Equity - At-The-Market Offerings of Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands				3 Months Ended				12 Months Ended
	Nov. 18, 2016	Mar. 28, 2016	Aug. 18, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Remaining offering amount available				$ 236,100				$ 236,100
Number of shares (shares)	6,330	9,780	16,330
Gross proceeds	$ 80,900	$ 105,800	$ 163,000					$ 170,323	$ 200,402	$ 162,963
Maximum
Class of Stock [Line Items]
Authorized aggregate offering price of common stock						$ 300,000
At-The-Market Offering
Class of Stock [Line Items]
Number of shares (shares)				0	0	3,399	7,175	10,574
Average sales price per share (usd per share)				$ 0.00	$ 0.00	$ 18.82	$ 15.31	$ 16.43
Gross proceeds				$ 0	$ 0	$ 63,947	$ 109,813	$ 173,760