Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets:
Total assets measured at fair value	$ 131,271	$ 112,401
Liabilities:
Total liabilities measured at fair value	764
Embedded derivative liability
Liabilities:
Total liabilities measured at fair value	764
Level 1
Assets:
Total assets measured at fair value	19,960	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	111,311	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	764
Level 3 | Embedded derivative liability
Liabilities:
Total liabilities measured at fair value	764
Money market
Assets:
Total assets measured at fair value	19,960	192
Money market | Level 1
Assets:
Total assets measured at fair value	19,960	192
Corporate debt
Assets:
Total assets measured at fair value	43,120	51,233
Corporate debt | Level 2
Assets:
Total assets measured at fair value	43,120	51,233
U.S. Treasury notes
Assets:
Total assets measured at fair value	68,191	60,976
U.S. Treasury notes | Level 2
Assets:
Total assets measured at fair value	$ 68,191	$ 60,976