Derivative Instruments (Narrative) (Details)	9 Months Ended				0 Months Ended							9 Months Ended
	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2014 Cash Flow Hedging [Member] USD ($)	Sep. 06, 2013 Interest Rate Swap [Member] USD ($) swap_agreements	Jan. 05, 2012 Interest Rate Swap [Member] USD ($) swap_agreements	Sep. 30, 2014 Interest Rate Swap [Member] USD ($)	Sep. 30, 2014 Interest Rate Risk [Member] USD ($)	Sep. 30, 2014 Cross Currency Swaps [Member] CAD	Sep. 30, 2014 Cross Currency Swaps 2018 [Member] USD ($)	Sep. 30, 2014 Cross Currency Swaps 2018 [Member] CAD	Sep. 30, 2014 Currency Forward Agreements [Member] Net Investment Hedging [Member] USD ($) properties	Sep. 30, 2014 Currency Forward Agreements [Member] Net Investment Hedging [Member] CAD properties	Feb. 28, 2014 Currency Forward Agreements [Member] Net Investment Hedging [Member] CAD	Feb. 28, 2014 Currency Forward Agreements 2018 [Member] Net Investment Hedging [Member] CAD
credit risk related contingent features default on debt amount	$ 25,000,000
Derivative Liability, Fair Value, Gross Liability	5,100,000		4,500,000
Derivative Asset	3,400,000		1,600,000
Derivative Asset, Fair Value, Gross Asset	8,500,000		6,100,000
Number of entered into interest rate swap agreements (in interest rate swaps)					3	3
Amount of hedged term loan						240,000,000
Derivative fixed interest rate					2.38%	2.51%
Number of Canadian properties exposed to foreign currency exchange risk (in properties)												4	4
Derivative, Notional Amount					240,000,000	240,000,000				98,100,000	100,000,000	94,300,000	100,000,000	100,000,000	88,100,000
Net exchange rate, CAD to US dollar									1.05				1.06		1.13
Estimated amount to be reclassified from accumulated other comprehensive income to other expense in the next twelve months				900,000				1,600,000
Fair value of derivatives in a liability position							5,100,000
Assets needed to settle obligations under the agreements							5,300,000
Proceeds from settlement of derivative	$ (5,725,000)	$ 0