Derivative Instruments (Narrative) (Details) $ in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($) swap_agreements	Jun. 30, 2019 CAD ($)	Dec. 31, 2018 USD ($)
credit risk related contingent features default on debt amount		$ 50.0
Derivative Liability, Fair Value, Gross Liability	$ 4.7		$ 0.0
Derivative Asset, Fair Value, Gross Asset	0.6		$ 10.6
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	4.3
Cash Flow Hedging [Member]
Estimated amount to be reclassified from accumulated other comprehensive income to other expense in the next twelve months	0.4
Interest Rate Risk [Member]
Estimated amount to be reclassified from accumulated other comprehensive income to other expense in the next twelve months	0.9
Cross Currency Swaps 2020 [Member]
Derivative, Notional Amount	$ 79.5	$ 100.0
Net exchange rate, CAD to US dollar	1.26	1.26
Foreign currency exposure	$ 13.5
Currency Forward Agreements [Member]
Number of Canadian properties exposed to foreign currency exchange risk (in properties)	4
Cross Currency Swap 2023 [Member] | Net Investment Hedging [Member]
Derivative, Notional Amount		$ 200.0
Net exchange rate, CAD to US dollar	1.32	1.32
Foreign currency exposure	$ 4.5
Number of Foreign Currency Derivatives Held	2	2
interest rate swap 3.15percent [Member] | Interest Rate Swap [Member]
Number of entered into interest rate swap agreements (in interest rate swaps) | swap_agreements	3
Derivative fixed interest rate	3.15%	3.15%
interest rate swap 3.15percent [Member] | Maximum [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount	$ 350.0
interest rate swap 3.35 percent [Member] | Interest Rate Swap [Member]
Derivative fixed interest rate	3.35%	3.35%
interest rate swap 3.35 percent [Member] | Minimum [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount	$ 50.0