Commitments and Contingencies Loss Contingency (Details) CAD in Millions		1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 12, 2014 CAD	Dec. 31, 2009 CAD	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 1990 USD ($)	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Guarantor Obligations, Current Carrying Value			$ 20,600,000		$ 20,600,000			$ 16,900,000
Guarantees
Letter of credit provided to entity | CAD		CAD 10
Litigation and Other Disputes
Accrued liabilities, litigations and disputes			11,900,000		11,900,000			13,100,000
Loss contingency, possible loss, portion not accrued			103,000,000		$ 103,000,000
Kaiser Tax, Civil and Labor Indemnity Reserve [Member]
Loss Contingencies [Line Items]
Equity interest sold (as a percent)					68.00%
Environmental matters Lowry [Member]
Environmental
Environmental remediation expense, pretax charge							$ 30,000,000
Environmental remediation threshold, assumed remediation cost			$ 120,000,000		$ 120,000,000
Inflation rate assumption, future costs (as a percent)			2.50%		2.50%
Risk free rate of return assumption (as a percent)			1.96%		1.96%
Site contingency, accrual, present value			$ 4,000,000		$ 4,000,000
Site contingency, accrual, undiscounted amount			6,000,000		6,000,000
David Hughes and 631992 Ontario Inc [Member]
Litigation and Other Disputes
Loss contingency, damages sought | CAD	CAD 1,400
Cervejarias Kaiser Brasil S.A. [Member]
Loss Contingencies [Line Items]
Gain (loss) discontinued operations			0	$ 2,900,000	(2,300,000)	$ 4,500,000
Other Noncurrent Liabilities [Member] | Canada [Member]
Loss Contingencies [Line Items]
Guarantor Obligations, Current Carrying Value			4,600,000		4,600,000			$ 4,400,000
Minimum [Member]
Litigation and Other Disputes
Loss Contingency, Estimate of Possible Loss			0		0
Maximum [Member]
Litigation and Other Disputes
Loss Contingency, Estimate of Possible Loss			128,000,000		128,000,000
Maximum [Member] | Kaiser purchased tax credits indemnity reserve, category two
Litigation and Other Disputes
Loss Contingency, Estimate of Possible Loss			107,300,000		107,300,000
Maximum [Member] | Kaiser Tax, Civil and Labor Indemnity Reserve [Member]
Litigation and Other Disputes
Loss Contingency, Estimate of Possible Loss			$ 68,000,000		$ 68,000,000