Derivative Financial Instruments Outstanding Interest Rate Swap Agreements (Details) $ in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 AUD ($)
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 [Member]
Derivative [Line Items]
Derivative, Settlement Date	Sep. 30, 2026
Interest Rate Swap 1 [Member]
Derivative [Line Items]
Effective Date	Sep. 30, 2016
Expiration Date	Sep. 30, 2026
Notional Amount	$ 100,000
Pay Fixed Rate	2.76%	2.76%
Interest Rate Swap 2 [Member]
Derivative [Line Items]
Effective Date	Sep. 30, 2016
Expiration Date	Sep. 30, 2026
Notional Amount	$ 100,000
Pay Fixed Rate	2.74%	2.74%
Interest Rate Swap 3 [Member]
Derivative [Line Items]
Effective Date	Sep. 30, 2016
Expiration Date	Sep. 30, 2026
Notional Amount	$ 100,000
Pay Fixed Rate	2.73%	2.73%
Interest Rate Swap 4 [Member]
Derivative [Line Items]
Effective Date	Dec. 01, 2016
Expiration Date	Dec. 01, 2021
Notional Amount		$ 93,150
Pay Fixed Rate	2.44%	2.44%
Interest Rate Swap 5 [Member]
Derivative [Line Items]
Effective Date	Dec. 01, 2016
Expiration Date	Dec. 01, 2021
Notional Amount		$ 93,150
Pay Fixed Rate	2.44%	2.44%
Interest Rate Swap 6 [Member]
Derivative [Line Items]
Effective Date	Dec. 01, 2016
Expiration Date	Dec. 01, 2021
Notional Amount		$ 93,150
Pay Fixed Rate	2.44%	2.44%
Interest Rate Swap 7 [Member]
Derivative [Line Items]
Effective Date	Dec. 01, 2016
Expiration Date	Dec. 01, 2021
Notional Amount		$ 93,150
Pay Fixed Rate	2.44%	2.44%
Interest Rate Swap 8 [Member]
Derivative [Line Items]
Effective Date	Dec. 01, 2016
Expiration Date	Dec. 01, 2021
Notional Amount		$ 55,373
Pay Fixed Rate	2.44%	2.44%
Interest Rate Swap 9 [Member]
Derivative [Line Items]
Effective Date	Dec. 01, 2016
Expiration Date	Dec. 01, 2021
Notional Amount		$ 55,373
Pay Fixed Rate	2.44%	2.44%
Interest Rate Swap 10 [Member]
Derivative [Line Items]
Effective Date	Dec. 01, 2016
Expiration Date	Dec. 01, 2021
Notional Amount		$ 34,155
Pay Fixed Rate	2.44%	2.44%