Earnings per Share Earnings per Share - Table (Detail) - USD ($) $ in Millions	3 Months Ended							9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2019	Dec. 31, 2017
Continuing operations
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest								$ 70.0	$ 29.1	$ 36.3	$ 6.3
Less: Net income attributable to noncontrolling interest	$ 0.1	$ 0.0	$ (2.7)	$ (0.6)	$ (0.3)	$ (0.9)	$ (0.1)	(1.2)	(0.1)	(3.3)	(2.0)
Net Income (Loss) Available to Common Stockholders, Basic								68.8		33.0	4.3
Discontinued operations
Income (loss) discontinued operations, net of taxes								$ 0.0	$ 38.8	$ 0.0	$ 46.0
Denominator:
Weighted average common shares outstanding - basic								41,938,987	42,200,000	41,974,429	42,142,690
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements								30,069		30,712	238,333
Weighted average common shares outstanding - diluted								41,969,056	42,500,000	42,005,141	42,381,023
Anti-dilutive awards excluded from diluted weighted average shares outstanding								0		0	1,093,394