Annual report pursuant to Section 13 and 15(d)

Quarterly Financial Information (Unaudited) - Summary of Quarterly Financial Information (Detail)

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Quarterly Financial Information (Unaudited) - Summary of Quarterly Financial Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]                      
Revenues $ 404.3 $ 429.9 $ 459.4 $ 471.8 $ 424.9 $ 359.4 $ 374.8 $ 419.5 $ 1,765.4 $ 1,578.6 $ 1,338.3
Gross profit 36.9 42.9 57.9 49.0 45.8 43.6 44.2 50.6 186.7 184.2 181.7
Income before provision for income taxes and equity income 8.8 16.8 29.1 21.2 19.3 19.0 24.6 22.7 75.9 85.6 49.5
Net income 6.7 14.9 21.1 15.9 5.8 12.4 15.8 17.7 58.6 51.7 32.8
Net income attributable to Global Brass and Copper Holdings, Inc. $ 6.6 $ 14.8 $ 21.0 $ 15.8 $ 5.6 $ 12.3 $ 15.7 $ 17.5 $ 58.2 $ 51.1 $ 32.2
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:                      
Basic (in usd per share) $ 0.30 $ 0.67 $ 0.95 $ 0.72 $ 0.26 $ 0.56 $ 0.72 $ 0.81 $ 2.66 $ 2.35 $ 1.50
Diluted (in usd per share) $ 0.29 $ 0.66 $ 0.94 $ 0.71 $ 0.25 $ 0.56 $ 0.71 $ 0.79 $ 2.61 $ 2.31 $ 1.49