Equity - Issuance of Common Stock (Details) - USD ($) $ in Thousands	1 Months Ended				12 Months Ended
	May 31, 2016	Oct. 31, 2015	Apr. 30, 2015	Apr. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Issuance of common stock
Net proceeds from issuance of common shares					$ 383,572	$ 793,559	$ 528,615
Common stock
Issuance of common stock
Shares issued	6,500,000	11,500,000	5,500,000	13,800,000
Underwriting discounts and offering costs	$ 12,100	$ 22,000	$ 1,400	$ 22,800
Net proceeds from issuance of common shares	$ 383,600	$ 517,100	$ 276,400	$ 528,600