6. Long-Term Debt (Details) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2014	Dec. 31, 2013	May 31, 2009	Sep. 30, 2014 Unsecured Debt [Member] Senior Notes 4.875% [Member]	Dec. 31, 2013 Unsecured Debt [Member] Senior Notes 4.875% [Member]	Dec. 31, 2010 Unsecured Debt [Member] Senior Notes 4.875% [Member]	Sep. 30, 2014 Capital Lease Obligations [Member] Debt 6 [Member]	Dec. 31, 2013 Capital Lease Obligations [Member] Debt 6 [Member]	Sep. 30, 2014 Line of Credit [Member]	Sep. 30, 2014 Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage					8.00%			4.875%
Long-term debt						$ 423,400,000	$ 423,200,000	$ 422,600,000
Long-term Debt and Capital Lease Obligations, Including Current Maturities									12,600,000	12,600,000
Debt and Capital Lease Obligations			436,000,000	435,800,000
Less Current Maturities			(300,000)	(200,000)
Debt Instrument, Interest Rate, Effective Percentage								4.946%
Long-term debt, net of current maturities			435,739,000	435,615,000
Face amount of debt sold								425,000,000
Loss on Extinguishment of Debt	15,600,000	15,600,000
Line of credit maximum borrowing capacity											200,000,000	5,000,000
Outstanding borrowings											$ 0
Line of Credit Facility, Interest Rate at Period End											1.50%