Fair Value Measurements (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands	Jun. 30, 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	$ 576,821	$ 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	135,653	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	1,000	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	13,000	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	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	427,168	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	0	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	1,253,050	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	15,672	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	7,002	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	51,262	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	346,179	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	375,444	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	16,217
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	21,230	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	23,791	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	361,775	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	34,498	21,721
Fair Value, Total [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	1,829,871	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	15,672	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	142,655	150,866
Fair Value, Total [Member] | Commercial paper [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	52,262	49,535
Fair Value, Total [Member] | Corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	346,179	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	388,444	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	16,217
Fair Value, Total [Member] | Federally insured corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	21,230	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	450,959	584,427
Fair Value, Total [Member] | Municipal bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	361,755	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	34,498	22,721
Fair Value, Inputs, Level 1 [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	911,348	960,653
Fair Value, Inputs, Level 1 [Member] | Auction rate securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 1 [Member] | Certificates of deposit [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 1 [Member] | Commercial paper [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 1 [Member] | Corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 1 [Member] | Debt securities of government sponsored entities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	388,444	332,051
Fair Value, Inputs, Level 1 [Member] | Equity Index Funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	16,217
Fair Value, Inputs, Level 1 [Member] | Federally insured corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	21,230	21,454
Fair Value, Inputs, Level 1 [Member] | Money market funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	450,959	584,427
Fair Value, Inputs, Level 1 [Member] | Municipal bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 1 [Member] | U.S. Treasury securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	34,498	22,721
Fair Value, Inputs, Level 2 [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	902,851	743,900
Fair Value, Inputs, Level 2 [Member] | Auction rate securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 2 [Member] | Certificates of deposit [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	142,655	150,866
Fair Value, Inputs, Level 2 [Member] | Commercial paper [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	52,262	49,535
Fair Value, Inputs, Level 2 [Member] | Corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	346,179	238,918
Fair Value, Inputs, Level 2 [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, Inputs, Level 2 [Member] | Equity Index Funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0
Fair Value, Inputs, Level 2 [Member] | Federally insured corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 2 [Member] | Money market funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 2 [Member] | Municipal bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	361,755	304,581
Fair Value, Inputs, Level 2 [Member] | U.S. Treasury securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 3 [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	15,672	21,293
Fair Value, Inputs, Level 3 [Member] | Auction rate securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	15,672	21,293
Fair Value, Inputs, Level 3 [Member] | Certificates of deposit [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 3 [Member] | Commercial paper [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 3 [Member] | Corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 3 [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, Inputs, Level 3 [Member] | Equity Index Funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0
Fair Value, Inputs, Level 3 [Member] | Federally insured corporate bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 3 [Member] | Money market funds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 3 [Member] | Municipal bonds [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 3 [Member] | U.S. Treasury securities [Member]
Assets measured at fair value on a recurring basis
Total assets measured at fair value	$ 0	$ 0