Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified			12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Auction rate securities	Dec. 31, 2011 Recurring Fair value	Dec. 31, 2010 Recurring Fair value	Dec. 31, 2011 Recurring Fair value Auction rate securities	Dec. 31, 2010 Recurring Fair value Auction rate securities	Dec. 31, 2011 Recurring Fair value Bonds	Dec. 31, 2010 Recurring Fair value Bonds	Dec. 31, 2011 Recurring Fair value Commercial paper	Dec. 31, 2010 Recurring Fair value Commercial paper	Dec. 31, 2011 Recurring Fair value Asset backed securities	Dec. 31, 2010 Recurring Fair value Asset backed securities	Dec. 31, 2011 Recurring Level 1	Dec. 31, 2010 Recurring Level 1	Dec. 31, 2011 Recurring Level 2	Dec. 31, 2010 Recurring Level 2	Dec. 31, 2011 Recurring Level 2 Bonds	Dec. 31, 2010 Recurring Level 2 Bonds	Dec. 31, 2011 Recurring Level 2 Commercial paper	Dec. 31, 2010 Recurring Level 2 Commercial paper	Dec. 31, 2011 Recurring Level 2 Asset backed securities	Dec. 31, 2010 Recurring Level 2 Asset backed securities	Dec. 31, 2011 Recurring Level 3	Dec. 31, 2010 Recurring Level 3
Fair Value Measurements
Marketable securities	$ 497,552	$ 670,739		$ 497,552	$ 670,739			$ 492,517	$ 665,023	$ 4,557	$ 4,998	$ 478	$ 718			$ 497,552	$ 670,739	$ 492,517	$ 665,023	$ 4,557	$ 4,998	$ 478	$ 718
Cash, Cash Equivalents and Restricted Cash				148,960	134,113									148,960	134,113
Other Assets						3,793	4,002																	3,793	4,002
Assets Measured at Fair Value				650,305	808,854									148,960	134,113	497,552	670,739							3,793	4,002
Changes in assets measured at fair value on a recurring basis using significant unobservable inputs (level 3)
Balance at the beginning of the period			4,002
Total realized and unrealized gains or (losses) Included in other comprehensive income			(209)
Balance at the end of the period			$ 3,793