Summary of Significant Accounting Policies (Details 3) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Summary of Significant Accounting Policies
Cash and cash equivalents/money market funds	$ 69,987	$ 61,349
Cash and cash equivalents/money market funds, Fair Value	69,987	61,349	$ 54,876	$ 29,766
Cash and money market funds, Amortized Cost		60,369
Cash and money market funds, Fair Value		60,369
Cash, cash equivalents and marketable securities, Amortized Cost	374,853	191,545
Cash, cash equivalents and marketable securities, gross unrealized gains	144	18
Cash, cash equivalents and marketable securities, gross unrealized losses	(128)	(34)
Cash, cash equivalents and marketable securities, Fair Value	374,869	191,529
Cash, Cash Equivalents and Marketable Securities:
Marketable securities, Amortized Cost	304,866	130,196
Marketable securities, Gross Unrealized Gains	(144)	18
Marketable securities, Gross Unrealized Losses	128	34
Marketable securities, Fair Value	304,882	130,180
Cash, Money Market Funds and Overnight Repurchase Agreement, at Carrying Value	69,987
Cash, Money Market Funds and Overnight Repurchase Agreement, Fair Value Disclosure	69,987
Corporate debt securities
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	102,148	71,669
Available for sale securities, Due in one year or less, Gross Unrealized Gains	3	3
Available for sale securities, Due in one year or less, Gross Unrealized Losses	(116)	(24)
Available for sale securities, Due in one year or less, Fair Value	102,035	71,648
Commercial paper obligations
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	145,205	15,237
Available for sale securities, Due in one year or less, Gross Unrealized Gains	139	13
Available for sale securities, Due in one year or less, Fair Value	145,344	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	57,513	25,751
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	(12)	(9)
Available for sale securities, Due in one year or less, Fair Value	$ 57,503	$ 25,742