Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ 56.6	$ 45.1	$ 68.0	$ 41.5	$ 49.2	$ 47.0	$ 38.8	$ 76.3	$ 211.2	$ 211.3	$ 502.0
Denominator:
Denominator for basic earnings per share — weighted average shares									35.7	37.6	38.8
Effect of dilutive securities:
Equity compensation plans									0.7	0.7	0.6
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion									36.4	38.3	39.4
Basic earnings per share (in dollars per share)	$ 1.62	$ 1.28	$ 1.89	$ 1.14	$ 1.32	$ 1.25	$ 1.03	$ 2.02	$ 5.92	$ 5.62	$ 12.95
Diluted earnings per share (in dollars per share)	$ 1.59	$ 1.25	$ 1.86	$ 1.12	$ 1.30	$ 1.22	$ 1.01	$ 1.98	$ 5.81	$ 5.52	$ 12.75