Investments (Summary of Short-Term Investments) (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Short term investment maturity weighted average (less than)	1 year
Amortized cost	$ 27,559	$ 61,236
Unrealized Gains	0	12
Unrealized Losses	(10)	(78)
Estimated fair value	$ 27,549	$ 61,170
Corporate debt securities
Investment [Line Items]
Maturity (or less) (in years)	1 year	2 years
Amortized cost	$ 21,264	$ 49,185
Unrealized Gains	0	12
Unrealized Losses	(9)	(77)
Estimated fair value	$ 21,255	$ 49,120
Certificates of deposit
Investment [Line Items]
Maturity (or less) (in years)	1 year	1 year
Amortized cost	$ 3,785	$ 9,291
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated fair value	$ 3,785	$ 9,291
Commercial paper
Investment [Line Items]
Maturity (or less) (in years)		1 year
Amortized cost		$ 1,247
Unrealized Gains		0
Unrealized Losses		0
Estimated fair value		$ 1,247
U.S. Treasury securities
Investment [Line Items]
Maturity (or less) (in years)	1 year	1 year
Amortized cost	$ 2,010	$ 1,001
Unrealized Gains	0	0
Unrealized Losses	(1)	(1)
Estimated fair value	$ 2,009	$ 1,000
Debt securities of U.S. government-sponsored agencies
Investment [Line Items]
Maturity (or less) (in years)	1 year	1 year
Amortized cost	$ 500	$ 512
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated fair value	$ 500	$ 512