Revenue Recognition - Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Other non-current assets	$ 2,976	$ 1,636
Liabilities:
Deferred income taxes	8,178	12,043
Stockholders' equity:
Retained earnings	48,905	$ 124,442
Difference between Revenue Guidance in Effect before and after Topic 606
Assets:
Other non-current assets	(1,152)
Liabilities:
Deferred income taxes	(128)
Stockholders' equity:
Retained earnings	(1,024)
Calculated under Revenue Guidance in Effect before Topic 606
Assets:
Other non-current assets	1,824
Liabilities:
Deferred income taxes	8,050
Stockholders' equity:
Retained earnings	$ 47,881