Shareholder Report, Line Graph (Details) - USD ($) |
8 Months Ended | 12 Months Ended | 20 Months Ended | 24 Months Ended | 32 Months Ended | 36 Months Ended | 44 Months Ended | 48 Months Ended | 56 Months Ended | 60 Months Ended | 68 Months Ended | 72 Months Ended | 80 Months Ended | 84 Months Ended | 92 Months Ended | 96 Months Ended | 104 Months Ended | 108 Months Ended | 120 Months Ended | ||||
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Sep. 01, 2017 |
Apr. 30, 2016 |
Apr. 30, 2018 |
Apr. 30, 2026 |
Apr. 30, 2017 |
Apr. 30, 2019 |
Apr. 30, 2018 |
Apr. 30, 2020 |
Apr. 30, 2019 |
Apr. 30, 2021 |
Apr. 30, 2020 |
Apr. 30, 2022 |
Apr. 30, 2021 |
Apr. 30, 2023 |
Apr. 30, 2022 |
Apr. 30, 2024 |
Dec. 30, 2022 |
Apr. 30, 2023 |
Apr. 30, 2025 |
Apr. 30, 2024 |
Apr. 30, 2026 |
Apr. 30, 2025 |
Apr. 30, 2026 |
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| C000006757 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | A with initial sales charge (2.50%) | ||||||||||||||||||||||
| Accumulated Value | $ 9,750 | $ 9,796 | $ 9,854 | $ 10,284 | $ 10,364 | $ 10,978 | $ 10,402 | $ 10,577 | $ 10,879 | $ 11,263 | $ 11,820 | ||||||||||||
| C000006758 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | B with CDSC (declining over six years from 4% to 0%) | ||||||||||||||||||||||
| Accumulated Value | 10,000 | 9,972 | 9,957 | 10,315 | 10,318 | 10,850 | 10,203 | 10,297 | 10,514 | 10,818 | 11,255 | ||||||||||||
| C000006759 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | C with CDSC (1% for 12 months) | ||||||||||||||||||||||
| Accumulated Value | 10,000 | 9,950 | 9,925 | 10,271 | 10,263 | 10,793 | 10,128 | 10,211 | 10,414 | 10,705 | 11,126 | ||||||||||||
| C000006760 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | A with initial sales charge (4.25%) | ||||||||||||||||||||||
| Accumulated Value | 9,575 | 9,731 | 9,709 | 10,216 | 10,962 | 11,460 | 10,491 | 10,391 | 10,400 | 11,247 | 11,742 | ||||||||||||
| C000006765 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | B with CDSC (declining over six years from 4% to 0%) | ||||||||||||||||||||||
| Accumulated Value | 10,000 | 10,087 | 9,999 | 10,442 | 11,121 | 11,540 | 10,486 | 10,310 | 10,242 | 10,994 | 11,392 | ||||||||||||
| C000006766 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | C with CDSC (1% for 12 months) | ||||||||||||||||||||||
| Accumulated Value | 10,000 | 10,077 | 9,979 | 10,411 | 11,077 | 11,473 | 10,423 | 10,227 | 10,150 | 10,884 | 11,267 | ||||||||||||
| C000006716 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | B with CDSC (declining over six years from 4% to 0%) | ||||||||||||||||||||||
| Accumulated Value | 10,000 | 10,204 | 10,135 | 10,695 | 11,680 | 12,167 | 10,743 | 10,692 | 10,771 | 11,479 | 11,987 | ||||||||||||
| C000006711 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | A with initial sales charge (4.25%) | ||||||||||||||||||||||
| Accumulated Value | 9,575 | 9,836 | 9,844 | 10,473 | 11,521 | 12,090 | 10,760 | 10,781 | 10,943 | 11,750 | 12,361 | ||||||||||||
| C000006717 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | C with CDSC (1% for 12 months) | ||||||||||||||||||||||
| Accumulated Value | 10,000 | 10,204 | 10,135 | 10,703 | 11,689 | 12,168 | 10,750 | 10,690 | 10,768 | 11,478 | 11,985 | ||||||||||||
| C000006745 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | A with initial sales charge (2.50%) | ||||||||||||||||||||||
| Accumulated Value | 9,750 | 9,859 | 9,895 | 10,234 | 10,460 | 10,940 | 10,547 | 10,692 | 11,096 | 11,897 | 12,377 | ||||||||||||
| C000006750 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | B with CDSC (declining over six years from 4% to 0%) | ||||||||||||||||||||||
| Accumulated Value | 10,000 | 10,035 | 9,995 | 10,278 | 10,427 | 10,825 | 10,357 | 10,421 | 10,734 | 11,423 | 11,816 | ||||||||||||
| C000006751 [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | C with CDSC (1% for 12 months) | ||||||||||||||||||||||
| Accumulated Value | 10,000 | 10,026 | 9,977 | 10,249 | 10,388 | 10,773 | 10,298 | 10,350 | 10,651 | 11,323 | 11,682 | ||||||||||||
| Bloomberg U S Aggregate Bond Index [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg U.S. Aggregate Bond Index | ||||||||||||||||||||||
| Accumulated Value | 10,000 | 10,083 | 10,051 | 10,582 | 11,730 | 11,698 | 10,703 | 10,657 | 10,500 | 11,343 | 11,803 | ||||||||||||
| Bloomberg Municipal Bond Index [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg Municipal Bond Index | ||||||||||||||||||||||
| Accumulated Value | $ 10,000 | 10,000 | $ 9,877 | 10,014 | $ 10,486 | 10,170 | $ 10,712 | 10,797 | $ 11,542 | 11,030 | $ 10,632 | 11,884 | $ 10,938 | 10,947 | $ 11,165 | $ 10,947 | 11,262 | $ 11,351 | 11,497 | $ 12,071 | 11,688 | 12,429 | |
| Bloomberg 1 3 Year U S Government Credit Bond Index [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg 1-3 Year U.S. Government/Credit Bond Index | ||||||||||||||||||||||
| Accumulated Value | 10,000 | 10,076 | 10,070 | 10,410 | 10,925 | 11,036 | 10,649 | 10,771 | 11,072 | 11,821 | 12,225 | ||||||||||||
| Bloomberg 1 9 Year Municipal Bond Index [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg 1-9 Year Municipal Bond Index | ||||||||||||||||||||||
| Accumulated Value | $ 10,000 | 10,000 | $ 9,829.828419 | 10,046 | $ 10,273.15408 | 10,040 | $ 10,495.9942 | 10,493 | $ 10,998.81103 | 10,721 | $ 10,391.24839 | 11,234 | $ 10,686.75826 | 10,614 | $ 10,871.02137 | 10,915 | $ 11,169.47043 | 11,104 | $ 11,717.79585 | 11,408 | 11,969 | ||
| Bloomberg U S Credit Index [Member] | |||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg U.S. Credit Index | ||||||||||||||||||||||
| Accumulated Value | $ 10,000 | $ 10,274 | $ 10,340 | $ 11,000 | $ 12,030 | $ 12,542 | $ 11,271 | $ 11,352 | $ 11,438 | $ 12,309 | $ 12,955 | ||||||||||||