Debt Textuals (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 6.3% Senior Notes	Dec. 31, 2012 3.67% Percent Notes	Dec. 31, 2012 $870 Credit Agreement	Dec. 31, 2012 $950 Credit Agreement	Dec. 31, 2012 Other Local Arrangements	Dec. 31, 2011 Tender Offer and Repayment of 4.85% Senior Notes	Dec. 31, 2010 Tender Offer and Repayment of 4.85% Senior Notes	Nov. 15, 2010 Tender Offer and Repayment of 4.85% Senior Notes
Debt Instrument
Senior Notes, Stated Interest Rate				6.30%	3.67%						4.85%
Senior Notes, Face Amount	$ 50,000,000			$ 100,000,000	$ 50,000,000						$ 150,000,000
Maturity Date				Jun. 25, 2015	Dec. 17, 2022	Dec. 20, 2016		Jul. 01, 2012	Nov. 15, 2010
Minimum Prepayment Percent				10.00%	10.00%
Percent of Price of Repayment				100.00%	100.00%
Percent of Principal Balance Paid due to Change in Control of Company				100.00%	100.00%
Consolidated Interest Coverage Ratio Covenant				3.5 to 1.0	3.5 to 1.0	3.5 to 1.0
Consolidated Leverage Ratio Covenant				3.5 to 1.0	3.5 to 1.0	3.25 to 1.0
Debt Instrument, Covenant Compliance				The Company was in compliance with these covenants at December 31, 2012.	The Company was in compliance with these covenants at December 31, 2012.	the Company was in compliance as of December 31, 2012
Debt issuance costs	363,000	3,144,000	0	700,000	400,000
Amortized Issuance Costs Period				six	ten	2016
Credit Agreement Borrowing Capacity						880,000,000	950,000,000
$880 million Credit Agreement, Basis Spread on Variable Rate					0.85%
Gains (Losses) on Extinguishment of Debt							300,000
Interest Costs, Capitalized During Period						3,100,000
Line of Credit Facility, Remaining Borrowing Capacity						677,900,000
Other Local Arrangements	41,600,000	28,300,000						10,000,000
Debt, Weighted Average Interest Rate	5.00%	4.00%
Tender Offer Expiration Date										May 12, 2009
Early Repayment of Senior Debt										75,000,000
Repayments of Senior Debt									$ 75,000,000