Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012		Dec. 31, 2011		Jun. 30, 2011	Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 437,624		$ 424,155		$ 418,428	$ 276,915
Short-term investments	100,738		127,039
Long-term investments	55,468		30,057
Long term investment maturities - begin	12 months from June 30, 2012
Long term investment maturities - end	21 months from June 30, 2012
Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	437,624		424,155
Fair Value | Level 1 | 6.95% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	366,624	[1]	364,162	[1]
Fair Value | Level 2 | 6.7% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	540,928	[1]	534,860	[1]
Fair Value | Government backed securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments	100,738	[2],[3],[4]	127,039	[2],[3],[4]
Long-term investments	55,468	[3],[4],[5]	30,140	[3],[4],[5]
Book Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	437,624		424,155
Book Value | 6.95% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	342,000	[1]	342,000	[1]
Book Value | 6.7% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	534,338	[1]	534,111	[1]
Book Value | Government backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments	100,738	[2],[3],[4]	127,039	[2],[3],[4]
Long-term investments	$ 55,468	[3],[4],[5]	$ 30,057	[3],[4],[5]

[1] Excludes capital lease obligations and current portion of Long-term debt.

[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] Designated as held-to-maturity investments and recorded at amortized cost in the Consolidated Balance Sheet.

[5] At June 30, 2012, maturities range between 12 and 21 months.