Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands						3 Months Ended	9 Months Ended
	May 26, 2017 USD ($) $ / shares shares	Jan. 02, 2017 USD ($)	Sep. 15, 2016 USD ($) $ / shares shares	Aug. 15, 2016 USD ($)	Jun. 28, 2016 shares	Sep. 30, 2016	Sep. 30, 2017 USD ($) Branch $ / shares shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 $ / shares shares	Jul. 31, 2016 shares
Class Of Stock [Line Items]
Description of stock split							the Company declared a 100-for-1 stock split, increasing the number of issued and authorized shares from 171,800 to 17,180,000 and 250,000 to 25,000,000, respectively.
Stock split ratio					0.01
Common stock, shares issued | shares							30,526,592		24,107,660
Common stock, shares authorized | shares							75,000,000		75,000,000
Proceeds from issuance of common stock, net of offering costs | $							$ 153,356	$ 115,525
Common stock sold and issued, par value | $ / shares							$ 1		$ 1
Percent of remaining principal allowed to buy back under GNMA optional repurchase programs							100.00%
GNMA
Class Of Stock [Line Items]
Mortgage loans held for sale measured at fair value | $							$ 13,575
ASC 860-50-35, Transfers and Servicing
Class Of Stock [Line Items]
Adjustment to retained earnings | $		$ 615
Pro forma
Class Of Stock [Line Items]
Effective tax rate						37.21%		37.33%
Initial Public Offering
Class Of Stock [Line Items]
Common stock sold and issued, shares | shares			6,764,704
Common stock sold and issued, price per share | $ / shares			$ 19
Proceeds from issuance of common stock, net of offering costs | $			$ 115,525
Distribution to majority shareholder from net proceeds | $			55,000
Repayments of aggregate principal amount of subordinated notes from net proceeds | $			$ 10,075	$ 10,075
Private Placement
Class Of Stock [Line Items]
Common stock sold and issued, shares | shares	4,806,710
Proceeds from issuance of common stock, net of offering costs | $	$ 152,721
Common stock sold and issued, par value | $ / shares	$ 1.00
Common stock purchase price per share | $ / shares	$ 33.00
Placement agent and other offering costs | $	$ 5,901
Minimum
Class Of Stock [Line Items]
Common stock, shares issued | shares					171,800
Common stock, shares authorized | shares					250,000					25,000,000
Maximum
Class Of Stock [Line Items]
Common stock, shares issued | shares					17,180,000
Common stock, shares authorized | shares					25,000,000					75,000,000
FirstBank
Class Of Stock [Line Items]
Number of bank branches | Branch							63