Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Total assets measured at fair value	$ 119,652	$ 112,401
Liabilities:
Total liabilities measured at fair value	233
Embedded derivative liability
Liabilities:
Total liabilities measured at fair value	233
Level 1
Assets:
Total assets measured at fair value	11,043	192
Liabilities:
Total liabilities measured at fair value	0
Level 1 | Embedded derivative liability
Liabilities:
Total liabilities measured at fair value	0
Level 2
Assets:
Total assets measured at fair value	108,609	112,209
Liabilities:
Total liabilities measured at fair value	0
Level 2 | Embedded derivative liability
Liabilities:
Total liabilities measured at fair value	0
Level 3
Liabilities:
Total liabilities measured at fair value	233
Level 3 | Embedded derivative liability
Liabilities:
Total liabilities measured at fair value	233
Money market
Assets:
Total assets measured at fair value	11,043	192
Money market | Level 1
Assets:
Total assets measured at fair value	11,043	192
Corporate debt
Assets:
Total assets measured at fair value	37,913	51,233
Corporate debt | Level 2
Assets:
Total assets measured at fair value	37,913	51,233
U.S. Treasury notes
Assets:
Total assets measured at fair value	70,696	60,976
U.S. Treasury notes | Level 2
Assets:
Total assets measured at fair value	$ 70,696	$ 60,976