Investments (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Amortized cost
Cash and cash equivalents, Amortized cost	$ 27,583,000	$ 39,564,000	$ 27,583,000	$ 39,564,000	$ 32,349,000	$ 24,870,000
Investments, Amortized Cost	30,340,000		30,340,000		48,519,000
Total cash, cash equivalents, and investments, Amortized Cost	57,923,000		57,923,000		80,868,000
Gross unrealized gains
Gross unrealized gains	6,000		6,000		53,000
Gross unrealized losses
Gross unrealized losses	(11,000)		(11,000)		(24,000)
Fair value
Cash and cash equivalents, Fair value	27,583,000		27,583,000		32,349,000
Short-term investments	30,335,000		30,335,000		48,548,000
Total cash, cash equivalents, and investments, Fair Value	57,918,000		57,918,000		$ 80,897,000
Realized gains or losses
Realized gains or losses on investments	$ 0	$ 0	$ 0	$ 0
Number of investments in unrealized loss position for more than 12 months | item	0		0		0
Number of investments in unrealized loss position for less than 12 months | item	8		8		14
Aggregate unrealized loss on these securities			$ 11,000		$ 24,000
Unrealized loss on fair value	$ 17,400,000		17,400,000		23,600,000
Cash and money market accounts
Amortized cost
Cash and cash equivalents, Amortized cost	27,583,000		27,583,000		22,349,000
Fair value
Cash and cash equivalents, Fair value	27,583,000		$ 27,583,000		22,349,000
Overnight repurchase agreements
Amortized cost
Cash and cash equivalents, Amortized cost					10,000,000
Fair value
Cash and cash equivalents, Fair value					$ 10,000,000
Corporate bonds and commercial paper
Investments
Maximum maturity period, investments			1 year		1 year
Amortized cost
Investments, Amortized Cost	30,340,000		$ 30,340,000		$ 48,519,000
Gross unrealized gains
Gross unrealized gains	6,000		6,000		53,000
Gross unrealized losses
Gross unrealized losses	(11,000)		(11,000)		(24,000)
Fair value
Short-term investments	$ 30,335,000		$ 30,335,000		$ 48,548,000