Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2008
Available for sale debt securities:
Trading securities - energy industry	$ 11,600	$ 19,630
Total short-term investments			46,000
Recurring [Member]
Short term investments:
Available-for-sale equity securities - energy industry	71,433
Available for sale debt securities:
Commercial paper and CDs	1,230
Corporate debt securities	51,300
Mortgage-backed debt securities	309
Mortgage-CMO debt securities	2,547
Asset-backed debt securities	2,495
Trading securities - energy industry	11,600
Total short-term investments	140,914
Liabilities:
Derivative contract	1,281
Recurring [Member] | Level 1 [Member]
Short term investments:
Available-for-sale equity securities - energy industry	60,923
Available for sale debt securities:
Commercial paper and CDs	1,230
Corporate debt securities	0
Mortgage-backed debt securities	0
Mortgage-CMO debt securities	0
Asset-backed debt securities	2,495
Trading securities - energy industry	11,600
Total short-term investments	76,248
Liabilities:
Derivative contract	0
Recurring [Member] | Level 2 [Member]
Short term investments:
Available-for-sale equity securities - energy industry	10,510
Available for sale debt securities:
Commercial paper and CDs	0
Corporate debt securities	51,300
Mortgage-backed debt securities	309
Mortgage-CMO debt securities	2,547
Asset-backed debt securities	0
Trading securities - energy industry	0
Total short-term investments	64,666
Liabilities:
Derivative contract	1,281
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