FAIR VALUE MEASUREMENTS - ASSETS MEASURED AT FAIR VALUE (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Mar. 31, 2017
Assets measured at fair value on a recurring basis
Total recurring fair value measurements, net	$ 1,040,380		$ 1,173,946
Cash and cash equivalents
Assets measured at fair value on a recurring basis
Corporate bonds	10,246
Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents	166,321		175,745
Corporate bonds	362,416		243,019
Prepaid expenses and other
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	134		2
Accrued expense and other current liabilities
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	(18)		(352)
Other assets
Assets measured at fair value on a recurring basis
Private equity	917		570
Other long-term liabilities
Assets measured at fair value on a recurring basis
Contingent consideration		$ (136)	(6,465)
Cross-currency swap | Accrued expense and other current liabilities
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	(8,626)
Money market funds | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents	419,642		646,386
Bank-time deposits | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents	53,598		46,605
Commercial paper | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents	17,294		38,268
Commercial paper | Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents	13,921		25,936
Mutual funds | Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents	4,671		4,232
Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Total recurring fair value measurements, net	639,561		868,736
Quoted prices in active markets for identical assets (level 1) | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Corporate bonds
Quoted prices in active markets for identical assets (level 1) | Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents	166,321		175,745
Corporate bonds			0
Quoted prices in active markets for identical assets (level 1) | Prepaid expenses and other
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	0		0
Quoted prices in active markets for identical assets (level 1) | Accrued expense and other current liabilities
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	0		0
Quoted prices in active markets for identical assets (level 1) | Other assets
Assets measured at fair value on a recurring basis
Private equity	0		0
Quoted prices in active markets for identical assets (level 1) | Other long-term liabilities
Assets measured at fair value on a recurring basis
Contingent consideration	0		0
Quoted prices in active markets for identical assets (level 1) | Cross-currency swap | Accrued expense and other current liabilities
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	0
Quoted prices in active markets for identical assets (level 1) | Money market funds | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents	419,642		646,386
Quoted prices in active markets for identical assets (level 1) | Bank-time deposits | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents	53,598		46,605
Quoted prices in active markets for identical assets (level 1) | Commercial paper | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents			0
Quoted prices in active markets for identical assets (level 1) | Commercial paper | Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents			0
Quoted prices in active markets for identical assets (level 1) | Mutual funds | Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents			0
Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Total recurring fair value measurements, net	400,038		311,105
Significant other observable inputs (level 2) | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Corporate bonds	10,246
Significant other observable inputs (level 2) | Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents			0
Corporate bonds	362,416		243,019
Significant other observable inputs (level 2) | Prepaid expenses and other
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	134		2
Significant other observable inputs (level 2) | Accrued expense and other current liabilities
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	(18)		(352)
Significant other observable inputs (level 2) | Other assets
Assets measured at fair value on a recurring basis
Private equity	0		0
Significant other observable inputs (level 2) | Other long-term liabilities
Assets measured at fair value on a recurring basis
Contingent consideration	0		0
Significant other observable inputs (level 2) | Cross-currency swap | Accrued expense and other current liabilities
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	(8,626)
Significant other observable inputs (level 2) | Money market funds | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents			0
Significant other observable inputs (level 2) | Bank-time deposits | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents			0
Significant other observable inputs (level 2) | Commercial paper | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents	17,294		38,268
Significant other observable inputs (level 2) | Commercial paper | Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents	13,921		25,936
Significant other observable inputs (level 2) | Mutual funds | Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents	4,671		4,232
Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Total recurring fair value measurements, net	781		(5,895)
Significant unobservable inputs (level 3) | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Corporate bonds	0
Significant unobservable inputs (level 3) | Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0		0
Corporate bonds	0		0
Significant unobservable inputs (level 3) | Prepaid expenses and other
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	0		0
Significant unobservable inputs (level 3) | Accrued expense and other current liabilities
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	0		0
Significant unobservable inputs (level 3) | Other assets
Assets measured at fair value on a recurring basis
Private equity	917		570
Significant unobservable inputs (level 3) | Other long-term liabilities
Assets measured at fair value on a recurring basis
Contingent consideration	(136)		(6,465)
Significant unobservable inputs (level 3) | Cross-currency swap | Accrued expense and other current liabilities
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	0
Significant unobservable inputs (level 3) | Money market funds | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0		0
Significant unobservable inputs (level 3) | Bank-time deposits | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0		0
Significant unobservable inputs (level 3) | Commercial paper | Cash and cash equivalents
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0		0
Significant unobservable inputs (level 3) | Commercial paper | Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0		0
Significant unobservable inputs (level 3) | Mutual funds | Short-term investments
Assets measured at fair value on a recurring basis
Cash and cash equivalents	$ 0		$ 0