Fair Value of Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Assets:
Marketable securities	$ 137,760	$ 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	17,732	24,213
Marketable securities	137,760	143,963
Total assets measured at fair value	155,492	168,176
Recurring | Level 1
Assets:
Cash equivalents	9,740	11,522
Total assets measured at fair value	9,740	11,522
Recurring | Level 2
Assets:
Cash equivalents	7,992	12,691
Marketable securities	137,760	143,963
Total assets measured at fair value	$ 145,752	$ 156,654