Income and Mining Taxes - Unrecognized tax benefit (Details) $ in Millions	Sep. 30, 2015 USD ($)
Total unrecognized tax benefit	$ 98
Unrecognized tax benefits affecting effective tax rate	35
Minimum
Significant change in unrecognized tax benefits is reasonably possible, estimated range of change	50
Maximum
Significant change in unrecognized tax benefits is reasonably possible, estimated range of change	$ 55