Derivative Instruments (Narrative) (Details)		0 Months Ended						3 Months Ended
	Mar. 31, 2013 Cash Flow Hedging [Member] USD ($)	Jan. 05, 2012 Interest Rate Swap [Member] USD ($) swap_agreements	Mar. 31, 2013 Interest Rate Swap [Member] USD ($)	Mar. 31, 2013 Interest Rate Risk [Member] USD ($)	Mar. 31, 2013 Cross Currency Swaps [Member] USD ($)	Mar. 31, 2013 Cross Currency Swaps [Member] CAD	Mar. 31, 2013 Cross Currency Swaps [Member] Cash Flow Hedging [Member] CAD	Mar. 31, 2013 Currency Forward Agreements [Member] Net Investment Hedging [Member] USD ($) properties	Mar. 31, 2013 Currency Forward Agreements [Member] Net Investment Hedging [Member] CAD properties	Oct. 11, 2012 Currency Forward Agreements [Member] Cash Flow Hedging [Member] USD ($)	Oct. 11, 2012 Minimum [Member] Currency Forward Agreements [Member] Cash Flow Hedging [Member] CAD	Oct. 11, 2012 Maximum [Member] Currency Forward Agreements [Member] Cash Flow Hedging [Member] CAD
Outstanding notional amount of interest rate swaps		$ 240,000,000
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		0.98	0.99
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	300,000			1,600,000
Net investment hedges notional value								96,100,000	100,000,000
Fair value of derivatives in a liability position			5,500,000
Assets needed to settle obligations under the agreements			$ 5,600,000