Investments - Investments in fixed income securities(Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 58,213	$ 65,167
Gains	8	2
Losses	(4)	(36)
Fair Value	58,217	65,133
Corporate debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	10,740	28,549
Gains		1
Losses	(2)	(30)
Fair Value	10,738	28,520
Commercial paper
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	47,473	24,187
Gains	8	1
Losses	(2)	(1)
Fair Value	$ 47,479	24,187
Securities and obligations of U.S. government agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost		12,431
Losses		(5)
Fair Value		$ 12,426