Derivatives and Fair Value of Financial Instruments (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Commodities
Assets:
Derivatives	$ 11,732,000	$ 5,144,000
Margin account	8,788,000	8,619,000
Derivative assets and liabilities, net basis	8,603,000	8,234,000
Liabilities:
Derivatives	11,917,000	5,529,000
Interest rate swaps
Liabilities:
Derivatives	13,566,000	11,268,000
Foreign currencies
Assets:
Derivatives	647,000	2,247,000
Liabilities:
Derivatives	1,133,000	3,380,000
Corporate bonds
Assets:
Available-for-sale securities	136,258,000	89,146,000
Money market funds and deposit accounts
Assets:
Available-for-sale securities	101,011,000	139,420,000
Enhanced cash mutual fund
Assets:
Available-for-sale securities	50,072,000
Collateralized mortgage obligations
Assets:
Available-for-sale securities	24,944,000	15,011,000
Fixed rate municipal notes and bonds
Assets:
Available-for-sale securities	23,592,000	17,788,000
U.S. Government agency securities
Assets:
Available-for-sale securities	22,572,000	9,757,000
U.S. Treasury securities
Assets:
Available-for-sale securities	20,526,000	4,905,000
Emerging markets debt mutual fund
Assets:
Available-for-sale securities	17,035,000	16,399,000
Asset backed debt securities
Assets:
Available-for-sale securities	15,514,000	3,533,000
High yield trading debt securities
Assets:
Trading securities	22,084,000	20,750,000
Emerging markets trading debt mutual fund
Assets:
Trading securities	2,919,000	2,487,000
Emerging markets trading debt securities
Assets:
Trading securities	2,073,000	2,355,000
Recurring basis | Balance at period end
Assets:
Total Assets	487,577,000	364,571,000
Liabilities:
Total Liabilities	26,616,000	20,177,000
Recurring basis | Balance at period end | Commodities
Assets:
Derivatives	11,732,000	5,144,000
Margin account	8,788,000	8,619,000
Derivative assets and liabilities, net basis	8,603,000	8,234,000
Liabilities:
Derivatives	11,917,000	5,529,000
Recurring basis | Balance at period end | Interest rate swaps
Liabilities:
Derivatives	13,566,000	11,268,000
Recurring basis | Balance at period end | Foreign currencies
Assets:
Derivatives	647,000	2,247,000
Liabilities:
Derivatives	1,133,000	3,380,000
Recurring basis | Balance at period end | Corporate bonds | Short-term investments
Assets:
Available-for-sale securities	136,258,000	89,146,000
Recurring basis | Balance at period end | Corporate bonds | Other current assets
Assets:
Trading securities	62,000	72,000
Recurring basis | Balance at period end | Money market funds and deposit accounts | Short-term investments
Assets:
Available-for-sale securities	101,011,000	139,420,000
Recurring basis | Balance at period end | Money market funds and deposit accounts | Other current assets
Assets:
Trading securities	3,078,000	4,521,000
Recurring basis | Balance at period end | Enhanced cash mutual fund | Short-term investments
Assets:
Available-for-sale securities	50,072,000
Recurring basis | Balance at period end | Collateralized mortgage obligations | Short-term investments
Assets:
Available-for-sale securities	24,944,000	15,011,000
Recurring basis | Balance at period end | Fixed rate municipal notes and bonds | Short-term investments
Assets:
Available-for-sale securities	23,592,000	17,788,000
Recurring basis | Balance at period end | U.S. Government agency securities | Short-term investments
Assets:
Available-for-sale securities	22,572,000	9,757,000
Recurring basis | Balance at period end | U.S. Government agency securities | Other current assets
Assets:
Trading securities	2,402,000	2,085,000
Recurring basis | Balance at period end | U.S. Treasury securities | Short-term investments
Assets:
Available-for-sale securities	20,526,000	4,905,000
Recurring basis | Balance at period end | U.S. Treasury securities | Other current assets
Assets:
Trading securities	1,433,000	1,474,000
Recurring basis | Balance at period end | Emerging markets debt mutual fund | Short-term investments
Assets:
Available-for-sale securities	17,035,000	16,399,000
Recurring basis | Balance at period end | Asset backed debt securities | Short-term investments
Assets:
Available-for-sale securities	15,514,000	3,533,000
Recurring basis | Balance at period end | Other | Short-term investments
Assets:
Available-for-sale securities	683,000	1,484,000
Recurring basis | Balance at period end | Other | Other current assets
Assets:
Trading securities	276,000	236,000
Recurring basis | Balance at period end | High yield trading debt securities | Short-term investments
Assets:
Trading securities	22,084,000	20,750,000
Recurring basis | Balance at period end | Emerging markets trading debt mutual fund | Short-term investments
Assets:
Trading securities	2,919,000	2,487,000
Recurring basis | Balance at period end | Emerging markets trading debt securities | Short-term investments
Assets:
Trading securities	2,073,000	2,355,000
Recurring basis | Balance at period end | Other debt securities | Short-term investments
Assets:
Trading securities	754,000	221,000
Recurring basis | Balance at period end | Domestic equity securities | Other current assets
Assets:
Trading securities	16,667,000	13,563,000
Recurring basis | Balance at period end | Foreign equity securities | Other current assets
Assets:
Trading securities	6,232,000	7,490,000
Recurring basis | Balance at period end | Fixed income mutual funds | Other current assets
Assets:
Trading securities	5,011,000	4,483,000
Recurring basis | Level 1
Assets:
Total Assets	211,331,000	190,167,000
Liabilities:
Total Liabilities	11,917,000	5,529,000
Recurring basis | Level 1 | Commodities
Assets:
Derivatives	11,732,000	5,144,000
Liabilities:
Derivatives	11,917,000	5,529,000
Recurring basis | Level 1 | Money market funds and deposit accounts | Short-term investments
Assets:
Available-for-sale securities	101,011,000	139,420,000
Recurring basis | Level 1 | Money market funds and deposit accounts | Other current assets
Assets:
Trading securities	3,078,000	4,521,000
Recurring basis | Level 1 | Enhanced cash mutual fund | Short-term investments
Assets:
Available-for-sale securities	50,072,000
Recurring basis | Level 1 | Emerging markets debt mutual fund | Short-term investments
Assets:
Available-for-sale securities	17,035,000	16,399,000
Recurring basis | Level 1 | Other | Other current assets
Assets:
Trading securities	233,000	159,000
Recurring basis | Level 1 | Emerging markets trading debt mutual fund | Short-term investments
Assets:
Trading securities	2,919,000	2,487,000
Recurring basis | Level 1 | Domestic equity securities | Other current assets
Assets:
Trading securities	16,667,000	13,563,000
Recurring basis | Level 1 | Foreign equity securities | Other current assets
Assets:
Trading securities	3,573,000	3,991,000
Recurring basis | Level 1 | Fixed income mutual funds | Other current assets
Assets:
Trading securities	5,011,000	4,483,000
Recurring basis | Level 2
Assets:
Total Assets	276,246,000	174,404,000
Liabilities:
Total Liabilities	14,699,000	14,648,000
Recurring basis | Level 2 | Interest rate swaps
Liabilities:
Derivatives	13,566,000	11,268,000
Recurring basis | Level 2 | Foreign currencies
Assets:
Derivatives	647,000	2,247,000
Liabilities:
Derivatives	1,133,000	3,380,000
Recurring basis | Level 2 | Corporate bonds | Short-term investments
Assets:
Available-for-sale securities	136,258,000	89,146,000
Recurring basis | Level 2 | Corporate bonds | Other current assets
Assets:
Trading securities	62,000	72,000
Recurring basis | Level 2 | Collateralized mortgage obligations | Short-term investments
Assets:
Available-for-sale securities	24,944,000	15,011,000
Recurring basis | Level 2 | Fixed rate municipal notes and bonds | Short-term investments
Assets:
Available-for-sale securities	23,592,000	17,788,000
Recurring basis | Level 2 | U.S. Government agency securities | Short-term investments
Assets:
Available-for-sale securities	22,572,000	9,757,000
Recurring basis | Level 2 | U.S. Government agency securities | Other current assets
Assets:
Trading securities	2,402,000	2,085,000
Recurring basis | Level 2 | U.S. Treasury securities | Short-term investments
Assets:
Available-for-sale securities	20,526,000	4,905,000
Recurring basis | Level 2 | U.S. Treasury securities | Other current assets
Assets:
Trading securities	1,433,000	1,474,000
Recurring basis | Level 2 | Asset backed debt securities | Short-term investments
Assets:
Available-for-sale securities	15,514,000	3,533,000
Recurring basis | Level 2 | Other | Short-term investments
Assets:
Available-for-sale securities	683,000	1,484,000
Recurring basis | Level 2 | Other | Other current assets
Assets:
Trading securities	43,000	77,000
Recurring basis | Level 2 | High yield trading debt securities | Short-term investments
Assets:
Trading securities	22,084,000	20,750,000
Recurring basis | Level 2 | Emerging markets trading debt securities | Short-term investments
Assets:
Trading securities	2,073,000	2,355,000
Recurring basis | Level 2 | Other debt securities | Short-term investments
Assets:
Trading securities	754,000	221,000
Recurring basis | Level 2 | Foreign equity securities | Other current assets
Assets:
Trading securities	$ 2,659,000	$ 3,499,000