Stockholders Equity (Deficit) and Noncontrolling Interest - Summary of Activity within Stockholders Equity Attributable to Real Industry and Noncontrolling Interest (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Stockholders Equity
Beginning balance			$ 34.5
Net earnings (loss)	$ (40.5)	$ (10.8)	(58.0)	$ (22.0)
Distribution to noncontrolling interest			(2.0)
Share-based compensation expense			2.0
Dividends on Redeemable Preferred Stock, in cash or accrued	(0.6)		(1.7)
Accretion of fair value adjustment to Redeemable Preferred Stock	(0.2)	$ (0.2)	(0.7)	$ (0.8)
Change in accumulated other comprehensive income (loss)			9.2
Ending balance	(16.7)		(16.7)
Equity (Deficit) Attributable to Real Industry, Inc.
Stockholders Equity
Beginning balance			33.4
Net earnings (loss)			(58.7)
Consolidation of noncontrolling interest			(0.5)
Share-based compensation expense			2.0
Dividends on Redeemable Preferred Stock, in cash or accrued			(1.7)
Accretion of fair value adjustment to Redeemable Preferred Stock			(0.7)
Change in accumulated other comprehensive income (loss)			9.2
Ending balance	(17.0)		(17.0)
Noncontrolling Interest
Stockholders Equity
Beginning balance			1.1
Net earnings (loss)			0.7
Distribution to noncontrolling interest			(2.0)
Consolidation of noncontrolling interest			0.5
Ending balance	$ 0.3		$ 0.3