Shareholders' Equity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Proceeds from issuance of common shares, net	$ 220,258	$ 4,530	$ 289,122
Common Stock, Shares Authorized	600,000,000	600,000,000
At-the-market equity issuance [Member]
Proceeds from issuance of common shares, net	$ 215,600	$ 4,500	$ 289,100
Stock Issued During Period, Shares, New Issues	8,400,000	233,000	16,400,000
Gross Proceeds [Member] | At-the-market equity issuance [Member]
Proceeds from issuance of common shares, net	$ 218,200	$ 5,000	$ 292,300
After Increase Approved by Shareholders [Member]
Common Stock, Shares Authorized			600,000,000
Prior to Increase Approved by Shareholders [Member]
Common Stock, Shares Authorized			400,000,000