Securities - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Estimated Fair Value
Less than 12 months	$ 11,330	$ 7,640
12 months or more	1,621	812
Total	12,951	8,452
Gross Unrealized Losses
Less than 12 months	(360)	(31)
12 months or more	(76)	(17)
Total	(436)	(48)
Federal agency mortgage-backed securities
Estimated Fair Value
Less than 12 months	6,078	4,130
12 months or more	1,621	812
Total	7,699	4,942
Gross Unrealized Losses
Less than 12 months	(185)	(20)
12 months or more	(76)	(17)
Total	(261)	(37)
Federal agency debt
Estimated Fair Value
Less than 12 months	5,252	3,510
Total	5,252	3,510
Gross Unrealized Losses
Less than 12 months	(175)	(11)
Total	$ (175)	$ (11)