Equity - Issuance of Common Stock (Details) - USD ($) $ in Thousands	1 Months Ended				12 Months Ended
	Mar. 31, 2017	May 31, 2016	Oct. 31, 2015	Apr. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Issuance of Common Stock
Net proceeds from issuance of common shares					$ 704,938	$ 383,572	$ 793,559
Common stock
Issuance of Common Stock
Shares issued	11,850,000	6,500,000	11,500,000	5,500,000
Underwriting discounts and offering costs	$ 29,800	$ 12,100	$ 22,000	$ 1,400
Net proceeds from issuance of common shares	$ 704,900	$ 383,600	$ 517,100	$ 276,400