Equity - Issuance of Common Stock (Details) - USD ($) $ in Thousands	1 Months Ended		9 Months Ended
	Mar. 31, 2017	May 31, 2016	Sep. 30, 2017	Sep. 30, 2016
Issuance of Common Stock
Net proceeds from issuance of common shares			$ 704,938	$ 383,572
Common stock
Issuance of Common Stock
Shares issued	11,850,000	6,500,000
Underwriting discounts and offering costs	$ 29,700	$ 12,100
Net proceeds from issuance of common shares	$ 704,900	$ 383,600