FAIR VALUE MEASUREMENTS - ASSETS MEASURED AT FAIR VALUE (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Mar. 31, 2018
Assets measured at fair value on a recurring basis
Short-term investments	$ 562,952	$ 615,406
Total recurring fair value measurements, net	1,131,102	1,128,364
Foreign currency forward contracts
Assets measured at fair value on a recurring basis
Prepaid expenses and other	311	12
Foreign currency forward contracts	(74)	(43)
Cross-currency swap
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	(6,817)	(15,659)
Corporate bonds
Assets measured at fair value on a recurring basis
Short-term investments	317,049	308,716
US Treasuries
Assets measured at fair value on a recurring basis
Short-term investments	53,305	59,725
Commercial paper
Assets measured at fair value on a recurring basis
Short-term investments	9,539	25,422
Mutual funds
Assets measured at fair value on a recurring basis
Short-term investments		4,880
Bank-time deposits
Assets measured at fair value on a recurring basis
Short-term investments	183,059	216,663
Money market funds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	151,696	516,626
Restricted cash	368,240
Bank-time deposits
Assets measured at fair value on a recurring basis
Cash and cash equivalents	23,910	21
Commercial paper
Assets measured at fair value on a recurring basis
Cash and cash equivalents	29,061	10,796
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	780,210	793,035
Quoted prices in active markets for identical assets (level 1) | Foreign currency forward contracts
Assets measured at fair value on a recurring basis
Prepaid expenses and other	0	0
Foreign currency forward contracts	0
Quoted prices in active markets for identical assets (level 1) | Cross-currency swap
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	0
Quoted prices in active markets for identical assets (level 1) | Corporate bonds
Assets measured at fair value on a recurring basis
Short-term investments		0
Quoted prices in active markets for identical assets (level 1) | US Treasuries
Assets measured at fair value on a recurring basis
Short-term investments	53,305	59,725
Quoted prices in active markets for identical assets (level 1) | Commercial paper
Assets measured at fair value on a recurring basis
Short-term investments	0	0
Quoted prices in active markets for identical assets (level 1) | Mutual funds
Assets measured at fair value on a recurring basis
Short-term investments		0
Quoted prices in active markets for identical assets (level 1) | Bank-time deposits
Assets measured at fair value on a recurring basis
Short-term investments	183,059	216,663
Quoted prices in active markets for identical assets (level 1) | Money market funds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	151,696	516,626
Restricted cash	368,240
Quoted prices in active markets for identical assets (level 1) | Bank-time deposits
Assets measured at fair value on a recurring basis
Cash and cash equivalents	23,910	21
Quoted prices in active markets for identical assets (level 1) | Commercial paper
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	349,069	334,124
Significant other observable inputs (level 2) | Foreign currency forward contracts
Assets measured at fair value on a recurring basis
Prepaid expenses and other	311	12
Foreign currency forward contracts	(74)	(43)
Significant other observable inputs (level 2) | Cross-currency swap
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	(6,817)	(15,659)
Significant other observable inputs (level 2) | Corporate bonds
Assets measured at fair value on a recurring basis
Short-term investments	317,049	308,716
Significant other observable inputs (level 2) | US Treasuries
Assets measured at fair value on a recurring basis
Short-term investments	0	0
Significant other observable inputs (level 2) | Commercial paper
Assets measured at fair value on a recurring basis
Short-term investments	9,539	25,422
Significant other observable inputs (level 2) | Mutual funds
Assets measured at fair value on a recurring basis
Short-term investments		4,880
Significant other observable inputs (level 2) | Bank-time deposits
Assets measured at fair value on a recurring basis
Short-term investments	0	0
Significant other observable inputs (level 2) | Money market funds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	0
Restricted cash	0
Significant other observable inputs (level 2) | Bank-time deposits
Assets measured at fair value on a recurring basis
Cash and cash equivalents		0
Significant other observable inputs (level 2) | Commercial paper
Assets measured at fair value on a recurring basis
Cash and cash equivalents	29,061	10,796
Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Total recurring fair value measurements, net	1,823	1,205
Significant unobservable inputs (level 3) | Foreign currency forward contracts
Assets measured at fair value on a recurring basis
Prepaid expenses and other	0	0
Foreign currency forward contracts	0
Significant unobservable inputs (level 3) | Cross-currency swap
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	0
Significant unobservable inputs (level 3) | Corporate bonds
Assets measured at fair value on a recurring basis
Short-term investments	0	0
Significant unobservable inputs (level 3) | US Treasuries
Assets measured at fair value on a recurring basis
Short-term investments	0	0
Significant unobservable inputs (level 3) | Commercial paper
Assets measured at fair value on a recurring basis
Short-term investments	0	0
Significant unobservable inputs (level 3) | Mutual funds
Assets measured at fair value on a recurring basis
Short-term investments		0
Significant unobservable inputs (level 3) | Bank-time deposits
Assets measured at fair value on a recurring basis
Short-term investments	0	0
Significant unobservable inputs (level 3) | Money market funds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	0
Restricted cash	0
Significant unobservable inputs (level 3) | Bank-time deposits
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	0
Significant unobservable inputs (level 3) | Commercial paper
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	0
Other assets
Assets measured at fair value on a recurring basis
Private equity	1,823	1,205
Other assets | Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Private equity	0	0
Other assets | Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Private equity	0	0
Other assets | Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Private equity	$ 1,823	$ 1,205