Borrowings (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Borrowings
Total long-term debt	$ 753,300,000		$ 752,500,000
Less current portion	(800,000)
Total long-term debt, net of current portion	752,500,000		752,500,000
Cash payments for interest	4,600,000	5,900,000
Interest income	100,000	100,000
3.75% senior notes due 2022
Borrowings
Total long-term debt	348,900,000		348,900,000
Interest rate (as a percent)	3.75%
Unamortized discount	(1,100,000)		(1,100,000)
Par value of senior notes	350,000,000
3.75% senior notes due 2022 | Significant Observable Inputs (Level 2)
Borrowings
Fair value of notes	347,100,000
5.125% senior notes due 2020
Borrowings
Total long-term debt	249,100,000		249,100,000
Interest rate (as a percent)	5.125%
Unamortized discount	(900,000)		(900,000)
Par value of senior notes	250,000,000
5.125% senior notes due 2020 | Significant Observable Inputs (Level 2)
Borrowings
Fair value of notes	273,800,000
6.125% senior notes due 2016
Borrowings
Total long-term debt	149,600,000		149,600,000
Interest rate (as a percent)	6.125%
Unamortized discount	(400,000)		(400,000)
Par value of senior notes	150,000,000
6.125% senior notes due 2016 | Significant Observable Inputs (Level 2)
Borrowings
Fair value of notes	169,400,000
Industrial development and revenue bonds through 2018
Borrowings
Total long-term debt	4,500,000		4,500,000
Other, including capital lease obligations
Borrowings
Total long-term debt	1,200,000		400,000
Revolving credit facility
Borrowings
Interest on borrowings	0
Remaining borrowing capacity	600,000,000
Average interest rate (as a percent)		1.34%
Uncommitted line of credit
Borrowings
Maximum borrowing capacity	$ 45,000,000		$ 45,000,000
Average interest rate (as a percent)	1.50%	1.50%