Derivative Financial Instruments Outstanding Interest Rate Swap Agreements (Details) $ in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 AUD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Derivatives [Line Items]
Long-term debt		$ 2,391,633	$ 2,256,312
Interest Rate Swap 5 [Member]
Derivatives [Line Items]
Effective Date	Dec. 01, 2016
Expiration Date	Dec. 01, 2021
Notional Amount		$ 372,600
Derivative, Forward Interest Rate	2.44%	2.44%
Interest Rate Swap 6 [Member]
Derivatives [Line Items]
Effective Date	Dec. 01, 2016
Expiration Date	Dec. 01, 2021
Notional Amount		$ 110,745
Derivative, Forward Interest Rate	2.44%	2.44%
Interest Rate Swap 7 [Member]
Derivatives [Line Items]
Effective Date	Dec. 01, 2016
Expiration Date	Dec. 01, 2021
Notional Amount	$ 34,155
Derivative, Forward Interest Rate	2.44%	2.44%
Interest Rate Swap 8 [Member]
Derivatives [Line Items]
Effective Date	Aug. 31, 2018
Expiration Date	Aug. 31, 2021
Notional Amount		$ 100,000
Derivative, Forward Interest Rate	2.70%	2.70%
Interest Rate Swap 9 [Member]
Derivatives [Line Items]
Effective Date	Aug. 31, 2018
Expiration Date	Aug. 31, 2021
Notional Amount		$ 100,000
Derivative, Forward Interest Rate	2.71%	2.71%
Interest Rate Swap 10 [Member]
Derivatives [Line Items]
Derivative, Mandatory Termination Date	Aug. 31, 2023
Effective Date	Aug. 31, 2018
Expiration Date	Aug. 31, 2048
Notional Amount		$ 100,000
Derivative, Forward Interest Rate	2.84%	2.84%
Interest Rate Swap 11 [Member]
Derivatives [Line Items]
Derivative, Mandatory Termination Date	Aug. 31, 2023
Effective Date	Aug. 31, 2018
Expiration Date	Aug. 31, 2048
Notional Amount		$ 100,000
Derivative, Forward Interest Rate	2.87%	2.87%
Interest Rate Swap 12 [Member]
Derivatives [Line Items]
Derivative, Mandatory Termination Date	Aug. 31, 2023
Effective Date	Aug. 31, 2018
Expiration Date	Aug. 31, 2048
Notional Amount		$ 100,000
Derivative, Forward Interest Rate	2.84%	2.84%
Fixed rate debt [Member]
Derivatives [Line Items]
Probable future borrowings		$ 300,000
Variable rate debt [Member]
Derivatives [Line Items]
Probable future borrowings		$ 300,000