Business Segment, Geographic and Major Customer Information (Revenue by Geographic Location) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue by geographic location
Revenue	$ 72,509	$ 79,458	$ 85,177	$ 79,954	$ 70,604	$ 64,337	$ 64,079	$ 63,812	$ 317,098	$ 262,832	$ 269,966
U.S.
Revenue by geographic location
Revenue									259,610	210,890	227,117
Other countries
Revenue by geographic location
Revenue									$ 57,488	$ 51,942	$ 42,849