Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash, Cash Equivalents and Marketable Securities:
Cash and cash equivalents/money market funds	$ 61,461	$ 61,349	$ 29,766	$ 52,990
Cash and Cash Equivalents, Fair Value Disclosure	61,461	61,349
Cash, cash equivalents and marketable securities, Amortized Cost	350,040	191,545
Cash, cash equivalents and marketable securities, gross unrealized gains	126	18
Cash, cash equivalents and marketable securities, gross unrealized losses	(122)	(34)
Cash, cash equivalents and marketable securities, Fair Value	350,044	191,529
Marketable securities, Amortized Cost	288,579	130,196
Marketable securities, Gross Unrealized Gains	126	18
Marketable securities, Gross Unrealized Losses	(122)	(34)
Marketable securities, Fair Value	288,583	130,180
Cash, money market funds and overnight repurchase agreements
Cash, Cash Equivalents and Marketable Securities:
Cash and cash equivalents/money market funds	61,461	60,369
Cash and Cash Equivalents, Fair Value Disclosure	61,461	60,369
U.S. government-sponsored enterprise securities:
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	24,285
Available for sale securities, Due in one year or less, Gross Unrealized Gains	5
Available for sale securities, Due in one year or less, Fair Value	24,290
Corporate debt securities
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	73,735	71,669
Available for sale securities, Due in one year or less, Gross Unrealized Gains	1	3
Available for sale securities, Due in one year or less, Gross Unrealized Losses	(84)	(24)
Available for sale securities, Due in one year or less, Fair Value	73,652	71,648
Commercial paper obligations
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	125,693	15,237
Available for sale securities, Due in one year or less, Gross Unrealized Gains	120	13
Available for sale securities, Due in one year or less, Gross Unrealized Losses	(8)
Available for sale securities, Due in one year or less, Fair Value	125,805	15,250
Foreign government bonds
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost		18,519
Available for sale securities, Due in one year or less, Gross Unrealized Gains		2
Available for sale securities, Due in one year or less, Gross Unrealized Losses		(1)
Available for sale securities, Due in one year or less, Fair Value		18,520
Asset-backed securities
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	64,866	25,751
Available for sale securities, Due in one year or less, Gross Unrealized Losses	(30)	(9)
Available for sale securities, Due in one year or less, Fair Value	$ 64,836	$ 25,742