Fair Value Measurements (Details) (USD $)			6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Auction rate securities	Jun. 30, 2013 Recurring Fair value	Dec. 31, 2012 Recurring Fair value	Jun. 30, 2013 Recurring Fair value Bonds	Dec. 31, 2012 Recurring Fair value Bonds	Jun. 30, 2013 Recurring Fair value Asset backed securities	Dec. 31, 2012 Recurring Fair value Asset backed securities	Dec. 31, 2012 Recurring Fair value Commercial paper	Jun. 30, 2013 Recurring Level 1	Dec. 31, 2012 Recurring Level 1	Jun. 30, 2013 Recurring Level 2	Dec. 31, 2012 Recurring Level 2	Jun. 30, 2013 Recurring Level 2 Bonds	Dec. 31, 2012 Recurring Level 2 Bonds	Jun. 30, 2013 Recurring Level 2 Asset backed securities	Dec. 31, 2012 Recurring Level 2 Asset backed securities	Dec. 31, 2012 Recurring Level 2 Commercial paper	Jun. 30, 2013 Recurring Level 3	Dec. 31, 2012 Recurring Level 3
Fair Value Measurements
Marketable securities	$ 505,516,000	$ 556,117,000		$ 505,516,000	$ 556,117,000	$ 505,293,000	$ 552,289,000	$ 223,000	$ 314,000	$ 3,514,000			$ 505,516,000	$ 556,117,000	$ 505,293,000	$ 552,289,000	$ 223,000	$ 314,000	$ 3,514,000
Cash, Cash Equivalents and Restricted Cash				160,105,000	153,325,000						160,105,000	153,325,000
Other Assets				2,248,000	3,844,000															2,248,000	3,844,000
Total Assets Measured at Fair Value				667,869,000	713,286,000						160,105,000	153,325,000	505,516,000	556,117,000						2,248,000	3,844,000
Changes in assets measured at fair value on a recurring basis using significant unobservable inputs (level 3)
Balance at the beginning of the period			3,844,000
Total realized and unrealized gains or (losses) Included in other comprehensive income			(68,000)
Settlements			(1,528,000)
Balance at the end of the period			2,248,000
Fair Value of Financial Instruments
Fair value of long-term debt													1,651,400,000	1,744,200,000
Carrying amount of long-term debt													$ 1,556,100,000	$ 1,642,000,000