Long Term Debt (Details Textual)	6 Months Ended
	Jun. 30, 2012
Revolver Under Credit Facility [Member]
Schedule of Long Term Debt Instruments (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	1.39%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	3.65%
Senior Notes due 2015 [Member]
Schedule of Long Term Debt Instruments (Textual) [Abstract]
Interest Rate	6.22%
Senior Notes due 2016 [Member]
Schedule of Long Term Debt Instruments (Textual) [Abstract]
Interest Rate	3.30%
Senior Notes due 2018 [Member]
Schedule of Long Term Debt Instruments (Textual) [Abstract]
Interest Rate	4.00%
Senior Notes due 2019 [Member]
Schedule of Long Term Debt Instruments (Textual) [Abstract]
Interest Rate	5.25%
Senior Notes due 2021 [Member]
Schedule of Long Term Debt Instruments (Textual) [Abstract]
Interest Rate	4.64%
Tax-exempt bonds [Member]
Schedule of Long Term Debt Instruments (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	0.12%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	0.43%
2.50% to 10.35% notes payable to sellers in connection with acquisitions [Member]
Schedule of Long Term Debt Instruments (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	2.50%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	10.35%
6.7% to 10.9% Notes Payable to Third Parties [Member]
Schedule of Long Term Debt Instruments (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	6.70%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	10.90%