Annual report pursuant to Section 13 and 15(d)

Earnings Per Share - Schedule of Basic and Diluted Earning Per Shares (Detail)

v3.10.0.1
Earnings Per Share - Schedule of Basic and Diluted Earning Per Shares (Detail) - USD ($)
$ / shares in Units, shares in Millions, $ 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
Numerator                      
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
Denominator                      
Weighted-average common shares outstanding (in shares)                 21.9 21.7 21.4
Effect of potentially dilutive securities:                      
Stock options and nonvested share awards (in shares)                 0.4 0.4 0.2
Weighted-average common shares outstanding, assuming dilution (in shares)                 22.3 22.1 21.6
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
Stock Compensation Plan [Member]                      
Effect of potentially dilutive securities:                      
Anti-dilutive shares excluded from above (in shares)                 0.1 0.1 0.1