Derivative Instruments (Narrative) (Details)	0 Months Ended		12 Months Ended							3 Months Ended	12 Months Ended
	Jan. 05, 2012 swap_agreements	Feb. 07, 2011 swap_agreements	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($) swap_agreements	Dec. 31, 2011 Cash Flow Hedging [Member] USD ($)	Jan. 05, 2012 Interest Rate Swap [Member] USD ($)	Dec. 31, 2011 Cross Currency Swaps [Member] USD ($)	Dec. 31, 2011 Cross Currency Swaps [Member] CAD	Dec. 31, 2011 Cross Currency Swaps [Member] Cash Flow Hedging [Member] CAD	Mar. 29, 2011 Currency Forward Agreements [Member] Net Investment Hedging [Member] USD ($)	Dec. 31, 2011 Currency Forward Agreements [Member] Net Investment Hedging [Member] USD ($) properties	Dec. 31, 2011 Currency Forward Agreements [Member] Net Investment Hedging [Member] CAD properties	Feb. 03, 2011 Currency Forward Agreements [Member] Net Investment Hedging [Member] USD ($)	Feb. 03, 2011 Currency Forward Agreements [Member] Net Investment Hedging [Member] CAD	Dec. 31, 2011 Currency Forward Agreements [Member] Cash Flow Hedging [Member] CAD	Feb. 29, 2012 New Contract [Member] Currency Forward Agreements [Member] Cash Flow Hedging [Member] USD ($)	Feb. 29, 2012 New Contract [Member] Currency Forward Agreements [Member] Cash Flow Hedging [Member] CAD
Number of terminated interest rate swap agreements		6		3
Outstanding notional amount of interest rate swaps			$ 87,700,000	$ 118,600,000		$ 240,000,000
Derivative, Fixed Interest Rate						2.66%
Interest rate swap reclassified into earning as expenses			4,600,000	(8,700,000)
Number of entered into interest rate swap agreements	3
Amount of hedged term loan						240,000,000
Fixed notional value of cross-currency swaps							71,500,000	76,000,000
Net exchange rate, CAD to US dollar								1.05				1.04			0.99		1.00
Monthly CAD Denominated Cash Flows Properties Under Hedges of Foreign Exchange Risk									500,000							500,000
Number of Canadian properties exposed to foreign currency exchange risk											4	4
Cash flows on the properties									13,000,000
Estimated amount to be reclassified from accumulated other comprehensive income to other expense in the next twelve months					300,000
Net investment hedges notional value											96,100,000	100,000,000
Notional value of foreign currency sell contract													201,500,000	200,000,000
Loss on settlement of derivative contract conjunction with sale of property										$ 4,300,000