Issuance of Common Stock (Details) - USD ($) $ in Thousands	1 Months Ended		6 Months Ended
	Apr. 30, 2015	Apr. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Issuance of common stock
Net proceeds from issuance of common shares			$ 276,430	$ 528,627
Common stock
Issuance of common stock
Shares issued	5,500,000	13,800,000
Common stock purchased by the underwriters (in shares)		1,800,000
Underwriting discounts and offering costs	$ 1,400	$ 22,800
Net proceeds from issuance of common shares	$ 276,400	$ 528,600