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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income	$ 39.2	$ 44.1	$ 70.6	$ 82.8
Income allocated to participating securities, basic	(0.5)	(0.9)	(1.0)	(1.6)
Net income for basic earnings per share	38.7	43.2	69.6	81.2
Net income for diluted earnings per share	$ 38.7	$ 43.2	$ 69.6	$ 81.2
Denominator:
Basic weighted average shares (in shares)	77.6	77.1	77.5	77.1
Effect of:
Employee stock options and stock purchase plan (in shares)	0.2	0.1	0.2	0.1
Diluted weighted average shares (in shares)	82.1	80.7	82.6	80.6
Basic
Basic earnings per share (in dollars per share)	$ 0.50	$ 0.56	$ 0.90	$ 1.05
Diluted
Diluted earnings per share (in dollars per share)	$ 0.47	$ 0.54	$ 0.84	$ 1.01
2 1/4% Notes
Effect of:
2.25% Convertible Senior Notes (21/4% Notes) (in shares)	4.3	3.5	4.9	3.4
Convertible Debt | 2 1/4% Notes
Effect of:
Debt instrument interest rate stated percentage	2.25%		2.25%