Derivative Instruments - Foreign Currency Derivative Contracts Outstanding (Details) - Cash Flow Hedges NZD in Millions, AUD in Millions	Sep. 30, 2015 NZD	Sep. 30, 2015 AUD
Foreign exchange forward contracts | AUD
Derivative contracts
Derivative notional amount		AUD 325
Average rate	0.95	0.95
Foreign exchange forward contracts | NZD
Derivative contracts
Derivative notional amount | NZD	NZD 21
Average rate	0.80	0.80
Expected Maturity Date - 2015 | AUD
Derivative contracts
Derivative notional amount		AUD 56
Average rate	0.97	0.97
Expected hedge ratio	17.00%	17.00%
Expected Maturity Date - 2015 | NZD
Derivative contracts
Derivative notional amount | NZD	NZD 10
Average rate	0.80	0.80
Expected hedge ratio	29.00%	29.00%
Expected Maturity Date - 2016 | AUD
Derivative contracts
Derivative notional amount		AUD 158
Average rate	0.95	0.95
Expected hedge ratio	12.00%	12.00%
Expected Maturity Date - 2016 | NZD
Derivative contracts
Derivative notional amount | NZD	NZD 11
Average rate	0.80	0.80
Expected hedge ratio	15.00%	15.00%
Expected Maturity Date - 2017 | AUD
Derivative contracts
Derivative notional amount		AUD 105
Average rate	0.93	0.93
Expected hedge ratio	8.00%	8.00%
Expected Maturity Date - 2018 | AUD
Derivative contracts
Derivative notional amount		AUD 6
Average rate	0.92	0.92
Expected hedge ratio	4.00%	4.00%