Derivatives - Narrative (Details)	3 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) derivative_swap	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Derivatives, Fair Value [Line Items]
Threshold for collateral requirements		$ 250,000
Collateral posted		300,000	$ 301,000
Notional amount		$ 12,500,000	18,900,000
Number of instruments | derivative_swap		4
Derivative fee income		$ 26,000	0	$ 0
Commitments to originate loans held for sale		43,602,000	62,421,000
RML
Derivatives, Fair Value [Line Items]
Commitments to originate loans held for sale		43,600,000	$ 62,400,000
Variable to Fixed
Derivatives, Fair Value [Line Items]
Notional amount		$ 6,300,000
Number of instruments | derivative_swap		1
Fixed to Variable
Derivatives, Fair Value [Line Items]
Notional amount		$ 6,300,000
Number of instruments | derivative_swap		1
Interest rate swaps | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Cash pledged to collateralize fair value exposure on interest rate swap		$ 400,000
Unrealized gain on interest rate swap		$ 184,000
Subordinated Debt
Derivatives, Fair Value [Line Items]
Junior subordinated debenture	$ 10,000,000
Effective interest rate on junior subordinated debenture	3.72%
Debt instrument, interest rate during period		2.96%
Subordinated Debt | LIBOR
Derivatives, Fair Value [Line Items]
Debt instrument, basis spread on variable rate	1.37%