Fair Value of Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Assets:
Marketable securities	$ 143,963	$ 82,187
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	24,213	9,060
Marketable securities	143,963	82,187
Total assets measured at fair value	168,176	91,247
Recurring | Level 1
Assets:
Cash equivalents	11,522	8,810
Total assets measured at fair value	11,522	8,810
Recurring | Level 2
Assets:
Cash equivalents	12,691	250
Marketable securities	143,963	82,187
Total assets measured at fair value	$ 156,654	$ 82,437