Loans and the Allowance for Credit Losses - Additional Information (Detail)		6 Months Ended	12 Months Ended
	Apr. 05, 2018 USD ($)	Jun. 30, 2018 USD ($) Contract	Dec. 31, 2017 USD ($) Contract	Dec. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Deferred loan (costs) and fees		$ 2,400,000	$ 1,600,000
Unamortized discount on purchased loans from acquisitions		31,400,000	21,900,000
Aggregate amount of residential real estate, home equity and consumer loans classified as substandard		20,300,000	22,800,000
Internally assigned loan grades to residential real estate, home equity and consumer loans		1,300,000	2,500,000
Loans acquired with deteriorated credit quality		10,573,000	8,074,000
Acquired loans, sold book value		17,700,000
Acquired loans, sold		12,900,000
Allowance for loan losses		$ 47,638,000	$ 45,284,000		$ 44,909,000	$ 43,674,000
Number of restructured contracts greater than $0 million | Contract		0	0
Threshold for TDR		$ 1,000,000	$ 1,000,000
Accruing and non accrual TDR permitted interest-only payment period		3 months
Unfunded commitments to debtors for impaired loans		$ 300,000	100,000
Other real estate owned		4,334,000	5,195,000
First Sentry Bancshares, Inc. [Member]
Financing Receivable, Recorded Investment [Line Items]
Unamortized discount on purchased loans from acquisitions		11,900,000
Portfolio loans, net of unearned income	$ 465,900,000
Fair value of acquired loans	448,339,000
Fair market value adjustment of acquired loans	432,300,000
Fair market value adjustment of acquired loans	10,300,000
Loans acquired with deteriorated credit quality	5,500,000	3,000,000
Loans acquired with deteriorated credit quality, fair value	3,100,000
Loans acquired with deteriorated credit quality outstanding customer balance		5,400,000
Acquired impaired loans allowances for loan losses		0
First Sentry Bancshares, Inc. [Member] | Cost Recovery Method [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans acquired with deteriorated credit quality, fair value	$ 700,000
Other Acquired Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans acquired with deteriorated credit quality		7,600,000		$ 8,000,000
Loans acquired accretable		7,100,000
Allowance for loan losses		2,200,000		2,000,000
Other Acquired Loans [Member] | Cost Recovery Method [Member]
Financing Receivable, Recorded Investment [Line Items]
Fair value of acquired loans		4,100,000		$ 4,300,000
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans acquired with deteriorated credit quality		1,100,000	714,000
Allowance for loan losses		3,893,000	3,206,000		$ 3,717,000	$ 4,106,000
Other real estate owned		800,000	1,500,000
Foreclosure proceedings in process on residential real estate loans		$ 5,700,000	$ 3,500,000