Shareholders' Equity (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Proceeds from issuance of common shares, net		$ 4,530,000	$ 289,122,000	$ 649,690,000
Common Stock, Shares Authorized		600,000,000	600,000,000
Line of credit balance		$ 71,000,000	$ 106,000,000
Secondary Offering [Member]
Stock Issued During Period, Value, Other	$ 14.25
At-the-market equity issuance [Member]
Proceeds from issuance of common shares, net	$ 571,900,000	$ 4,500,000	$ 289,100,000	$ 77,800,000
Stock Issued During Period, Shares, New Issues	41,400,000	233,000	16,400,000	4,800,000
Gross Proceeds [Member] | At-the-market equity issuance [Member]
Proceeds from issuance of common shares, net	$ 590,000,000	$ 5,000,000	$ 292,300,000	$ 79,300,000
Unsecured Line of Credit DRLP [Member]
Line of credit balance					$ 285,000,000
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