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Redeemable Noncontrolling Interest - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2016
Noncontrolling Interest [Line Items]      
Change in redeemable noncontrolling interest $ 0.0 $ 0.0  
Redeemable noncontrolling interest 12.1   $ 0.0
Retained Earnings      
Noncontrolling Interest [Line Items]      
Change in redeemable noncontrolling interest $ (0.3) $ (0.3)  
ATI Flowform [Member]      
Noncontrolling Interest [Line Items]      
Noncontrolling interest held by others (percent) 15.00%