Borrowings (New Zealand Westpac Bank Corporate Credit Facility) (Narrative) (Details) $ in Thousands, $ in Millions			12 Months Ended
	Dec. 20, 2018 NZD ($)	Dec. 19, 2018 NZD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Contractual facility			$ 252,929	$ 271,732
Westpac Bank Corporate Credit Facility, Tranch 1 [Member]
Debt Instrument [Line Items]
Contractual facility	$ 32.0	$ 35.0
Interest rate	1.10%
Westpac Bank Corporate Credit Facility, Tranch 1 [Member] | Bank Bill Swap Bid Rate [Member]
Debt Instrument [Line Items]
Interest rate	1.75%
Westpac Bank Corporate Credit Facility, Tranch 2 [Member]
Debt Instrument [Line Items]
Contractual facility	$ 18.0