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Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value $ 2,285,840 $ 1,564,441
Fair Value 2,285,840 1,564,441
Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 2,521,465 2,737,842
Cash and cash equivalents
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Fair Value 92,556 254,732
Cash and cash equivalents | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 325,756 1,425,815
Cash and cash equivalents | Cash | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 27,409 65,935
Cash and cash equivalents | Money market funds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 205,791 1,105,148
Cash and cash equivalents | Commercial paper | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 70,873 247,206
Cash and cash equivalents | Corporate bonds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 21,683 7,526
Short-term restricted funds
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Fair Value 90,449 90,440
Short-term restricted funds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 90,449 90,440
Short-term restricted funds | U.S. Treasury bills | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 90,449 90,440
Short-term investments
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Fair Value 1,287,298 789,668
Short-term investments | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 1,287,298 789,668
Short-term investments | Commercial paper | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 579,633 315,769
Short-term investments | Corporate bonds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 682,639 466,898
Short-term investments | U.S. Treasury bills | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 26  
Short-term investments | Certificates of deposit | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 25,000 7,001
Long-term restricted funds
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Fair Value 302,783 393,334
Long-term restricted funds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 305,208 395,652
Long-term restricted funds | Money market funds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 2,425 2,318
Long-term restricted funds | U.S. Treasury bills | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total   178,216
Long-term restricted funds | U.S. Treasury notes | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 302,783 215,118
Long-term investments
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Fair Value 512,754 36,267
Long-term investments | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 512,754 36,267
Long-term investments | Corporate bonds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Total 512,754 36,267
Level 1 | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 235,625 1,173,401
Fair Value 235,625 1,173,401
Level 1 | Cash and cash equivalents | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 233,200 1,171,083
Fair Value 233,200 1,171,083
Level 1 | Cash and cash equivalents | Cash | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 27,409 65,935
Fair Value 27,409 65,935
Level 1 | Cash and cash equivalents | Money market funds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 205,791 1,105,148
Fair Value 205,791 1,105,148
Level 1 | Long-term restricted funds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 2,425 2,318
Fair Value 2,425 2,318
Level 1 | Long-term restricted funds | Money market funds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 2,425 2,318
Fair Value 2,425 2,318
Level 2 | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 2,285,840 1,564,441
Fair Value 2,285,840 1,564,441
Level 2 | Cash and cash equivalents | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 92,556 254,732
Fair Value 92,556 254,732
Level 2 | Cash and cash equivalents | Commercial paper | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 70,873 247,206
Fair Value 70,873 247,206
Level 2 | Cash and cash equivalents | Corporate bonds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 21,683 7,526
Fair Value 21,683 7,526
Level 2 | Short-term restricted funds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 90,449 90,440
Fair Value 90,449 90,440
Level 2 | Short-term restricted funds | U.S. Treasury bills | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 90,449 90,440
Fair Value 90,449 90,440
Level 2 | Short-term investments | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 1,287,298 789,668
Fair Value 1,287,298 789,668
Level 2 | Short-term investments | Commercial paper | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 579,633 315,769
Fair Value 579,633 315,769
Level 2 | Short-term investments | Corporate bonds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 682,639 466,898
Fair Value 682,639 466,898
Level 2 | Short-term investments | U.S. Treasury bills | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 26  
Fair Value 26  
Level 2 | Short-term investments | Certificates of deposit | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 25,000 7,001
Fair Value 25,000 7,001
Level 2 | Long-term restricted funds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 302,783 393,334
Fair Value 302,783 393,334
Level 2 | Long-term restricted funds | U.S. Treasury bills | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value   178,216
Fair Value   178,216
Level 2 | Long-term restricted funds | U.S. Treasury notes | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 302,783 215,118
Fair Value 302,783 215,118
Level 2 | Long-term investments | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 512,754 36,267
Fair Value 512,754 36,267
Level 2 | Long-term investments | Corporate bonds | Recurring basis
    
Significant financial instruments as categorized by the fair value measurement hierarchy    
Carrying Value 512,754 36,267
Fair Value $ 512,754 $ 36,267