: Average participating shares outstanding (in shares)									115,418	109,755	141,558
Denominator for Two-Class Method-Diluted (in shares)									34,844,794	34,787,493	33,954,548
Earnings per common share—basic (in USD per share)									$ 2.10	$ 2.06	$ 1.98
Earnings per common share—diluted (in USD per share)	$ 0.27	$ 0.65	$ 0.65	$ 0.52	$ 0.51	$ 0.59	$ 0.49	$ 0.46	2.09	2.05	1.98
Exercise price of warrants (in USD per share)	$ 31.53				$ 31.53				$ 31.53	$ 31.53	$ 31.53
Warrants
Denominators:
Anti-dilutive excluded from dilutive potential common shares (in shares)									438,681	517,012	517,012
Restricted Stock
Denominators:
Anti-dilutive excluded from dilutive potential common shares (in shares)									88,578	116,749	106,466

Fair Value Measurements - Asset

Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016
ASSETS
Securities available-for-sale, at fair value	$ 698,291		$ 693,487
U.S. Treasury securities
ASSETS
Securities available-for-sale, at fair value	19,789		24,811
Obligations of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	162,193		232,179
Collateralized mortgage obligations of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	108,688		129,777
Residential mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	32,854		37,358
Commercial mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	242,221	[1]	125,604
Obligations of states and political subdivisions
ASSETS
Securities available-for-sale, at fair value	127,402		132,509
Marketable equity securities
ASSETS
Securities available-for-sale, at fair value	5,144		11,249
Fair Value Measurements, Recurring
ASSETS
Securities available-for-sale, at fair value	698,291		693,487
Trading securities held in a Rabbi Trust	5,080		4,410
Total securities	703,371		697,897
Total Assets	706,671		705,093
LIABILITIES
Total Liabilities	3,060		6,985
Fair Value Measurements, Recurring | Interest rate swaps
ASSETS
Derivative financial assets	3,074		6,960
LIABILITIES
Derivative financial liabilities	3,055		6,958
Fair Value Measurements, Recurring | Interest rate lock commitments
ASSETS
Derivative financial assets	226		236
Fair Value Measurements, Recurring | Forward sale contracts
LIABILITIES
Derivative financial liabilities	5		27
Fair Value Measurements, Recurring | U.S. Treasury securities
ASSETS
Securities available-for-sale, at fair value	19,789		24,811
Fair Value Measurements, Recurring | Obligations of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	162,193		232,179
Fair Value Measurements, Recurring | Collateralized mortgage obligations of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	108,688		129,777
Fair Value Measurements, Recurring | Residential mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	32,854		37,358
Fair Value Measurements, Recurring | Commercial mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	242,221		125,604
Fair Value Measurements, Recurring | Obligations of states and political subdivisions
ASSETS
Securities available-for-sale, at fair value	127,402		132,509
Fair Value Measurements, Recurring | Marketable equity securities
ASSETS
Securities available-for-sale, at fair value	5,144		11,249
Fair Value Measurements, Recurring | Level 1
ASSETS
Trading securities held in a Rabbi Trust	5,080		4,410
Total securities	5,080		4,410
Total Assets	5,080		4,410
Fair Value Measurements, Recurring | Level 1 | Interest rate swaps
ASSETS
Derivative financial assets	0		0
LIABILITIES
Derivative financial liabilities	0		0
Fair Value Measurements, Recurring | Level 1 | Interest rate lock commitments
ASSETS
Derivative financial assets	0		0
Fair Value Measurements, Recurring | Level 1 | Forward sale contracts
LIABILITIES
Derivative financial liabilities	0		0
Fair Value Measurements, Recurring | Level 1 | U.S. Treasury securities
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 1 | Obligations of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 1 | Collateralized mortgage obligations of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 1 | Residential mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 1 | Commercial mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 1 | Obligations of states and political subdivisions
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 1 | Marketable equity securities
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 2
ASSETS
Securities available-for-sale, at fair value	698,291		693,487
Trading securities held in a Rabbi Trust	0		0
Total securities	698,291		693,487
Total Assets	701,591		700,683
LIABILITIES
Total Liabilities	3,060		6,985
Fair Value Measurements, Recurring | Level 2 | Interest rate swaps
ASSETS
Derivative financial assets	3,074		6,960
LIABILITIES
Derivative financial liabilities	3,055		6,958
Fair Value Measurements, Recurring | Level 2 | Interest rate lock commitments
ASSETS
Derivative financial assets	226		236
Fair Value Measurements, Recurring | Level 2 | Forward sale contracts
LIABILITIES
Derivative financial liabilities	5		27
Fair Value Measurements, Recurring | Level 2 | U.S. Treasury securities
ASSETS
Securities available-for-sale, at fair value	19,789		24,811
Fair Value Measurements, Recurring | Level 2 | Obligations of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	162,193		232,179
Fair Value Measurements, Recurring | Level 2 | Collateralized mortgage obligations of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	108,688		129,777
Fair Value Measurements, Recurring | Level 2 | Residential mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	32,854		37,358
Fair Value Measurements, Recurring | Level 2 | Commercial mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	242,221		125,604
Fair Value Measurements, Recurring | Level 2 | Obligations of states and political subdivisions
ASSETS
Securities available-for-sale, at fair value	127,402		132,509
Fair Value Measurements, Recurring | Level 2 | Marketable equity securities
ASSETS
Securities available-for-sale, at fair value	5,144		11,249
Fair Value Measurements, Recurring | Level 3
ASSETS
Trading securities held in a Rabbi Trust	0		0
Fair Value Measurements, Recurring | Level 3 | Interest rate swaps
ASSETS
Derivative financial assets	0		0
LIABILITIES
Derivative financial liabilities	0		0
Fair Value Measurements, Recurring | Level 3 | Interest rate lock commitments
ASSETS
Derivative financial assets	0		0
Fair Value Measurements, Recurring | Level 3 | Forward sale contracts
LIABILITIES
Derivative financial liabilities	0		0
Fair Value Measurements, Recurring | Level 3 | U.S. Treasury securities
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 3 | Obligations of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 3 | Collateralized mortgage obligations of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 3 | Residential mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 3 | Commercial mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 3 | Obligations of states and political subdivisions
ASSETS
Securities available-for-sale, at fair value	0		0
Fair Value Measurements, Recurring | Level 3 | Marketable equity securities
ASSETS
Securities available-for-sale, at fair value	$ 0		$ 0
[1]	Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017.

Fair Value Measurements - Ass60

Fair Value Measurements - Assets Measured at Estimated Fair Value on Nonrecurring Basis by Fair Value Hierarchy (Details) - USD ($)		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on nonrecurring basis		$ 0	$ 0
Fair Value, Measurements, Nonrecurring
ASSETS
Loans held for sale	[1]	0	1,802,000
Impaired loans	[1]	6,759,000	10,329,000
Other real estate owned	[1]	444,000	396,000
Mortgage servicing rights	[1]	178,000	538,000
Total Assets	[1]	7,381,000	13,065,000
Fair Value, Measurements, Nonrecurring | Level 1
ASSETS
Loans held for sale	[1]	0	0
Impaired loans	[1]	0	0
Other real estate owned	[1]	0	0
Mortgage servicing rights	[1]	0	0
Total Assets	[1]	0	0
Fair Value, Measurements, Nonrecurring | Level 2
ASSETS
Loans held for sale	[1]	0	0
Impaired loans	[1]	0	0
Other real estate owned	[1]	0	0
Mortgage servicing rights	[1]	0	0
Total Assets	[1]	0	0
Fair Value, Measurements, Nonrecurring | Level 3
ASSETS
Loans held for sale	[1]	0	1,802,000
Impaired loans	[1]	6,759,000	10,329,000
Other real estate owned	[1]	444,000	396,000
Mortgage servicing rights	[1]	178,000	538,000
Total Assets	[1]	$ 7,381,000	$ 13,065,000
[1]	This table presents only the nonrecurring items that are recorded at fair value in our financial statements.

Fair Value Measurements - Carry

Fair Value Measurements - Carrying Values and Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Securities available-for-sale, at fair value		$ 698,291	$ 693,487
FHLB and other restricted stock		29,270	31,817
LIABILITIES
Junior subordinated debt securities		45,619	45,619	$ 45,619
Carrying Value
ASSETS
Cash and due from banks, including interest-bearing deposits	[1]	117,152	139,486
Securities available-for-sale, at fair value	[1]	698,291	693,487
Loans held for sale	[1]	4,485	3,793
Portfolio loans, net of unearned income	[1]	5,761,449	5,611,419
Bank owned life insurance	[1]	72,150	72,081
FHLB and other restricted stock	[1]	29,270	31,817
Trading securities held in a Rabbi Trust	[1]	5,080	4,410
Mortgage servicing rights	[1]	4,133	3,744
LIABILITIES
Deposits	[1]	5,427,891	5,272,377
Securities sold under repurchase agreements	[1]	50,161	50,832
Short-term borrowings	[1]	540,000	660,000
Long-term borrowings	[1]	47,301	14,713
Junior subordinated debt securities	[1]	45,619	45,619
Carrying Value | Interest rate swaps
ASSETS
Derivative financial assets	[1]	3,074	6,960
LIABILITIES
Derivative financial liabilities	[1]	3,055	6,958
Carrying Value | Interest rate lock commitments
ASSETS
Derivative financial assets	[1]	226	236
Carrying Value | Forward sale contracts
LIABILITIES
Derivative financial liabilities	[1]	5	27
Fair Value Measurements
ASSETS
Cash and due from banks, including interest-bearing deposits		117,152	139,486
Securities available-for-sale, at fair value		698,291	693,487
Loans held for sale		4,583	3,815
Portfolio loans, net of unearned income		5,690,292	5,551,266
Bank owned life insurance		72,150	72,081
FHLB and other restricted stock		29,270	31,817
Trading securities held in a Rabbi Trust		5,080	4,410
Mortgage servicing rights		4,571	4,098
LIABILITIES
Deposits		5,426,928	5,276,499
Securities sold under repurchase agreements		50,161	50,832
Short-term borrowings		540,000	660,000
Long-term borrowings		47,618	15,267
Junior subordinated debt securities		45,619	45,619
Fair Value Measurements | Interest rate swaps
ASSETS
Derivative financial assets		3,074	6,960
LIABILITIES
Derivative financial liabilities		3,055	6,958
Fair Value Measurements | Interest rate lock commitments
ASSETS
Derivative financial assets		226	236
Fair Value Measurements | Forward sale contracts
LIABILITIES
Derivative financial liabilities		5	27
Fair Value Measurements | Level 1
ASSETS
Cash and due from banks, including interest-bearing deposits		117,152	139,486
Securities available-for-sale, at fair value		0	0
Loans held for sale		0	0
Portfolio loans, net of unearned income		0	0
Bank owned life insurance		0	0
FHLB and other restricted stock		0	0
Trading securities held in a Rabbi Trust		5,080	4,410
Mortgage servicing rights		0	0
LIABILITIES
Deposits		0	0
Securities sold under repurchase agreements		0	0
Short-term borrowings		0	0
Long-term borrowings		0	0
Junior subordinated debt securities		0	0
Fair Value Measurements | Level 1 | Interest rate swaps
ASSETS
Derivative financial assets		0	0
LIABILITIES
Derivative financial liabilities		0	0
Fair Value Measurements | Level 1 | Interest rate lock commitments
ASSETS
Derivative financial assets		0	0
Fair Value Measurements | Level 1 | Forward sale contracts
LIABILITIES
Derivative financial liabilities		0	0
Fair Value Measurements | Level 2
ASSETS
Cash and due from banks, including interest-bearing deposits		0	0
Securities available-for-sale, at fair value		698,291	693,487
Loans held for sale		0	0
Portfolio loans, net of unearned income		0	0
Bank owned life insurance		72,150	72,081
FHLB and other restricted stock		0	0
Trading securities held in a Rabbi Trust		0	0
Mortgage servicing rights		0	0
LIABILITIES
Deposits		0	0
Securities sold under repurchase agreements		0	0
Short-term borrowings		0	0
Long-term borrowings		0	0
Junior subordinated debt securities		0	0
Fair Value Measurements | Level 2 | Interest rate swaps
ASSETS
Derivative financial assets		3,074	6,960
LIABILITIES
Derivative financial liabilities		3,055	6,958
Fair Value Measurements | Level 2 | Interest rate lock commitments
ASSETS
Derivative financial assets		226	236
Fair Value Measurements | Level 2 | Forward sale contracts
LIABILITIES
Derivative financial liabilities		5	27
Fair Value Measurements | Level 3
ASSETS
Cash and due from banks, including interest-bearing deposits		0	0
Securities available-for-sale, at fair value		0	0
Loans held for sale		4,583	3,815
Portfolio loans, net of unearned income		5,690,292	5,551,266
Bank owned life insurance		0	0
FHLB and other restricted stock		29,270	31,817
Trading securities held in a Rabbi Trust		0	0
Mortgage servicing rights		4,571	4,098
LIABILITIES
Deposits		5,426,928	5,276,499
Securities sold under repurchase agreements		50,161	50,832
Short-term borrowings		540,000	660,000
Long-term borrowings		47,618	15,267
Junior subordinated debt securities		45,619	45,619
Fair Value Measurements | Level 3 | Interest rate swaps
ASSETS
Derivative financial assets		0	0
LIABILITIES
Derivative financial liabilities		0	0
Fair Value Measurements | Level 3 | Interest rate lock commitments
ASSETS
Derivative financial assets		0	0
Fair Value Measurements | Level 3 | Forward sale contracts
LIABILITIES
Derivative financial liabilities		$ 0	$ 0
[1]	As reported in the Consolidated Balance Sheets

Restrictions on Cash and Due 62

Restrictions on Cash and Due from Bank Accounts - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Abstract]
Reserved cash maintained at Federal Reserve	$ 36.2	$ 36.8	$ 44.1

Dividend and Loan Restrictions

Dividend and Loan Restrictions (Details)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Percentage of collateralized loans	10.00%

Securities Available-for-Sale -

Securities Available-for-Sale - Composition of Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 695,284		$ 686,205
Gross Unrealized Gains	6,975		10,754
Gross Unrealized Losses	(3,968)		(3,472)
Fair Value	698,291		693,487
Security purchased pending settlement	5,884		0	$ 0
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,943		24,891
Gross Unrealized Gains	0		47
Gross Unrealized Losses	(154)		(127)
Fair Value	19,789		24,811
Obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	162,045		230,989
Gross Unrealized Gains	341		1,573
Gross Unrealized Losses	(193)		(383)
Fair Value	162,193		232,179
Collateralized mortgage obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	109,916		130,046
Gross Unrealized Gains	93		465
Gross Unrealized Losses	(1,321)		(734)
Fair Value	108,688		129,777
Residential mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,388		36,606
Gross Unrealized Gains	679		984
Gross Unrealized Losses	(213)		(232)
Fair Value	32,854		37,358
Commercial mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	244,018	[1]	127,311
Gross Unrealized Gains	247	[1]	243
Gross Unrealized Losses	(2,044)	[1]	(1,950)
Fair Value	242,221	[1]	125,604
Security purchased pending settlement	5,900
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	123,159		128,783
Gross Unrealized Gains	4,285		3,772
Gross Unrealized Losses	(42)		(46)
Fair Value	127,402		132,509
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	691,469		678,626
Gross Unrealized Gains	5,645		7,084
Gross Unrealized Losses	(3,967)		(3,472)
Fair Value	693,147		682,238
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,815		7,579
Gross Unrealized Gains	1,330		3,670
Gross Unrealized Losses	(1)		0
Fair Value	$ 5,144		$ 11,249
[1]	Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017.

Securities Available-for-Sale65

Securities Available-for-Sale - Gross and Net Realized Gains and Losses on Sale of Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Gross realized gains	$ 3,986	$ 0	$ 0
Gross realized losses	(986)	0	(34)
Net Realized (Losses) Gains	$ 3,000	$ 0	$ (34)

Securities Available-for-Sale66

Securities Available-for-Sale - Fair Value and Age of Gross Unrealized Losses by Investment Category (Details) $ in Thousands	Dec. 31, 2017USD ($)security	Dec. 31, 2016USD ($)security
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Securities | security	37	31
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 271,922	$ 267,500
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Unrealized Losses	$ (1,374)	$ (3,472)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Securities | security	15	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 113,210	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Unrealized Losses	$ (2,594)	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Number of Securities | security	52	31
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	$ 385,132	$ 267,500
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses, Total	$ (3,968)	$ (3,472)
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Securities | security	3	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 19,789	$ 9,811
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Unrealized Losses	$ (154)	$ (127)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Securities | security	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Unrealized Losses	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Number of Securities | security	3	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	$ 19,789	$ 9,811
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses, Total	$ (154)	$ (127)
Obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Securities | security	9	7
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 63,635	$ 62,483
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Unrealized Losses	$ (144)	$ (383)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Securities | security	1	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 10,017	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Unrealized Losses	$ (49)	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Number of Securities | security	10	7
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	$ 73,652	$ 62,483
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses, Total	$ (193)	$ (383)
Collateralized mortgage obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Securities | security	7	10
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 47,465	$ 83,031
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Unrealized Losses	$ (248)	$ (734)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Securities | security	7	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 45,809	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Unrealized Losses	$ (1,073)	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Number of Securities | security	14	10
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	$ 93,274	$ 83,031
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses, Total	$ (1,321)	$ (734)
Residential mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Securities | security	1	2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 2,333	$ 10,022
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Unrealized Losses	$ (10)	$ (232)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Securities | security	2	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 8,638	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Unrealized Losses	$ (203)	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Number of Securities | security	3	2
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	$ 10,971	$ 10,022
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses, Total	$ (213)	$ (232)
Commercial mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Securities | security	14	10
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 128,300	$ 96,576
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Unrealized Losses	$ (775)	$ (1,950)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Securities | security	5	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 48,746	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Unrealized Losses	$ (1,269)	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Number of Securities | security	19	10
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	$ 177,046	$ 96,576
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses, Total	$ (2,044)	$ (1,950)
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Securities | security	2	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 10,330	$ 5,577
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Unrealized Losses	$ (42)	$ (46)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Securities | security	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Unrealized Losses	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Number of Securities | security	2	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	$ 10,330	$ 5,577
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses, Total	$ (42)	$ (46)
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Securities | security	36	31
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 271,852	$ 267,500
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Unrealized Losses	$ (1,373)	$ (3,472)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Securities | security	15	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 113,210
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Unrealized Losses	$ (2,594)
Available-for-sale Securities, Continuous Unrealized Loss Position, Number of Securities | security	51	31
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	$ 385,062	$ 267,500
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses, Total	$ (3,967)	$ (3,472)
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Securities | security	1	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 70	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Unrealized Losses	$ (1)	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Securities | security	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Unrealized Losses	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Number of Securities | security	1	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	$ 70	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses, Total	$ (1)	$ 0

Securities Available-for-Sale67

Securities Available-for-Sale - Additional Information (Details) $ in Millions	Dec. 31, 2017USD ($)security	Dec. 31, 2016USD ($)security
Schedule of Available-for-sale Securities [Line Items]
Number of debt securities on which unrealized losses were primarily attributable to changes in interest	51
Number of marketable equity securities in unrealized loss position	52	31
Carrying values of pledged securities | $	$ 249	$ 342
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Number of marketable equity securities in unrealized loss position	1	0

Securities Available-for-Sale68

Securities Available-for-Sale - Unrealized Gains (Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Total unrealized gains/(losses) on securities available for sale, Gross Unrealized Gains	$ 6,975	$ 10,754
Total unrealized gains/(losses) on securities available for sale, Gross Unrealized Losses	(3,968)	(3,472)
Total unrealized gains/(losses) on securities available for sale, Net Unrealized Gains	3,007	7,282
Income tax expense/(benefit), Gross Unrealized Gains	(2,450)	(3,776)
Income tax expense/(benefit), Gross Unrealized Losses	1,394	1,219
Income tax expense/(benefit), Net Unrealized Gains	(1,056)	(2,557)
Net unrealized gains/(losses), net of tax included in accumulated other comprehensive income/(loss)	4,525	6,978
Net unrealized gains/(losses), net of tax included in accumulated other comprehensive income/(loss)	(2,574)	(2,253)
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	$ 1,951	$ 4,725

Securities Available-for-Sale69

Securities Available-for-Sale - Amortized Cost and Fair Value of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 695,284		$ 686,205
Fair Value	698,291		693,487
Obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	58,501
Due after one year through five years, Amortized Cost	141,646
Due after five years through ten years, Amortized Cost	70,584
Due after ten years, Amortized Cost	34,416
Available-for-sale Securities, Debt Maturities, Amortized Cost	305,147
Due in one year or less, Fair Value	58,438
Due after one year through five years, Fair Value	142,859
Due after five years through ten years, Fair Value	72,132
Due after ten years, Fair Value	35,955
Fair Value, Debt securities	309,384
Collateralized mortgage obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	109,916		130,046
Fair Value	108,688		129,777
Residential mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,388		36,606
Fair Value	32,854		37,358
Commercial mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	244,018	[1]	127,311
Fair Value	242,221	[1]	125,604
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	691,469		678,626
Fair Value	693,147		682,238
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,815		7,579
Fair Value	$ 5,144		$ 11,249
[1]	Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017.

Loans and Loans Held for Sale -

Loans and Loans Held for Sale - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)loan	Dec. 31, 2016USD ($)loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unearned income on loans and leases receivable	$ 5,200	$ 5,200
Net unamortized discount related to purchase accounting fair value adjustments	2,800	7,100
Loans receivable, net	$ 5,765,934	$ 5,615,212
Number of loan modified | loan	15
Number of commitments to lend additional funds on TDRs | loan	2
Commitment to lend additional funds on TDRs	$ 200
Number of TDRs returned back to accruing status | loan	1	5
Financial receivable trouble debt restructuring reclassified to accruing trouble debt restructuring status	$ 2,000	$ 900
Minimum period of loan payment defaults following restructure for TDRs to be in default	90 days
Number of defaulted TDRs that were restructured within twelve months prior to defaulting | loan	0	0
Decrease in nonperforming assets	$ 18,900
Nonperforming assets	$ 24,407	$ 43,314
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of commercial loans in total portfolio loans	78.00%	78.00%
Loans recorded investment	$ 4,503,594	$ 4,355,395
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	$ 2,685,994	2,498,476
Number of loan modified | loan	5
Loans modified not considered to be troubled debt restructuring	$ 7,500
Commercial real estate | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans recorded investment	2,685,994	2,498,476
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	$ 1,433,266	1,401,035
Number of loan modified | loan	10
Loans modified not considered to be troubled debt restructuring	$ 10,800
Number of nonperforming, impaired loans paid off during the period | loan	2
Balance of nonperforming, impaired loans paid off during the period	$ 10,500
Commercial and industrial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans recorded investment	1,433,266	1,401,035
Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	384,334	455,884
Commercial construction | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans recorded investment	384,334	455,884
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	698,774	701,982
Consumer construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	$ 4,551	$ 5,906
CRE and Commercial Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Out-of-state exposure of combined portfolio	5.20%	5.20%
Percentage of total loans out-of-state	2.80%	2.70%
CRE and Commercial Construction | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans recorded investment	$ 3,100,000	$ 3,000,000
Combined percentage of commercial real estate and commercial construction in total commercial loans	68.00%	68.00%
Combined percentage of commercial real estate and commercial construction in total portfolio loans	53.00%	53.00%
Concentration risk percentage	0.00%	0.00%
Maximum concentration of commercial real estate and commercial construction portfolio in loans	14.00%	14.00%
Integrity Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	$ 387,000	$ 543,000
Integrity Bancshares, Inc. | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	209,000	273,000
Integrity Bancshares, Inc. | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	92,100	141,000
Integrity Bancshares, Inc. | Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	11,100	33,000
Integrity Bancshares, Inc. | Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	57,500	74,000
Integrity Bancshares, Inc. | Home Equity, Installment and Other Consumer Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	$ 17,300	$ 22,000

Loans and Loans Held for Sale71

Loans and Loans Held for Sale - Composition of Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Composition of Loans
Portfolio loans, net of unearned income	$ 5,761,449	$ 5,611,419
Loans held for sale	4,485	3,793
Total Loans	5,765,934	5,615,212
Commercial real estate
Composition of Loans
Portfolio loans, net of unearned income	2,685,994	2,498,476
Total Loans	2,685,994	2,498,476
Commercial and industrial
Composition of Loans
Portfolio loans, net of unearned income	1,433,266	1,401,035
Total Loans	1,433,266	1,401,035
Commercial construction
Composition of Loans
Portfolio loans, net of unearned income	384,334	455,884
Total Loans	384,334	455,884
Residential mortgage
Composition of Loans
Total Loans	698,774	701,982
Home equity
Composition of Loans
Total Loans	487,326	482,284
Installment and other consumer
Composition of Loans
Total Loans	67,204	65,852
Consumer construction
Composition of Loans
Total Loans	4,551	5,906
Commercial
Composition of Loans
Portfolio loans, net of unearned income	4,503,594	4,355,395
Commercial | Commercial real estate
Composition of Loans
Portfolio loans, net of unearned income	2,685,994	2,498,476
Commercial | Commercial and industrial
Composition of Loans
Portfolio loans, net of unearned income	1,433,266	1,401,035
Commercial | Commercial construction
Composition of Loans
Portfolio loans, net of unearned income	384,334	455,884
Consumer
Composition of Loans
Portfolio loans, net of unearned income	1,257,855	1,256,024
Consumer | Residential mortgage
Composition of Loans
Portfolio loans, net of unearned income	698,774	701,982
Consumer | Home equity
Composition of Loans
Portfolio loans, net of unearned income	487,326	482,284
Consumer | Installment and other consumer
Composition of Loans
Portfolio loans, net of unearned income	67,204	65,852
Consumer | Consumer construction
Composition of Loans
Portfolio loans, net of unearned income	$ 4,551	$ 5,906

Loans and Loans Held for Sale72

Loans and Loans Held for Sale - Restructured Loans for Periods Presented (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Modifications [Line Items]
Restructured loans	[1]	$ 6,142	$ 12,192
Performing TDRs
Financing Receivable, Modifications [Line Items]
Restructured loans		14,901	13,423
Performing TDRs | Commercial real estate
Financing Receivable, Modifications [Line Items]
Restructured loans		2,579	2,994
Performing TDRs | Commercial and industrial
Financing Receivable, Modifications [Line Items]
Restructured loans		3,946	1,387
Performing TDRs | Commercial construction
Financing Receivable, Modifications [Line Items]
Restructured loans		2,420	2,966
Performing TDRs | Residential mortgage
Financing Receivable, Modifications [Line Items]
Restructured loans		2,039	2,375
Performing TDRs | Home equity
Financing Receivable, Modifications [Line Items]
Restructured loans		3,885	3,683
Performing TDRs | Installment and other consumer
Financing Receivable, Modifications [Line Items]
Restructured loans		32	18
Nonperforming TDRs
Financing Receivable, Modifications [Line Items]
Restructured loans		11,150	11,598
Nonperforming TDRs | Commercial real estate
Financing Receivable, Modifications [Line Items]
Restructured loans		967	646
Nonperforming TDRs | Commercial and industrial
Financing Receivable, Modifications [Line Items]
Restructured loans		3,197	4,493
Nonperforming TDRs | Commercial construction
Financing Receivable, Modifications [Line Items]
Restructured loans		2,413	430
Nonperforming TDRs | Residential mortgage
Financing Receivable, Modifications [Line Items]
Restructured loans		3,585	5,068
Nonperforming TDRs | Home equity
Financing Receivable, Modifications [Line Items]
Restructured loans		979	954
Nonperforming TDRs | Installment and other consumer
Financing Receivable, Modifications [Line Items]
Restructured loans		9	7
Total TDRs
Financing Receivable, Modifications [Line Items]
Restructured loans		26,051	25,021
Total TDRs | Commercial real estate
Financing Receivable, Modifications [Line Items]
Restructured loans		3,546	3,640
Total TDRs | Commercial and industrial
Financing Receivable, Modifications [Line Items]
Restructured loans		7,143	5,880
Total TDRs | Commercial construction
Financing Receivable, Modifications [Line Items]
Restructured loans		4,833	3,396
Total TDRs | Residential mortgage
Financing Receivable, Modifications [Line Items]
Restructured loans		5,624	7,443
Total TDRs | Home equity
Financing Receivable, Modifications [Line Items]
Restructured loans		4,864	4,637
Total TDRs | Installment and other consumer
Financing Receivable, Modifications [Line Items]
Restructured loans		$ 41	$ 25
[1]	Excludes loans that were fully paid off or fully charged-off by period end. The pre-modification balance represents the balance outstanding prior to modification. The post-modification balance represents the outstanding balance at period end

Loans and Loans Held for Sale73

Loans and Loans Held for Sale - Restructured Loans by Type of Concession (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2017USD ($)loan	Dec. 31, 2016USD ($)loan
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		36	53
Pre-Modification Outstanding Recorded Investment	[1]	$ 5,757	$ 14,074
Post-Modification Outstanding Recorded Investment	[1]	6,142	12,192
Total Difference in Recorded Investment		$ 385	$ (1,882)
Chapter 7 bankruptcy
Financing Receivable, Modifications [Line Items]
Number of Loans | loan	[2]	26	29
Pre-Modification Outstanding Recorded Investment	[1],[2]	$ 770	$ 1,840
Post-Modification Outstanding Recorded Investment	[1],[2]	709	1,712
Total Difference in Recorded Investment	[2]	$ (61)	$ (128)
Interest rate reduction
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		0	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 250
Post-Modification Outstanding Recorded Investment	[1]	0	242
Total Difference in Recorded Investment		$ 0	$ (8)
Maturity date extension
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		3	13
Pre-Modification Outstanding Recorded Investment	[1]	$ 905	$ 6,795
Post-Modification Outstanding Recorded Investment	[1]	1,406	5,197
Total Difference in Recorded Investment		$ 501	$ (1,598)
Maturity date extension and interest rate reduction
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		3	3
Pre-Modification Outstanding Recorded Investment	[1]	$ 1,973	$ 884
Post-Modification Outstanding Recorded Investment	[1]	1,918	877
Total Difference in Recorded Investment		$ (55)	$ (7)
Principal deferral
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		2	7
Pre-Modification Outstanding Recorded Investment	[1]	$ 113	$ 4,305
Post-Modification Outstanding Recorded Investment	[1]	113	4,164
Total Difference in Recorded Investment		$ 0	$ (141)
Principal forgiveness
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		2	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 1,996	$ 0
Post-Modification Outstanding Recorded Investment	[1]	1,996	0
Total Difference in Recorded Investment		$ 0	$ 0
Commercial real estate | Chapter 7 bankruptcy
Financing Receivable, Modifications [Line Items]
Number of Loans | loan	[2]	0	1
Pre-Modification Outstanding Recorded Investment	[1],[2]	$ 0	$ 709
Post-Modification Outstanding Recorded Investment	[1],[2]	0	646
Total Difference in Recorded Investment	[2]	$ 0	$ (63)
Commercial real estate | Interest rate reduction
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		0	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 250
Post-Modification Outstanding Recorded Investment	[1]	0	242
Total Difference in Recorded Investment		$ 0	$ (8)
Commercial real estate | Maturity date extension
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		1	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 400	$ 0
Post-Modification Outstanding Recorded Investment	[1]	398	0
Total Difference in Recorded Investment		$ (2)	$ 0
Commercial and industrial | Maturity date extension
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		1	4
Pre-Modification Outstanding Recorded Investment	[1]	$ 274	$ 4,756
Post-Modification Outstanding Recorded Investment	[1]	777	3,334
Total Difference in Recorded Investment		$ 503	$ (1,422)
Commercial and industrial | Maturity date extension and interest rate reduction
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		2	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 1,800	$ 0
Post-Modification Outstanding Recorded Investment	[1]	1,805	0
Total Difference in Recorded Investment		$ 5	$ 0
Commercial and industrial | Principal deferral
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		2	5
Pre-Modification Outstanding Recorded Investment	[1]	$ 113	$ 985
Post-Modification Outstanding Recorded Investment	[1]	113	986
Total Difference in Recorded Investment		$ 0	$ 1
Commercial construction | Maturity date extension
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		0	3
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1,251
Post-Modification Outstanding Recorded Investment	[1]	0	1,151
Total Difference in Recorded Investment		$ 0	$ (100)
Commercial construction | Principal forgiveness
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		2	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 1,996	$ 0
Post-Modification Outstanding Recorded Investment	[1]	1,996	0
Total Difference in Recorded Investment		$ 0	$ 0
Residential mortgage | Chapter 7 bankruptcy
Financing Receivable, Modifications [Line Items]
Number of Loans | loan	[2]	1	7
Pre-Modification Outstanding Recorded Investment	[1],[2]	$ 33	$ 439
Post-Modification Outstanding Recorded Investment	[1],[2]	31	413
Total Difference in Recorded Investment	[2]	$ (2)	$ (26)
Residential mortgage | Maturity date extension
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		0	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 483
Post-Modification Outstanding Recorded Investment	[1]	0	414
Total Difference in Recorded Investment		$ 0	$ (69)
Residential mortgage | Maturity date extension and interest rate reduction
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		0	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 280
Post-Modification Outstanding Recorded Investment	[1]	0	279
Total Difference in Recorded Investment		$ 0	$ (1)
Residential mortgage | Principal deferral
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		0	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 3,273
Post-Modification Outstanding Recorded Investment	[1]	0	3,133
Total Difference in Recorded Investment		$ 0	$ (140)
Home equity | Chapter 7 bankruptcy
Financing Receivable, Modifications [Line Items]
Number of Loans | loan	[2]	21	19
Pre-Modification Outstanding Recorded Investment	[1],[2]	$ 689	$ 676
Post-Modification Outstanding Recorded Investment	[1],[2]	643	643
Total Difference in Recorded Investment	[2]	$ (46)	$ (33)
Home equity | Maturity date extension
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		1	5
Pre-Modification Outstanding Recorded Investment	[1]	$ 231	$ 305
Post-Modification Outstanding Recorded Investment	[1]	231	298
Total Difference in Recorded Investment		$ 0	$ (7)
Home equity | Maturity date extension and interest rate reduction
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		1	2
Pre-Modification Outstanding Recorded Investment	[1]	$ 173	$ 604
Post-Modification Outstanding Recorded Investment	[1]	113	598
Total Difference in Recorded Investment		$ (60)	$ (6)
Home equity | Principal deferral
Financing Receivable, Modifications [Line Items]
Number of Loans | loan		0	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 47
Post-Modification Outstanding Recorded Investment	[1]	0	45
Total Difference in Recorded Investment		$ 0	$ (2)
Installment and other consumer | Chapter 7 bankruptcy
Financing Receivable, Modifications [Line Items]
Number of Loans | loan	[2]	4	2
Pre-Modification Outstanding Recorded Investment	[1],[2]	$ 48	$ 16
Post-Modification Outstanding Recorded Investment	[1],[2]	35	10
Total Difference in Recorded Investment	[2]	$ (13)	$ (6)
[1]	Excludes loans that were fully paid off or fully charged-off by period end. The pre-modification balance represents the balance outstanding prior to modification. The post-modification balance represents the outstanding balance at period end
[2]	Chapter 7 bankruptcy loans where the debt has been legally discharged through the bankruptcy court and not reaffirmed.

Loans and Loans Held for Sale74

Loans and Loans Held for Sale - Summary of Nonperforming Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Nonperforming Assets
Nonaccrual loans	$ 12,788	$ 31,037
Nonaccrual TDRs	11,150	11,598
Total nonaccrual loans	23,938	42,635
OREO	469	679
Total Nonperforming Assets	$ 24,407	$ 43,314

Loans and Loans Held for Sale75

Loans and Loans Held for Sale - Summary of Aggregate Amount of Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loans Receivable from Officers and Directors [Roll Forward]
Balance at beginning of year	$ 25,167	$ 24,517
New loans	25,203	22,740
Repayments or no longer considered a related party	(40,300)	(22,090)
Balance at end of year	$ 10,070	$ 25,167

Allowance for Loan Losses - Age

Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 5,730,074	$ 5,563,123
Past Due	35,860	52,089
Non- performing	23,938	42,635
Total Loans	5,765,934	5,615,212
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,681,395	2,479,513
Past Due	4,599	18,963
Total Loans	2,685,994	2,498,476
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,426,754	1,391,475
Past Due	6,512	9,560
Total Loans	1,433,266	1,401,035
Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	377,968	450,410
Past Due	6,366	5,474
Total Loans	384,334	455,884
Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	687,195	689,635
Past Due	11,579	12,347
Total Loans	698,774	701,982
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	480,956	476,866
Past Due	6,370	5,418
Total Loans	487,326	482,284
Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	66,770	65,525
Past Due	434	327
Total Loans	67,204	65,852
Consumer construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,551	5,906
Past Due	0	0
Total Loans	4,551	5,906
Loans held for sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,485	3,793
Past Due	0	0
Total Loans	4,485	3,793
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	9,123	6,598
30-59 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	997	2,032
30-59 Days Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	420	1,061
30-59 Days Past Due | Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,473	547
30-59 Days Past Due | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,975	1,312
30-59 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,065	1,470
30-59 Days Past Due | Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	193	176
30-59 Days Past Due | Consumer construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
30-59 Days Past Due | Loans held for sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,799	2,856
60-89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	134	759
60-89 Days Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	446	428
60-89 Days Past Due | Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	20	0
60-89 Days Past Due | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,439	1,117
60-89 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	590	509
60-89 Days Past Due | Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	170	43
60-89 Days Past Due | Consumer construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60-89 Days Past Due | Loans held for sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non- performing	23,938	42,635
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non- performing	3,468	16,172
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non- performing	5,646	8,071
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non- performing	3,873	4,927
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non- performing	7,165	9,918
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non- performing	3,715	3,439
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non- performing	71	108
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non- performing	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Loans held for sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non- performing	$ 0	$ 0

Allowance for Loan Losses - Rec

Allowance for Loan Losses - Recorded Investment in Commercial Loan Classes by Internally Assigned Risk Ratings (Details) - Commercial - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 4,503,594	$ 4,355,395
% of Total	100.00%	100.00%
Pass
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 4,302,762	$ 4,169,721
% of Total	95.50%	95.70%
Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 130,101	$ 88,198
% of Total	2.90%	2.00%
Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 70,731	$ 97,476
% of Total	1.60%	2.30%
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 2,685,994	$ 2,498,476
% of Total	100.00%	100.00%
Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 2,588,847	$ 2,423,742
% of Total	96.40%	97.00%
Commercial real estate | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 66,436	$ 33,098
% of Total	2.50%	1.30%
Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 30,711	$ 41,636
% of Total	1.10%	1.70%
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 1,433,266	$ 1,401,035
% of Total	100.00%	100.00%
Commercial and industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 1,345,810	$ 1,315,507
% of Total	93.90%	93.90%
Commercial and industrial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 54,320	$ 40,409
% of Total	3.80%	2.90%
Commercial and industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 33,136	$ 45,119
% of Total	2.30%	3.20%
Commercial construction
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 384,334	$ 455,884
% of Total	100.00%	100.00%
Commercial construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 368,105	$ 430,472
% of Total	95.80%	94.40%
Commercial construction | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 9,345	$ 14,691
% of Total	2.40%	3.20%
Commercial construction | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 6,884	$ 10,721
% of Total	1.80%	2.40%

Allowance for Loan Losses - R78

Allowance for Loan Losses - Recorded Investment in Consumer Loan Classes by Performing and Nonperforming Status (Details) - Consumer - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 1,257,855	$ 1,256,024
% of Total	100.00%	100.00%
Performing
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 1,246,904	$ 1,242,559
% of Total	99.10%	98.90%
Nonperforming
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 10,951	$ 13,465
% of Total	0.90%	1.10%
Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 698,774	$ 701,982
% of Total	100.00%	100.00%
Residential mortgage | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 691,609	$ 692,064
% of Total	99.00%	98.60%
Residential mortgage | Nonperforming
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 7,165	$ 9,918
% of Total	1.00%	1.40%
Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 487,326	$ 482,284
% of Total	100.00%	100.00%
Home equity | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 483,611	$ 478,845
% of Total	99.20%	99.30%
Home equity | Nonperforming
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 3,715	$ 3,439
% of Total	0.80%	0.70%
Installment and other consumer
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 67,204	$ 65,852
% of Total	100.00%	100.00%
Installment and other consumer | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 67,133	$ 65,744
% of Total	99.90%	99.80%
Installment and other consumer | Nonperforming
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 71	$ 108
% of Total	0.10%	0.20%
Consumer construction
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 4,551	$ 5,906
% of Total	100.00%	100.00%
Consumer construction | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 4,551	$ 5,906
% of Total	100.00%	100.00%
Consumer construction | Nonperforming
Financing Receivable, Recorded Investment [Line Items]
Loans recorded investment	$ 0	$ 0
% of Total	0.00%	0.00%

Allowance for Loan Losses - Add

Allowance for Loan Losses - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Minimum period interest and principal of loans past due considered nonperforming (in days)	90 days
Evaluation for impairment of substandard and nonaccrual commercial loans	$ 500,000
Impaired financing receivables	26,823,000	$ 41,935,000
Loans receivable, net	5,765,934,000	5,615,212,000
Integrity Bancshares, Inc.
Financing Receivable, Allowance for Credit Losses [Line Items]
Acquired loans that experienced credit deterioration since acquisition	5,100,000
Allowance for loan losses of acquired loans		0
Loans receivable, net	$ 387,000,000	$ 543,000,000

Allowance for Loan Losses - Inv

Allowance for Loan Losses - Investments in Loans Considered to be Impaired and Related Information on Impaired Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
With a related allowance recorded, Recorded Investment	$ 1,783	$ 991
With a related allowance recorded, Unpaid Principal Balance	1,835	2,460
Without a related allowance recorded, Recorded Investment	25,040	40,944
Without a related allowance, Unpaid Principal Balance	31,302	48,539
Impaired financing receivables	26,823	41,935
Impaired Financing Receivable, Unpaid Principal Balance, Total	33,137	50,999
Impaired Financing Receivable, Related Allowance, Total	77	798
With a related allowance recorded, Average Recorded Investment	1,025	2,468
With a related allowance recorded, Interest Income Recognized	56	2
Without a related allowance recorded, Average Recorded Investment	34,421	43,334
Without a related allowance recorded, Interest Income Recognized	1,174	990
Impaired Financing Receivable, Average Recorded Investment, Total	35,446	45,802
Impaired Financing Receivable, Interest Income Recognized, Total	1,230	992
Commercial real estate
Financing Receivable, Impaired [Line Items]
With a related allowance recorded, Recorded Investment	0	0
With a related allowance recorded, Unpaid Principal Balance	0	0
Without a related allowance recorded, Recorded Investment	3,546	16,352
Without a related allowance, Unpaid Principal Balance	3,811	17,654
Impaired financing receivables	3,546	16,352
Impaired Financing Receivable, Unpaid Principal Balance, Total	3,811	17,654
Impaired Financing Receivable, Related Allowance, Total	0	0
With a related allowance recorded, Average Recorded Investment	0	0
With a related allowance recorded, Interest Income Recognized	0	0
Without a related allowance recorded, Average Recorded Investment	6,636	17,496
Without a related allowance recorded, Interest Income Recognized	177	144
Impaired Financing Receivable, Average Recorded Investment, Total	6,636	17,496
Impaired Financing Receivable, Interest Income Recognized, Total	177	144
Commercial and industrial
Financing Receivable, Impaired [Line Items]
With a related allowance recorded, Recorded Investment	1,735	964
With a related allowance recorded, Unpaid Principal Balance	1,787	2,433
Without a related allowance recorded, Recorded Investment	5,549	5,902
Without a related allowance, Unpaid Principal Balance	7,980	7,699
Impaired financing receivables	7,284	6,866
Impaired Financing Receivable, Unpaid Principal Balance, Total	9,767	10,132
Impaired Financing Receivable, Related Allowance, Total	29	771
With a related allowance recorded, Average Recorded Investment	968	2,438
With a related allowance recorded, Interest Income Recognized	52	0
Without a related allowance recorded, Average Recorded Investment	9,897	6,141
Without a related allowance recorded, Interest Income Recognized	257	160
Impaired Financing Receivable, Average Recorded Investment, Total	10,865	8,579
Impaired Financing Receivable, Interest Income Recognized, Total	309	160
Commercial construction
Financing Receivable, Impaired [Line Items]
With a related allowance recorded, Recorded Investment	0	0
With a related allowance recorded, Unpaid Principal Balance	0	0
Without a related allowance recorded, Recorded Investment	5,464	6,613
Without a related allowance, Unpaid Principal Balance	8,132	10,306
Impaired financing receivables	5,464	6,613
Impaired Financing Receivable, Unpaid Principal Balance, Total	8,132	10,306
Impaired Financing Receivable, Related Allowance, Total	0	0
With a related allowance recorded, Average Recorded Investment	0	0
With a related allowance recorded, Interest Income Recognized	0	0
Without a related allowance recorded, Average Recorded Investment	6,828	7,723
Without a related allowance recorded, Interest Income Recognized	253	162
Impaired Financing Receivable, Average Recorded Investment, Total	6,828	7,723
Impaired Financing Receivable, Interest Income Recognized, Total	253	162
Consumer real estate
Financing Receivable, Impaired [Line Items]
With a related allowance recorded, Recorded Investment	21	26
With a related allowance recorded, Unpaid Principal Balance	21	26
Without a related allowance recorded, Recorded Investment	10,467	12,053
Without a related allowance, Unpaid Principal Balance	11,357	12,849
Impaired financing receivables	10,488	12,079
Impaired Financing Receivable, Unpaid Principal Balance, Total	11,378	12,875
Impaired Financing Receivable, Related Allowance, Total	21	26
With a related allowance recorded, Average Recorded Investment	23	28
With a related allowance recorded, Interest Income Recognized	2	2
Without a related allowance recorded, Average Recorded Investment	11,037	11,939
Without a related allowance recorded, Interest Income Recognized	487	523
Impaired Financing Receivable, Average Recorded Investment, Total	11,060	11,967
Impaired Financing Receivable, Interest Income Recognized, Total	489	525
Other consumer
Financing Receivable, Impaired [Line Items]
With a related allowance recorded, Recorded Investment	27	1
With a related allowance recorded, Unpaid Principal Balance	27	1
Without a related allowance recorded, Recorded Investment	14	24
Without a related allowance, Unpaid Principal Balance	22	31
Impaired financing receivables	41	25
Impaired Financing Receivable, Unpaid Principal Balance, Total	49	32
Impaired Financing Receivable, Related Allowance, Total	27	1
With a related allowance recorded, Average Recorded Investment	34	2
With a related allowance recorded, Interest Income Recognized	2	0
Without a related allowance recorded, Average Recorded Investment	23	35
Without a related allowance recorded, Interest Income Recognized	0	1
Impaired Financing Receivable, Average Recorded Investment, Total	57	37
Impaired Financing Receivable, Interest Income Recognized, Total	$ 2	$ 1

Allowance for Loan Losses - Sum

Allowance for Loan Losses - Summary of Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				$ 52,775				$ 48,147	$ 52,775	$ 48,147
Charge-offs									(13,496)	(15,561)
Recoveries									3,228	2,224
Net (Charge-offs) Recoveries									(10,268)	(13,337)
Provision for loan losses	$ 982	$ 2,850	$ 4,869	5,183	$ 5,586	$ 2,516	$ 4,848	5,014	13,883	17,965	$ 10,388
Balance at End of Year	56,390				52,775				56,390	52,775	48,147
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				19,976				15,043	19,976	15,043
Charge-offs									(2,304)	(3,114)
Recoveries									810	692
Net (Charge-offs) Recoveries									(1,494)	(2,422)
Provision for loan losses									8,753	7,355
Balance at End of Year	27,235				19,976				27,235	19,976	15,043
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				10,810				10,853	10,810	10,853
Charge-offs									(4,709)	(6,810)
Recoveries									654	722
Net (Charge-offs) Recoveries									(4,055)	(6,088)
Provision for loan losses									2,211	6,045
Balance at End of Year	8,966				10,810				8,966	10,810	10,853
Commercial construction
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				13,999				12,625	13,999	12,625
Charge-offs									(2,571)	(1,877)
Recoveries									851	21
Net (Charge-offs) Recoveries									(1,720)	(1,856)
Provision for loan losses									888	3,230
Balance at End of Year	13,167				13,999				13,167	13,999	12,625
Consumer real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				6,095				8,400	6,095	8,400
Charge-offs									(2,274)	(1,657)
Recoveries									342	433
Net (Charge-offs) Recoveries									(1,932)	(1,224)
Provision for loan losses									1,316	(1,081)
Balance at End of Year	5,479				6,095				5,479	6,095	8,400
Other consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				$ 1,895				$ 1,226	1,895	1,226
Charge-offs									(1,638)	(2,103)
Recoveries									571	356
Net (Charge-offs) Recoveries									(1,067)	(1,747)
Provision for loan losses									715	2,416
Balance at End of Year	$ 1,543				$ 1,895				$ 1,543	$ 1,895	$ 1,226

Allowance for Loan Losses - S82

Allowance for Loan Losses - Summary of Allowance for Loan Losses and Recorded Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment	$ 77	$ 798
Allowance for Loan Losses, Collectively Evaluated for Impairment	56,313	51,977
Total Allowance for Loan Losses	56,390	52,775	$ 48,147
Portfolio Loans, Individually Evaluated for Impairment	26,823	41,935
Portfolio Loans, Collectively Evaluated for Impairment	5,734,626	5,569,484
Total Portfolio Loans	5,761,449	5,611,419
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment	0	0
Allowance for Loan Losses, Collectively Evaluated for Impairment	27,235	19,976
Total Allowance for Loan Losses	27,235	19,976	15,043
Portfolio Loans, Individually Evaluated for Impairment	3,546	16,352
Portfolio Loans, Collectively Evaluated for Impairment	2,682,448	2,482,124
Total Portfolio Loans	2,685,994	2,498,476
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment	29	771
Allowance for Loan Losses, Collectively Evaluated for Impairment	8,937	10,039
Total Allowance for Loan Losses	8,966	10,810	10,853
Portfolio Loans, Individually Evaluated for Impairment	7,284	6,866
Portfolio Loans, Collectively Evaluated for Impairment	1,425,982	1,394,169
Total Portfolio Loans	1,433,266	1,401,035
Commercial construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment	0	0
Allowance for Loan Losses, Collectively Evaluated for Impairment	13,167	13,999
Total Allowance for Loan Losses	13,167	13,999	12,625
Portfolio Loans, Individually Evaluated for Impairment	5,464	6,613
Portfolio Loans, Collectively Evaluated for Impairment	378,870	449,271
Total Portfolio Loans	384,334	455,884
Consumer real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment	21	26
Allowance for Loan Losses, Collectively Evaluated for Impairment	5,458	6,069
Total Allowance for Loan Losses	5,479	6,095	8,400
Portfolio Loans, Individually Evaluated for Impairment	10,488	12,079
Portfolio Loans, Collectively Evaluated for Impairment	1,180,163	1,178,093
Total Portfolio Loans	1,190,651	1,190,172
Other consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment	27	1
Allowance for Loan Losses, Collectively Evaluated for Impairment	1,516	1,894
Total Allowance for Loan Losses	1,543	1,895	$ 1,226
Portfolio Loans, Individually Evaluated for Impairment	41	25
Portfolio Loans, Collectively Evaluated for Impairment	67,163	65,827
Total Portfolio Loans	$ 67,204	$ 65,852

Premises and Equipment - Summar

Premises and Equipment - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	$ 99,544	$ 98,714
Accumulated depreciation	(56,842)	(53,715)
Premises and equipment, net	42,702	44,999
Land
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	6,266	6,397
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	51,799
Premises
Property, Plant and Equipment [Line Items]
Premises and equipment, gross		52,696
Furniture and equipment
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	34,836	32,328
Leasehold improvements
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	$ 6,643	$ 7,293

Premises and Equipment - Additi

Premises and Equipment - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)contractdirector	Dec. 31, 2016USD ($)director	Dec. 31, 2015USD ($)director
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 5.1	$ 5	$ 4.7
Lease expiration year	Jan. 1, 2054
Capital Lease Obligations, Number of Contracts | contract	1
Rental expense	$ 4	$ 4.1	$ 3.9
Number of Directors | director	2	2	2
Directors
Property, Plant and Equipment [Line Items]
Rental expense for premises leases with two directors	$ 0.2	$ 0.3	$ 0.3

Premises and Equipment - Minimu

Premises and Equipment - Minimum Annual Rental and Renewal Option Payments (Details) $ in Thousands	Dec. 31, 2017USD ($)
Operating
Operating, 2018	$ 3,257
Operating, 2019	3,277
Operating, 2020	3,312
Operating, 2021	3,351
Operating, 2022	3,425
Operating, Thereafter	55,000
Operating, Total	71,622
Capital
Capital, 2018	76
Capital, 2019	77
Capital, 2020	77
Capital, 2021	76
Capital, 2022	77
Capital, Thereafter	457
Capital, Total	840
Total
2,018	3,333
2,019	3,354
2,020	3,389
2,021	3,427
2,022	3,502
Thereafter	55,457
Total	$ 72,462

Goodwill and Other Intangible86

Goodwill and Other Intangible Assets - Roll Forward of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Balance at beginning of year	$ 291,670	$ 291,764
Additions	0	0
Other adjustments	0	(94)
Balance at End of Year	$ 291,670	$ 291,670

Goodwill and Other Intangible87

Goodwill and Other Intangible Assets - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill, decrease during measurement period	$ 0	$ 94
Core deposit intangible	3,400
Acquisition of insurance contract relationships	300
Amortization expense on finite-lived intangible assets	$ 1,200	$ 1,600	$ 1,800

Goodwill and Other Intangible88

Goodwill and Other Intangible Assets - Summary of Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Gross carrying amount at beginning of year	$ 22,114	$ 22,114
Additions	0	0
Accumulated amortization	(18,437)	(17,204)
Gross carrying amount at end of year	$ 3,677	$ 4,910

Goodwill and Other Intangible89

Goodwill and Other Intangible Assets - Summary of Expected Amortization Expense for Finite-Lived Intangibles Assets (Details) $ in Thousands	Dec. 31, 2017USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2,018	$ 1,013
2,019	655
2,020	554
2,021	477
2,022	359
Thereafter	619
Total	$ 3,677

Derivative Instruments and He90

Derivative Instruments and Hedging Activities - Value of Derivative Assets and Derivative Liabilities (Details) - Not Designated as Hedging Instruments - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other Assets
Derivatives not Designated as Hedging Instruments
Fair value, Derivatives (included in Other Assets)	$ 3,074	$ 6,960
Other Assets | Interest Rate Swap Contracts—Commercial Loans
Derivatives not Designated as Hedging Instruments
Fair value, Derivatives (included in Other Assets)	3,074	6,960
Notional amount, Derivatives (included in Other Assets)	263,841	282,930
Collateral posted, Derivatives	0	0
Other Assets | Interest Rate Lock Commitments—Mortgage Loans
Derivatives not Designated as Hedging Instruments
Fair value, Derivatives (included in Other Assets)	226	236
Notional amount, Derivatives (included in Other Assets)	6,860	8,490
Other Assets | Forward Sale Contracts—Mortgage Loans
Derivatives not Designated as Hedging Instruments
Fair value, Derivatives (included in Other Assets)	0	0
Notional amount, Derivatives (included in Other Assets)	0	0
Other Liabilities
Derivatives not Designated as Hedging Instruments
Fair value, Derivatives (included in Other Liabilities)	3,055	6,958
Other Liabilities | Interest Rate Swap Contracts—Commercial Loans
Derivatives not Designated as Hedging Instruments
Fair value, Derivatives (included in Other Liabilities)	3,055	6,958
Notional amount, Derivatives (included in Other Liabilities)	263,841	282,930
Collateral posted, Derivatives	1,448	14,340
Other Liabilities | Interest Rate Lock Commitments—Mortgage Loans
Derivatives not Designated as Hedging Instruments
Fair value, Derivatives (included in Other Liabilities)	0	0
Notional amount, Derivatives (included in Other Liabilities)	0	0
Other Liabilities | Forward Sale Contracts—Mortgage Loans
Derivatives not Designated as Hedging Instruments
Fair value, Derivatives (included in Other Liabilities)	5	27
Notional amount, Derivatives (included in Other Liabilities)	$ 6,580	$ 8,216

Derivative Instruments and He91

Derivative Instruments and Hedging Activities - Schedule of Gross Amounts of Derivative Assets and Derivative Liabilities (Details) - Not Designated as Hedging Instruments - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other Assets
Derivatives (included in Other Assets)
Gross amounts recognized	$ 4,974	$ 8,590
Gross amounts offset	(1,900)	(1,630)
Net amounts presented in the Consolidated Balance Sheets	3,074	6,960
Gross amounts not offset	0	0
Net Amount	3,074	6,960
Other Liabilities
Derivatives (included in Other Liabilities)
Gross amounts recognized	4,955	8,588
Gross amounts offset	(1,900)	(1,630)
Net amounts presented in the Consolidated Balance Sheets	3,055	6,958
Gross amounts not offset	(1,448)	(14,340)
Net Amount	$ 1,607	$ (7,382)

Derivative Instruments and He92

Derivative Instruments and Hedging Activities - Amount of Gain or Loss Recognized in Income on Derivatives (Details) - Not Designated as Hedging Instruments - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Total Derivative (Loss) Gain	$ 58	$ (63)	$ 70
Interest Rate Swap Contracts—Commercial Loans
Derivative Instruments, Gain (Loss) [Line Items]
Total Derivative (Loss) Gain	17	(16)	(8)
Interest Rate Lock Commitments—Mortgage Loans
Derivative Instruments, Gain (Loss) [Line Items]
Total Derivative (Loss) Gain	(11)	(25)	26
Forward Sale Contracts—Mortgage Loans
Derivative Instruments, Gain (Loss) [Line Items]
Total Derivative (Loss) Gain	$ 52	$ (22)	$ 52

Mortgage Servicing Rights - Add

Mortgage Servicing Rights - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mortgage Loan Activity [Line Items]
Sale of 1-4 family mortgage loans	$ 93,991	$ 108,209	$ 99,458
Total servicing portfolio	441,000	407,000	361,000
Fannie Mae
Mortgage Loan Activity [Line Items]
Sale of 1-4 family mortgage loans	$ 78,800	$ 93,900	$ 76,800

Mortgage Servicing Rights - Net

Mortgage Servicing Rights - Net Carrying Values (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Servicing Rights
Beginning Balance	$ 3,858	$ 3,426
Additions	918	1,047
Amortization	(584)	(615)
Ending Balance	4,192	3,858
Valuation Allowance
Beginning Balance	(114)	(189)
Temporary recapture (impairment)	55	75
Ending Balance	(59)	(114)
Net Carrying Value
Beginning balance	3,744	3,237
Additions	918	1,047
Amortization	(584)	(615)
Temporary recapture (impairment)	55	75
Ending balance	$ 4,133	$ 3,744

Qualified Affordable Housing (D

Qualified Affordable Housing (Details)	12 Months Ended
	Dec. 31, 2017USD ($)commitment	Dec. 31, 2016USD ($)commitment	Dec. 31, 2015USD ($)
Schedule of Equity Method Investments [Line Items]
Investments in qualified affordable housing projects	$ 9,000,000	$ 11,700,000
Number of open commitments to fund future investments in qualified affordable housing | commitment	1	0
Open commitments to fund future investments qualified affordable housing projects	$ 800,000	$ 0
Tax credits to offset amortization expense of investments in affordable housing projects	3,400,000	3,700,000	$ 4,000,000
Noninterest Expense
Schedule of Equity Method Investments [Line Items]
Amortization expense of investments in qualified affordable housing projects	$ 3,000,000	$ 3,300,000	$ 3,600,000

Deposits - Composition of Depos

Deposits - Composition of Deposits and Interest Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	$ 5,427,891	$ 5,272,377	$ 4,876,611
Interest Expense	25,330	19,692	12,944
Noninterest-bearing demand
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	1,387,712	1,263,833	1,227,766
Interest-bearing demand
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	603,141	638,300	616,188
Interest Expense	67	111	818
Money market
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	1,146,156	936,461	605,184
Interest Expense	9,204	4,199	1,299
Savings
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	893,119	1,050,131	1,061,265
Interest Expense	2,081	2,002	1,712
Certificates of deposit
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	1,397,763	1,383,652	1,366,208
Interest Expense	$ 13,978	$ 13,380	$ 9,115

Deposits - Additional Informati

Deposits - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Banking and Thrift [Abstract]
Certificates of deposits over $100,000, including brokered CDs	$ 585	$ 672
Certificates of deposits over $250,000, including brokered CDs	$ 228	$ 303

Deposits - Scheduled Maturities

Deposits - Scheduled Maturities of Certificates of Deposit (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Banking and Thrift [Abstract]
2,018	$ 1,015,515
2,019	233,932
2,020	43,461
2,021	65,563
2,022	33,172
Thereafter	6,120
Total	$ 1,397,763	$ 1,383,652

Short-Term Borrowings - Additio

Short-Term Borrowings - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Securities available-for-sale, amortized cost	$ 695,284	$ 686,205
Securities available-for-sale, at fair value	698,291	693,487
Securities Pledged as Collateral
Short-term Debt [Line Items]
Securities available-for-sale, amortized cost	57,500	53,200
Securities available-for-sale, at fair value	$ 56,800	$ 52,900

Short-Term Borrowings - Balance

Short-Term Borrowings - Balance, Weighted Average and Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Balance	$ 590,161	$ 710,832	$ 418,086
Weighted Average Interest Rate	1.38%	0.70%	0.44%
Interest Expense	$ 7,453	$ 2,718	$ 936
REPOs
Short-term Debt [Line Items]
Balance	$ 50,161	$ 50,832	$ 62,086
Weighted Average Interest Rate	0.39%	0.01%	0.01%
Interest Expense	$ 54	$ 5	$ 4
FHLB advances
Short-term Debt [Line Items]
Balance	$ 540,000	$ 660,000	$ 356,000
Weighted Average Interest Rate	1.47%	0.76%	0.52%
Interest Expense	$ 7,399	$ 2,713	$ 932

Long-Term Borrowings and Sub101

Long-Term Borrowings and Subordinated Debt - Additional Information (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Long-term borrowings at FHLB	$ 47,200,000	$ 14,700,000
Loans pledged as collateral at FHLB	3,500,000,000
Maximum eligible borrowing based on qualifying collateral at FHLB	1,800,000,000
Maximum borrowing capacity at FHLB	$ 2,500,000,000
Percentage of equity owned	100.00%

Long-Term Borrowings and Sub102

Long-Term Borrowings and Subordinated Debt - Interest Expense and Weighted Average Interest Rates (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Long-term borrowings	$ 47,301	$ 14,713	$ 117,043
Weighted average interest rate	1.88%	2.91%	0.81%
Interest expense	$ 463	$ 670	$ 790

Long-Term Borrowings and Sub103

Long-Term Borrowings and Subordinated Debt - Scheduled Annual Maturities and Average Interest Rates (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
2018 Balance	$ 2,496
2019 Balance	37,514
2020 Balance	2,004
2021 Balance	1,057
2022 Balance	529
Thereafter Balance	3,701
Total Long-term Balance	$ 47,301	$ 14,713	$ 117,043
2018 Average Rate	3.60%
2019 Average Rate	1.60%
2020 Average Rate	3.22%
2021 Average Rate	3.44%
2022 Average Rate	4.50%
Thereafter Average Rate	1.96%
Total Average Rate	1.88%

Long-Term Borrowings and Sub104

Long-Term Borrowings and Subordinated Debt - Junior Subordinated Debt Securities and Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Junior Subordinated Debentures [Line Items]
Balance	$ 45,619	$ 45,619	$ 45,619
Interest Expense	1,663	1,434	1,327
2006 Junior Subordinated Debt
Junior Subordinated Debentures [Line Items]
Balance	25,000	25,000	25,000
Interest Expense	708	580	554
2008 Junior subordinated debt—trust preferred securities
Junior Subordinated Debentures [Line Items]
Balance	20,619	20,619	20,619
Interest Expense	$ 955	$ 854	$ 773

Long-Term Borrowings and Sub105

Long-Term Borrowings and Subordinated Debt - Key Terms of Junior Subordinated Debt Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Junior Subordinated Debentures [Line Items]
Junior subordinated debt securities	$ 45,619	$ 45,619	$ 45,619
December 31, 2017	1.88%	2.91%	0.81%
2006 Junior Subordinated Debt
Junior Subordinated Debentures [Line Items]
Junior subordinated debt securities	$ 25,000	$ 25,000	$ 25,000
Stated Maturity Date	Dec. 15, 2036
Optional redemption date at par	Any time after 9/15/2011
Regulatory Capital	Tier 2
December 31, 2017	2.92%
2006 Junior Subordinated Debt | 3-Month LIBOR
Junior Subordinated Debentures [Line Items]
Basis spread over variable rate	160.00%
2008 Trust Preferred Securities
Junior Subordinated Debentures [Line Items]
Junior subordinated debt securities	$ 20,619	$ 20,619	$ 20,619
Stated Maturity Date	Mar. 15, 2038
Optional redemption date at par	Any time after 3/15/2013
Regulatory Capital	Tier 1
December 31, 2017	4.82%
2008 Trust Preferred Securities | 3-Month LIBOR
Junior Subordinated Debentures [Line Items]
Basis spread over variable rate	350.00%

Commitments and Contingencies -

Commitments and Contingencies - Commitments and Letters of Credit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Commitment And Contingencies [Line Items]
Total Commitments	$ 1,501,346	$ 1,594,230
Standby letters of credit
Commitment And Contingencies [Line Items]
Total Commitments	80,918	84,534
Commitments to extend credit
Commitment And Contingencies [Line Items]
Total Commitments	$ 1,420,428	$ 1,509,696

Commitments and Contingencie107

Commitments and Contingencies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Commitments [Line Items]
Data processing and communication expense	$ 10,400,000	$ 10,400,000	$ 11,700,000
Merger-related expenses included in data processing and communication charges	0	0	$ 1,300,000
Unfunded Loan Commitment
Other Commitments [Line Items]
Allowance for unfunded commitments	$ 2,200,000	$ 2,600,000

- Future Estimated Payments Rel

- Future Estimated Payments Related to Data Processing and Communication (Details) $ in Thousands	Dec. 31, 2017USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,018	$ 12,237
2,019	12,633
2,020	13,047
2,021	13,465
2,022	13,926
Total	$ 65,308

Income Taxes - Income Tax Expen

Income Taxes - Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Federal
Current									$ 32,282	$ 24,521	$ 24,825
Deferred									13,980	665	(427)
Total Federal									46,262	25,186	24,398
State
Current									323	248	0
Deferred									(148)	(129)	0
Total State									175	119	0
Total Federal and State	$ 22,255	$ 8,883	$ 8,604	$ 6,695	$ 6,510	$ 7,367	$ 5,496	$ 5,931	$ 46,437	$ 25,305	$ 24,398

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Income Taxes [Line Items]
Income tax expense due to re-measurement of deferred tax assets and liabilities as result of changes in federal tax rate	$ 13,400
Increase in income tax expense for re-measurement due to change in corporate tax rate	13,400
Valuation allowance related to gross deferred tax assets	3,598	$ 3,114
Deferred tax assets, net operating loss carry forwards	$ 36,000
Unrecognized tax benefits, accrued interest		$ 100	$ 100
Minimum
Schedule Of Income Taxes [Line Items]
Deferred tax assets, net operating loss carry forwards, expiration date	2,020
Maximum
Schedule Of Income Taxes [Line Items]
Deferred tax assets, net operating loss carry forwards, expiration date	2,037
PENNSYLVANIA
Schedule Of Income Taxes [Line Items]
Valuation allowance related to gross deferred tax assets	$ 3,600

Income Taxes - Statutory to Eff

Income Taxes - Statutory to Effective Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Statutory tax rate	35.00%	35.00%	35.00%
Low income housing tax credits	(2.90%)	(3.80%)	(4.40%)
Tax-exempt interest	(4.00%)	(4.40%)	(4.10%)
Bank owned life insurance	(0.80%)	(0.80%)	(0.80%)
Other	0.30%	0.20%	1.00%
Adjustment to net deferred tax assets for enacted changes in tax laws and rates	11.30%	0.00%	0.00%
Effective Tax Rate	38.90%	26.20%	26.70%

Income Taxes - Significant Comp

Income Taxes - Significant Components of Temporary Differences (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets:
Allowance for loan losses	$ 12,440	$ 19,446
Purchase accounting adjustments	0	365
Other employee benefits	3,095	3,983
Low income housing partnerships	3,213	4,845
Net adjustment to funded status of pension	6,481	10,018
Impairment of securities	300	1,318
State net operating loss carryforwards	3,598	3,114
Other	2,355	4,984
Gross Deferred Tax Assets	31,482	48,073
Less: Valuation allowance	(3,598)	(3,114)
Total Deferred Tax Assets	27,884	44,959
Deferred Tax Liabilities:
Net unrealized holding gains on securities available-for-sale	(638)	(2,557)
Prepaid pension	(1,749)	(2,770)
Deferred loan income	(2,937)	(3,815)
Purchase accounting adjustments	(100)	0
Depreciation on premises and equipment	(480)	(1,239)
Other	(1,401)	(1,766)
Total Deferred Tax liabilities	(7,305)	(12,147)
Net Deferred Tax Asset	$ 20,579	$ 32,812

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Change in Federal and State Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 804	$ 1,102	$ 284
Prior period tax positions, Increase	0	0	818
Prior period tax positions, Decrease	(37)	(449)	0
Current period tax positions	142	151	0
Reductions for statute of limitations expirations	0	0	0
Balance at End of Year	909	804	1,102
Amount That Would Impact the Effective Tax Rate if Recognized	$ 770	$ 610	$ 542

Tax Effects on Other Compreh114

Tax Effects on Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pre-Tax Amount
Other Comprehensive Income (Loss)	$ (6,267)	$ 4,075	$ (4,180)
Tax (Expense) Benefit
Other Comprehensive Income (Loss)	1,624	(1,402)	1,556
Net of Tax Amount
Other comprehensive income (loss), net of tax	(4,643)	2,673	(2,624)
Accumulated Net Investment Gain (Loss) Attributable to Parent
Pre-Tax Amount
Net change in unrealized gains on securities available-for-sale	(1,275)	(2,899)	(663)
Net available-for-sale securities losses (gains) reclassified into earnings	(3,000)	0	34
Tax (Expense) Benefit
Net change in unrealized gains on securities available-for-sale	448	1,006	232
Net available-for-sale securities losses (gains) reclassified into earnings	1,054	0	(12)
Net of Tax Amount
Net change in unrealized gains on securities available-for-sale	(827)	(1,893)	(431)
Net available-for-sale securities losses (gains) reclassified into earnings	(1,946)	0	22
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent
Pre-Tax Amount
Other Comprehensive Income (Loss)	(1,992)	6,974	(3,551)
Tax (Expense) Benefit
Other Comprehensive Income (Loss)	122	(2,408)	1,336
Net of Tax Amount
Other comprehensive income (loss), net of tax	$ (1,870)	$ 4,566	$ (2,215)

Employee Benefits - Additional

Employee Benefits - Additional Information (Details)	Dec. 19, 2017USD ($)retiree	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of consecutive years of employee's compensation		5 years
Number of total years of employee's compensation		10 years
Net actuarial loss included in accumulated other comprehensive losses expected to be recognized		$ 2,200,000
Future prior service credit to be recognized due to freeze of accrual benefits		0
Accumulated benefit obligation		106,700,000	$ 105,800,000
Number of retirees covered by annuity contract | retiree	124
Number of retirees covered by annuity contract receiving death benefits | retiree	30
Defined benefit obligations, death benefits	$ 2,000
Total premium paid out of plan's assets	$ 1,500,000	8,244,000	5,250,000
Pension contributions		0
Projected benefit obligation		106,664,000	105,834,000	$ 109,747,000
Net periodic benefit cost		(347,000)	296,000	1,736,000
Transfers of plan assets between levels 1 and 2 on recurring basis		0	0
Purchases, transfers or sales of level 3 plan assets		0
Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation		5,300,000	4,700,000
Net periodic benefit cost		500,000	500,000	600,000
Amounts before tax reflected in accumulated other comprehensive income (loss)		$ 2,700,000	2,500,000	2,100,000
Thrift Plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to the Thrift Plan		3.50%
Compensation expense		$ 1,800,000	$ 1,700,000	$ 1,500,000
Marketable equity securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation percentage		50.00%
Marketable equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation percentage		70.00%
Fixed Income | Minimum
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation percentage		30.00%
Fixed Income | Maximum
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation percentage		50.00%

Employee Benefits - Benefit Obl

Employee Benefits - Benefit Obligation and Plan Assets Deriving Funded Status (Details) - USD ($) $ in Thousands	Dec. 19, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in Projected Benefit Obligation
Projected benefit obligation at beginning of year		$ 105,834	$ 109,747
Service cost		0	463	$ 2,601
Interest cost		4,100	4,296	4,425
Actuarial loss		4,974	3,575
Curtailments		0	(6,997)
Benefits paid		(8,244)	(5,250)
Projected Benefit Obligation at End of Year		106,664	105,834	109,747
Change in Plan Assets
Fair value of plan assets at beginning of year		87,711	84,585
Actual return on plan assets		7,687	8,376
Benefits paid	$ (1,500)	(8,244)	(5,250)
Fair Value of Plan Assets at End of Year		87,154	87,711	$ 84,585
Funded Status		$ (19,510)	$ (18,123)

Employee Benefits - Accumulated

Employee Benefits - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Prior service credit	$ 0	$ 0
Net actuarial loss	(27,825)	(26,013)
Total (Before Tax Effects)	$ (27,825)	$ (26,013)

Employee Benefits - Actuarial W

Employee Benefits - Actuarial Weighted Average Assumptions Used in Determining Benefit Obligation (Details)		Dec. 31, 2017		Dec. 31, 2016
Retirement Benefits [Abstract]
Discount rate		3.75%		4.00%
Rate of compensation increase	[1]	0.00%	[2]	0.00%
[1]	Rate of compensation increase is not applicable for 2017 and 2016 due to the amendment to freeze benefit accruals under the qualified and nonqualified defined benefit pension plans effective March 31, 2016
[2]	Refer to Note 1 Summary of Significant Accounting Policies, Fair Value Measurements for a description of levels within the fair value hierarchy.

Employee Benefits - Components

Employee Benefits - Components of Net Periodic Pension Cost and Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of Net Periodic Pension Cost
Service cost—benefits earned during the period	$ 0	$ 463	$ 2,601
Interest cost on projected benefit obligation	4,100	4,296	4,425
Expected return on plan assets	(6,313)	(5,780)	(7,180)
Amortization of prior service credit	0	(11)	(138)
Recognized net actuarial loss	1,866	2,345	2,028
Curtailment gain	0	(1,017)	0
Net Periodic Pension Expense	(347)	296	1,736
Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income (Loss)
Net actuarial (gain) loss	3,678	(6,018)	5,678
Recognized net actuarial loss	(1,866)	(2,345)	(2,028)
Recognized prior service credit	0	1,029	138
Total (Before Tax Effects)	1,812	(7,334)	3,788
Total Recognized in Net Benefit Cost and Other Comprehensive (Loss)/Income (Before Tax Effects)	$ 1,465	$ (7,038)	$ 5,524

- Actuarial Weighted Average As

- Actuarial Weighted Average Assumptions Used in Determining Net Periodic Pension Cost (Details)		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Discount rate		4.00%		4.25%	4.00%
Rate of compensation increase	[1]	0.00%	[2]	0.00%
Rate of compensation increase(1)	[2]			3.00%	3.00%
Expected return on assets		7.50%		7.50%	8.00%
[1]	Rate of compensation increase is not applicable for 2017 and 2016 due to the amendment to freeze benefit accruals under the qualified and nonqualified defined benefit pension plans effective March 31, 2016
[2]	Refer to Note 1 Summary of Significant Accounting Policies, Fair Value Measurements for a description of levels within the fair value hierarchy.

Employee Benefits - Estimated F

Employee Benefits - Estimated Future Benefit Payments (Details) $ in Thousands	Dec. 31, 2017USD ($)
Retirement Benefits [Abstract]
2,018	$ 7,175
2,019	7,090
2,020	6,956
2,021	7,157
2,022	7,106
2023-2027	$ 32,658

Employee Benefits - Pension Pla

Employee Benefits - Pension Plan Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value Measurements, Recurring - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	$ 87,074	$ 87,613
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[2]	1,780	3,336
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[3]	27,738	27,279
Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[4]	4,016	3,362
Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[5]	53,540	53,636
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	85,294	84,277
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[2]	0	0
Level 1 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[3]	27,738	27,279
Level 1 | Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[4]	4,016	3,362
Level 1 | Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[5]	53,540	53,636
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	1,780	3,336
Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[2]	1,780	3,336
Level 2 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[3]	0	0
Level 2 | Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[4]	0	0
Level 2 | Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[5]	0	0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	0	0
Level 3 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[2]	0	0
Level 3 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[3]	0	0
Level 3 | Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[4]	0	0
Level 3 | Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[5]	$ 0	$ 0
[1]	Refer to Note 1 Summary of Significant Accounting Policies, Fair Value Measurements for a description of levels within the fair value hierarchy.
[2]	This asset class includes FDIC insured money market instruments.
[3]	This asset class includes a variety of fixed income mutual funds which primarily invest in investment grade rated securities. Investment managers have discretion to invest in fixed income related securities including futures, options and other derivatives. Investments may be made in currencies other than the U.S. dollar.
[4]	The sole investment within this asset class is MSCI EAFE Index iShares.
[5]	This asset class includes individual domestic equities invested in an active all-cap strategy. It may also include convertible bonds.

Incentive and Restricted Sto123

Incentive and Restricted Stock Plan and Dividend Reinvestment Plan - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 19, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonstatutory stock options granted since 2006 (in shares)		0
Number of shares expired (in shares)	155,500
Weighted average exercise price of shares expired (in dollars per share)	$ 37.86
Unrecognized compensation expense to be recognized		$ 2.7
Weighted average compensation expense recognize period		1 year 7 months 26 days
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 3	$ 2.5	$ 1.7
Tax benefit realized on compensation expense		$ 1.1	$ 0.9	$ 0.6
2014 Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of common stock authorized (in shares)		750,000
Stock plan expiration period		10 years
Number of nonstatutory stock options outstanding (in shares)		0
2014 Stock Plan | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted (in shares)		90,115	110,643
2014 Stock Plan | Restricted Stock | Employees and Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted (in shares)		366,570
2014 Stock Plan | Restricted Stock | Outside Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted (in shares)		12,728	15,613	16,142
Awards vesting period		1 year
LTIP | Time-based and Performance-based Restricted Stocks | Senior Management
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted (in shares)		77,387	95,030	71,699
LTIP | Time-based Restricted Shares | Senior Management
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock award vesting after year two, percent		50.00%
Restricted stock award vesting after year three, percent		50.00%
LTIP | Performance-based Restricted Shares | Senior Management
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards vesting period		3 years

Incentive and Restricted Sto124

Incentive and Restricted Stock Plan and Dividend Reinvestment Plan - Non-vested Restricted Stock Granted (Details) - 2014 Stock Plan - Restricted Stock - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock
Non-vested at beginning of the year (in shares)	225,500	150,356
Granted (in shares)	90,115	110,643
Vested (in shares)	83,958	32,164
Forfeited (in shares)	11,089	3,335
Non-vested at end of the year (in shares)	220,568	225,500	150,356
Weighted Average Grant Date Fair Value
Non-vested at beginning of the year (in USD per share)	$ 30.19	$ 26.16	$ 26.34
Granted (in USD per share)	35.19	25.58
Vested (in USD per share)	24.82	25.03
Forfeited (in USD per share)	29.56	26.04
Non-vested at end of the year (in USD per share)	$ 30.19	$ 26.16	$ 26.34

Parent Company Condensed Fin125

Parent Company Condensed Financial Information - Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Cash	$ 117,152	$ 139,486	$ 99,399	$ 109,580
Other assets	95,799	102,166
Total Assets	7,060,255	6,943,053
LIABILITIES
Other liabilities	65,252	57,556
Total Liabilities	6,176,224	6,101,097
Total Shareholders’ Equity	884,031	841,956	792,237	608,389
Total Liabilities and Shareholders’ Equity	7,060,255	6,943,053
Parent Company
ASSETS
Cash	21,310	17,057	$ 12,595	$ 38,028
Investments in bank subsidiary	857,293	819,531
Investments in nonbank subsidiaries	19,569	21,980
Other assets	7,272	4,694
Total Assets	905,444	863,262
LIABILITIES
Long-term debt	20,619	20,619
Other liabilities	794	687
Total Liabilities	21,413	21,306
Total Shareholders’ Equity	884,031	841,956
Total Liabilities and Shareholders’ Equity	$ 905,444	$ 863,262

Parent Company Condensed Fin126

Parent Company Condensed Financial Information - Statements of Net Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Income Statements, Captions [Line Items]
Total Income	$ 67,855	$ 66,723	$ 64,914	$ 61,150	$ 59,096	$ 57,808	$ 55,850	$ 55,019	$ 260,642	$ 227,774	$ 203,548
Interest expense on long-term debt									463	670	790
Total Expense	37,947	36,553	36,597	36,808	35,625	34,439	34,753	38,416	147,907	143,232	136,717
Provision for income taxes	$ 22,255	$ 8,883	$ 8,604	$ 6,695	$ 6,510	$ 7,367	$ 5,496	$ 5,931	46,437	25,305	24,398
Net Income									72,968	71,392	67,081
Parent Company
Condensed Income Statements, Captions [Line Items]
Dividends from subsidiaries									36,169	34,134	75,413
Investment income									22	17	19
Total Income									36,191	34,151	75,432
Interest expense on long-term debt									955	854	773
Other expenses									3,801	4,012	3,687
Total Expense									4,756	4,866	4,460
Income before income tax and undistributed net income of subsidiaries									31,435	29,285	70,972
Provision for income taxes									(1,596)	(1,697)	(1,549)
Income before income tax and undistributed net income of subsidiaries									33,031	30,982	72,521
Bank subsidiary									40,877	40,051	(5,064)
Nonbank subsidiaries									(940)	359	(376)
Net Income									$ 72,968	$ 71,392	$ 67,081

Parent Company Condensed Fin127

Parent Company Condensed Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING ACTIVITIES
Net Income	$ 72,968	$ 71,392	$ 67,081
Tax benefit from stock-based compensation	0	(9)	(53)
Net Cash Provided by Operating Activities	114,236	96,796	60,584
INVESTING ACTIVITIES
Net Cash Used in Investing Activities	(212,415)	(616,653)	(405,737)
FINANCING ACTIVITIES
(Purchase) Sale of treasury shares, net	(689)	(115)	(182)
Cash dividends paid to common shareholders	(28,569)	(26,784)	(24,487)
Tax benefit from stock-based compensation	0	9	53
Net Cash Provided by Financing Activities	75,845	559,944	334,972
Net increase (decrease) in cash	(22,334)	40,087	(10,181)
Cash and cash equivalents at beginning of year	139,486	99,399	109,580
Cash and Cash Equivalents at End of Year	117,152	139,486	99,399
Parent Company
OPERATING ACTIVITIES
Net Income	72,968	71,392	67,081
Equity in undistributed (earnings) losses of subsidiaries	(39,937)	(40,410)	5,440
Tax benefit from stock-based compensation	0	(9)	(53)
Other	480	379	3,129
Net Cash Provided by Operating Activities	33,511	31,352	75,597
INVESTING ACTIVITIES
Net investments in subsidiaries	0	0	(38,404)
Acquisitions	0	0	(29,510)
Net Cash Used in Investing Activities	0	0	(67,914)
FINANCING ACTIVITIES
Repayment of junior subordinated debt	0	0	(8,500)
(Purchase) Sale of treasury shares, net	(689)	(115)	(182)
Cash dividends paid to common shareholders	(28,569)	(26,784)	(24,487)
Tax benefit from stock-based compensation	0	9	53
Net Cash Provided by Financing Activities	(29,258)	(26,890)	(33,116)
Net increase (decrease) in cash	4,253	4,462	(25,433)
Cash and cash equivalents at beginning of year	17,057	12,595	38,028
Cash and Cash Equivalents at End of Year	$ 21,310	$ 17,057	$ 12,595

Regulatory Matters - Additional

Regulatory Matters - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Banking and Thrift [Abstract]
Total trust preferred securities	$ 20
Junior subordinated debt, included in Tier 2 capital	$ 25

Regulatory Matters - Summary of

Regulatory Matters - Summary of Risk-Based Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
S&T
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Leverage Ratio, Actual Amount	$ 628,876	$ 582,155
Leverage Ratio, Actual Ratio	9.17%	8.98%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 274,254	$ 259,170
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	$ 342,818	$ 323,963
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%
Tier One Common Equity Capital	$ 608,876	$ 562,155
Tier One Common Equity Capital to Risk Weighted Assets	10.71%	10.04%
Tier One Common Equity Capital Required for Capital Adequacy	$ 255,778	$ 252,079
Tier One Common Equity Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Tier One Common Equity Capital Required to be Well Capitalized	$ 369,457	$ 364,114
Tier One Common Equity Capital Required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	$ 628,876	$ 582,155
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	11.06%	10.39%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 341,037	$ 336,105
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	6.00%	6.00%
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	$ 454,717	$ 448,140
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 713,056	$ 664,184
Total Capital (to Risk-Weighted Assets), Actual Ratio	12.55%	11.86%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 454,717	$ 448,140
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	$ 568,396	$ 560,175
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	10.00%	10.00%
S&T Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Leverage Ratio, Actual Amount	$ 582,929	$ 542,048
Leverage Ratio, Actual Ratio	8.52%	8.39%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 273,538	$ 258,460
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	$ 341,922	$ 323,075
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%
Tier One Common Equity Capital	$ 582,929	$ 542,048
Tier One Common Equity Capital to Risk Weighted Assets	10.29%	9.71%
Tier One Common Equity Capital Required for Capital Adequacy	$ 255,024	$ 251,213
Tier One Common Equity Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Tier One Common Equity Capital Required to be Well Capitalized	$ 368,368	$ 362,864
Tier One Common Equity Capital Required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	$ 582,929	$ 542,048
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	10.29%	9.71%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 340,032	$ 334,951
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	6.00%	6.00%
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	$ 453,375	$ 446,601
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 666,560	$ 622,469
Total Capital (to Risk-Weighted Assets), Actual Ratio	11.76%	11.15%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 453,375	$ 446,602
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	$ 566,719	$ 558,252
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	10.00%	10.00%

Selected Financial Data (Detail

Selected Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 67,855	$ 66,723	$ 64,914	$ 61,150	$ 59,096	$ 57,808	$ 55,850	$ 55,019	$ 260,642	$ 227,774	$ 203,548
Interest expense	10,027	9,267	8,344	7,272	6,638	6,353	6,142	5,382	34,909	24,515	15,997
Provision for loan losses	982	2,850	4,869	5,183	5,586	2,516	4,848	5,014	13,883	17,965	10,388
Net Interest Income After Provision for Loan Losses	56,846	54,606	51,701	48,695	46,872	48,939	44,860	44,623	211,850	185,294	177,163
Security (losses) gains, net	(986)	0	3,617	370	0	0	0	0
Noninterest income	13,636	13,551	12,648	12,626	12,922	13,448	12,448	15,817	55,462	54,635	51,033
Noninterest expense	37,947	36,553	36,597	36,808	35,625	34,439	34,753	38,416	147,907	143,232	136,717
Income Before Taxes	31,549	31,604	31,369	24,883	24,169	27,948	22,555	22,024
Provision for income taxes	22,255	8,883	8,604	6,695	6,510	7,367	5,496	5,931	46,437	25,305	24,398
Net Income Allocated to Common Shareholders	$ 9,294	$ 22,721	$ 22,765	$ 18,188	$ 17,659	$ 20,581	$ 17,059	$ 16,093	$ 72,726	$ 71,167	$ 66,801
Per Share Data
Common earnings per share-diluted (in USD per share)	$ 0.27	$ 0.65	$ 0.65	$ 0.52	$ 0.51	$ 0.59	$ 0.49	$ 0.46	$ 2.09	$ 2.05	$ 1.98
Dividends declared per common share (in USD per share)	0.22	0.20	0.20	0.20	0.20	0.19	0.19	0.19	$ 0.82	$ 0.77	$ 0.73
Common book value (in USD per share)	$ 25.28	$ 25.37	$ 24.90	$ 24.45	$ 24.12	$ 24.02	$ 23.63	$ 23.23

Subsequent Event (Details)

Subsequent Event (Details) - USD ($) $ in Millions	Jan. 01, 2018	Feb. 28, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Percentage of equity owned			100.00%
Subsequent Event | Subsidiary | S&T Evergreen Insurance, LLC
Subsequent Event [Line Items]
Proceeds from sale of majority interest in subsidiary	$ 5
Recognized gain on sale of subsidiary		$ 1
Subsequent Event | S&T Evergreen Insurance, LLC
Subsequent Event [Line Items]
Percentage of equity owned	30.00%