Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Auction rate securities	Dec. 31, 2012 Recurring Fair value	Dec. 31, 2011 Recurring Fair value	Dec. 31, 2012 Recurring Fair value Auction rate securities	Dec. 31, 2011 Recurring Fair value Auction rate securities	Dec. 31, 2012 Recurring Fair value Bonds	Dec. 31, 2011 Recurring Fair value Bonds	Dec. 31, 2012 Recurring Fair value Commercial paper	Dec. 31, 2011 Recurring Fair value Commercial paper	Dec. 31, 2012 Recurring Fair value Asset backed securities	Dec. 31, 2011 Recurring Fair value Asset backed securities	Dec. 31, 2012 Recurring Level 1	Dec. 31, 2011 Recurring Level 1	Dec. 31, 2012 Recurring Level 2	Dec. 31, 2011 Recurring Level 2	Dec. 31, 2012 Recurring Level 2 Bonds	Dec. 31, 2011 Recurring Level 2 Bonds	Dec. 31, 2012 Recurring Level 2 Commercial paper	Dec. 31, 2011 Recurring Level 2 Commercial paper	Dec. 31, 2012 Recurring Level 2 Asset backed securities	Dec. 31, 2011 Recurring Level 2 Asset backed securities	Dec. 31, 2012 Recurring Level 3	Dec. 31, 2011 Recurring Level 3
Fair Value Measurements
Marketable securities	$ 556,117	$ 497,552		$ 556,117	$ 497,552			$ 552,289	$ 492,517	$ 3,514	$ 4,557	$ 314	$ 478			$ 556,117	$ 497,552	$ 552,289	$ 492,517	$ 3,514	$ 4,557	$ 314	$ 478
Cash, Cash Equivalents and Restricted Cash				153,325	148,960									153,325	148,960
Other Assets						3,844	3,793																	3,844	3,793
Total Assets Measured at Fair Value				713,286	650,305									153,325	148,960	556,117	497,552							3,844	3,793
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
Total realized and unrealized gains or (losses) Included in other comprehensive income			51
Balance at the end of the period			$ 3,844