Stockholders' Equity (Details) (USD $)	6 Months Ended			6 Months Ended		1 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2012 Cantor Agreement	Jan. 31, 2011 Cantor Agreement	Sep. 30, 2012 Cantor Amendment	Jun. 30, 2013 Cantor Amendment
Stockholders' Equity
Common stock issued (in shares)	80,977,696		64,359,513	2,450,000			2,433,608
Common stock available for sale (in shares)					5,000,000
Net proceeds from sale of common stock	$ 114,187,000	$ 51,957,000		$ 8,500,000			$ 17,100,000
Aggregate offering price of additional shares which the entity can issue as per amended agreement						44,000,000	4,400,000
Fixed commission rate (as a percent)						3.00%
Notice period for termination of agreement						10 days
Common stock in an underwritten public offering issued (in shares)	13,800,000	12,075,000
Shares issued on exercise of over-allotment option by underwriters (in shares)	1,800,000	1,575,000
Net proceeds from sale of stock in an underwritten public offering	$ 97,000,000	$ 43,400,000