Fair Value Measurements (Details) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis
Letters of credit outstanding	$ 2,100,000	$ 2,800,000
Liabilities, Fair Value Adjustment	0	0
Total
Fair Value, Assets and Liabilities Measured on Recurring Basis
Cash and cash equivalents	40,402,000	40,315,000
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis
Line of Credit Facility, Fair Value of Amount Outstanding	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis
Line of Credit Facility, Fair Value of Amount Outstanding	2,100,000	2,800,000
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis
Line of Credit Facility, Fair Value of Amount Outstanding	0	0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis
Assets, Fair Value Adjustment	0	0
Fair Value, Measurements, Recurring [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis
Cash and cash equivalents	40,402,000	40,315,000
Fair Value, Measurements, Recurring [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis
Cash and cash equivalents	$ 0	$ 0