Quarterly report pursuant to Section 13 or 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 6 Months Ended
Jul. 31, 2018
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Numerator          
Net income attributable to Global Brass and Copper Holdings, Inc.   $ 21.0 $ 15.7 $ 36.8 $ 33.2
Denominator          
Weighted-average common shares outstanding (in shares)   22.0 21.8 21.9 21.6
Effect of potentially dilutive securities:          
Stock options and nonvested share awards (in shares)   0.3 0.3 0.3 0.5
Weighted-average common shares outstanding, assuming dilution (in shares)   22.3 22.1 22.2 22.1
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:          
Basic (in usd per share)   $ 0.95 $ 0.72 $ 1.68 $ 1.54
Diluted (in usd per share)   $ 0.94 $ 0.71 $ 1.66 $ 1.50
Stock Compensation Plan [Member]          
Effect of potentially dilutive securities:          
Anti-dilutive shares excluded from above (in shares)   0.1 0.2 0.1 0.2
Subsequent Event [Member]          
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:          
Stock Repurchase Program, Authorized Amount $ 35.0        
Stock Repurchase Program Expiration Date Sep. 30, 2020