Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities
Estimated Fair Value	$ 187,263	$ 40,831
Contractual maturities of investments:
One year or less	178,021	26,352
One year through three years	9,242	14,479
Estimated Fair Value	187,263	40,831
Short-term investments
Available-for-sale Securities
Amortized Cost	178,095	26,353
Gross Unrealized Gains	3	11
Gross Unrealized Losses	(77)	(12)
Estimated Fair Value	178,021	26,352
Contractual maturities of investments:
Estimated Fair Value	178,021	26,352
Short-term investments \| Corporate debt securities
Available-for-sale Securities
Amortized Cost	54,911	13,371
Gross Unrealized Gains	3	2
Gross Unrealized Losses	(52)	(12)
Estimated Fair Value	54,862	13,361
Contractual maturities of investments:
Estimated Fair Value	54,862	13,361
Short-term investments \| Agency securities
Available-for-sale Securities
Amortized Cost	10,781	7,957
Gross Unrealized Gains	0	6
Gross Unrealized Losses	(14)	0
Estimated Fair Value	10,767	7,963
Contractual maturities of investments:
Estimated Fair Value	10,767	7,963
Short-term investments \| Commercial paper
Available-for-sale Securities
Amortized Cost	101,546	1,727
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	101,546	1,727
Contractual maturities of investments:
Estimated Fair Value	101,546	1,727
Short-term investments \| U.S. Government securities
Available-for-sale Securities
Amortized Cost	10,857	3,298
Gross Unrealized Gains	0	3
Gross Unrealized Losses	(11)	0
Estimated Fair Value	10,846	3,301
Contractual maturities of investments:
Estimated Fair Value	10,846	3,301
Long-term investments
Available-for-sale Securities
Amortized Cost	9,285	14,523
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(43)	(46)
Estimated Fair Value	9,242	14,479
Contractual maturities of investments:
Estimated Fair Value	9,242	14,479
Long-term investments \| Corporate debt securities
Available-for-sale Securities
Amortized Cost	6,287	6,208
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(25)	(20)
Estimated Fair Value	6,262	6,189
Contractual maturities of investments:
Estimated Fair Value	6,262	6,189
Long-term investments \| Agency securities
Available-for-sale Securities
Amortized Cost	2,000	5,359
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(17)	(20)
Estimated Fair Value	1,983	5,339
Contractual maturities of investments:
Estimated Fair Value	1,983	5,339
Long-term investments \| U.S. Government securities
Available-for-sale Securities
Amortized Cost	998	2,956
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(1)	(6)
Estimated Fair Value	997	2,951
Contractual maturities of investments:
Estimated Fair Value	$ 997	$ 2,951

Available-for-sale Securities, Debt Maturities, Year One Through Three, Fair Value

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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