Derivative Instruments and Hedging Activities - Narrative (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) instrument	Dec. 31, 2014 USD ($)	Dec. 31, 2018 USD ($)
Derivative [Line Items]
Loans receivable with fixed rates of interest maturity period		10 years
Loans receivable with fixed rates		$ 29,100,000
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	$ 18,216,000		$ 18,422,000
Derivative Assets	24,000		189,000
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	195,226,000		167,549,000
Derivative Assets	$ 455,000		490,000
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Loans receivable with fixed rates of interest maturity period	15 years
Derivative fixed interest rate	7.43%
Notional Amount	$ 390,000		418,000
Loans receivable fixed interest rate (percentage)	7.43%
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative Assets	$ 0		0
Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	$ 150,118,000		122,410,000
Derivative number of instruments held | instrument	22
Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative Assets	$ 0		0
Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Liabilities [Member]
Derivative [Line Items]
Underlying derivative at fair value	79,000
Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative asset notional amount		$ 20,000,000
Derivative fixed interest rate		2.10%
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months	81,000
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	16,881,000		17,076,000
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative Assets	$ 24,000		185,000
Fair Value Hedging [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Loans receivable with fixed rates of interest maturity period	10 years
Derivative fixed interest rate	5.83%
Notional Amount	$ 1,335,000		1,346,000
Loans receivable fixed interest rate (percentage)	5.83%
Fair Value Hedging [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative Assets	$ 0		$ 4,000
Minimum [Member] | Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative remaining maturity	1 year
Maximum [Member] | Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative remaining maturity	10 years
One-Month LIBOR [Member] | Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative basis spread on variable rate	2.24%
One-Month LIBOR [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative basis spread on variable rate	3.50%
Paying [Member] | Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	$ 87,700,000
Receiving [Member] | Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	62,400,000
Customer [Member] | Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Underlying derivative at fair value	$ 2,765,000