Commitments and Contingencies Loss Contingency (Details) $ in Millions, CAD in Billions		3 Months Ended			6 Months Ended		12 Months Ended
	Dec. 12, 2014 CAD	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 1990 USD ($)
Loss Contingencies [Line Items]
Loss Contingency Accrual, Period Increase (Decrease)			$ 50.0
Guarantor Obligations, Current Carrying Value		$ 49.4			$ 49.4		$ 36.1
Litigation and Other Disputes
Accrued liabilities, litigations and disputes		20.2			20.2		27.7
Loss contingency, possible loss, portion not accrued		141.0			$ 141.0
Loss Contingency Accrual, Provision							$ 50.0
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								$ 30.0
Environmental remediation threshold, assumed remediation cost		$ 120.0			$ 120.0
Inflation rate assumption, future costs (as a percent)		2.50%			2.50%
Risk free rate of return assumption (as a percent)		2.58%			2.58%
Site contingency, accrual, present value		$ 3.0			$ 3.0
Site contingency, accrual, undiscounted amount		6.0			6.0
David Hughes and 631992 Ontario Inc [Member]
Litigation and Other Disputes
Loss contingency, damages sought | CAD	CAD 1.4
Cervejarias Kaiser Brasil S.A. [Member]
Loss Contingencies [Line Items]
Gain (loss) discontinued operations		1.6		$ (1.8)	1.0	$ (2.3)
Maximum [Member] | Kaiser purchased tax credits indemnity reserve, category two
Litigation and Other Disputes
Loss Contingency, Estimate of Possible Loss		105.8			105.8
Maximum [Member] | Kaiser Tax, Civil and Labor Indemnity Reserve [Member]
Litigation and Other Disputes
Loss Contingency, Estimate of Possible Loss		$ 68.0			$ 68.0