Fair Value Measurements (Details) (USD $)			6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Auction rate securities	Jun. 30, 2012 Recurring Fair value	Jun. 30, 2012 Recurring Fair value Bond and bond funds	Jun. 30, 2012 Recurring Fair value Commercial paper	Jun. 30, 2012 Recurring Fair value Asset backed securities	Jun. 30, 2012 Recurring Level 1	Jun. 30, 2012 Recurring Level 2	Jun. 30, 2012 Recurring Level 2 Bond and bond funds	Jun. 30, 2012 Recurring Level 2 Commercial paper	Jun. 30, 2012 Recurring Level 2 Asset backed securities	Jun. 30, 2012 Recurring Level 3
Fair Value Measurements
Marketable securities	$ 501,332,000	$ 497,552,000		$ 501,332,000	$ 497,410,000	$ 3,543,000	$ 379,000		$ 501,332,000	$ 497,410,000	$ 3,543,000	$ 379,000
Cash, Cash Equivalents and Restricted Cash				128,159,000				128,159,000
Other Assets				3,825,000									3,825,000
Assets Measured at Fair Value				633,316,000				128,159,000	501,332,000				3,825,000
Changes in assets measured at fair value on a recurring basis using significant unobservable inputs (level 3)
Balance at the beginning of the period			3,793,000
Total realized and unrealized gains or (losses) Included in other comprehensive income			32,000
Balance at the end of the period			3,825,000
Fair Value of Financial Instruments
Fair value of long-term debt									1,834,500,000
Carrying amount of long-term debt	$ 1,730,800,000