Summary of Activity within Stockholders Equity Attributable to Real Industry and Noncontrolling Interest (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Class Of Stock [Line Items]
Beginning balance			$ 142.4	$ 85.6	$ 85.6
Net earnings (loss)	$ (10.8)	$ 1.3	(22.0)	(3.0)	(6.8)
Common stock issued, net					63.3
Common stock acquired					(0.1)
Common stock options exercised			0.1		1.2
Warrants exercised			0.2		0.2
Noncontrolling interest acquired in business combination					0.8
Share-based compensation expense			3.0		1.5
Dividends and accretion of fair value adjustment to Redeemable Preferred Stock			(2.2)		(2.3)
Change in accumulated other comprehensive income (loss)			1.7		(1.0)
Ending balance	123.2		123.2		142.4
Equity Attributable to Real Industry, Inc.
Class Of Stock [Line Items]
Beginning balance			141.6	85.7	85.7
Net earnings (loss)			(22.5)		(6.9)
Common stock issued, net					63.3
Common stock acquired					(0.1)
Common stock options exercised			0.1		1.2
Warrants exercised			0.2		0.2
Noncontrolling interest acquired in business combination					0.0
Share-based compensation expense			3.0		1.5
Dividends and accretion of fair value adjustment to Redeemable Preferred Stock			(2.2)		(2.3)
Change in accumulated other comprehensive income (loss)			1.7		(1.0)
Ending balance	121.9		121.9		141.6
Noncontrolling Interest
Class Of Stock [Line Items]
Beginning balance			0.8	$ (0.1)	(0.1)
Net earnings (loss)			0.5		0.1
Common stock issued, net					0.0
Common stock acquired					0.0
Common stock options exercised			0.0		0.0
Warrants exercised			0.0		0.0
Noncontrolling interest acquired in business combination					0.8
Share-based compensation expense			0.0		0.0
Dividends and accretion of fair value adjustment to Redeemable Preferred Stock			0.0		0.0
Change in accumulated other comprehensive income (loss)			0.0		0.0
Ending balance	$ 1.3		$ 1.3		$ 0.8