Quarterly report pursuant to Section 13 or 15(d)

Acquisition Pro Forma (Details)

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Acquisition Pro Forma (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Net sales:        
As reported $ 459.4 $ 374.8 $ 931.2 $ 794.3
Pro forma   409.4   861.0
Net income attributable to Global Brass and Copper Holdings, Inc.:        
As reported $ 21.0 15.7 $ 36.8 33.2
Pro forma   $ 17.0   $ 35.4
Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Basic:        
As reported (in usd per share) $ 0.95 $ 0.72 $ 1.68 $ 1.54
Pro forma (in usd per share)   0.78   1.64
Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Diluted:        
As reported (in usd per share) $ 0.94 0.71 $ 1.66 1.50
Pro forma (in usd per share)   $ 0.77   $ 1.60