Loss Per Share - Computation of Basic and Diluted loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Per Share
Loss from continuing operations										$ (120.8)	$ (103.2)	$ (31.7)
Earnings (loss) from discontinued operations, net of income taxes											0.6	24.9
Net loss									$ (58.0)	(120.8)	(102.6)	(6.8)
Earnings from continuing operations attributable to noncontrolling interest										0.9	0.3	0.1
Net loss attributable to Real Industry, Inc.	$ (63.0)	$ (40.8)	$ (6.5)	$ (11.4)	$ (80.4)	$ (10.9)	$ (1.5)	$ (10.1)		(121.7)	(102.9)	(6.9)
Dividends on Redeemable Preferred Stock, in-kind											(2.0)	(1.5)
Dividends on Redeemable Preferred Stock, in cash or accrued										(2.4)
Accretion of fair value adjustment to Redeemable Preferred Stock										(3.6)	(1.0)	(0.8)
Net loss available to common stockholders	$ (66.6)	$ (41.6)	$ (7.3)	$ (12.2)	$ (81.2)	$ (11.7)	$ (2.2)	$ (10.8)		$ (127.7)	$ (105.9)	$ (9.2)
Denominator for basic and diluted loss per share-Weighted average shares outstanding										28,968,363	28,719,098	26,657,832
Basic and diluted loss per share:
Continuing operations										$ (4.41)	$ (3.68)	$ (0.35)
Basic and diluted loss per share	$ (2.29)	$ (1.43)	$ (0.25)	$ (0.43)	$ (2.84)	$ (0.40)	$ (0.07)	$ (0.38)		$ (4.41)	$ (3.68)	$ (0.35)