FAIR VALUE MEASUREMENTS - ASSETS MEASURED AT FAIR VALUE (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Mar. 31, 2019
Assets measured at fair value on a recurring basis
Short-term investments	$ 742,613	$ 744,485
Restricted cash and cash equivalents	661,397	565,461
Total recurring fair value measurements, net	1,850,247	1,766,768
Cross-currency swap
Assets measured at fair value on a recurring basis
Prepaid expenses and other	6,683	791
Foreign currency forward contracts
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	(168)	(423)
Bank-time deposits
Assets measured at fair value on a recurring basis
Short-term investments	381,000	387,720
Commercial paper
Assets measured at fair value on a recurring basis
Short-term investments	58,645	4,990
Corporate bonds
Assets measured at fair value on a recurring basis
Short-term investments	274,027	296,141
US Treasuries
Assets measured at fair value on a recurring basis
Short-term investments	28,941	55,634
Money market funds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	320,146	389,936
Bank-time deposits
Assets measured at fair value on a recurring basis
Cash and cash equivalents	10,333
Commercial paper
Assets measured at fair value on a recurring basis
Cash and cash equivalents	104,612	39,246
Corporate bonds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	2,565
US Treasuries
Assets measured at fair value on a recurring basis
Cash and cash equivalents		25,449
Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Restricted cash and cash equivalents	661,397	565,461
Total recurring fair value measurements, net	1,401,817	1,424,200
Quoted prices in active markets for identical assets (level 1) | Cross-currency swap
Assets measured at fair value on a recurring basis
Prepaid expenses and other	0	0
Quoted prices in active markets for identical assets (level 1) | Foreign currency forward contracts
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) | Bank-time deposits
Assets measured at fair value on a recurring basis
Short-term investments	381,000	387,720
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) | Corporate bonds
Assets measured at fair value on a recurring basis
Short-term investments	0	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	28,941	55,634
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	320,146	389,936
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	10,333
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	0
Quoted prices in active markets for identical assets (level 1) | Corporate bonds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0
Quoted prices in active markets for identical assets (level 1) | US Treasuries
Assets measured at fair value on a recurring basis
Cash and cash equivalents		25,449
Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Restricted cash and cash equivalents	0	0
Total recurring fair value measurements, net	446,364	340,745
Significant other observable inputs (level 2) | Cross-currency swap
Assets measured at fair value on a recurring basis
Prepaid expenses and other	6,683	791
Significant other observable inputs (level 2) | Foreign currency forward contracts
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	(168)	(423)
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) | Commercial paper
Assets measured at fair value on a recurring basis
Short-term investments	58,645	4,990
Significant other observable inputs (level 2) | Corporate bonds
Assets measured at fair value on a recurring basis
Short-term investments	274,027	296,141
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) | Money market funds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	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	104,612	39,246
Significant other observable inputs (level 2) | Corporate bonds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	2,565
Significant other observable inputs (level 2) | US Treasuries
Assets measured at fair value on a recurring basis
Cash and cash equivalents		0
Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Restricted cash and cash equivalents	0	0
Total recurring fair value measurements, net	2,066	1,823
Significant unobservable inputs (level 3) | Cross-currency swap
Assets measured at fair value on a recurring basis
Prepaid expenses and other	0	0
Significant unobservable inputs (level 3) | Foreign currency forward contracts
Assets measured at fair value on a recurring basis
Foreign currency forward contracts	0	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) | Commercial paper
Assets measured at fair value on a recurring basis
Short-term investments	0	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) | Money market funds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	0
Significant unobservable inputs (level 3) | Bank-time deposits
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0
Significant unobservable inputs (level 3) | Commercial paper
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	0
Significant unobservable inputs (level 3) | Corporate bonds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0
Significant unobservable inputs (level 3) | US Treasuries
Assets measured at fair value on a recurring basis
Cash and cash equivalents		0
Other assets
Assets measured at fair value on a recurring basis
Private equity	2,066	1,823
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	$ 2,066	$ 1,823