Derivative Instruments (Narrative) (Details)	0 Months Ended	3 Months Ended		0 Months Ended						3 Months Ended
	Feb. 07, 2011 swap_agreements	Mar. 31, 2012 USD ($)	Mar. 31, 2012 Cash Flow Hedging [Member] USD ($)	Jan. 05, 2012 Interest Rate Swap [Member] USD ($) swap_agreements	Mar. 31, 2012 Interest Rate Swap [Member] USD ($)	Mar. 31, 2012 Interest Rate Risk [Member] USD ($)	Mar. 31, 2012 Cross Currency Swaps [Member] USD ($)	Mar. 31, 2012 Cross Currency Swaps [Member] CAD	Mar. 31, 2012 Cross Currency Swaps [Member] Cash Flow Hedging [Member] CAD	Mar. 31, 2012 Currency Forward Agreements [Member] Net Investment Hedging [Member] USD ($) properties	Mar. 31, 2012 Currency Forward Agreements [Member] Net Investment Hedging [Member] CAD properties	Mar. 29, 2011 Currency Forward Agreements [Member] Net Investment Hedging [Member] USD ($)	Feb. 03, 2011 Currency Forward Agreements [Member] Net Investment Hedging [Member] USD ($)	Feb. 03, 2011 Currency Forward Agreements [Member] Net Investment Hedging [Member] CAD	Feb. 29, 2012 Currency Forward Agreements [Member] Cash Flow Hedging [Member] USD ($)	Feb. 29, 2012 Minimum [Member] Currency Forward Agreements [Member] Cash Flow Hedging [Member] CAD	Feb. 29, 2012 Maximum [Member] Currency Forward Agreements [Member] Cash Flow Hedging [Member] CAD
Number of terminated interest rate swap agreements (in interest rate swaps)	6
Outstanding notional amount of interest rate swaps		$ 87,700,000		$ 240,000,000
Interest rate swap reclassified into earning as expenses		4,600,000
Number of entered into interest rate swap agreements (in interest rate swaps)				3
Amount of hedged term loan				240,000,000
Derivative fixed interest rate				2.66%
Number of Canadian properties exposed to foreign currency exchange risk (in properties)										4	4
Fixed notional value of cross-currency swaps							71,500,000	76,000,000
Net exchange rate, CAD to US dollar								1.05			1.04					1	1.01
Cash flows on the properties									13,000,000
Monthly CAD denominated cash flows															500,000
Estimated amount to be reclassified from accumulated other comprehensive income to other expense in the next twelve months			600,000			1,500,000
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
Fair value of derivatives in a liability position					4,500,000
Assets needed to settle obligations under the agreements					$ 4,600,000