Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 item	Jun. 30, 2011
Segment Information
Number of business segments in which the entity operates			1
Segment information
Total revenues	$ 57,631	$ 50,846	$ 116,098	$ 100,656
United States
Segment information
Total revenues	51,820	46,065	105,234	91,888
Outside of the United States
Segment information
Total revenues	$ 5,811	$ 4,781	$ 10,864	$ 8,768

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

No definition available.

Name:	us-gaap_NumberOfOperatingSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration