Common Stock (Details) (USD $)	1 Months Ended	6 Months Ended			0 Months Ended		3 Months Ended
	Feb. 11, 2014	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Feb. 11, 2014 Common Stock	Jan. 10, 2014 Common Stock	Jun. 30, 2014 At-the-Market	May 28, 2014 At-the-Market	May 28, 2014 Maximum At-the-Market
Class of Stock [Line Items]
Issuance of shares of common stock (in shares)		11,996,800			1,325,000	10,500,000	171,800
Stock offering price (dollars per share)					$ 18.15	$ 18.15
Proceeds from stock offerings, net of offering costs	$ 205,200,000	$ 208,429,000	$ 208,934,000				$ 3,500,000
Common stock, par value (in dollars per share)		$ 0.001		$ 0.001
Amount of gross proceeds authorized								150,000,000
Percentage of sales commissions on stock sale price									2.00%
Sales commissions							$ 53,000