Derivative Instruments (Narrative) (Details) $ in Thousands, CAD in Millions			12 Months Ended
	Aug. 12, 2015 USD ($) swap_agreements	Sep. 06, 2013 USD ($) swap_agreements	Dec. 31, 2016 USD ($) properties CAD / $	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2016 CAD CAD / $	Feb. 28, 2014 CAD CAD / $
credit risk related contingent features default on debt amount			$ 25,000
Derivative Liability, Fair Value, Gross Liability			2,500	$ 5,700
Derivative Asset			33,400	36,500
Derivative Asset, Fair Value, Gross Asset			35,900	42,200
Proceeds from settlement of derivative			0	$ 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]
Derivative, Fixed Interest Rate	2.94%	3.78%
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net			2,800
Number of entered into interest rate swap agreements | swap_agreements	2	3
Derivative, Notional Amount		$ 240,000
Fair value of derivatives in a liability position			2,500
Assets Needed for Immediate Settlement, Aggregate Fair Value			$ 448
ERROR in label resolution.
Derivative, Forward Exchange Rate | CAD / $			1.05			1.05
Monthly CAD Denominated Cash Flows Properties Under Hedges of Foreign Exchange Risk			$ 13,500
Number of Canadian properties exposed to foreign currency exchange risk | properties			4
Derivative, Notional Amount			$ 98,100			CAD 100.0
Minimum [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount	$ 60,000
Maximum [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount	$ 300,000
Canada, Dollars | Currency Forward Agreements Member | Net Investment Hedging [Member]
Derivative, Forward Exchange Rate | CAD / $			1.06			1.06	1.13
Derivative, Notional Amount | CAD						CAD 100.0	CAD 100.0
United States of America, Dollars | Currency Forward Agreements Member | Net Investment Hedging [Member]
Derivative, Notional Amount			$ 94,300				CAD 88.1