SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended	9 Months Ended
	Dec. 13, 2017	Nov. 13, 2017	Nov. 10, 2017	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
SUBSEQUENT EVENTS
Repayment of note payable					$ 3,322	$ 2,214
Capital to risk based assets ratio					14.10%		12.90%
Subsequent Event
SUBSEQUENT EVENTS
Proceeds from stock offering			$ 64,500
Subsequent Event | Frost Bank | Line of Credit
SUBSEQUENT EVENTS
Revolving line of credit available	$ 30,000
Revolving line of credit draw period	12 months
Debt term	60 months
Line of credit payment terms	due and payable as accrued in four quarterly installments in the first 12 months and thereafter payments of principal in 20 quarterly installments are required together with accrued interest over a term of 60 months
Subsequent Event | Frost Bank | Line of Credit | Minimum
SUBSEQUENT EVENTS
Free cash flow coverage ratio	125.00%
Tangible net worth	$ 240,000
Subsequent Event | Frost Bank | Line of Credit | Maximum
SUBSEQUENT EVENTS
Capital to risk based assets ratio	12.00%
Subsequent Event | Frost Bank | Line of Credit | TEXAS | Maximum
SUBSEQUENT EVENTS
Capital to risk based assets ratio	15.00%
Notes payable
SUBSEQUENT EVENTS
Loan amount					$ 31,000
Debt term					7 years
Notes payable | Subsequent Event
SUBSEQUENT EVENTS
Repayment of note payable		$ 23,283
Change of Control and Non‑Competition Agreements | VB Texas, Inc.
SUBSEQUENT EVENTS
Deferred compensation expense					$ 0
Change of Control and Non‑Competition Agreements | VB Texas, Inc. | Subsequent Event
SUBSEQUENT EVENTS
Deferred compensation expense				$ 2,500
Initial public offering | Subsequent Event
SUBSEQUENT EVENTS
Stock issued (in shares)			2,760,000
Share price (in dollars per share)			$ 26.00
Underwriter option | Subsequent Event
SUBSEQUENT EVENTS
Stock issued (in shares)			360,000