Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income	$ 49.2	$ 47.0	$ 38.8	$ 76.3	$ 342.1	$ 49.0	$ 53.4	$ 57.5	$ 211.3	$ 502.0	$ 257.1
Denominator:
Denominator for basic earnings per share — weighted average shares									37.6	38.8	40.5
Effect of dilutive securities:
Equity compensation plans									0.7	0.6	0.4
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion									38.3	39.4	40.9
Basic earnings per share (in dollars per share)	$ 1.32	$ 1.25	$ 1.03	$ 2.02	$ 8.98	$ 1.27	$ 1.37	$ 1.46	$ 5.62	$ 12.95	$ 6.35
Diluted earnings per share (in dollars per share)	$ 1.30	$ 1.22	$ 1.01	$ 1.98	$ 8.83	$ 1.25	$ 1.35	$ 1.44	$ 5.52	$ 12.75	$ 6.29