Business Combinations - Additional Information (Detail)	Dec. 11, 2017 USD ($) $ / shares	Dec. 08, 2017 USD ($) $ / shares	Mar. 18, 2016 USD ($) Branches	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Common stock, closing price | $ / shares		$ 41.64
Goodwill recognized				$ 64,311,000	$ 64,311,000
Merger Agreement [Member] | FNBB [Member]
Business Acquisition [Line Items]
Ownership percentage of combined entity post merger		24.00%
Bank of America [Member]
Business Acquisition [Line Items]
Total consideration value			$ 3,204,000
Number of branches | Branches			3
Acquisition, completion date			Oct. 28, 2015
Cash consideration			$ 3,204,000
Deposits			161,231,000
Loans			289,000
Cash			159,520,000
Goodwill recognized			849,000
Increase in net book value of land and buildings acquired			309,000
Recognized intangible related to acquisition			2,046,000
Bank of America [Member] | Core Deposit Intangibles [Member]
Business Acquisition [Line Items]
Recognized intangible related to acquisition			$ 2,046,000
Percentage of acquired intangibles			1.50%
Finite lived intangible asset estimated useful life			7 years
FNBB [Member]
Business Acquisition [Line Items]
Consideration value, per share | $ / shares		$ 40.81
Total consideration value		$ 315,300,000
FNBB [Member] | Merger Agreement [Member]
Business Acquisition [Line Items]
Average share price of the Company's Stock, percentage	120.00%
Initial price | $ / shares	$ 41.48
Merger Agreement termination conditions	(i) the average share price of the Company’s common stock for the 20 day period up to and including the fifth day prior to the closing date (the “Average Closing Share Price”) is greater than $49.78, which equals 120% of the average share price of the Company’s Stock for the 20 trading-day period up to and including December 8, 2017, which was $41.48 (the “Initial Price”) and (ii) the Company’s common stock outperforms the KBW Regional Banking Index by more than 20%, unless FNBB agrees that the Exchange Ratio will be reduced and fewer shares of the Company’s common stock will be issued to FNBB shareholders on a per share basis. Conversely, FNBB can terminate the Merger Agreement if both (i) the Average Closing Share Price is less than $33.18, which is equivalent to 80% of the Initial Price, and (ii) the Company’s common stock underperforms the KBW Regional Banking Index by more than 20%, unless the Company agrees that the Exchange Ratio will be increased and more shares of the Company common stock will be issued to FNBB shareholders on a per share basis.
Percentage of initial price	80.00%
FNBB [Member] | Merger Agreement [Member] | KBW Regional Banking Index [Member]
Business Acquisition [Line Items]
Percentage of common stock outperforms in excess of KBW Regional Banking Index	20.00%
FNBB [Member] | Merger Agreement [Member] | Minimum [Member]
Business Acquisition [Line Items]
Consideration value, per share | $ / shares	$ 49.78
FNBB [Member] | Merger Agreement [Member] | Maximum [Member]
Business Acquisition [Line Items]
Consideration value, per share | $ / shares	$ 33.18
Number of trading days	20 days
Shares held by acquiree	$ 119,000,000
FNBB [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Shares exchange ratio	0.98