Fair Value Measurements (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fair Value of Cash Equivalents [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	$ 539,512	$ 741,835
Fair Value of Cash Equivalents [Member] | Auction rate securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value of Cash Equivalents [Member] | Certificates of deposit [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	105,017	137,215
Fair Value of Cash Equivalents [Member] | Commercial paper [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	26,617	34,742
Fair Value of Cash Equivalents [Member] | Corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	1,002
Fair Value of Cash Equivalents [Member] | Debt securities of government sponsored entities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value of Cash Equivalents [Member] | Equity index funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0
Fair Value of Cash Equivalents [Member] | Federally insured corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value		0
Fair Value of Cash Equivalents [Member] | Money market funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	404,757	564,112
Fair Value of Cash Equivalents [Member] | Municipal bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	3,121	3,764
Fair Value of Cash Equivalents [Member] | U.S. Treasury securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	1,000
Fair Value of Available-for-Sale Securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	1,640,536	984,011
Fair Value of Available-for-Sale Securities [Member] | Auction rate securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	11,551	21,293
Fair Value of Available-for-Sale Securities [Member] | Certificates of deposit [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	118,094	13,651
Fair Value of Available-for-Sale Securities [Member] | Commercial paper [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	172,564	14,793
Fair Value of Available-for-Sale Securities [Member] | Corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	566,724	237,916
Fair Value of Available-for-Sale Securities [Member] | Debt securities of government sponsored entities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	290,543	332,051
Fair Value of Available-for-Sale Securities [Member] | Equity index funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	40,843
Fair Value of Available-for-Sale Securities [Member] | Federally insured corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value		21,454
Fair Value of Available-for-Sale Securities [Member] | Money market funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	15,067	20,315
Fair Value of Available-for-Sale Securities [Member] | Municipal bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	399,026	300,817
Fair Value of Available-for-Sale Securities [Member] | U.S. Treasury securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	26,124	21,721
Fair Value, Total [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	2,180,048	1,725,846
Fair Value, Total [Member] | Auction rate securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	11,551	21,293
Fair Value, Total [Member] | Certificates of deposit [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	223,111	150,866
Fair Value, Total [Member] | Commercial paper [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	199,181	49,535
Fair Value, Total [Member] | Corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	566,724	238,918
Fair Value, Total [Member] | Debt securities of government sponsored entities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	290,543	332,051
Fair Value, Total [Member] | Equity index funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	40,843
Fair Value, Total [Member] | Federally insured corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value		21,454
Fair Value, Total [Member] | Money market funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	419,824	584,427
Fair Value, Total [Member] | Municipal bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	402,147	304,581
Fair Value, Total [Member] | U.S. Treasury securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	26,124	22,721
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets measured at fair value on a recurring basis
Total assets measured at fair value	777,334	960,653
Quoted Prices in Active Markets for Identical Assets (Level 1) | Auction rate securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Certificates of deposit [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial paper [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Debt securities of government sponsored entities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	290,543	332,051
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity index funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	40,843
Quoted Prices in Active Markets for Identical Assets (Level 1) | Federally insured corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value		21,454
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	419,824	584,427
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	26,124	22,721
Significant Other Observable Inputs (Level 2)
Assets measured at fair value on a recurring basis
Total assets measured at fair value	1,391,163	743,900
Significant Other Observable Inputs (Level 2) | Auction rate securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Significant Other Observable Inputs (Level 2) | Certificates of deposit [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	223,111	150,866
Significant Other Observable Inputs (Level 2) | Commercial paper [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	199,181	49,535
Significant Other Observable Inputs (Level 2) | Corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	566,724	238,918
Significant Other Observable Inputs (Level 2) | Debt securities of government sponsored entities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Significant Other Observable Inputs (Level 2) | Equity index funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0
Significant Other Observable Inputs (Level 2) | Federally insured corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value		0
Significant Other Observable Inputs (Level 2) | Money market funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Significant Other Observable Inputs (Level 2) | Municipal bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	402,147	304,581
Significant Other Observable Inputs (Level 2) | U.S. Treasury securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Significant Unobservable Inputs (Level 3)
Assets measured at fair value on a recurring basis
Total assets measured at fair value	11,551	21,293
Significant Unobservable Inputs (Level 3) | Auction rate securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	11,551	21,293
Significant Unobservable Inputs (Level 3) | Certificates of deposit [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Significant Unobservable Inputs (Level 3) | Commercial paper [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Significant Unobservable Inputs (Level 3) | Corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Significant Unobservable Inputs (Level 3) | Debt securities of government sponsored entities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Significant Unobservable Inputs (Level 3) | Equity index funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0
Significant Unobservable Inputs (Level 3) | Federally insured corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value		0
Significant Unobservable Inputs (Level 3) | Money market funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Significant Unobservable Inputs (Level 3) | Municipal bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Significant Unobservable Inputs (Level 3) | U.S. Treasury securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	$ 0	$ 0