Nature of Business and Significant Accounting Policies - Barter (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Barter transactions:
Barter revenue	$ 710	$ 934	$ 811
Barter expense	(676)	(756)	(791)
Barter revenue and expense	34	178	20
Advertising and Marketing costs:
Advertising and marketing costs	$ 3,300	$ 3,800	$ 3,500