Subsequent Events (Details) (USD $)	6 Months Ended			0 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 At-the-market equity offering programs	Jul. 31, 2013 Subsequent event At-the-market equity offering programs
Subsequent Events
Common stock, shares issued			95,000	165,718
Aggregate gross proceeds from shares sold			$ 8,700,000	$ 15,200,000
Net proceeds from sale of common stock	8,478,000	201,307,000	8,500,000	14,900,000
Aggregate value of shares available for issuance			$ 36,700,000	$ 21,500,000