Equity - Offerings of Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands				12 Months Ended
	Nov. 18, 2016	Mar. 28, 2016	Aug. 18, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Issuance of common stock, net (shares)	6,330	9,780	16,330
Proceeds from the issuance of common stock, net	$ 80,900	$ 105,800	$ 163,000	$ 170,323	$ 200,402	$ 162,963
Share price to the public (usd per share)	$ 13.35	$ 11.35	$ 10.50
Over-Allotment Option
Class of Stock [Line Items]
Issuance of common stock, net (shares)	830	1,280	2,130