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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net income	$ 42.6	$ 31.7	$ 105.7	$ 102.3
Income allocated to participating securities	(0.3)	(0.3)	(0.7)	(1.3)
Net income for basic earnings per share	42.3	31.4	105.0	101.0
Net income for diluted earnings per share	$ 42.3	$ 31.4	$ 105.0	$ 101.0
Denominator:
Basic weighted average shares (in shares)	79.9	77.6	79.0	77.6
Effect of:
2 1/4% Notes (in shares)	2.4	4.2	2.5	4.7
Awards issued under equity plans (in shares)	0.0	0.1	0.0	0.1
Diluted weighted average shares (in shares)	82.3	81.9	81.5	82.4
Basic
Basic earnings per share (in dollars per share)	$ 0.53	$ 0.40	$ 1.33	$ 1.30
Diluted
Diluted earnings per share (in dollars per share)	$ 0.52	$ 0.38	$ 1.29	$ 1.23
Convertible Debt | 2 1/4% Notes
Diluted
Debt instrument interest rate stated percentage	2.25%		2.25%