Borrowings (New Zealand Westpac Bank Corporate Credit Facility) (Narrative) (Details) $ in Thousands, $ in Millions				1 Months Ended			12 Months Ended
	Apr. 26, 2017 NZD ($)	Apr. 26, 2017 USD ($)	May 21, 2015 NZD ($)	Oct. 31, 2016 NZD ($)	Oct. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 NZD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 NZD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2007	Dec. 31, 2016 USD ($)	May 21, 2015 USD ($)
Debt Instrument [Line Items]
Contractual facility							$ 271,732		$ 266,134
Previous New Zealand Corporate Credit Facility [Member]
Debt Instrument [Line Items]
Contractual facility			$ 50.0
Maturity date							Mar. 31, 2018
Number of tranches | item							2
Contractual facility														$ 34,200
Debt Instrument, Interest Rate, Stated Percentage														1.75%
Westpac Bank Corporate Credit Facility, Tranch 1 [Member]
Debt Instrument [Line Items]
Line of credit facility, increase in maximum borrowing capacity				$ 3.0	$ 2,100
Contractual facility	$ 53.0	$ 37,600		$ 50.0	$ 34,800					$ 35.0	$ 23,900
Maturity date	Mar. 31, 2018	Mar. 31, 2018					Dec. 31, 2019					Dec. 31, 2018
Contractual facility								$ 53.0					$ 36,900
Westpac Bank Corporate Credit Facility, Tranch 2 [Member]
Debt Instrument [Line Items]
Contractual facility						$ 10,400		$ 18.0	$ 12,500	$ 15.0	$ 10,300