Fair Value of Financial Instruments - Cash and Cash Equivalents, Available-for-sale Securities Reconciliation (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	$ 187,114	$ 166,656
Unrealized Gains	42	37
Unrealized Losses	(15)	(27)
Fair Value	187,141	166,666
Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	84,055	76,504
Unrealized Gains	3	0
Unrealized Losses	0	0
Fair Value	84,058	76,504
Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	74,030	50,716
Unrealized Gains	28	10
Unrealized Losses	(6)	(12)
Fair Value	74,052	50,714
Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	29,029	39,436
Unrealized Gains	11	27
Unrealized Losses	(9)	(15)
Fair Value	29,031	39,448
Cash | Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	41,300	36,024
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	41,300	36,024
Money market funds | Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	21,061	39,180
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	21,061	39,180
Commercial paper | Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	21,694	1,000
Unrealized Gains	3	0
Unrealized Losses	0	0
Fair Value	21,697	1,000
Commercial paper | Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	15,929	4,998
Unrealized Gains	7	1
Unrealized Losses	0	0
Fair Value	15,936	4,999
Corporate bonds | Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	12,688	12,438
Unrealized Gains	1	6
Unrealized Losses	(6)	(6)
Fair Value	12,683	12,438
Corporate bonds | Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	7,090	3,101
Unrealized Gains	1	0
Unrealized Losses	(5)	(2)
Fair Value	7,086	3,099
Asset backed securities | Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	71
Unrealized Gains	0
Unrealized Losses	0
Fair Value	71
Asset backed securities | Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	9,999	16,092
Unrealized Gains	1	5
Unrealized Losses	0	(3)
Fair Value	10,000	16,094
US government agencies | Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost		300
Unrealized Gains		0
Unrealized Losses		0
Fair Value		300
US government agencies | Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	45,342	33,280
Unrealized Gains	20	3
Unrealized Losses	0	(6)
Fair Value	45,362	33,277
US government agencies | Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	11,940	20,243
Unrealized Gains	9	22
Unrealized Losses	(4)	(10)
Fair Value	$ 11,945	$ 20,255