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 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Net sales:        
As reported $ 429.9 $ 359.4 $ 1,361.1 $ 1,153.7
Pro forma   391.7   1,252.7
Net income attributable to Global Brass and Copper Holdings, Inc.:        
As reported $ 14.8 12.3 $ 51.6 45.5
Pro forma   $ 13.3   $ 48.7
Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Basic:        
As reported (in usd per share) $ 0.67 $ 0.56 $ 2.35 $ 2.10
Pro forma (in usd per share)   0.61   2.24
Net income attributable to Global Brass and Copper Holdings, Inc. per common share - Diluted:        
As reported (in usd per share) $ 0.66 0.56 $ 2.31 2.06
Pro forma (in usd per share)   $ 0.60   $ 2.20