Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	$ 482,396	$ 153,961
Total long-term investments	5,471	5,196
Assets and liabilities measured at fair value on recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	107,869	69,432
Total short-term investments	62,494	12,416
Total assets at fair value	170,363	81,848
Assets and liabilities measured at fair value on recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	107,869	69,432
Assets and liabilities measured at fair value on recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Government debt securities
Financial assets and liabilities measured at fair value on a recurring basis
Total short-term investments	62,494	12,416
Assets and liabilities measured at fair value on recurring basis | Significant Other Observable Inputs (Level 2)
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	30,263	11,057
Total short-term investments	419,902	141,545
Derivative instruments		1,808
Other assets, net		1,808
Total assets at fair value	450,165	154,410
Assets and liabilities measured at fair value on recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt securities
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	21,317	7,153
Total short-term investments	197,626	44,442
Assets and liabilities measured at fair value on recurring basis | Significant Other Observable Inputs (Level 2) | Government debt securities
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	8,946	3,904
Total short-term investments	222,276	97,103
Assets and liabilities measured at fair value on recurring basis | Significant Unobservable Inputs (Level 3)
Financial assets and liabilities measured at fair value on a recurring basis
Total long-term investments	5,471	5,196
Total assets at fair value	5,471	5,196
Contingent consideration, noncurrent	4,154
Total liabilities at fair value	4,154
Assets and liabilities measured at fair value on recurring basis | Significant Unobservable Inputs (Level 3) | Auction rate securities
Financial assets and liabilities measured at fair value on a recurring basis
Total long-term investments	5,471	5,196
Assets and liabilities measured at fair value on recurring basis | Fair Value
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	138,132	80,489
Total short-term investments	482,396	153,961
Total long-term investments	5,471	5,196
Derivative instruments		1,808
Other assets, net		1,808
Total assets at fair value	625,999	241,454
Contingent consideration, noncurrent	4,154
Total liabilities at fair value	4,154
Assets and liabilities measured at fair value on recurring basis | Fair Value | Money market funds
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	107,869	69,432
Assets and liabilities measured at fair value on recurring basis | Fair Value | Corporate debt securities
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	21,317	7,153
Total short-term investments	197,626	44,442
Assets and liabilities measured at fair value on recurring basis | Fair Value | Government debt securities
Financial assets and liabilities measured at fair value on a recurring basis
Total cash equivalents	8,946	3,904
Total short-term investments	284,770	109,519
Assets and liabilities measured at fair value on recurring basis | Fair Value | Auction rate securities
Financial assets and liabilities measured at fair value on a recurring basis
Total long-term investments	$ 5,471	$ 5,196