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Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2012
Minimum
Dec. 31, 2012
Maximum
Dec. 31, 2012
6.7% Senior Notes
Minimum
Dec. 31, 2012
6.7% Senior Notes
Maximum
Dec. 31, 2012
Fair Value
Level 1
Dec. 31, 2011
Fair Value
Level 1
Dec. 31, 2012
Fair Value
Level 1
6.95% Senior Notes
Dec. 31, 2011
Fair Value
Level 1
6.95% Senior Notes
Dec. 31, 2012
Fair Value
Level 2
6.7% Senior Notes
Dec. 31, 2011
Fair Value
Level 2
6.7% Senior Notes
Dec. 31, 2012
Fair Value
Government backed securities
Level 1
Dec. 31, 2011
Fair Value
Government backed securities
Level 1
Dec. 31, 2012
Book Value
Dec. 31, 2011
Book Value
Dec. 31, 2012
Book Value
6.95% Senior Notes
Dec. 31, 2011
Book Value
6.95% Senior Notes
Dec. 31, 2012
Book Value
6.7% Senior Notes
Dec. 31, 2011
Book Value
6.7% Senior Notes
Dec. 31, 2012
Book Value
Government backed securities
Dec. 31, 2011
Book Value
Government backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]                                                
Cash and cash equivalents $ 378,358 $ 424,155 $ 276,915 $ 302,991         $ 378,358 $ 424,155             $ 378,358 $ 424,155            
Short-term investments 100,676 127,039                         100,676 [1],[2],[3] 127,039 [1],[2],[3]             100,676 [1],[2],[3] 127,039 [1],[2],[3]
Long-term investments 50,305 30,057                         50,339 [1],[3],[4] 30,140 [1],[3],[4] 50,305 [1],[3],[4] 30,057 [1],[3],[4]            
Long-term debt                     $ 376,610 [5] $ 364,162 [5] $ 582,744 [5] $ 534,860 [5]         $ 342,000 [5] $ 342,000 [5] $ 532,194 [5] $ 534,111 [5]    
Long term investment maturities         Mar. 15, 2014 Dec. 15, 2014                                    
Fair value assumption, interest rate             6.09% 6.85%                                
[1] Designated as held-to-maturity investments and are recorded at amortized cost in the Consolidated Balance Sheet.
[2] Maturities are less than twelve months from the respective balance sheet dates.
[3] Includes U.S. treasuries and corporate notes guaranteed under the Federal Deposit Insurance Corporation’s Temporary Liquidity Guarantee Program.
[4] Maturities range between 14 and 23 months from the balance sheet date.
[5] Excludes capital lease obligations and current portion of Long-term debt.