Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions - Fair Value of Derivatives (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($) item
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Amount to be reclassified as a reduction to interest expense during the next twelve months	$ 75,000
Designated as Hedging Instrument [Member]
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative liabilities designated as hedging instruments, fair value	$ 1,131,000	$ 58,000
Designated as Hedging Instrument [Member] \| Interest Rate Products
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative, number of transactions \| item	1	1
Notional Amount	$ 25,774,000	$ 25,774,000
Derivated liabilities designated as hedging instruments, classified in other liabilities	1,131,000	58,000
Designated as Hedging Instrument [Member] \| Interest Rate Products \| OSBC Affiliates
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Notional Amount	25,800,000
Not Designated as Hedging Instrument [Member]
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative assets not designated as hedging instruments, fair value	909,000	831,000
Total Derivative liabilities not designated as hedging instruments, fair value	$ 774,000	$ 698,000
Not Designated as Hedging Instrument [Member] \| Interest Rate Products \| Commercial Loan Customers
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative, number of transactions \| item	25	25
Notional Amount	$ 184,186,000	$ 188,931,000
Interest rate derivatives, derivative assets not designated as hedging instruments, classified in other assets	742,000	672,000
Interest rate derivatives, derivative not designated as hedging instruments, classified in other liabilities	$ 742,000	$ 672,000
Weighted average maturity	6 years 3 months 18 days	6 years 7 months 6 days
Not Designated as Hedging Instrument [Member] \| Interest Rate Swap [Member]
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Cash collateral pledged with financial institutions, dollars	$ 273,000	$ 260,000
Investment securities pledged with financial institutions, dollars	$ 2,200,000	$ 0
Number of correspondent financial institutions where investment securities are pledged \| item	1	1
Number of financial institutions where investment securities are required to be pledged \| item	2
Not Designated as Hedging Instrument [Member] \| Interest Rate Lock Commitments and Forward Contracts
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative, number of transactions \| item	107	63
Notional Amount	$ 33,185,000	$ 18,130,000
Interest rate derivatives, derivative assets not designated as hedging instruments, classified in other assets	$ 167,000	$ 159,000
Not Designated as Hedging Instrument [Member] \| Other Contracts
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative, number of transactions \| item	3	3
Notional Amount	$ 18,101,000	$ 18,155,000
Interest rate derivatives, derivative not designated as hedging instruments, classified in other liabilities	$ 32,000	$ 26,000

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

Represents the number of financial institutions where investment securities are pledged.

+ References

No definition available.

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Number of financial institutions where investment securities are required to be pledged.

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- Definition

Carrying amount as of the balance sheet date of cash collateral held for borrowed securities, for which the cash is restricted as to withdrawal or usage.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.1)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.(a),2)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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- Definition

Fair value of all liability derivatives designated as cash flow hedging instruments.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

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- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-Paragraph 4B
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- Definition

Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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The number of derivative instruments of a particular group held by the entity.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

Fair value as of the balance sheet date of all interest rate derivative liabilities designated as cash flow hedging instruments.

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Fair value as of the balance sheet date of all derivative assets not designated as hedging instruments.

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Fair value as of the balance sheet date of all derivative liabilities not designated as hedging instruments.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for debt facilities other than FHLB loans, repurchase agreements and letter of credit arrangements, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

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