Debt (Westpac Bank Corporate Credit Facility) (Narrative) (Details) $ in Thousands, $ in Millions				6 Months Ended	12 Months Ended
	Apr. 26, 2017 NZD ($)	Apr. 26, 2017 USD ($)	Apr. 25, 2017	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Contractual facility				$ 270,746	$ 271,732
Westpac Bank Corporate Credit Facility, Tranch 1 [Member]
Debt Instrument [Line Items]
Contractual facility	$ 53.0	$ 35,900
Maturity date	Dec. 31, 2018	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2019