Annual report pursuant to Section 13 and 15(d)

Acquisition Narrative (Details)

v3.10.0.1
Acquisition Narrative (Details) - USD ($)
$ in Millions
2 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2017
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
Business Acquisition [Line Items]                        
Effective date of acquisition                   Nov. 01, 2017    
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
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
Alumet [Member]                        
Business Acquisition [Line Items]                        
Revenues $ 19.0                      
Net income attributable to Global Brass and Copper Holdings, Inc. $ 0.5                      
Consideration transferred                   41.7    
Finite-lived intangibles acquired   1.7               1.7    
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