Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 65,167	$ 49,788
Gains	2	7
Losses	(36)	(28)
Fair Value	65,133	49,767
Corporate debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	28,549	19,756
Gains	1	3
Losses	(30)	(21)
Fair Value	28,520	19,738
Commercial paper
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	24,187	16,391
Gains	1	2
Losses	(1)
Fair Value	24,187	16,393
Securities and obligations of U.S. government agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	12,431	13,641
Gains		2
Losses	(5)	(7)
Fair Value	$ 12,426	$ 13,636