DEBT (Details) (USD $)			12 Months Ended					12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Unsecured lines of credit	Dec. 31, 2010 Unsecured lines of credit	Dec. 31, 2011 Revolving credit facility	Dec. 31, 2010 Revolving credit facility	Jan. 31, 2012 New revolving credit facility	Dec. 31, 2011 Notes payable 2.1% - 16.5%, due in monthly and annual installments through 2015	Dec. 31, 2010 Notes payable 2.1% - 16.5%, due in monthly and annual installments through 2015	Dec. 31, 2011 Senior unsecured Notes 6.6%, due in installments through 2011	Dec. 31, 2010 Senior unsecured Notes 6.6%, due in installments through 2011	Dec. 31, 2011 Senior unsecured notes 5.1%, due in 2011	Dec. 31, 2010 Senior unsecured notes 5.1%, due in 2011	Dec. 31, 2011 Senior unsecured notes 5.4%, due in 2013	Dec. 31, 2010 Senior unsecured notes 5.4%, due in 2013	Dec. 31, 2011 Senior unsecured notes 2.3%, due in 2015	Dec. 31, 2010 Senior unsecured notes 2.3%, due in 2015	Dec. 31, 2011 Senior unsecured notes 6.0% due in 2016	Dec. 31, 2010 Senior unsecured notes 6.0% due in 2016	Dec. 31, 2011 Senior unsecured notes 6.0%, due in 2018	Dec. 31, 2010 Senior unsecured notes 6.0%, due in 2018	Dec. 31, 2011 Senior unsecured notes 3.8%, due in 2020	Dec. 31, 2010 Senior unsecured notes 3.8%, due in 2020	Dec. 31, 2011 Capital lease obligations	Dec. 31, 2010 Capital lease obligations
Debt
Average borrowings			$ 112,800,000	$ 106,500,000
Average annual interest rate on short-term notes payable (as a percent)	0.70%	1.00%
Revolving credit, maximum borrowing facility					200,000,000		300,000,000
Revolving credit facility, outstanding amount					165,000,000	42,000,000
Long-term obligations including current maturities	259,026,000	308,899,000						5,654,000	7,916,000		21,555,000		25,000,000	25,000,000	25,000,000	16,000,000	16,000,000	50,000,000	50,000,000	75,000,000	75,000,000	84,000,000	84,000,000	3,372,000	4,428,000
Current maturities of long-term obligations	(4,116,000)	(50,126,000)
Total long-term obligations	254,910,000	258,773,000
Interest rate on notes payable, low end of range (as a percent)								2.10%	2.10%
Interest rate on notes payable, high end of range (as a percent)								16.50%	16.50%
Interest rate on senior unsecured notes (as a percent)										6.60%	6.60%	5.10%	5.10%	5.40%	5.40%	2.30%	2.30%	6.00%	6.00%	6.00%	6.00%	3.80%	3.80%
Aggregate long-term maturities, excluding capital lease obligations
2012	3,441,000
2013	25,826,000
2014	697,000
2015	16,720,000
2016	50,000,000
Thereafter	$ 159,000,000