Shareholders' Equity (Details) (USD $)	1 Months Ended	12 Months Ended
	Jan. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Proceeds from issuance of common shares, net		$ 289,122,000	$ 649,690,000	$ 315,295,000
Common Stock, Shares Authorized		600,000,000	400,000,000
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	289,100,000	77,800,000	315,300,000
Stock Issued During Period, Shares, New Issues	41,400,000	16,400,000	4,800,000	22,700,000
Gross Proceeds [Member] | At-the-market equity issuance [Member]
Proceeds from issuance of common shares, net	590,000,000	292,300,000	79,300,000	322,200,000
Unsecured Line of Credit DRLP [Member]
Line of Credit Facility, Amount Outstanding		$ 106,000,000		$ 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