FAIR VALUE MEASUREMENTS - Gross holding gains and fair value of held-to-maturity securities (Details) - Fair Value, Recurring - Level 1 - USD ($)	Mar. 31, 2021	Dec. 31, 2020
UBS Select Treasury Preferred Fund
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Amortized Cost	$ 1,035,984,935
Fair Value	$ 1,035,984,935
U.S. Treasury Securities
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Amortized Cost		$ 1,035,848,884
Gross Holding Gain		22,518
Fair Value		$ 1,035,871,402