Shareholder Report, Line Graph (Details) - USD ($) |
12 Months Ended | 47 Months Ended | 60 Months Ended | 114 Months Ended | 120 Months Ended | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Mar. 31, 2025 |
Mar. 31, 2025 |
Mar. 31, 2025 |
Mar. 31, 2025 |
Mar. 31, 2025 |
Sep. 30, 2024 |
Mar. 31, 2024 |
Sep. 30, 2023 |
Mar. 31, 2023 |
Sep. 30, 2022 |
Mar. 31, 2022 |
Sep. 30, 2021 |
May 05, 2021 |
Mar. 31, 2021 |
Mar. 31, 2020 |
Mar. 31, 2019 |
Mar. 31, 2018 |
Mar. 31, 2017 |
Mar. 31, 2016 |
Sep. 21, 2015 |
Mar. 31, 2015 |
|
| C000003891 [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | BATS: Series C Portfolio | ||||||||||||||||||||
| Account Value | $ 13,024 | $ 13,024 | $ 13,024 | $ 13,024 | $ 13,024 | $ 12,435 | $ 11,846 | $ 12,459 | $ 12,961 | $ 11,924 | $ 11,217 | $ 10,677 | $ 10,385 | $ 10,071 | $ 10,000 | ||||||
| Average Annual Return, Percent | 4.73% | 1.78% | 2.68% | ||||||||||||||||||
| C000003892 [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | BATS: Series M Portfolio | ||||||||||||||||||||
| Account Value | $ 11,615 | 11,615 | $ 11,615 | 11,615 | $ 11,615 | 10,962 | 10,727 | 11,262 | 11,852 | 11,542 | 10,904 | 10,391 | 10,297 | 10,244 | 10,000 | ||||||
| Average Annual Return, Percent | 5.96% | 0.13% | 1.51% | ||||||||||||||||||
| C000003893 [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | BATS: Series S Portfolio | ||||||||||||||||||||
| Account Value | $ 12,953 | 12,953 | $ 12,953 | 12,953 | $ 12,953 | 12,184 | 11,606 | 11,534 | 11,894 | 11,033 | 10,996 | 10,562 | 10,442 | 10,118 | 10,000 | ||||||
| Average Annual Return, Percent | 6.31% | 3.26% | 2.62% | ||||||||||||||||||
| C000032703 [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | BATS: Series P Portfolio | ||||||||||||||||||||
| Account Value | $ 12,873 | 12,873 | $ 12,873 | 12,873 | $ 12,873 | 12,059 | 10,810 | 9,825 | 9,539 | 8,694 | 10,011 | 10,249 | 10,011 | 9,552 | 10,000 | ||||||
| Average Annual Return, Percent | 6.75% | 8.17% | 2.56% | ||||||||||||||||||
| C000144517 [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | BATS: Series E Portfolio | ||||||||||||||||||||
| Account Value | $ 15,483 | 15,483 | $ 15,483 | 15,483 | $ 15,483 | 14,738 | 13,881 | 14,492 | 14,648 | 12,610 | 12,569 | 11,808 | 11,013 | 10,715 | 10,000 | ||||||
| Average Annual Return, Percent | 5.05% | 4.19% | 4.47% | ||||||||||||||||||
| C000146955 [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | BATS: Series A Portfolio | ||||||||||||||||||||
| Account Value | $ 15,493 | 15,493 | $ 15,493 | $ 15,493 | $ 15,493 | 14,331 | 13,188 | 13,243 | 13,322 | 11,691 | 12,334 | 11,825 | 11,203 | 10,207 | $ 10,000 | ||||||
| Average Annual Return, Percent | 8.11% | 5.79% | 4.70% | ||||||||||||||||||
| C000227189 [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | BATS: Series V Portfolio | ||||||||||||||||||||
| Account Value | $ 10,875 | $ 10,875 | $ 10,875 | $ 10,875 | 10,875 | $ 10,728 | 10,535 | $ 10,357 | 10,184 | $ 10,051 | 10,001 | $ 10,005 | $ 10,000 | ||||||||
| Average Annual Return, Percent | 3.23% | 2.17% | |||||||||||||||||||
| Bloomberg U S Aggregate Bond Index [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg U.S. Aggregate Bond Index | ||||||||||||||||||||
| Account Value | $ 11,564 | $ 11,564 | $ 11,564 | 11,564 | $ 11,564 | 11,026 | 10,842 | 11,386 | 11,880 | 11,796 | 10,829 | 10,364 | 10,241 | 10,196 | 10,000 | ||||||
| Average Annual Return, Percent | 4.88% | (0.40%) | 1.46% | ||||||||||||||||||
| Bloomberg Municipal Bond Index [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg Municipal Bond Index | ||||||||||||||||||||
| Account Value | $ 12,341 | $ 12,341 | $ 12,341 | 12,341 | $ 12,341 | 10,050 | 12,193 | 9,105 | 11,823 | 8,869 | 11,793 | 10,021 | 10,000 | 12,344 | 11,700 | 11,266 | 10,691 | 10,414 | 10,398 | 10,000 | |
| Average Annual Return, Percent | 1.22% | (0.24%) | 1.07% | 2.13% | |||||||||||||||||
| Bloomberg U S Universal Index [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg U.S. Universal Index | ||||||||||||||||||||
| Account Value | $ 12,100 | $ 12,100 | $ 12,100 | $ 12,100 | $ 12,100 | 11,497 | 11,198 | 11,739 | 12,257 | 11,906 | 11,111 | 10,630 | 10,471 | 10,274 | 10,000 | ||||||
| Average Annual Return, Percent | 5.24% | 0.32% | 2.02% | ||||||||||||||||||
| Bloomberg U S Credit Index [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg U.S. Credit Index | ||||||||||||||||||||
| Account Value | $ 12,568 | 12,568 | $ 12,568 | $ 12,568 | $ 12,568 | 11,984 | 11,507 | 12,152 | 12,679 | 11,753 | 11,183 | 10,662 | 10,393 | 10,093 | 10,000 | ||||||
| Average Annual Return, Percent | 4.87% | 1.35% | 2.31% | ||||||||||||||||||
| Reference Benchmark [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | Reference Benchmark | ||||||||||||||||||||
| Account Value | $ 12,624 | 12,624 | $ 12,624 | $ 12,624 | $ 12,624 | 11,853 | 11,250 | 11,503 | 11,931 | 11,286 | 10,939 | 10,448 | 10,329 | 10,201 | $ 10,000 | ||||||
| Average Annual Return, Percent | 6.51% | 2.27% | 2.48% | ||||||||||||||||||
| Bloomberg MBS Index [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg MBS Index | ||||||||||||||||||||
| Account Value | $ 11,163 | 11,163 | $ 11,163 | $ 11,163 | $ 11,163 | 10,592 | 10,447 | 10,979 | 11,547 | 11,557 | 10,798 | 10,341 | 10,261 | 10,243 | 10,000 | ||||||
| Average Annual Return, Percent | 5.39% | (0.69%) | 1.11% | ||||||||||||||||||
| ICE BofA 1 3 Year U S Treasury Index [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | ICE BofA 1-3 Year U.S. Treasury Index | ||||||||||||||||||||
| Account Value | $ 11,610 | 11,610 | $ 11,610 | 11,610 | $ 11,610 | 11,012 | 10,695 | 10,667 | 10,985 | 10,958 | 10,395 | 10,120 | 10,117 | 10,092 | 10,000 | ||||||
| Average Annual Return, Percent | 5.43% | 1.16% | 1.50% | ||||||||||||||||||
| Bloomberg U S Treasury 7 10 Year Bond Index [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg U.S. Treasury 7-10 Year Bond Index | ||||||||||||||||||||
| Account Value | $ 10,902 | 10,902 | $ 10,902 | 10,902 | $ 10,902 | 10,417 | 10,563 | 11,195 | 11,658 | 12,387 | 10,662 | 10,081 | 10,111 | 10,375 | 10,000 | ||||||
| Average Annual Return, Percent | 4.65% | (2.52%) | 0.87% | ||||||||||||||||||
| Bloomberg U S Bellwether 10 Year Swap Index [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg U.S. Bellwether 10 Year Swap Index | ||||||||||||||||||||
| Account Value | $ 10,700 | 10,700 | $ 10,700 | 10,700 | $ 10,700 | 10,265 | 10,462 | 11,193 | 11,632 | 12,701 | 10,683 | 10,052 | 10,142 | 10,627 | 10,000 | ||||||
| Average Annual Return, Percent | 4.24% | (3.37%) | 0.68% | ||||||||||||||||||
| Bloomberg Municipal High Yield Bond Index [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg Municipal High Yield Bond Index | ||||||||||||||||||||
| Account Value | $ 15,165 | 15,165 | $ 15,165 | 15,165 | $ 15,165 | 14,362 | 13,309 | 13,935 | $ 14,126 | $ 12,281 | $ 12,373 | $ 11,442 | $ 10,791 | $ 10,345 | $ 10,000 | ||||||
| Average Annual Return, Percent | 5.59% | 4.31% | 4.25% | ||||||||||||||||||
| Customized Reference Benchmark prior [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | Customized Reference Benchmark (prior) | ||||||||||||||||||||
| Average Annual Return, Percent | 2.73% | 2.21% | |||||||||||||||||||
| Customized Reference Benchmark [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | Customized Reference Benchmark | ||||||||||||||||||||
| Average Annual Return, Percent | 4.05% | 3.31% | |||||||||||||||||||
| SIFMA Municipal Swap Index [Member] | |||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||
| Line Graph and Table Measure Name | SIFMA Municipal Swap Index | ||||||||||||||||||||
| Account Value | $ 10,880 | $ 10,880 | $ 10,880 | $ 10,880 | $ 10,880 | $ 10,725 | $ 10,543 | $ 10,364 | $ 10,194 | $ 10,059 | $ 10,007 | $ 10,001 | $ 10,000 | ||||||||
| Average Annual Return, Percent | 3.21% | 2.19% |