Stockholders' Equity (Details) (USD $)	3 Months Ended
	Mar. 31, 2013
Stockholders' Equity
Common stock issued (in shares)	13,800,000
Net proceeds from sale of common stock	$ 114,187,000
Net proceeds from sale of stock in an underwritten public offering	97,000,000
Cantor Amendment
Stockholders' Equity
Common stock issued (in shares)	2,433,608
Net proceeds from sale of common stock	$ 17,100,000