Derivative Instruments (Narrative) (Details) $ in Thousands, CAD in Millions	9 Months Ended
	Sep. 30, 2016 USD ($) swap_agreements properties CAD / $	Sep. 30, 2016 CAD CAD / $	Dec. 31, 2015 USD ($)
credit risk related contingent features default on debt amount	$ 25,000
Derivative Liability, Fair Value, Gross Liability	7,000		$ 5,700
Derivative Asset	26,200		36,500
Derivative Asset, Fair Value, Gross Asset	33,200		$ 42,200
Cash Flow Hedging [Member]
Estimated amount to be reclassified from accumulated other comprehensive income to other expense in the next twelve months	2,600
Interest Rate Swap [Member]
Fair value of derivatives in a liability position	7,000
Assets needed to settle obligations under the agreements	704
Interest Rate Risk [Member]
Estimated amount to be reclassified from accumulated other comprehensive income to other expense in the next twelve months	$ 4,100
Cross Currency Swaps [Member]
Net exchange rate, CAD to US dollar | CAD / $	1.05	1.05
Cross Currency Swaps 2018 [Member]
Derivative, Notional Amount	$ 98,100	CAD 100.0
Description of Foreign Currency Exposure	13.5
Currency Forward Agreements [Member] | Net Investment Hedging [Member]
Number of Canadian properties exposed to foreign currency exchange risk (in properties) | properties	4
Derivative, Notional Amount	$ 94,300	CAD 100.0
Net exchange rate, CAD to US dollar | CAD / $	1.06	1.06
Currency Forward Agreements 2018 [Member] | Net Investment Hedging [Member]
Derivative, Notional Amount | CAD		CAD 88.1
Net exchange rate, CAD to US dollar | CAD / $	1.13	1.13
interest rate swap 3.78percent [Member] | Interest Rate Swap [Member]
Number of entered into interest rate swap agreements (in interest rate swaps) | swap_agreements	3
Derivative fixed interest rate	3.78%	3.78%
Derivative, Notional Amount	$ 240,000
interest rate swap 2.94percent [Member] [Member] | Interest Rate Swap [Member]
Number of entered into interest rate swap agreements (in interest rate swaps) | swap_agreements	2
Derivative fixed interest rate	2.94%	2.94%
interest rate swap 2.94percent [Member] [Member] | Minimum [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount	$ 60,000
interest rate swap 2.94percent [Member] [Member] | Maximum [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount	$ 300,000