Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Contingency
Income tax provision	$ 7,000	$ 8,000	$ 7,000
Provision for income taxes related to foreign income taxes	6,000	11,000	6,000
Provision for income taxes related to state minimum income taxes	1,000	1,000	1,000
Undistributed earnings of foreign subsidiaries	400,000
Valuation allowance for deferred tax assets	61,766,000	54,420,000
Increase in valuation allowance	7,400,000	9,400,000
Unrecognized tax benefits excluding interest	4,212,000	3,757,000	3,261,000	$ 2,656,000
Accrued interest and penalties related to unrecognized tax benefits	$ 0	$ 0	$ 0
Research and Development Expense
Income Tax Contingency
Tax credit carryforward, term	20 years
Internal Revenue Service (IRS)
Income Tax Contingency
Deferred tax assets, U.S. net operating loss carryforwards	$ 105,500,000
Adjustments to additional paid in capital, income tax benefit from share-based compensation	$ 14,600,000
Operating loss carryforwards, expiration start year	2026
Internal Revenue Service (IRS) | Research and Development Expense
Income Tax Contingency
Tax credit carryforward	$ 10,000,000
State Jurisdiction
Income Tax Contingency
Deferred tax assets, U.S. net operating loss carryforwards	96,600,000
Adjustments to additional paid in capital, income tax benefit from share-based compensation	$ 13,600,000
Operating loss carryforwards, expiration start year	2018
State Jurisdiction | Research and Development Expense
Income Tax Contingency
Tax credit carryforward	$ 11,100,000