Note 2 - Significant Accounting Policies - Activity in Silver Bullion (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) oz		Dec. 31, 2019 USD ($) oz		Dec. 31, 2018 USD ($) oz
Beginning balance (Ounce) | oz	362,616,711		317,233,610		320,629,662
Beginning balance, average cost	$ 6,294,157,046		$ 5,641,558,156		$ 5,831,748,196
Beginning balance, fair value	$ 6,543,418,555	[1]	$ 4,906,017,787		$ 5,407,419,250
Silver bullion contributed (Ounce) | oz	371,438,229		113,045,188		60,236,118
Silver bullion contributed, average cost	$ 7,299,222,832		$ 1,836,353,140		$ 954,051,929
Silver bullion contributed, fair value	$ 7,299,222,832		$ 1,836,353,140		$ 954,051,929
Silver bullion distributed (Ounce) | oz	(173,055,462)		(65,991,660)		(62,004,144)
Silver bullion distributed, average cost	$ (3,080,813,670)		$ (1,154,479,083)		$ (1,114,932,000)
Silver bullion distributed, fair value	(4,002,727,459)		(1,101,564,648)		(982,882,736)
Silver bullion distributed, realized gain (loss)	$ 921,913,789		$ (52,914,435)		$ (132,049,264)
Silver bullion sold to pay expenses (Ounce) | oz	(2,283,596)		(1,670,427)		(1,628,026)
Silver bullion sold to pay expenses, average cost	$ (40,091,905)		$ (29,275,167)		$ (29,309,969)
Silver bullion sold to pay expenses, fair value	(47,505,972)		(27,086,721)		(25,579,758)
Silver bullion sold to pay expenses, realized gain (loss)	7,414,067		(2,188,446)		(3,730,211)
Net realized gain (loss), fair value	929,327,856		(55,102,881)		(135,779,475)
Net change in unrealized appreciation/depreciation, fair value	$ 4,075,854,334		$ 984,801,878		$ (311,211,423)
Ending balance (Ounce) | oz	558,715,882		362,616,711		317,233,610
Ending balance, average cost	$ 10,472,474,303		$ 6,294,157,046		$ 5,641,558,156
Ending balance, fair value	14,797,590,146	[1]	6,543,418,555	[1]	4,906,017,787
Ending balance, realized gain (loss)	$ 929,327,856		$ (55,102,881)		$ (135,779,475)

[1] Cost of investment in silver bullion: $10,472,474,303 and $6,294,157,046, respectively.