Fair Values of Financial Instruments - Assets Measured at Fair Value on a Non-recurring Basis (Details) - Nonrecurring - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	$ 0	$ 1,103,000
Level 1
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		0
Level 2
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		0
Level 3
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		1,103,000
Impaired residential mortgage loan
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		58,000
Impaired residential mortgage loan | Level 1
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		0
Impaired residential mortgage loan | Level 2
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		0
Impaired residential mortgage loan | Level 3
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		58,000
Impaired commercial real estate loan
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		463,000
Impaired commercial real estate loan | Level 1
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		0
Impaired commercial real estate loan | Level 2
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		0
Impaired commercial real estate loan | Level 3
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		463,000
Impaired commercial business loan
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		582,000
Impaired commercial business loan | Level 1
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		0
Impaired commercial business loan | Level 2
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		0
Impaired commercial business loan | Level 3
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		$ 582,000