Income Per Share of Common Stock - Reconciliation of Numerator and Denominator (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net income	$ 14.0	$ 5.9
Income allocated to participating securities, basic	(0.3)	(0.1)
Income allocated to participating securities, diluted	(0.3)	(0.1)
Net income for basic earnings per share	13.7	5.8
Net income for diluted earnings per share	$ 13.7	$ 5.8
Denominator:
Basic weighted average shares (in shares)	73.7	72.3
Effect of:
Convertible subordinated notes	0.9	0.0
Employee stock options and stock purchase plan (in shares)	0.1	0.0
Diluted weighted average shares (in shares)	74.7	72.3
Basic
Basic net income per share (in USD per share)	$ 0.19	$ 0.08
Diluted
Diluted net income per share (in USD per share)	$ 0.18	$ 0.08
2 1/4% Notes
Diluted
Debt instrument interest rate stated percentage	2.25%