Commitments and Contingencies Loss Contingency (Details) CAD in Millions, $ in Millions		1 Months Ended	3 Months Ended		12 Months Ended
	Dec. 12, 2014 CAD	Dec. 31, 2009 CAD	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 1990 USD ($)	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Loss contingency, possible loss, maximum			$ 121.0
Guarantor Obligations, Current Carrying Value			30.4			$ 16.9
Guarantees
Letter of credit provided to entity | CAD		CAD 10
Litigation and Other Disputes
Accrued liabilities, litigations and disputes			12.3			13.1
Loss contingency, possible loss, portion not accrued			114.0
Loss Contingency, Range of Possible Loss, Minimum			0.0
Kaiser purchased tax credits indemnity reserve, category two
Loss Contingencies [Line Items]
Loss contingency, possible loss, maximum			$ 97.4
Kaiser Tax, Civil and Labor Indemnity Reserve [Member]
Loss Contingencies [Line Items]
Equity interest sold (as a percent)			68.00%
Loss contingency, possible loss, maximum			$ 68.0
Environmental matters Lowry [Member]
Environmental
Environmental remediation expense, pretax charge					$ 30.0
Environmental remediation threshold, assumed remediation cost			$ 120.0
Inflation rate assumption, future costs (as a percent)			2.50%
Risk free rate of return assumption (as a percent)			2.20%
Site contingency, accrual, present value			$ 3.4
Site contingency, accrual, undiscounted amount			6.2
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			(0.5)	$ 1.9
Other Noncurrent Liabilities [Member] | Canada [Member]
Loss Contingencies [Line Items]
Guarantor Obligations, Current Carrying Value			$ 4.7			$ 4.4