Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	$ 173,753	$ 94,964
Total short-term investments	138,147	212,526
Total long-term investments	11,028	17,477
Auction rate securities
Financial assets and liabilities measured at fair value on a recurring basis
Total assets at fair value	11,028
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	66,158	50,851
Total short-term investments	19,805	11,107
Total assets at fair value	85,963	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	49,917	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	16,241
Total short-term investments	7,150	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	12,655	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	118,342	201,419
Total assets at fair value	118,342	201,419
Derivative instruments	1,256	1,998
Total liabilities at fair value	1,256	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	63,154	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	32,532	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	8,981	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	900	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	10,665	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	2,110	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	11,028	17,477
Total assets at fair value	11,028	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	11,028	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	66,158	50,851
Total short-term investments	138,147	212,526
Total long-term investments	11,028	17,477
Total assets at fair value	215,333	280,854
Derivative instruments	1,256	1,998
Contingent consideration		876
Total liabilities at fair value	1,256	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	49,917	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	63,154	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	32,532	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	8,981	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	900	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	16,241
Total short-term investments	7,150	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	10,665	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	12,655	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	2,110	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	$ 11,028	$ 17,477