FAIR VALUE MEASUREMENTS - ASSETS MEASURED AT FAIR VALUE (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019
Assets measured at fair value on a recurring basis
Short-term investments	$ 644,003	$ 744,485
Private equity	2,759
Total recurring fair value measurements, net	2,212,804	1,766,768
Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Private equity	0
Total recurring fair value measurements, net	1,669,165	1,424,200
Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Private equity	0
Total recurring fair value measurements, net	540,880	340,745
Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Private equity	2,759
Total recurring fair value measurements, net	2,759	1,823
Currency Swap
Assets measured at fair value on a recurring basis
Prepaid expenses and other	11,275
Other assets, accrued expenses and other current liabilities		(423)
Currency Swap | Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Prepaid expenses and other	0
Other assets, accrued expenses and other current liabilities		0
Currency Swap | Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Prepaid expenses and other	11,275
Other assets, accrued expenses and other current liabilities		(423)
Currency Swap | Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Prepaid expenses and other	0
Other assets, accrued expenses and other current liabilities		0
Foreign Exchange Forward
Assets measured at fair value on a recurring basis
Prepaid expenses and other		791
Other assets, accrued expenses and other current liabilities	(11)	1,823
Foreign Exchange Forward | Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Prepaid expenses and other		0
Other assets, accrued expenses and other current liabilities	0	0
Foreign Exchange Forward | Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Prepaid expenses and other		791
Other assets, accrued expenses and other current liabilities	(11)	0
Foreign Exchange Forward | Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Prepaid expenses and other		0
Other assets, accrued expenses and other current liabilities	0	1,823
Money market funds
Assets measured at fair value on a recurring basis
Cash and cash equivalents	497,861	389,936
Restricted cash and cash equivalents	546,604
Long-term restricted cash and cash equivalents	89,124
Money market funds | Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	497,861	389,936
Restricted cash and cash equivalents	546,604
Long-term restricted cash and cash equivalents	89,124
Money market funds | Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	0
Restricted cash and cash equivalents	0
Long-term restricted cash and cash equivalents	0
Money market funds | Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	0
Restricted cash and cash equivalents	0
Long-term restricted cash and cash equivalents	0
Bank time deposits
Assets measured at fair value on a recurring basis
Cash and cash equivalents	313,757	39,246
Bank time deposits | Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	313,757	0
Bank time deposits | Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	39,246
Bank time deposits | Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	0
Commercial paper
Assets measured at fair value on a recurring basis
Cash and cash equivalents	97,544	25,449
Commercial paper | Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	25,449
Commercial paper | Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	97,544	0
Commercial paper | Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0	0
Commercial paper
Assets measured at fair value on a recurring basis
Short-term investments	87,553	55,634
Commercial paper | Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Short-term investments	0	55,634
Commercial paper | Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Short-term investments	87,553	0
Commercial paper | Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Short-term investments	0	0
US Treasuries
Assets measured at fair value on a recurring basis
Cash and cash equivalents	9,888
Short-term investments	30,819	387,720
US Treasuries | Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0
Short-term investments	30,819	387,720
US Treasuries | Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	9,888
Short-term investments	0	0
US Treasuries | Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Cash and cash equivalents	0
Short-term investments	0	0
Bank time deposits
Assets measured at fair value on a recurring basis
Short-term investments	191,000	296,141
Bank time deposits | Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Short-term investments	191,000	0
Bank time deposits | Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Short-term investments	0	296,141
Bank time deposits | Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Short-term investments	0	0
Corporate bonds
Assets measured at fair value on a recurring basis
Short-term investments	334,631	565,461
Corporate bonds | Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Short-term investments	0	565,461
Corporate bonds | Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Short-term investments	334,631	0
Corporate bonds | Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Short-term investments	$ 0	0
Commercial paper
Assets measured at fair value on a recurring basis
Short-term investments		4,990
Commercial paper | Quoted prices in active markets for identical assets (level 1)
Assets measured at fair value on a recurring basis
Short-term investments		0
Commercial paper | Significant other observable inputs (level 2)
Assets measured at fair value on a recurring basis
Short-term investments		4,990
Commercial paper | Significant unobservable inputs (level 3)
Assets measured at fair value on a recurring basis
Short-term investments		$ 0