Accounting Pronouncements Recently Issued Narrative - Impact of implementation of new accounting standards (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounting Standards Update 2016-01 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 0
Accounting Standards Update 2016-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 67
Retained Earnings [Member] | Accounting Standards Update 2016-01 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ (157)
Retained Earnings [Member] | Accounting Standards Update 2016-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption		(117)
Common Stock [Member] | Accounting Standards Update 2016-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 184