Commitments, Contingencies and Credit Risk - Financial Instruments Whose Contractual Amounts Represent Credit Risk (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Concentration Of Risk Financial Statement Captions [Line Items]
Contractual amounts represent credit risk	$ 94,798	$ 63,704
Commitments to Grant Mortgage Loans
Fair Value Concentration Of Risk Financial Statement Captions [Line Items]
Contractual amounts represent credit risk	54,423	33,813
Unfunded Commitments Under Lines of Credit
Fair Value Concentration Of Risk Financial Statement Captions [Line Items]
Contractual amounts represent credit risk	33,641	27,404
Standby Letters of Credit
Fair Value Concentration Of Risk Financial Statement Captions [Line Items]
Contractual amounts represent credit risk	$ 6,734	$ 2,487

This item represents disclosure of all significant concentrations of credit risk or market risk arising from the subject financial instrument (as defined), whether from an individual counterparty or groups of counterparties.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(25))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(19))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(17))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_FairValueConcentrationOfRiskCommitments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant