Financial Instruments and Fair Value Measurements - Fair Value of Assets and Liabilities (Details) (Fair value measurements on recurring basis [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	$ 356,829	$ 368,189
Contingent Consideration Fair Value Disclosure, Current	(604)	(616)
Acquisition-related liabilities, non-current		(596)
Contingent Consideration:
Acquisition-related liabilities	(604)	(1,212)
Money Market Funds [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	59,433	72,514
Certificates of deposit [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	1,115	1,116
Corporate notes [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	128,800	103,946
Commercial paper [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	19,985	19,973
U.S. government treasury securities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	41,986	52,390
Securities of government-sponsored entities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	105,510	118,250
Quoted price in active market (Level 1) [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	102,534	126,020
Contingent Consideration Fair Value Disclosure, Current	0	0
Acquisition-related liabilities, non-current		0
Contingent Consideration:
Acquisition-related liabilities	0	0
Quoted price in active market (Level 1) [Member] | Money Market Funds [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	59,433	72,514
Quoted price in active market (Level 1) [Member] | Certificates of deposit [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	1,115	1,116
Quoted price in active market (Level 1) [Member] | Corporate notes [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Quoted price in active market (Level 1) [Member] | Commercial paper [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
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	41,986	52,390
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
Significant other observable inputs (Level 2) [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	254,295	242,169
Contingent Consideration Fair Value Disclosure, Current	0	0
Acquisition-related liabilities, non-current		0
Contingent Consideration:
Acquisition-related liabilities	0	0
Significant other observable inputs (Level 2) [Member] | Money Market Funds [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Significant other observable inputs (Level 2) [Member] | Certificates of deposit [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Significant other observable inputs (Level 2) [Member] | Corporate notes [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	128,800	103,946
Significant other observable inputs (Level 2) [Member] | Commercial paper [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	19,985	19,973
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
Significant other observable inputs (Level 2) [Member] | Securities of government-sponsored entities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	105,510	118,250
Significant unobservable inputs (Level 3) [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Contingent Consideration Fair Value Disclosure, Current	(604)	(616)
Acquisition-related liabilities, non-current		(596)
Contingent Consideration:
Acquisition-related liabilities	(604)	(1,212)
Significant unobservable inputs (Level 3) [Member] | Money Market Funds [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Significant unobservable inputs (Level 3) [Member] | Certificates of deposit [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Significant unobservable inputs (Level 3) [Member] | Corporate notes [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
Significant unobservable inputs (Level 3) [Member] | Commercial paper [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	0	0
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
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