FAIR VALUE MEASUREMENTS - ASSETS MEASURED AT FAIR VALUE (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019
Assets measured at fair value on a recurring basis
Short-term investments	$ 557,653	$ 744,485
Restricted cash	620,397	565,461
Total recurring fair value measurements, net	1,838,728	1,766,768
Cross-currency swap
Assets measured at fair value on a recurring basis
Prepaid expenses and other	950	791
Foreign currency forward contracts
Assets measured at fair value on a recurring basis
Private equity	(112)	(423)
Corporate bonds
Assets measured at fair value on a recurring basis
Short-term investments	260,752	296,141
Bank-time deposits
Assets measured at fair value on a recurring basis
Short-term investments	254,417	387,720
US Treasuries
Assets measured at fair value on a recurring basis
Short-term investments	35,448	55,634
Commercial paper
Assets measured at fair value on a recurring basis
Short-term investments	7,036	4,990
Money market funds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	284,236	389,936
Bank-time deposits
Assets measured at fair value on a recurring basis
Cash and cash equivalents	150,000
Commercial paper
Assets measured at fair value on a recurring basis
Cash and cash equivalents	180,196	39,246
US Treasuries
Assets measured at fair value on a recurring basis
Cash and cash equivalents	24,987	25,449
Corporate bonds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	18,598
Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Restricted cash	620,397	565,461
Total recurring fair value measurements, net	1,369,485	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
Private equity	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) | Bank-time deposits
Assets measured at fair value on a recurring basis
Short-term investments	254,417	387,720
Quoted prices in active markets for identical assets (level 1) | US Treasuries
Assets measured at fair value on a recurring basis
Short-term investments	35,448	55,634
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) | Money market funds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	284,236	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	150,000
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) | US Treasuries
Assets measured at fair value on a recurring basis
Cash and cash equivalents	24,987	25,449
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
Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Restricted cash	0	0
Total recurring fair value measurements, net	467,420	340,745
Significant other observable inputs (level 2) | Cross-currency swap
Assets measured at fair value on a recurring basis
Prepaid expenses and other	950	791
Significant other observable inputs (level 2) | Foreign currency forward contracts
Assets measured at fair value on a recurring basis
Private equity	(112)	(423)
Significant other observable inputs (level 2) | Corporate bonds
Assets measured at fair value on a recurring basis
Short-term investments	260,752	296,141
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) | 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	7,036	4,990
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	180,196	39,246
Significant other observable inputs (level 2) | US Treasuries
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	0
Significant other observable inputs (level 2) | Corporate bonds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	18,598
Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Restricted cash	0	0
Total recurring fair value measurements, net	1,823	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
Private equity	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) | Bank-time deposits
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) | 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) | US Treasuries
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
Other assets
Assets measured at fair value on a recurring basis
Private equity	1,823	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	$ 1,823	$ 1,823