Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Marketable Securities and Restricted Investments
Contingent Consideration Non Current	$ (596)	$ (1,074)
Fair value measurements on recurring basis [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	368,189	397,341
Contingent Consideration:
Acquisition-related liabilities	(1,212)	(1,074)
Fair value measurements on recurring basis [Member] | Money Market Funds [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	72,514	89,101
Fair value measurements on recurring basis [Member] | Certificates of deposit [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	1,116	998
Fair value measurements on recurring basis [Member] | Corporate notes [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	103,946	42,447
Fair value measurements on recurring basis [Member] | Commercial paper [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	19,973	9,997
Fair value measurements on recurring basis [Member] | U.S. government treasury securities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	52,390	56,472
Fair value measurements on recurring basis [Member] | Securities of government-sponsored entities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	118,250	198,326
Fair value measurements on recurring basis [Member] | Quoted price in active market (Level 1) [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	126,020	146,571
Contingent Consideration Current	0
Contingent Consideration Non Current	0
Contingent Consideration:
Acquisition-related liabilities	0	0
Fair value measurements on recurring basis [Member] | Quoted price in active market (Level 1) [Member] | Money Market Funds [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	72,514	89,101
Fair value measurements on recurring basis [Member] | Quoted price in active market (Level 1) [Member] | Certificates of deposit [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	1,116	998
Fair value measurements on recurring basis [Member] | Quoted price in active market (Level 1) [Member] | Corporate notes [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Fair value measurements on recurring basis [Member] | Quoted price in active market (Level 1) [Member] | Commercial paper [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Fair value measurements on recurring basis [Member] | Quoted price in active market (Level 1) [Member] | U.S. government treasury securities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	52,390	56,472
Fair value measurements on recurring basis [Member] | Quoted price in active market (Level 1) [Member] | Securities of government-sponsored entities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Fair value measurements on recurring basis [Member] | Significant other observable inputs (Level 2) [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	242,169	250,770
Contingent Consideration Current	0
Contingent Consideration Non Current	0
Contingent Consideration:
Acquisition-related liabilities	0	0
Fair value measurements on recurring basis [Member] | Significant other observable inputs (Level 2) [Member] | Money Market Funds [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Fair value measurements on recurring basis [Member] | Significant other observable inputs (Level 2) [Member] | Certificates of deposit [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Fair value measurements on recurring basis [Member] | Significant other observable inputs (Level 2) [Member] | Corporate notes [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	103,946	42,447
Fair value measurements on recurring basis [Member] | Significant other observable inputs (Level 2) [Member] | Commercial paper [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	19,973	9,997
Fair value measurements on recurring basis [Member] | Significant other observable inputs (Level 2) [Member] | U.S. government treasury securities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Fair value measurements on recurring basis [Member] | Significant other observable inputs (Level 2) [Member] | Securities of government-sponsored entities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	118,250	198,326
Fair value measurements on recurring basis [Member] | Significant unobservable inputs (Level 3) [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Contingent Consideration Current	(616)
Contingent Consideration Non Current	(596)
Contingent Consideration:
Acquisition-related liabilities	(1,212)	(1,074)
Fair value measurements on recurring basis [Member] | Significant unobservable inputs (Level 3) [Member] | Money Market Funds [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Fair value measurements on recurring basis [Member] | Significant unobservable inputs (Level 3) [Member] | Certificates of deposit [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Fair value measurements on recurring basis [Member] | Significant unobservable inputs (Level 3) [Member] | Corporate notes [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Fair value measurements on recurring basis [Member] | Significant unobservable inputs (Level 3) [Member] | Commercial paper [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Fair value measurements on recurring basis [Member] | Significant unobservable inputs (Level 3) [Member] | U.S. government treasury securities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Fair value measurements on recurring basis [Member] | Significant unobservable inputs (Level 3) [Member] | Securities of government-sponsored entities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	$ 0	$ 0