Derivative Financial Instruments Outstanding Interest Rate Swap Agreements (Details) $ in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 AUD ($)
Fixed rate debt [Member]
Derivative [Line Items]
Probable future debt	$ 300,000
Variable rate debt [Member]
Derivative [Line Items]
Probable future debt	$ 300,000
Interest Rate Swap 4 [Member]
Derivative [Line Items]
Effective Date	Dec. 01, 2016
Settlement Date	Dec. 01, 2021
Notional Amount		$ 372,600
Pay Fixed Rate	2.44%	2.44%
Interest Rate Swap 5 [Member]
Derivative [Line Items]
Effective Date	Dec. 01, 2016
Settlement Date	Dec. 01, 2021
Notional Amount		$ 110,746
Pay Fixed Rate	2.44%	2.44%
Interest Rate Swap 6 [Member]
Derivative [Line Items]
Effective Date	Dec. 01, 2016
Settlement Date	Dec. 01, 2021
Notional Amount		$ 34,155
Pay Fixed Rate	2.44%	2.44%
Interest Rate Swap 7 [Member]
Derivative [Line Items]
Effective Date	Aug. 31, 2018
Settlement Date	Aug. 31, 2021
Notional Amount	$ 100,000
Pay Fixed Rate	2.70%	2.70%
Interest Rate Swap 8 [Member]
Derivative [Line Items]
Effective Date	Aug. 31, 2018
Settlement Date	Aug. 31, 2021
Notional Amount	$ 100,000
Pay Fixed Rate	2.71%	2.71%
Interest Rate Swap 9 [Member]
Derivative [Line Items]
Effective Date	Aug. 31, 2018
Settlement Date	Aug. 31, 2048
Notional Amount	$ 100,000
Pay Fixed Rate	2.84%	2.84%
Mandatory Termination Date	Aug. 31, 2023
Interest Rate Swap 10 [Member]
Derivative [Line Items]
Effective Date	Aug. 31, 2018
Settlement Date	Aug. 31, 2048
Notional Amount	$ 100,000
Pay Fixed Rate	2.87%	2.87%
Mandatory Termination Date	Aug. 31, 2023
Interest Rate Swap 11 [Member]
Derivative [Line Items]
Effective Date	Aug. 31, 2018
Settlement Date	Aug. 31, 2048
Notional Amount	$ 100,000
Pay Fixed Rate	2.84%	2.84%
Mandatory Termination Date	Aug. 31, 2023