Investments - Investments in fixed income securities(Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 58,681	$ 58,213
Gains		8
Losses	(4)	(4)
Fair Value	58,677	58,217
Corporate debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,525	10,740
Losses		(2)
Fair Value	5,525	10,738
Commercial paper
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	52,256	47,473
Gains		8
Losses	(4)	(2)
Fair Value	52,252	$ 47,479
Government debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	900
Fair Value	$ 900