Unaudited Quarterly Results (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
Unaudited Quarterly Results [Abstract]
Net sales	$ 191,698	$ 173,437	$ 165,547	$ 174,862	$ 175,934	$ 172,802	$ 155,775	$ 137,352	$ 146,436	$ 112,486	$ 108,278	$ 105,458
Operating income	16,780	19,950	22,230	32,465	29,560	36,753	20,229	8,228	8,187	3,307	30,466	12,326	91,425	94,770	54,286
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 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 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