Fair Value of Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Assets:
Marketable securities	$ 137,789	$ 143,963
Transfers from Level 1 to Level 2	0	0
Transfers from Level 2 to Level 1	0	0
Transfers into or out of Level 3	0	0
Recurring
Assets:
Cash equivalents	30,589	24,213
Marketable securities	137,789	143,963
Total assets measured at fair value	168,378	168,176
Recurring | Level 1
Assets:
Cash equivalents	23,596	11,522
Total assets measured at fair value	23,596	11,522
Recurring | Level 2
Assets:
Cash equivalents	6,993	12,691
Marketable securities	137,789	143,963
Total assets measured at fair value	$ 144,782	$ 156,654