Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 segment product	Mar. 31, 2014
Segment Information
Number of business segments in which the entity operates	1
Number of products from which the majority of the entity's product revenues have been derived	1
Segment information
Total revenues	$ 68,152	$ 67,002
United States
Segment information
Total revenues	57,717	55,468
Outside of the United States
Segment information
Total revenues	$ 10,435	$ 11,534