Shareholders' Equity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Proceeds from issuance of common shares, net	$ 13,383	$ 220,258	$ 4,530
At-the-market equity issuance [Member]
Proceeds from issuance of common shares, net		$ 215,600	$ 4,500
Stock Issued During Period, Shares, New Issues		8,400,000	233,000
At-the-market equity issuance [Member] | Gross Proceeds [Member]
Proceeds from issuance of common shares, net		$ 218,200	$ 5,000