Schedules of Investments (Parenthetical) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
U.S. Equity Cumulative Dividends Fund-Series 2027
Coupon rate for this security		2.125%
Percentages are based on net assets	$ 24,290,529	$ 4,526,860
Total market value of securities pledged as collateral	$ 2,728,628	$ 513,627
Percentage of net assets	11.20%	11.30%
U.S. Equity Cumulative Dividends Fund-Series 2027 | Investments
Investment percentage	97.10%
Coupon rate for this security		99.30%
Cost, total	$ 23,480,622	$ 4,453,987
U.S. Equity Cumulative Dividends Fund-Series 2027 | U.S. Treasury Obligations
Investment percentage	97.10%
Coupon rate for this security		99.30%
Cost, total	$ 23,480,622	$ 4,453,987
U.S. Equity Ex-Dividend Fund-Series 2027
Investment percentage	97.10%
Coupon rate for this security		2.00%
Cost, total	$ 23,480,622
Percentages are based on net assets	26,941,802	$ 12,495,827
Total market value of securities pledged as collateral	$ 7,254,010	$ 3,059,980
Percentage of net assets	26.90%	24.50%
U.S. Equity Ex-Dividend Fund-Series 2027 | Investments
Investment percentage	95.10%	90.00%
Cost, total	$ 25,836,894	$ 11,249,881
U.S. Equity Ex-Dividend Fund-Series 2027 | U.S. Treasury Obligations
Investment percentage	95.10%	90.00%
Cost, total	$ 25,836,894	$ 11,249,881
6.125%, 11/15/2027 | U.S. Equity Cumulative Dividends Fund-Series 2027
Coupon rate for this security	6.125%	1.537%
2.250%, 12/31/2023 | U.S. Equity Cumulative Dividends Fund-Series 2027
Investment percentage		95.10%
Coupon rate for this security	2.25%	2.25%
Cost, total		$ 25,836,894
2.250%, 11/15/2024 | U.S. Equity Cumulative Dividends Fund-Series 2027
Investment percentage		95.10%
Coupon rate for this security	2.25%	2.25%
Cost, total		$ 25,836,894
2.250%, 11/15/2025 | U.S. Equity Cumulative Dividends Fund-Series 2027
Coupon rate for this security	2.25%	2.25%
Percentages are based on net assets		$ 26,941,802
Total market value of securities pledged as collateral		$ 7,254,010
Percentage of net assets		26.90%
2.125%, 12/31/2022 | U.S. Equity Cumulative Dividends Fund-Series 2027
Coupon rate for this security	2.125%	6.125%
2.000%, 12/31/2021 | U.S. Equity Cumulative Dividends Fund-Series 2027
Coupon rate for this security	2.00%	2.25%
2.000%, 11/15/2026 | U.S. Equity Cumulative Dividends Fund-Series 2027
Coupon rate for this security	2.00%	2.25%
1.750%, 12/31/2020 | U.S. Equity Cumulative Dividends Fund-Series 2027
Coupon rate for this security	1.75%	2.25%
1.537%, 05/28/2020 | U.S. Equity Ex-Dividend Fund-Series 2027
Investment percentage	97.10%
Coupon rate for this security	1.537%
Cost, total	$ 23,480,622
2.414%, 4/11/2019 | U.S. Equity Ex-Dividend Fund-Series 2027
Coupon rate for this security		2.414%