Stockholders' Equity (Details) (USD $)	2 Months Ended	6 Months Ended			1 Months Ended		6 Months Ended	12 Months Ended
	Mar. 31, 2012	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jul. 31, 2012 Controlled equity offering programme	Jan. 31, 2012 Controlled equity offering programme	Jun. 30, 2012 Controlled equity offering programme	Dec. 31, 2011 Controlled equity offering programme
Stockholders' Equity
Common stock the company may issue or sell (in shares)		297,000,000		297,000,000			5,000,000
Rate of commission of gross proceeds on each sale, maximum (as a percent)							5.00%
Notice period for termination of agreement							10 days
Common stock sold (in shares)		58,753,571		44,210,636	538,500	2,450,000		575,000
Net proceeds from sale of common stock		$ 51,957,000	$ 35,889,000		$ 2,800,000	$ 8,500,000		$ 2,200,000
Common stock available for sale (in shares)							1,975,000
Common stock in an underwritten public offering issued (in shares)	12,075,000
Shares issued on exercise of over-allotment option by underwriters (in shares)	1,575,000
Net proceeds from sale of stock in an underwritten public offering		$ 43,400,000