Business Combinations - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Dec. 11, 2017	Dec. 08, 2017
Business Acquisition [Line Items]
Common stock, closing price		$ 41.64
Merger Agreement [Member] | FNBB [Member]
Business Acquisition [Line Items]
Ownership percentage of combined entity post merger		24.00%
FNBB [Member]
Business Acquisition [Line Items]
Consideration value, per share		$ 40.81
Total consideration value		$ 315.3
FNBB [Member] | Merger Agreement [Member]
Business Acquisition [Line Items]
Average share price of the Company's Stock, percentage	120.00%
Initial price	$ 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	$ 49.78
FNBB [Member] | Merger Agreement [Member] | Maximum [Member]
Business Acquisition [Line Items]
Consideration value, per share	$ 33.18
Number of trading days	20 days
Shares held by acquiree	$ 119.0
FNBB [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Shares exchange ratio	0.98