Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 342.1	$ 49.0	$ 53.4	$ 57.5	$ 30.9	$ 95.7	$ 61.2	$ 69.3	$ 502.0	$ 257.1	$ 205.3
Denominator:
Denominator for basic earnings per share — weighted average shares									38.8	40.5	43.1
Effect of dilutive securities:
Equity compensation plans									0.6	0.4	0.7
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion									39.4	40.9	43.8
Basic earnings per share (in dollars per share)	$ 8.98	$ 1.27	$ 1.37	$ 1.46	$ 0.78	$ 2.39	$ 1.51	$ 1.67	$ 12.95	$ 6.35	$ 4.76
Diluted earnings per share (in dollars per share)	$ 8.83	$ 1.25	$ 1.35	$ 1.44	$ 0.77	$ 2.36	$ 1.49	$ 1.66	$ 12.75	$ 6.29	$ 4.69