Marketable Securities - Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 348,876	$ 261,791
Gross Unrealized Gains	230	1
Gross Unrealized Losses	(87)	(575)
Fair Value	349,019	261,217
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	111,690	105,105
Gross Unrealized Gains	170	1
Gross Unrealized Losses	(23)	(477)
Fair Value	111,837	104,629
U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	170,519	42,801
Gross Unrealized Gains	60
Gross Unrealized Losses	(64)	(98)
Fair Value	170,515	42,703
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	66,667	113,885
Fair Value	$ 66,667	$ 113,885