Long-term debt (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Long-term debt
Long-term Debt	$ 887,792	$ 1,178,657
Less - current portion	(4,618)	(5,899)
Long-term Debt, Noncurrent	883,174	1,172,758
Revolver Under Credit Facility [Member]
Long-term debt
Long-term Debt	230,000	519,000
Senior Notes due 2015 [Member]
Long-term debt
Long-term Debt	175,000	175,000
Senior Notes due 2016 [Member]
Long-term debt
Long-term Debt	100,000	100,000
Senior Notes due 2018 [Member]
Long-term debt
Long-term Debt	50,000	50,000
Senior Notes due 2019 [Member]
Long-term debt
Long-term Debt	175,000	175,000
Senior Notes due 2021 [Member]
Long-term debt
Long-term Debt	100,000	100,000
Tax-exempt bonds [Member]
Long-term debt
Long-term Debt	38,460	38,460
2.50% to 10.35% notes payable to sellers in connection with acquisitions [Member]
Long-term debt
Long-term Debt	16,548	18,356
6.7% to 10.9% Notes Payable to Third Parties [Member]
Long-term debt
Long-term Debt	$ 2,784	$ 2,841