Derivative Instruments (Outstanding Interest Rate Derivatives) (Details) $ in Thousands	Sep. 30, 2019 USD ($) $ / $	Dec. 31, 2018 USD ($) $ / $
Unsecured Term Loans | Canadian dollar (CAD)
Currently-paying contracts
Exchange rate | $ / $	0.76	0.73
Interest Rate Swap, 1.611
Currently-paying contracts
Notional Amount	$ 206,000	$ 206,000
Strike Rate	1.611%
Interest Rate Swap, 1.611 | Level 2
Currently-paying contracts
Fair value of derivatives	$ 178	1,976
Interest Rate Swap, 1.605
Currently-paying contracts
Notional Amount	$ 54,905	54,905
Strike Rate	1.605%
Interest Rate Swap, 1.605 | Level 2
Currently-paying contracts
Fair value of derivatives	$ 46	517
Interest Rate Swap 1.016
Currently-paying contracts
Notional Amount	$ 75,000	75,000
Strike Rate	1.016%
Interest Rate Swap 1.016 | Level 2
Currently-paying contracts
Fair value of derivatives	$ 547	2,169
Interest Rate Swap, 1.164
Currently-paying contracts
Notional Amount	$ 75,000	75,000
Strike Rate	1.164%
Interest Rate Swap, 1.164 | Level 2
Currently-paying contracts
Fair value of derivatives	$ 410	1,970
Interest Rate Swap, 1.435
Currently-paying contracts
Notional Amount	$ 300,000	300,000
Strike Rate	1.435%
Interest Rate Swap, 1.435 | Level 2
Currently-paying contracts
Fair value of derivatives	$ (379)	11,463
Interest Rate Swap, 0.779
Currently-paying contracts
Notional Amount	$ 74,397	72,220
Strike Rate	0.779%
Interest Rate Swap, 0.779 | Level 2
Currently-paying contracts
Fair value of derivatives	$ 1,052	2,024
Interest Rate Swap
Currently-paying contracts
Notional Amount	785,302	783,125
Interest Rate Swap | Level 2
Currently-paying contracts
Fair value of derivatives	$ 1,854	$ 20,119