Derivative Instruments (Narrative) (Details) $ in Thousands, CAD in Millions				9 Months Ended
	Aug. 12, 2015 USD ($) swap_agreements	Sep. 06, 2013 USD ($) swap_agreements	Jan. 05, 2012 USD ($) swap_agreements	Sep. 30, 2015 USD ($) properties CAD / $	Sep. 30, 2014 USD ($)	Sep. 30, 2015 CAD CAD / $	Dec. 31, 2014 USD ($)	Feb. 28, 2014 CAD CAD / $
credit risk related contingent features default on debt amount				$ 25,000
Derivative Liability, Fair Value, Gross Liability				8,600			$ 5,100
Derivative Asset				29,900			9,700
Derivative Asset, Fair Value, Gross Asset				38,500			$ 14,800
Proceeds from settlement of derivative				0	$ (5,725)
Cash Flow Hedging [Member]
Estimated amount to be reclassified from accumulated other comprehensive income to other expense in the next twelve months				2,800
Interest Rate Swap [Member]
Number of entered into interest rate swap agreements (in interest rate swaps) | swap_agreements	2	3	3
Amount of hedged term loan			$ 240,000
Derivative fixed interest rate	2.94%	2.38%	2.51%
Derivative, Notional Amount		$ 240,000	$ 240,000
Fair value of derivatives in a liability position				8,600
Assets needed to settle obligations under the agreements				8,800
Interest Rate Risk [Member]
Estimated amount to be reclassified from accumulated other comprehensive income to other expense in the next twelve months				$ 4,600
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
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		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
Minimum [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount	$ 60,000
Maximum [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount	$ 300,000