Securities - Unrealized Loss Positions (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($)
Fair value
Less Than 12 Months, Fair Value	$ 78,732,000	$ 92,173,000
12 Months or More, Fair Value	45,109,000	2,562,000
Total Fair Value	123,841,000	94,735,000
Unrealized Loss
Less Than 12 Months, Unrealized Loss	489,000	1,318,000
12 Months or More, Unrealized Loss	867,000	42,000
Total Unrealized Loss	$ 1,356,000	1,360,000
Other information
Number of positions considered temporarily impaired | security	200
Other than temporary impairment	$ 0
U.S. government agencies and corporations
Fair value
Less Than 12 Months, Fair Value	2,972,000	16,111,000
12 Months or More, Fair Value	13,201,000
Total Fair Value	16,173,000	16,111,000
Unrealized Loss
Less Than 12 Months, Unrealized Loss	31,000	414,000
12 Months or More, Unrealized Loss	310,000
Total Unrealized Loss	341,000	414,000
Mortgage-backed securities
Fair value
Less Than 12 Months, Fair Value	57,116,000	54,227,000
12 Months or More, Fair Value	22,545,000	671,000
Total Fair Value	79,661,000	54,898,000
Unrealized Loss
Less Than 12 Months, Unrealized Loss	341,000	621,000
12 Months or More, Unrealized Loss	420,000	1,000
Total Unrealized Loss	761,000	622,000
Obligations of states and political subdivisions
Fair value
Less Than 12 Months, Fair Value	18,644,000	21,835,000
12 Months or More, Fair Value	9,363,000	1,891,000
Total Fair Value	28,007,000	23,726,000
Unrealized Loss
Less Than 12 Months, Unrealized Loss	117,000	283,000
12 Months or More, Unrealized Loss	137,000	41,000
Total Unrealized Loss	$ 254,000	$ 324,000
Standard & Poor's, A Rating | U.S. government agencies and corporations
Other information
Percentage of portfolio with rating	98.00%
Percentage of portfolio with rating in an unrealized loss positions	99.00%
Not A Rating Or Better | U.S. government agencies and corporations
Other information
Percentage of portfolio with rating in an unrealized loss positions	1.00%