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Commitments and Contingencies Loss Contingency (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2017
Sep. 30, 2018
Dec. 31, 2017
Loss Contingencies [Line Items]      
Loss Contingency Accrual, Period Increase (Decrease) $ 50.0    
Guarantor Obligations, Current Carrying Value   $ 48.8 $ 42.8
Litigation and Other Disputes      
Accrued liabilities, litigations and disputes   13.5 17.8
Kaiser Tax, Civil and Labor Indemnity Reserve [Member]      
Loss Contingencies [Line Items]      
Loss Contingency Accrual   14.1 $ 17.3
Maximum [Member] | Kaiser purchased tax credits indemnity reserve, category two      
Litigation and Other Disputes      
Loss Contingency, Estimate of Possible Loss   $ 86.4