Stockholders' Equity (Details) (USD $)	6 Months Ended
	Jun. 30, 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
Controlled Equity Offering Program Amended [Member]
Stockholders' Equity
Common stock issued (in shares)	2,433,608
Net proceeds from sale of common stock	$ 17,100,000