Earnings Per Share ("EPS") of Common Stock - Reconciliation of Numerator and Denominator (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income	$ 31.4	$ 38.7
Income allocated to participating securities, basic	(0.5)	(0.7)
Net income for basic earnings per share	30.9	38.0
Net income for diluted earnings per share	$ 30.9	$ 38.0
Denominator:
Basic weighted average shares (in shares)	77.5	77.1
Effect of:
Employee stock options and stock purchase plan (in shares)	0.2	0.1
Diluted weighted average shares (in shares)	83.1	80.6
Basic
Basic earnings per share (in dollars per share)	$ 0.40	$ 0.49
Diluted
Diluted earnings per share (in dollars per share)	$ 0.37	$ 0.47
2 1/4% Notes
Effect of:
2.25% Convertible Senior Notes (21/4% Notes) (in shares)	5.4	3.4
Convertible Debt | 2 1/4% Notes
Effect of:
Debt instrument interest rate stated percentage	2.25%