Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Measurements Disclosure [Line Items]
Percentage of securities using market-based information	100.00%
Minimum reserves for commercial loan	$ 500,000
Allocated allowance for loan losses	3,600,000
Fair value of allocated allowance	4,100,000
Estimated costs to sell	400,000
Provision for fair value measurements included in allowance for loan losses	3,700,000
Carrying amount of OREO	23,300,000
Written down of OREO	18,700,000
Fair value of OREO	21,300,000
Estimated cost to sell OREO	2,600,000
Loss from OREO included in earnings	$ 4,600,000
Short-term borrowings approximate fair value for amounts that mature, days, less than	90 days
Fair value, measurements, nonrecurring
Fair Value Measurements Disclosure [Line Items]
Impaired loans, carrying amount	$ 7,300,000
Allocated allowance for loan losses	4,110,000	$ 6,383,000
Fair value of OREO	21,336,000	14,198,000
Assets Still Held
Fair Value Measurements Disclosure [Line Items]
Change in unrealized gains or losses included in earnings relating to assets still held	0	$ 0
Equity securities
Fair Value Measurements Disclosure [Line Items]
Transfer of equity security to non-marketable equity securities, included in other assets	600,000
Yadkin Financial Corporation (YDKN) | Other debt securities
Fair Value Measurements Disclosure [Line Items]
Certain loans acquired in transfer not accounted for as debt securities, outstanding balance	$ 12,000,000