Borrowings (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Borrowings
Total long-term debt	$ 751.3		$ 751.0
Total long-term debt, net of current portion	751.3		751.0
Cash payments for interest	22.2	22.0
Interest income	1.1	0.3
3.75% senior notes due 2022
Borrowings
Total long-term debt	349.1		349.0
Interest rate (as a percent)	3.75%
Unamortized discount	(0.9)		(1.0)
Par value of senior notes	350
3.75% senior notes due 2022 | Significant Observable Inputs (Level 2)
Borrowings
Fair value of notes	343.1
5.125% senior notes due 2020
Borrowings
Total long-term debt	249.3		249.2
Interest rate (as a percent)	5.125%
Unamortized discount	(0.7)		(0.8)
Par value of senior notes	250
5.125% senior notes due 2020 | Significant Observable Inputs (Level 2)
Borrowings
Fair value of notes	267.9
6.125% senior notes due 2016
Borrowings
Total long-term debt	149.8		149.7
Interest rate (as a percent)	6.125%
Unamortized discount	(0.2)		(0.3)
Par value of senior notes	150
6.125% senior notes due 2016 | Significant Observable Inputs (Level 2)
Borrowings
Fair value of notes	160.5
Revolving credit facility
Borrowings
Remaining borrowing capacity	600.0
Amount outstanding	0	0
Industrial development and revenue bonds through 2018
Borrowings
Total long-term debt	3.0		3.0
Other, including capital lease obligations
Borrowings
Total long-term debt	0.1		0.1
Uncommitted line of credit
Borrowings
Maximum borrowing capacity	45.0		45.0
Amount outstanding	$ 0	$ 0