Debt (Westpac Bank Corporate Credit Facility) (Narrative) (Details) $ in Thousands, NZD in Millions			9 Months Ended	12 Months Ended
	Apr. 26, 2017 NZD	Apr. 26, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Contractual facility			$ 272,717	$ 266,134
Westpac Bank Corporate Credit Facility, Tranch 1 [Member]
Debt Instrument [Line Items]
Contractual facility	NZD 35.0	$ 24,500
Maturity date	Mar. 31, 2018	Mar. 31, 2018	Dec. 31, 2018