Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Available for sale debt securities:
Trading securities - energy industry	$ 17,638	$ 11,600
Recurring [Member]
Short term investments:
Available-for-sale equity securities - energy industry	81,227
Available for sale debt securities:
Commercial paper and CDs	1,150
Corporate debt securities	34,950
Mortgage-backed debt securities	257
Mortgage-CMO debt securities	2,421
Asset-backed debt securities	2,307
Trading securities - energy industry	17,638
Total short-term investments	139,950
Liabilities:
Derivative contract	819
Recurring [Member] | Level 1 [Member]
Short term investments:
Available-for-sale equity securities - energy industry	72,903
Available for sale debt securities:
Commercial paper and CDs	1,150
Corporate debt securities
Mortgage-backed debt securities
Mortgage-CMO debt securities
Asset-backed debt securities	2,307
Trading securities - energy industry	17,638
Total short-term investments	93,998
Liabilities:
Derivative contract
Recurring [Member] | Level 2 [Member]
Short term investments:
Available-for-sale equity securities - energy industry	8,324
Available for sale debt securities:
Commercial paper and CDs
Corporate debt securities	34,950
Mortgage-backed debt securities	257
Mortgage-CMO debt securities	2,421
Asset-backed debt securities
Trading securities - energy industry
Total short-term investments	45,952
Liabilities:
Derivative contract	819
Recurring [Member] | Level 3 [Member]
Short term investments:
Available-for-sale equity securities - energy industry
Available for sale debt securities:
Commercial paper and CDs
Corporate debt securities
Mortgage-backed debt securities
Mortgage-CMO debt securities
Asset-backed debt securities
Trading securities - energy industry
Total short-term investments
Liabilities:
Derivative contract