Fair Value Measurements (Hierarchy) (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financial Assets:
Total financial assets	$ 209,639
Current Liabilities:
Warrants	3,741	$ 2,232
Long‑term Liabilities:
Senior secured term loan	21,276	20,964
Total financial liabilities	25,017	23,196
Money market deposits
Financial Assets:
Total financial assets	183,250
U.S. Treasury securities
Financial Assets:
Total financial assets	18,273
U.S. agency securities
Financial Assets:
Total financial assets	8,116
Level 1
Financial Assets:
Total financial assets	201,523
Level 1 | Money market deposits
Financial Assets:
Total financial assets	183,250
Level 1 | U.S. Treasury securities
Financial Assets:
Total financial assets	18,273
Level 2
Financial Assets:
Total financial assets	8,116
Level 2 | U.S. agency securities
Financial Assets:
Total financial assets	8,116
Level 3
Current Liabilities:
Warrants	3,741	2,232
Long‑term Liabilities:
Senior secured term loan	21,276	20,964
Total financial liabilities	$ 25,017	$ 23,196