Summary of Activity within Stockholders Equity Attributable to Real Industry and Noncontrolling Interest (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Class Of Stock [Line Items]
Beginning balance			$ 34.5
Net earnings (loss)	$ (6.2)	$ (1.2)	(17.5)	$ (11.2)
Distribution to noncontrolling interest			(0.9)
Share-based compensation expense			1.4
Dividends on Redeemable Preferred Stock, in cash or accrued	(0.5)	0.0	(1.1)	0.0
Accretion of fair value adjustment to Redeemable Preferred Stock	(0.3)	$ (0.2)	(0.5)	$ (0.5)
Change in accumulated other comprehensive loss			5.7
Ending balance	21.6		21.6
Equity Attributable to Real Industry, Inc.
Class Of Stock [Line Items]
Beginning balance			33.4
Net earnings (loss)			(17.9)
Consolidation of noncontrolling interest			(0.5)
Share-based compensation expense			1.4
Dividends on Redeemable Preferred Stock, in cash or accrued			(1.1)
Accretion of fair value adjustment to Redeemable Preferred Stock			(0.5)
Change in accumulated other comprehensive loss			5.7
Ending balance	20.5		20.5
Noncontrolling Interest
Class Of Stock [Line Items]
Beginning balance			1.1
Net earnings (loss)			0.4
Distribution to noncontrolling interest			(0.9)
Consolidation of noncontrolling interest			0.5
Ending balance	$ 1.1		$ 1.1