Long-Term Debt (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010
Long-term debt
Long-term Debt	$ 1,179,802	$ 912,635
Less - current portion	(5,302)	(2,657)
Long-term Debt, Noncurrent	1,174,500	909,978
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Long-term Debt	1,179,802	912,635
Discount rate of notes	Current market standard for United States treasury bills plus 0.50%
Maximum limit of aggregate principal amount of the Notes outstanding	750,000
Base rate for day fluctuating rate per annum equal federal funds rate	0.50%
Base rate for day fluctuating rate per annum equal LIBOR rate	1.00%
0.81% to 3.65% Revolver under credit facility [Member]
Long-term debt
Long-term Debt	519,000	511,000
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	0.81%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	3.65%
Long-term Debt	519,000	511,000
Senior Notes [Member]
Long-term debt
Long-term Debt	600,000
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Long-term Debt	600,000
6.22% Senior Notes due 2015 [Member]
Long-term debt
Long-term Debt	175,000	175,000
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage	6.22%
Debt Instrument, Maturity Date	2015
Long-term Debt	175,000	175,000
3.30% Senior Notes due 2016 [Member]
Long-term debt
Long-term Debt	100,000	0
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage	3.30%
Debt Instrument, Maturity Date	2016
Long-term Debt	100,000	0
4.00% Senior Notes due 2018 [Member]
Long-term debt
Long-term Debt	50,000	0
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Debt Instrument, Maturity Date	2018
Long-term Debt	50,000	0
5.25% Senior Notes due 2019 [Member]
Long-term debt
Long-term Debt	175,000	175,000
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage	5.25%
Debt Instrument, Maturity Date	2019
Long-term Debt	175,000	175,000
4.64% Senior Notes due 2021 [Member]
Long-term debt
Long-term Debt	100,000	0
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage	4.64%
Debt Instrument, Maturity Date	2021
Long-term Debt	100,000	0
0.08% to 0.42% Tax-exempt bonds [Member]
Long-term debt
Long-term Debt	38,460	39,420
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	0.08%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	0.42%
Long-term Debt	38,460	39,420
2.50% to 10.35% Notes payable to sellers in connection with acquisitions [Member]
Long-term debt
Long-term Debt	19,446	9,159
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%
Long-term Debt	19,446	9,159
6.7% to 10.9% Notes payable to third parties [Member]
Long-term debt
Long-term Debt	2,896	3,056
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%
Long-term Debt	2,896	3,056
Senior Uncollateralized Notes [Member]
Long-term debt
Long-term Debt	250,000
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Long-term Debt	250,000
New credit agreement [Member]
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Credit facility	1,200,000
Revolving credit facility maturing	2016, july 11
Maximum amount of increase in commitments under the revolving credit facility	1,500,000
Interest rate in addition to LIBOR Rate	1.40%
Interest rate increase	0.775%
New credit agreement [Member] | Maximum [Member]
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Commitment fee	0.35%
Margins for LIBOR loans	2.00%
Margins for base rate loans	1.00%
New credit agreement [Member] | Minimum [Member]
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Commitment fee	0.20%
Margins for LIBOR loans	1.15%
Margins for base rate loans	0.15%
Previous credit facility [Member]
Schedule of Long-term Debt Instruments (Textual) [Abstract]
Credit facility	$ 845,000
Revolving credit facility maturing	2012, September 27