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Cash and Investments (Summary by Security Type) (Details) - USD ($) $ in Thousands | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Investment [Line Items] |  |
Amortized Cost | $ 368,344 | $ 460,274
Gross Unrealized Gains | 41 | 14
Gross Unrealized Losses | (160) | (322)
Fair Value | 368,225 | 459,966
Money market funds [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 48,253 | 71,457
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 48,253 | 71,457
Commercial paper [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 303,877 | 165,375
Gross Unrealized Gains | 41 | 0
Gross Unrealized Losses | (132) | 0
Fair Value | 303,786 | 165,375
Corporate bonds [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 550 | 152,712
Gross Unrealized Gains | 0 | 3
Gross Unrealized Losses | 0 | (308)
Fair Value | 550 | 152,407
U.S. Treasury and government sponsored enterprises [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 15,664 | 70,730
Gross Unrealized Gains | 0 | 11
Gross Unrealized Losses | (28) | (14)
Fair Value | $ 15,636 | $ 70,727

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