Derivative Instruments (Narrative) (Details) CAD in Millions, $ in Millions			3 Months Ended
	Aug. 12, 2015 USD ($) swap_agreements	Sep. 06, 2013 USD ($) swap_agreements	Mar. 31, 2016 USD ($) properties CAD / $	Mar. 31, 2016 CAD CAD / $	Dec. 31, 2015 USD ($)	Feb. 28, 2014 CAD CAD / $
credit risk related contingent features default on debt amount			$ 25.0
Derivative Liability, Fair Value, Gross Liability			9.2		$ 5.7
Derivative Asset			23.4		36.5
Derivative Asset, Fair Value, Gross Asset			32.6		$ 42.2
Cash Flow Hedging [Member]
Estimated amount to be reclassified from accumulated other comprehensive income to other expense in the next twelve months			2.5
Interest Rate Swap [Member]
Number of entered into interest rate swap agreements (in interest rate swaps) | swap_agreements	2	3
Derivative fixed interest rate	2.94%	3.78%
Derivative, Notional Amount		$ 240.0
Fair value of derivatives in a liability position			1.2
Assets needed to settle obligations under the agreements			1.2
Interest Rate Risk [Member]
Estimated amount to be reclassified from accumulated other comprehensive income to other expense in the next twelve months			$ 5.0
Cross Currency Swaps [Member]
Net exchange rate, CAD to US dollar | CAD / $			1.05	1.05
Cross Currency Swaps 2018 [Member]
Derivative, Notional Amount			$ 98.1	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			$ 94.3	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	$ 60.0
Maximum [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount	$ 300.0