Issuance of Common Stock (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Oct. 31, 2015	Apr. 30, 2015	Apr. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Issuance of common stock
Net proceeds from issuance of common shares				$ 793,559	$ 528,615	$ 1,133,574
Common stock
Issuance of common stock
Shares issued	11,500,000	5,500,000	13,800,000
Common stock purchased by the underwriters (in shares)	1,500,000		1,800,000
Underwriting discounts and offering costs	$ 22,000	$ 1,400	$ 22,800
Net proceeds from issuance of common shares	$ 517,100	276,400	$ 528,600
Previous credit facility		$ 1,500,000