Summary of Significant Accounting Policies (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Summary of Significant Accounting Policies
Cash and cash equivalents, amortized cost	$ 26,514	$ 52,990
Cash and cash equivalents/money market funds	26,514	52,990	68,327	49,245
Cash and cash equivalents/money market funds, Fair Value	26,514	52,990
Cash, cash equivalents and marketable securities, Amortized Cost	300,138	340,492
Cash, cash equivalents and marketable securities, Fair Value	300,160	340,603
Cash, cash equivalents and marketable securities, gross unrealized gains	122	155
Cash, cash equivalents and marketable securities, gross unrealized losses	(100)	(44)
Cash, Cash Equivalents and Marketable Securities:
Marketable securities, Amortized Cost	273,624	287,502
Marketable securities, Gross Unrealized Gains	122	155
Marketable securities, Gross Unrealized Losses	(100)	(44)
Marketable securities, Fair Value	273,646	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	11,002	6,000
Available for sale securities, Due in two years or less, Amortized Cost	42,806	45,195
Available for sale securities, Due in one year or less, Gross Unrealized Gains	7
Available for sale securities, Due in two years or less, Gross Unrealized Gains	5	30
Available for sale securities, Due in two years or less, Gross Unrealized Losses	(27)
Available for sale securities, Due in one year or less, Fair Value	11,009	6,000
Available for sale securities, Due in two years or less, Fair Value	42,784	45,225
Corporate debt securities
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	100,796	76,500
Available for sale securities, Due in two years or less, Amortized Cost	19,070	82,363
Available for sale securities, Due in one year or less, Gross Unrealized Gains	6	19
Available for sale securities, Due in two years or less, Gross Unrealized Gains	1	72
Available for sale securities, Due in one year or less, Gross Unrealized Losses	(47)	(11)
Available for sale securities, Due in two years or less, Gross Unrealized Losses	(25)	(30)
Available for sale securities, Due in one year or less, Fair Value	100,755	76,508
Available for sale securities, Due in two years or less, Fair Value	19,046	82,405
Commercial paper obligations
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	66,560	65,104
Available for sale securities, Due in one year or less, Gross Unrealized Gains	74	34
Available for sale securities, Due in one year or less, Fair Value	66,634	65,138
Foreign government bonds
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	31,788	7,390
Available for sale securities, Due in two years or less, Amortized Cost	1,602	4,950
Available for sale securities, Due in one year or less, Gross Unrealized Gains	29
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	31,816	7,389
Available for sale securities, Due in two years or less, Fair Value	$ 1,602	$ 4,948