Computation of Basic and Diluted Earnings (loss) Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Earnings Per Share [Abstract]
Earnings (loss) from continuing operations	$ (10.8)	$ 2.0	$ (22.1)	$ (29.5)
Earnings (loss) from discontinued operations, net of income taxes	0.0	(0.7)	0.1	26.5
Net earnings (loss)	(10.8)	1.3	(22.0)	(3.0)	$ (6.8)
Earnings from continuing operations attributable to noncontrolling interest	0.1	0.1	0.5	0.3
Net earnings (loss) attributable to Real Industry, Inc.	(10.9)	1.2	(22.5)	(3.3)
Dividends on Redeemable Preferred Stock, in-kind	(0.5)	(0.5)	(1.4)	(1.0)
Accretion of fair value adjustment to Redeemable Preferred Stock	(0.2)	(0.2)	(0.8)	(0.6)
Net earnings (loss) available to common stockholders	$ (11.6)	$ 0.5	$ (24.7)	$ (4.9)
Denominator for basic earnings (loss) per share—Weighted average shares outstanding	29,268,515	28,556,383	29,196,598	25,993,660
Effect of dilutive securities	0	1,257,090	0	0
Denominator for diluted earnings (loss) per share—Weighted average shares outstanding	29,268,515	29,813,473	29,196,598	25,993,660
Basic earnings (loss) per share:
Continuing operations	$ (0.40)	$ 0.04	$ (0.85)	$ (1.21)
Discontinued operations	0	(0.02)	0	1.02
Basic earnings (loss) per share	(0.40)	0.02	(0.85)	(0.19)
Diluted earnings (loss) per share:
Continuing operations	(0.40)	0.04	(0.85)	(1.21)
Discontinued operations	0	(0.02)	0	1.02
Diluted earnings (loss) per share	$ (0.40)	$ 0.02	$ (0.85)	$ (0.19)