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Cash and Cash Equivalents and Short-term Investments (Details 1) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Cash and cash equivalents and short-term investments    
Cash and cash equivalents, fair value $ 354,022 $ 398,575
Cash and cash equivalents, gross unrealized holding gains     
Cash and cash equivalents, gross unrealized holding losses     
Short-term investments:    
Trading equity securities, fair value 17,638 11,600
Trading equity securities, gross unrealized holding gains 11,913 5,876
Trading Securities, Unrealized Holding Loss     
Available-for-sale debt securities:    
Total available-for-sale debt securities, fair value 41,085 57,881
Available-for-sale equities securities    
Available-for-sale equity securities - energy industry 122,312 129,314
Available-for-sale equity securities, gross unrealized holding gains 56,080 55,592
Available-for-sale equity securities, gross unrealized holding losses (3,076) (2,348)
Total short-term investments 139,950 140,914
Total cash, cash equivalents and short-term investments, fair value 493,972 539,489
Total cash, cash equivalents and short-term investments, gross unrealized gains 67,993 61,468
Total cash, cash equivalents and short-term investments, gross unrealized losses (3,076) (2,348)
Debt Securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities, fair value 41,085 57,881
Total available-for-sale debt securities, gross unrealized gains 13,211 22,517
Available-for-sale equities securities    
Available-for-sale equity securities, gross unrealized holding losses (3,076) (2,348)
Equity Securities [Member]
   
Available-for-sale equities securities    
Available-for-sale equity securities - energy industry 81,227 71,433
Available-for-sale equity securities, gross unrealized holding gains 42,869 33,075
Available-for-sale equity securities, gross unrealized holding losses    0
Commercial paper and CDs [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities, fair value 1,150 1,230
Total available-for-sale debt securities, gross unrealized gains     
Total available-for-sale debt securities, gross unrealized losses     
Corporate debt securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities, fair value 34,950 51,300
Total available-for-sale debt securities, gross unrealized gains 13,183 22,494
Available-for-sale equities securities    
Available-for-sale equity securities, gross unrealized holding losses (2,852) (2,095)
Mortgage-backed debt securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities, fair value 257 309
Total available-for-sale debt securities, gross unrealized gains 9 10
Available-for-sale equities securities    
Available-for-sale equity securities, gross unrealized holding losses     
Mortgage-CMO debt securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities, fair value 2,421 2,547
Total available-for-sale debt securities, gross unrealized gains 19 13
Available-for-sale equities securities    
Available-for-sale equity securities, gross unrealized holding losses (4) (15)
Asset-backed debt securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities, fair value 2,307 2,495
Total available-for-sale debt securities, gross unrealized gains     
Available-for-sale equities securities    
Available-for-sale equity securities, gross unrealized holding losses (220) (238)
Short-term Investments [Member]
   
Available-for-sale equities securities    
Available-for-sale equity securities, gross unrealized holding losses (3,076) (2,348)
Total short-term investments 139,950 140,914
Total short-term investments, gross unrealized gains $ 67,993 $ 61,468