Summary of Activity within Stockholders Equity Attributable to Real Industry and Noncontrolling Interest (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Class Of Stock [Line Items]
Beginning balance	$ 34.5	$ 142.4	$ 142.4
Net earnings (loss)	(11.3)	(10.0)	(102.6)
Distribution to noncontrolling interest	(0.9)
Dividends on Redeemable Preferred Stock, in-kind	0.0	(0.5)	(2.0)
Dividends on Redeemable Preferred Stock, accrued	(0.6)	0.0
Accretion of fair value adjustment to Redeemable Preferred Stock	(0.2)	(0.2)	(1.0)
Share-based compensation expense	0.7		3.6
Common stock options exercised			0.1
Warrants exercised			0.1
Change in accumulated other comprehensive loss	0.7		(6.1)
Ending balance	22.9		34.5
Equity Attributable to Real Industry, Inc.
Class Of Stock [Line Items]
Beginning balance	33.4	141.6	141.6
Net earnings (loss)	(11.4)		(102.9)
Consolidation of noncontrolling interest	(0.5)
Dividends on Redeemable Preferred Stock, in-kind			(2.0)
Dividends on Redeemable Preferred Stock, accrued	(0.6)
Accretion of fair value adjustment to Redeemable Preferred Stock	(0.2)		(1.0)
Share-based compensation expense	0.7		3.6
Common stock options exercised			0.1
Warrants exercised			0.1
Change in accumulated other comprehensive loss	0.7		(6.1)
Ending balance	22.1		33.4
Noncontrolling Interest
Class Of Stock [Line Items]
Beginning balance	1.1	$ 0.8	0.8
Net earnings (loss)	0.1		0.3
Distribution to noncontrolling interest	(0.9)
Consolidation of noncontrolling interest	0.5
Ending balance	$ 0.8		$ 1.1