Basis of Presentation - Additional Information (Details)			3 Months Ended		9 Months Ended
	Oct. 22, 2018 USD ($) $ / shares	Jan. 01, 2018 USD ($)	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 shares	Sep. 30, 2018 USD ($) Branch $ / shares	Dec. 31, 2017 USD ($)
Class Of Stock [Line Items]
Quarterly dividends declared, per share | $ / shares			$ 0.06		$ 0.12
ASC 860-50-35, Transfers and Servicing
Class Of Stock [Line Items]
Percent of remaining principal allowed to buy back under GNMA optional repurchase programs					100.00%
ASU 2014-09
Class Of Stock [Line Items]
Adjustment to retained earnings		$ 0
ASU 2016-01
Class Of Stock [Line Items]
Adjustment to retained earnings		$ 109,000
GNMA
Class Of Stock [Line Items]
Residential mortgage loans transferred by bank			$ 0		$ 0	$ 43,035,000
Delinquent GNMA loans			$ 58,435,000		$ 58,435,000
Subsequent Event
Class Of Stock [Line Items]
Quarterly dividends declared, per share | $ / shares	$ 0.08
Dividends , Date of record	Nov. 01, 2018
Dividends, Date to be paid	Nov. 15, 2018
Dividends, Date declared	Oct. 22, 2018
Dividends declared, Total	$ 2,548,000
Common stock repurchase expiration date	Oct. 22, 2019
Common stock repurchase value	$ 50,000,000
Subsequent Event | Maximum
Class Of Stock [Line Items]
Common stock repurchase amount	$ 50,000,000
James W. Ayers
Class Of Stock [Line Items]
Percentage of voting power of issued and outstanding common stock				44.00%
Secondary Offering | Common Stock
Class Of Stock [Line Items]
Shares issued during period | shares				3,680,000
FirstBank
Class Of Stock [Line Items]
Number of bank branches | Branch					56