Fair Value Measurements (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Available for sale debt securities:
Trading securities - energy industry	$ 12,970	$ 19,630
Recurring [Member]
Short term investments:
Available-for-sale equity securities - energy industry	68,580
Available for sale debt securities:
Commercial paper and CDs	1,216
Corporate debt securities	55,300
Mortgage-backed debt securities	370
Mortgage-CMO debt securities	2,754
Asset-backed debt securities	3,030
Trading securities - energy industry	12,970
Total short-term investments	144,220
Liabilities:
Derivative contract	2,578
Recurring [Member] | Level 1 [Member]
Short term investments:
Available-for-sale equity securities - energy industry	68,580
Available for sale debt securities:
Commercial paper and CDs	1,216
Corporate debt securities	0
Mortgage-backed debt securities	0
Mortgage-CMO debt securities	0
Asset-backed debt securities	3,030
Trading securities - energy industry	12,970
Total short-term investments	85,796
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	55,300
Mortgage-backed debt securities	370
Mortgage-CMO debt securities	2,754
Asset-backed debt securities	0
Trading securities - energy industry	0
Total short-term investments	58,424
Liabilities:
Derivative contract	2,578
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