Debt (Schedule of Long-term Debt Instruments) (Details) $ in Thousands				3 Months Ended		6 Months Ended
	Jul. 01, 2015 USD ($) shares	Apr. 24, 2015 USD ($)	Mar. 06, 2015 USD ($) properties	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jul. 24, 2015 USD ($)	Mar. 31, 2015 USD ($) shares	Mar. 16, 2015 USD ($)	Dec. 31, 2014 shares
Debt Instrument [Line Items]
Costs associated with loan refinancing				$ 243	$ 0	$ 243	$ 0
Extinguishment of Debt, Amount			$ 30,400
Number of Properties Securing Mortgage Note | properties			1
Common Shares, shares issued | shares									59,204,909		53,361,261
Net proceeds from issuance of common shares						240	$ 79,669
Term loan payable, due April 24,2020 [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 350,000
Debt Instrument, Basis Spread on Variable Rate		1.40%
Term loan payable, due July 23, 2018 [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity									$ 285,000
Unsecured Revolving Variable Rate Credit Facility, Variable Rate, Due July 23, 2017 [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity									$ 535,000
Line of Credit Facility, Amount Outstanding				100,000		$ 100,000
Senior Unsecured Notes Payable, 4.50 Percent, Due April 1, 2025 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount										$ 300,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate										0.00%
Senior Unsecured Notes Payable, Percent of Principal Amount Issued										0
Debt Covenant, Debt to Adjusted Total Assets Ratio, Maximum						0
Debt Covenant, Secured Debt to Adjusted Total Assets Ratio, Maximum						0
Debt Covenant, Debt Service Coverage Ratio, Minimum						0
Debt Covenant, Total Unencumbered Assets to Outstanding Unsecured Debt Ratio, Minimum						0.00%
Unsecured Revolving Variable Rate Credit Facility, Variable Rate, Due April 24,2019 [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Commitment Fee Percentage		0.25%
Costs associated with loan refinancing				$ 243
Line of Credit Facility, Current Borrowing Capacity		$ 650,000
Debt Instrument, Basis Spread on Variable Rate		1.25%
Combined unsecured revolving credit and term loan facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 1,000,000
Line of Credit Facility, Maximum Borrowing Capacity		$ 2,000,000
Subsequent Event [Member]
Debt Instrument [Line Items]
Common Shares, shares issued | shares	580,251
Net proceeds from issuance of common shares	$ 32,400
Subsequent Event [Member] | Term loan payable, due April 24,2020 [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Amount Outstanding								$ 65,000