Credit Facility and Accrued Interest Expense (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	1 Months Ended				0 Months Ended			1 Months Ended	12 Months Ended
	Sep. 30, 2013	Feb. 28, 2011 Revolving working capital credit facility	Dec. 31, 2012 Revolving working capital credit facility	Dec. 31, 2011 Revolving working capital credit facility	Feb. 07, 2010 Revolving working capital credit facility	Jul. 30, 2013 Revolving working capital credit facility Subsequent event	Feb. 07, 2010 Revolving working capital credit facility Minimum	Feb. 28, 2011 Revolving working capital credit facility Minimum	Feb. 28, 2011 Revolving working capital credit facility Prime rate	Dec. 31, 2011 Revolving working capital credit facility Prime rate
Credit facility and accrued interest expense
Maximum borrowing capacity				$ 25,000	$ 7,000
Ratio of current assets to current liabilities (excluding deferred revenue)							1.25
Variable base rate									prime rate	prime rate
Interest rate (as a percent)									1.00%	0.50%
Fee for unused portion (as a percent)		0.20%
Average principal amount of borrowings per quarter for which no fee charged								10,000
Amount of fee charged if average principal amount of borrowing per quarter is maintained above a specified amount		0
Borrowing base as a percentage of eligible accounts receivable				80.00%
Amount outstanding			6,000
Repayments of principal borrowings and accrued interest expense	$ 6,000					$ 6,033