Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Available-for-sale Securities
Available-for-sale securities, less than one year	$ 23,620	$ 22,918
Available-for-sale securities, between one and three years	24,832	29,005
Estimated fair value	48,452	51,923
Long-term Investments
Available-for-sale Securities
Gross amortized cost	24,780	29,102
Gross unrealized gains	55	1
Gross unrealized losses	(3)	(98)
Estimated fair value	24,832	29,005
Long-term Investments \| Corporate Debt Securities
Available-for-sale Securities
Gross amortized cost	8,368	10,132
Gross unrealized gains	15	0
Gross unrealized losses	(2)	(37)
Estimated fair value	8,381	10,095
Long-term Investments \| Agency Securities
Available-for-sale Securities
Gross amortized cost	10,239	13,421
Gross unrealized gains	25	1
Gross unrealized losses	(1)	(43)
Estimated fair value	10,263	13,379
Long-term Investments \| US Treasury and Government
Available-for-sale Securities
Gross amortized cost	6,173	5,549
Gross unrealized gains	15	0
Gross unrealized losses	0	(18)
Estimated fair value	6,188	5,531
Short-term Investments
Available-for-sale Securities
Gross amortized cost	23,605	22,932
Gross unrealized gains	19	2
Gross unrealized losses	(4)	(16)
Estimated fair value	23,620	22,918
Short-term Investments \| Corporate Debt Securities
Available-for-sale Securities
Gross amortized cost	14,119	16,187
Gross unrealized gains	12	2
Gross unrealized losses	(4)	(14)
Estimated fair value	14,127	16,175
Short-term Investments \| Agency Securities
Available-for-sale Securities
Gross amortized cost	6,613	6,246
Gross unrealized gains	6	0
Gross unrealized losses	0	(2)
Estimated fair value	6,619	6,244
Short-term Investments \| Commercial Paper
Available-for-sale Securities
Gross amortized cost	2,523	499
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	2,523	$ 499
Short-term Investments \| US Treasury and Government
Available-for-sale Securities
Gross amortized cost	350
Gross unrealized gains	1
Gross unrealized losses	0
Estimated fair value	$ 351

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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