Debt and Capital Markets (Schedule of Long-term Debt Instruments) (Details) $ in Thousands						3 Months Ended		9 Months Ended
	Oct. 01, 2015 USD ($) shares	Aug. 06, 2015 USD ($) properties	Jul. 31, 2015 USD ($) properties	Apr. 24, 2015 USD ($)	Mar. 06, 2015 USD ($) properties	Sep. 30, 2015 USD ($) shares	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) shares	Sep. 30, 2014 USD ($)	Jul. 24, 2015 USD ($)	Mar. 31, 2015 USD ($)	Mar. 16, 2015 USD ($)	Dec. 31, 2014 shares
Debt Instrument [Line Items]
Costs associated with loan refinancing or payoff						$ 18	$ 0	$ 261	$ 0
Extinguishment of Debt, Amount		$ 61,500	$ 4,800		$ 30,400
Number of Properties Securing Mortgage Note | properties		6	1		1
Common Shares, shares issued | shares						61,498,390		61,498,390					58,952,404
Net proceeds from issuance of common shares								$ 99,760	$ 264,008
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%
Line of Credit Facility, Amount Outstanding										$ 65,000
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						$ 196,000		$ 196,000
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 or payoff						$ 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
direct share purchase plan [Member]
Debt Instrument [Line Items]
Common Shares, shares issued | shares						1,862,582		1,862,582
Net proceeds from issuance of common shares						$ 99,400
direct share purchase plan [Member] | Subsequent Event [Member]
Debt Instrument [Line Items]
Common Shares, shares issued | shares	595,506
Net proceeds from issuance of common shares	$ 31,700