Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified			12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Auction rate securities	Dec. 31, 2013 Recurring Fair value	Dec. 31, 2012 Recurring Fair value	Dec. 31, 2013 Recurring Fair value Bonds	Dec. 31, 2012 Recurring Fair value Bonds	Dec. 31, 2012 Recurring Fair value Commercial paper	Dec. 31, 2013 Recurring Fair value Asset backed securities	Dec. 31, 2012 Recurring Fair value Asset backed securities	Dec. 31, 2013 Recurring Level 1	Dec. 31, 2012 Recurring Level 1	Dec. 31, 2013 Recurring Level 2	Dec. 31, 2012 Recurring Level 2	Dec. 31, 2013 Recurring Level 2 Bonds	Dec. 31, 2012 Recurring Level 2 Bonds	Dec. 31, 2012 Recurring Level 2 Commercial paper	Dec. 31, 2013 Recurring Level 2 Asset backed securities	Dec. 31, 2012 Recurring Level 2 Asset backed securities	Dec. 31, 2013 Recurring Level 3	Dec. 31, 2012 Recurring Level 3
Fair Value Measurements
Marketable securities	$ 487,239	$ 556,117		$ 487,239	$ 556,117	$ 487,049	$ 552,289	$ 3,514	$ 190	$ 314			$ 487,239	$ 556,117	$ 487,049	$ 552,289	$ 3,514	$ 190	$ 314
Cash, Cash Equivalents and Restricted Cash				182,855	153,325						182,855	153,325
Other Assets				2,245	3,844															2,245	3,844
Total Assets Measured at Fair Value				672,339	713,286						182,855	153,325	487,239	556,117						2,245	3,844
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
Total realized and unrealized gains or (losses) Included in other comprehensive income			(71)
Settlements			(1,528)
Balance at the end of the period			$ 2,245