Stockholders' Equity (Details) (USD $)	1 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Class of stock
Common stock, shares authorized to issue		200,000,000	200,000,000
Common stock, shares issued		116,471,371	101,481,134
Common stock, shares outstanding		116,471,371	101,481,134
Proceeds from issuance of common stock	$ 173,300,000	$ 173,292,000
Net proceeds from transactions		62,958,000	9,470,000	3,567,000
Preferred Stock
Class of stock
Preferred stock, shares authorized to issue		15,000,000
Preferred stock, shares outstanding		0
Series C Junior Participating Preferred Stock
Class of stock
Preferred stock, shares issued		0
Preferred stock, shares outstanding		0
Common stock
Class of stock
Common stock, shares authorized to issue		200,000,000	200,000,000
Common stock, shares issued		116,471,371	101,481,134
Common stock, shares outstanding		116,471,371	101,481,134
Common shares reserved for future issuance		19,810,897
Shares sold	9,617,869
Issuance price per share (in dollars per share)	$ 19.00
Additional shares sold pursuant to option granted to the underwriters	617,869
Underwriting discounts and commissions and other estimated offering expenses	9,500,000
Options exercises and employee stock purchase plan issuances (in shares)		5,372,000	1,438,000	646,000
Net proceeds from transactions		$ 63,000,000	$ 9,500,000	$ 3,600,000