Summary of Company's Marketable Securities Excluding Regulus (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,060,817	$ 750,537
Gross Unrealized Gains	12	57
Gross Unrealized Losses	(1,653)	(1,610)
Fair Value	1,059,176	748,984
Certificate of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,200	17,999
Fair Value	30,200	17,999
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	56,951	59,340
Fair Value	56,951	59,340
Corporate notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	373,736	334,266
Gross Unrealized Gains	11	47
Gross Unrealized Losses	(495)	(441)
Fair Value	373,252	333,872
U.S. government-sponsored enterprise securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	399,281	298,910
Gross Unrealized Gains		9
Gross Unrealized Losses	(983)	(1,146)
Fair Value	398,298	297,773
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	200,649	40,022
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(175)	(23)
Fair Value	$ 200,475	$ 40,000