Earnings (Loss) Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Sep. 30, 2009	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2010
Basic earnings per share computation - numerator [Abstract]
Net income attributable to Globe Specialty Metals, Inc.	$ 8,820	$ 11,613	$ 13,444	$ 20,693	$ 15,545	$ 23,393	$ 11,708	$ 2,162	$ 6,609	$ 516	$ 18,534	$ 8,442	$ 54,570	$ 52,808	$ 34,101
Basic earnings per share computation - denominator [Abstract]
Weighted average basic shares outstanding (in shares)													75,038,674	74,924,947	73,511,696
Basic earnings (loss) per common share (in dollars per share)	$ 0.12	$ 0.15	$ 0.18	$ 0.28	$ 0.21	$ 0.31	$ 0.16	$ 0.03	$ 0.09	$ 0.01	$ 0.25	$ 0.12	$ 0.73	$ 0.70	$ 0.46
Diluted earnings per share computation - numerator [Abstract]
Net income attributable to Globe Specialty Metals, Inc.	$ 8,820	$ 11,613	$ 13,444	$ 20,693	$ 15,545	$ 23,393	$ 11,708	$ 2,162	$ 6,609	$ 516	$ 18,534	$ 8,442	$ 54,570	$ 52,808	$ 34,101
Diluted earnings per share computation - denominator [Abstract]
Weighted average basic shares outstanding (in shares)													75,038,674	74,924,947	73,511,696
Effect of dilutive securities (in shares)													1,585,218	1,699,398	1,258,451
Weighted average diluted shares outstanding (in shares)													76,623,892	76,624,345	74,770,147
Diluted earnings per common share (in dollars per share)	$ 0.12	$ 0.15	$ 0.18	$ 0.27	$ 0.20	$ 0.30	$ 0.15	$ 0.03	$ 0.09	$ 0.01	$ 0.25	$ 0.12	$ 0.71	$ 0.69	$ 0.46
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares (in shares)													1,101,079	66,667	160,000