Computation of Basic and Diluted Earnings (loss) Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Loss from continuing operations									$ (103.2)	$ (31.7)	$ (0.1)
Earnings from discontinued operations, net of income taxes									0.6	24.9	5.5
Net earnings (loss)									(102.6)	(6.8)	5.4
Earnings (loss) from continuing operations attributable to noncontrolling interest									0.3	0.1	(0.1)
Net earnings (loss) attributable to Real Industry, Inc.	$ (80.4)	$ (10.9)	$ (1.5)	$ (10.1)	$ (3.9)	$ 1.2	$ (10.9)	$ 6.7	(102.9)	(6.9)	5.5
Dividends on Redeemable Preferred Stock, in-kind									(2.0)	(1.5)	0.0
Accretion of fair value adjustment to Redeemable Preferred Stock									(1.0)	(0.8)	0.0
Net earnings (loss) available to common stockholders	$ (81.2)	$ (11.7)	$ (2.2)	$ (10.8)	$ (4.6)	$ 0.5	$ (11.6)	$ 6.5	$ (105.9)	$ (9.2)	$ 5.5
Denominator for basic and diluted earnings (loss) per share— Weighted average shares outstanding									28,719,098	26,657,832	13,403,083
Basic and diluted earnings (loss) per share:
Continuing operations									$ (3.68)	$ (0.35)	$ 0
Discontinued operations									0	0	0.41
Basic and diluted earnings (loss) per share									$ (3.68)	$ (0.35)	$ 0.41