Net Income Per Share (Details) (USD $) In Millions, except Per Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Net Income Per Share
Net income	$ 117.4	$ 88.7	$ 221.5	$ 174.0
Weighted-average shares outstanding	39.2	39.3	39.2	39.1
Basic net income per share	$ 2.99	$ 2.26	$ 5.65	$ 4.45
Weighted-average shares outstanding used in basic calculation	39.2	39.3	39.2	39.1
Add common stock equivalents	1.1	1.2	1.0	1.3
Weighted-average shares used in computing diluted net income per share	40.3	40.5	40.2	40.4
Diluted net income per share	$ 2.91	$ 2.19	$ 5.51	$ 4.31
Employee stock options excluded from computation of diluted net income per share because the effect of including such shares would have been antidilutive	2.5	1.4	2.2	1.1