Debt Schedule of Long-Term Debt Instruments (Details) (USD $)																						12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Senior Notes due 2022 at a fixed interest rate of 3.75% [Member]	Dec. 31, 2011 Senior Notes due 2022 at a fixed interest rate of 3.75% [Member]	Dec. 31, 2012 Senior Exchangeable Notes due 2012 at a fixed interest rate of 3.75% (retired in October 2012)	Dec. 31, 2011 Senior Exchangeable Notes due 2012 at a fixed interest rate of 3.75% (retired in October 2012)	Oct. 01, 2007 Senior Exchangeable Notes due 2012 at a fixed interest rate of 3.75% (retired in October 2012)	Dec. 31, 2012 Senior Exchangeable Notes due 2015 at a fixed interest rate of 4.50%		Dec. 31, 2011 Senior Exchangeable Notes due 2015 at a fixed interest rate of 4.50%		Dec. 31, 2012 Installment note due 2014 at a fixed interest rate of 8.64%	Dec. 31, 2011 Installment note due 2014 at a fixed interest rate of 8.64%	Dec. 31, 2012 Mortgage notes due 2017 at fixed interest rates of 4.35%		Dec. 31, 2011 Mortgage notes due 2017 at fixed interest rates of 4.35%		Dec. 31, 2012 Polution control bomd (retired in May 2012) and solid waste bond due 2020 at a variable interest rate of 1.57% at December 31, 2012	Dec. 31, 2011 Polution control bomd (retired in May 2012) and solid waste bond due 2020 at a variable interest rate of 1.57% at December 31, 2012	Dec. 31, 2011 Revolving credit facility borrowings due 2016 at variable interest rates of 1.19% to 3.25% at December 31, 2012		Dec. 31, 2012 Revolving credit facility borrowings due 2016 as amended October 2012 at variable interest rates of 1.19% to 3.25% at December 31, 2012		Dec. 31, 2012 Term Credit Agreement borrowings due 2019 at a variable interest rate of 1.71% at December 31, 2012	Dec. 31, 2011 Term Credit Agreement borrowings due 2019 at a variable interest rate of 1.71% at December 31, 2012
Debt Instrument [Line Items]
Total debt	$ 1,270,052,000	$ 847,339,000	$ 325,000,000	$ 0	$ 0	$ 294,622,000		$ 165,821,000	[1]	$ 163,525,000	[1]	$ 112,500,000	$ 112,500,000	$ 76,731,000	[2]	$ 88,582,000	[2]	$ 15,000,000	$ 38,110,000	$ 150,000,000	[3]	$ 275,000,000	[3]	$ 300,000,000	$ 0
Less: Current maturities of long-term debt	(150,000,000)	(28,110,000)																				(150,000,000)
Long-term debt	1,120,052,000	819,229,000
Unamortized discount					0	5,378,000		6,679,000		8,975,000
Amount due upon maturity			325,000,000		0	300,000,000	300,000,000	172,500,000		172,500,000				73,500,000										300,000,000
Unamortized premium														$ 3,200,000		$ 4,600,000
Fixed interest rate			3.75%		3.75%			4.50%				8.64%		4.35%
Variable interest rate																		1.57%						1.71%
Variable interest rate, minimum																						1.19%
Variable interest rate, maximum																						3.25%

[1] Our Senior Exchangeable Notes maturing in 2015 were discounted by $6.7 million and $9.0 million as of December 31, 2012 and 2011, respectively, but upon maturity the liability will be $172.5 million.

[2] The mortgage notes due in 2017 were recorded at a premium of $3.2 million and $4.6 million as of December 31, 2012 and 2011, respectively. Upon maturity the liability will be $73.5 million.

[3] Borrowings under the Revolving Credit Facility include $150 million classified in current liabilities due to the Company's intent to repay this amount in January 2013.