Fair Value Measurments (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Derivative Assets [Abstract]
Derivative Assets, Current	$ 146,510	$ 133,991
Derivative Assets, Noncurrent	17,149	2,066
Derivative Liabilities [Abstract]
Derivative Liabilities, Current	0	718
Derivative Liabilities, Noncurrent	0	5,337
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	1,560,000	1,560,000
Fair Value, Inputs, Level 2 [Member]
Derivative Assets [Abstract]
Derivative Assets, Current	146,510
Derivative Assets, Noncurrent	17,149
Derivative Liabilities [Abstract]
Derivative Liabilities, Current	0
Derivative Liabilities, Noncurrent	0
Notes Due 2015 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	100,000	100,000
Notes Payable, Fair Value Disclosure	111,002	108,572
Notes Due 2018 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	200,000	200,000
Notes Payable, Fair Value Disclosure	227,987	212,660
Notes Due 2016 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	62,000	62,000
Notes Payable, Fair Value Disclosure	74,248	72,153
Notes Due 2019 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	173,000	173,000
Notes Payable, Fair Value Disclosure	216,361	203,051
Notes Due 2017 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	116,000	116,000
Notes Payable, Fair Value Disclosure	127,118	119,385
Notes Due January 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	207,000	207,000
Notes Payable, Fair Value Disclosure	225,461	206,233
Notes Due January 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	87,000	87,000
Notes Payable, Fair Value Disclosure	92,497	84,818
Notes Due January 2025 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	90,000	90,000
Notes Payable, Fair Value Disclosure	96,942	87,211
Notes Due October 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	315,000	315,000
Notes Payable, Fair Value Disclosure	316,205	284,207
Notes Due October 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	35,000	35,000
Notes Payable, Fair Value Disclosure	34,340	30,989
Notes Due October 2025 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	175,000	175,000
Notes Payable, Fair Value Disclosure	$ 171,555	$ 152,064