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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income	$ 31.7	$ 32.9	$ 102.3	$ 115.7
Income allocated to participating securities	(0.3)	(0.6)	(1.3)	(2.2)
Net income for basic earnings per share	31.4	32.3	101.0	113.5
Net income for diluted earnings per share	$ 31.4	$ 32.3	$ 101.0	$ 113.5
Denominator:
Basic weighted average shares (in shares)	77.6	77.3	77.6	77.1
Effect of:
2.25% Convertible Senior Notes (21/4% Notes) (in shares)	4.2	5.4	4.7	4.3
Awards issued under equity plans (in shares)	0.1	0.1	0.1	0.1
Diluted weighted average shares (in shares)	81.9	82.8	82.4	81.5
Basic
Basic earnings per share (in dollars per share)	$ 0.40	$ 0.42	$ 1.30	$ 1.47
Diluted
Diluted earnings per share (in dollars per share)	$ 0.38	$ 0.39	$ 1.23	$ 1.39
Convertible Debt | 2 1/4% Notes
Diluted
Debt instrument interest rate stated percentage	2.25%		2.25%