Stock-Based Employee Benefit Plans - Summary of Option Activity (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares
Shares
Outstanding at October 1, 2016 (in shares) | shares	1,521
Granted (in shares) | shares	34
Exercised (in shares) | shares	(325)
Outstanding at September 30, 2017 (in shares) | shares	1,230
Options exercisable at September 30, 2017 (in shares) | shares	946
Vested and expected to vest at September 30, 2017 (in shares) | shares	1,223
Weighted- average Exercise Price
Outstanding at October 1, 2016 (in dollars per share) | $ / shares	$ 52.37
Granted (in dollars per share) | $ / shares	128.80
Exercised (in dollars per share) | $ / shares	44.52
Outstanding at September 30, 2017 (in dollars per share) | $ / shares	56.54
Options exercisable at September 30, 2015 (in dollars per share) | $ / shares	50.29
Vested and expected to vest at September 30, 2017 (in dollars per share) | $ / shares	$ 56.43
Weighted- average Remaining Contractual Term
Outstanding at September 30, 2017	3 years 1 month 15 days
Exercisable at September 30, 2017	2 years 8 months 26 days
Vested and expected to vest at September 30, 2017	3 years 1 month 8 days
Aggregate Intrinsic Value
Outstanding at September 30, 2017 | $	$ 103,275
Exercisable at September 30, 2017 | $	85,354
Vested and expected to vest at September 30, 2017 | $	$ 102,843