Summary of Significant Accounting Policies (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Summary of Significant Accounting Policies
Cash and cash equivalents, amortized cost	$ 43,288	$ 52,990
Cash and cash equivalents/money market funds	43,288	52,990	38,657	49,245
Cash and cash equivalents/money market funds, Fair Value	43,288	52,990
Cash, cash equivalents and marketable securities, Amortized Cost	324,158	340,492
Cash, cash equivalents and marketable securities, Fair Value	324,204	340,603
Cash, cash equivalents and marketable securities, gross unrealized gains	127	155
Cash, cash equivalents and marketable securities, gross unrealized losses	(81)	(44)
Cash, Cash Equivalents and Marketable Securities:
Marketable securities, Amortized Cost	280,870	287,502
Marketable securities, Gross Unrealized Gains	127	155
Marketable securities, Gross Unrealized Losses	(81)	(44)
Marketable securities, Fair Value	280,916	287,613
U.S. Government-sponsored enterprise obligations
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	4,253	6,000
Available for sale securities, Due in two years or less, Amortized Cost	63,526	45,195
Available for sale securities, Due in one year or less, Gross Unrealized Gains	3
Available for sale securities, Due in two years or less, Gross Unrealized Gains	18	30
Available for sale securities, Due in two years or less, Gross Unrealized Losses	(12)
Available for sale securities, Due in one year or less, Fair Value	4,256	6,000
Available for sale securities, Due in two years or less, Fair Value	63,532	45,225
Corporate debt securities
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	108,993	76,500
Available for sale securities, Due in two years or less, Amortized Cost	36,541	82,363
Available for sale securities, Due in one year or less, Gross Unrealized Gains	33	19
Available for sale securities, Due in two years or less, Gross Unrealized Gains	27	72
Available for sale securities, Due in one year or less, Gross Unrealized Losses	(39)	(11)
Available for sale securities, Due in two years or less, Gross Unrealized Losses	(29)	(30)
Available for sale securities, Due in one year or less, Fair Value	108,987	76,508
Available for sale securities, Due in two years or less, Fair Value	36,539	82,405
Commercial paper obligations
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	57,588	65,104
Available for sale securities, Due in one year or less, Gross Unrealized Gains	45	34
Available for sale securities, Due in one year or less, Fair Value	57,633	65,138
Foreign government bonds
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	8,366	7,390
Available for sale securities, Due in two years or less, Amortized Cost	1,603	4,950
Available for sale securities, Due in one year or less, Gross Unrealized Gains	1
Available for sale securities, Due in one year or less, Gross Unrealized Losses	(1)	(1)
Available for sale securities, Due in two years or less, Gross Unrealized Losses		(2)
Available for sale securities, Due in one year or less, Fair Value	8,366	7,389
Available for sale securities, Due in two years or less, Fair Value	$ 1,603	$ 4,948