Quarterly report pursuant to Section 13 or 15(d)

Acquisition Narrative (Details)

v3.10.0.1
Acquisition Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Business Acquisition [Line Items]          
Effective date of acquisition     Nov. 01, 2017    
Net sales $ 459.4 $ 374.8 $ 931.2 $ 794.3  
Net income attributable to Global Brass and Copper Holdings, Inc. 21.0 $ 15.7 36.8 $ 33.2  
Goodwill 4.5   4.5   $ 4.5
Alumet [Member]          
Business Acquisition [Line Items]          
Consideration transferred     41.6    
Finite-lived intangibles acquired 1.7   1.7    
Goodwill 0.1   0.1    
Customer Relationships [Member] | Alumet [Member]          
Business Acquisition [Line Items]          
Finite-lived intangibles acquired 1.0   $ 1.0    
Acquired finite-lived intangible assets, weighted average useful life     15 years    
Trade Names [Member] | Alumet [Member]          
Business Acquisition [Line Items]          
Finite-lived intangibles acquired 0.3   $ 0.3    
Acquired finite-lived intangible assets, weighted average useful life     15 years    
Noncompete Agreements [Member] | Alumet [Member]          
Business Acquisition [Line Items]          
Finite-lived intangibles acquired 0.2   $ 0.2    
Acquired finite-lived intangible assets, weighted average useful life     2 years    
Leases, Acquired-in-Place, Market Adjustment [Member] | Alumet [Member]          
Business Acquisition [Line Items]          
Finite-lived intangibles acquired $ 0.2   $ 0.2    
Acquired finite-lived intangible assets, weighted average useful life     2 years 6 months