Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	$ 98,038	$ 94,964
Total short-term investments	235,299	212,526
Total long-term investments	17,729	17,477
Assets and liabilities measured at fair value on recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	54,787	50,851
Total short-term investments	17,988	11,107
Total assets at fair value	72,775	61,958
Assets and liabilities measured at fair value on recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	42,887	50,851
Assets and liabilities measured at fair value on recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury bills
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	11,900
Total short-term investments	5,349	8,600
Assets and liabilities measured at fair value on recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. government bonds
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	12,639	2,507
Assets and liabilities measured at fair value on recurring basis | Significant Other Observable Inputs (Level 2)
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	217,311	201,419
Total assets at fair value	217,311	201,419
Derivative instruments	1,696	1,998
Total liabilities at fair value	1,696	1,998
Assets and liabilities measured at fair value on recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	83,599	75,060
Assets and liabilities measured at fair value on recurring basis | Significant Other Observable Inputs (Level 2) | Municipal bonds
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	67,339	56,984
Assets and liabilities measured at fair value on recurring basis | Significant Other Observable Inputs (Level 2) | Variable-rate demand notes
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	48,310	41,280
Assets and liabilities measured at fair value on recurring basis | Significant Other Observable Inputs (Level 2) | U.S. government agency
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	5,902	19,836
Assets and liabilities measured at fair value on recurring basis | Significant Other Observable Inputs (Level 2) | Asset-backed securities
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	10,042	5,739
Assets and liabilities measured at fair value on recurring basis | Significant Other Observable Inputs (Level 2) | Certificates of deposit
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments		1,570
Assets and liabilities measured at fair value on recurring basis | Significant Other Observable Inputs (Level 2) | International government bonds
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	2,119	950
Assets and liabilities measured at fair value on recurring basis | Significant Unobservable Inputs (Level 3)
Financial assets and liabilities measured at fair value on a recurring basis
Total long-term investments	17,729	17,477
Total assets at fair value	17,729	17,477
Contingent consideration		876
Total liabilities at fair value		876
Assets and liabilities measured at fair value on recurring basis | Significant Unobservable Inputs (Level 3) | Auction rate securities
Financial assets and liabilities measured at fair value on a recurring basis
Total long-term investments	17,729	17,477
Assets and liabilities measured at fair value on recurring basis | Fair Value
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	54,787	50,851
Total short-term investments	235,299	212,526
Total long-term investments	17,729	17,477
Total assets at fair value	307,815	280,854
Derivative instruments	1,696	1,998
Contingent consideration		876
Total liabilities at fair value	1,696	2,874
Assets and liabilities measured at fair value on recurring basis | Fair Value | Money market funds
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	42,887	50,851
Assets and liabilities measured at fair value on recurring basis | Fair Value | Corporate bonds
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	83,599	75,060
Assets and liabilities measured at fair value on recurring basis | Fair Value | Municipal bonds
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	67,339	56,984
Assets and liabilities measured at fair value on recurring basis | Fair Value | Variable-rate demand notes
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	48,310	41,280
Assets and liabilities measured at fair value on recurring basis | Fair Value | U.S. government agency
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	5,902	19,836
Assets and liabilities measured at fair value on recurring basis | Fair Value | U.S. Treasury bills
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	11,900
Total short-term investments	5,349	8,600
Assets and liabilities measured at fair value on recurring basis | Fair Value | Asset-backed securities
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	10,042	5,739
Assets and liabilities measured at fair value on recurring basis | Fair Value | U.S. government bonds
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	12,639	2,507
Assets and liabilities measured at fair value on recurring basis | Fair Value | Certificates of deposit
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments		1,570
Assets and liabilities measured at fair value on recurring basis | Fair Value | International government bonds
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	2,119	950
Assets and liabilities measured at fair value on recurring basis | Fair Value | Auction rate securities
Financial assets and liabilities measured at fair value on a recurring basis
Total long-term investments	$ 17,729	$ 17,477