Stock Based Compensation Plans - Non-employee Directors Compensation Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Goods and Nonemployee Services Transaction [Line Items]
Non-employee director stock at lower cost	85.00%
Non-employee share based compensation expense	$ 922	$ 922	$ 905
Shares issued under non-employee director plan (in shares)	17,829	24,443	18,570
Shares Issued - Deferred [Member]
Share-based Goods and Nonemployee Services Transaction [Line Items]
Shares issued under non-employee director plan (in shares)	10,974	13,644	10,406
Non employee director stock, cumulative deferred shares (in shares)	171,519
Shares Issued - Not Deferred [Member]
Share-based Goods and Nonemployee Services Transaction [Line Items]
Shares issued under non-employee director plan (in shares)	6,855	10,799	8,164