Fair Value Measurements (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Available for sale debt securities:
Trading securities - energy industry	$ 11,576	$ 19,630
Recurring [Member]
Short term investments:
Available-for-sale equity securities - energy industry	56,654
Available for sale debt securities:
Commercial paper and CDs	1,160
Corporate debt securities	44,600
Mortgage-backed debt securities	367
Mortgage-CMO debt securities	2,797
Asset-backed debt securities	2,705
Trading securities - energy industry	11,576
Total short-term investments	119,859
Liabilities:
Derivative contract	(1,900)
Recurring [Member] | Level 1 [Member]
Short term investments:
Available-for-sale equity securities - energy industry	56,654
Available for sale debt securities:
Commercial paper and CDs	1,160
Corporate debt securities	0
Mortgage-backed debt securities	0
Mortgage-CMO debt securities	0
Asset-backed debt securities	2,705
Trading securities - energy industry	11,576
Total short-term investments	72,095
Liabilities:
Derivative contract	0
Recurring [Member] | Level 2 [Member]
Short term investments:
Available-for-sale equity securities - energy industry	0
Available for sale debt securities:
Commercial paper and CDs	0
Corporate debt securities	44,600
Mortgage-backed debt securities	367
Mortgage-CMO debt securities	2,797
Asset-backed debt securities	0
Trading securities - energy industry	0
Total short-term investments	47,764
Liabilities:
Derivative contract	(1,900)
Recurring [Member] | Level 3 [Member]
Short term investments:
Available-for-sale equity securities - energy industry	0
Available for sale debt securities:
Commercial paper and CDs	0
Corporate debt securities	0
Mortgage-backed debt securities	0
Mortgage-CMO debt securities	0
Asset-backed debt securities	0
Trading securities - energy industry	0
Total short-term investments	0
Liabilities:
Derivative contract	$ 0