Federal Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Composition of income tax expense (benefit)
Current	$ 1,199	$ 1,256	$ 916
Deferred	(28)	(129)	303
Change in corporate tax rate	1,246
Federal income tax expense (benefit)	$ 2,417	$ 1,127	$ 1,219