v3.26.1
Shareholder Report, Average Annual Return (Details)
 12 Months Ended 60 Months Ended 104 Months Ended 120 Months Ended
Apr. 30, 2026
Apr. 30, 2026
Apr. 30, 2026
 [9]
Apr. 30, 2026
C000006757 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A with initial sales charge (2.50%)      
Average Annual Return, Percent 2.33% 0.98%   1.69%
C000006758 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name B with CDSC (declining over six years from 4% to 0%)      
Average Annual Return, Percent [1] 0.04% 0.36%   1.19%
C000006759 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C with CDSC (1% for 12 months)      
Average Annual Return, Percent [2] 2.94% 0.61%   1.07%
C000006760 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A with initial sales charge (4.25%)      
Average Annual Return, Percent (0.03%) (0.38%)   1.62%
C000006765 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name B with CDSC (declining over six years from 4% to 0%)      
Average Annual Return, Percent [3] (0.37%) (0.60%)   1.31%
C000006766 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C with CDSC (1% for 12 months)      
Average Annual Return, Percent [4] 2.52% (0.36%)   1.20%
C000006716 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name B with CDSC (declining over six years from 4% to 0%)      
Average Annual Return, Percent [5] 0.42% (0.63%)   1.83%
C000006711 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A with initial sales charge (4.25%)      
Average Annual Return, Percent 0.73% (0.42%)   2.14%
C000006717 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C with CDSC (1% for 12 months)      
Average Annual Return, Percent [6] 3.42% (0.30%)   1.83%
C000006745 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A with initial sales charge (2.50%)      
Average Annual Return, Percent 1.44% 1.98%   2.16%
C000006750 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name B with CDSC (declining over six years from 4% to 0%)      
Average Annual Return, Percent [7] (0.55%) 1.41%   1.68%
C000006751 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C with CDSC (1% for 12 months)      
Average Annual Return, Percent [8] 2.17% 1.63%   1.57%
Without Sales Load [Member] | C000006757 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A without sales charge      
Average Annual Return, Percent 4.95% 1.49%   1.94%
Without Sales Load [Member] | C000006758 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name B without sales charge      
Average Annual Return, Percent 4.04% 0.74%   1.19%
Without Sales Load [Member] | C000006759 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C without sales charge      
Average Annual Return, Percent 3.94% 0.61%   1.07%
Without Sales Load [Member] | C000092113 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name I without sales charge      
Average Annual Return, Percent 4.98% 1.61%   2.09%
Without Sales Load [Member] | C000195152 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R6 without sales charge      
Average Annual Return, Percent 5.19% 1.71% 2.21%  
Without Sales Load [Member] | C000006760 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A without sales charge      
Average Annual Return, Percent 4.40% 0.49%   2.06%
Without Sales Load [Member] | C000006765 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name B without sales charge      
Average Annual Return, Percent 3.63% (0.26%)   1.31%
Without Sales Load [Member] | C000006766 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C without sales charge      
Average Annual Return, Percent 3.52% (0.36%)   1.20%
Without Sales Load [Member] | C000006767 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name I without sales charge      
Average Annual Return, Percent 4.56% 0.64%   2.21%
Without Sales Load [Member] | C000006769 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R1 without sales charge      
Average Annual Return, Percent 3.52% (0.34%)   1.20%
Without Sales Load [Member] | C000006771 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R2 without sales charge      
Average Annual Return, Percent 4.04% 0.14%   1.72%
Without Sales Load [Member] | C000006772 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R3 without sales charge      
Average Annual Return, Percent 4.30% 0.39%   1.96%
Without Sales Load [Member] | C000006761 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R4 without sales charge      
Average Annual Return, Percent 4.56% 0.64%   2.21%
Without Sales Load [Member] | C000034443 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R6 without sales charge      
Average Annual Return, Percent 4.66% 0.74%   2.32%
Without Sales Load [Member] | C000006716 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name B without sales charge      
Average Annual Return, Percent 4.42% (0.30%)   1.83%
Without Sales Load [Member] | C000006711 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A without sales charge      
Average Annual Return, Percent 5.20% 0.44%   2.59%
Without Sales Load [Member] | C000119776 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R6 without sales charge      
Average Annual Return, Percent 4.28% 2.73%   2.64%
Without Sales Load [Member] | C000006717 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C without sales charge      
Average Annual Return, Percent 4.42% (0.30%)   1.83%
Without Sales Load [Member] | C000006718 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name I without sales charge      
Average Annual Return, Percent 5.55% 0.71%   2.84%
Without Sales Load [Member] | C000006720 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R1 without sales charge      
Average Annual Return, Percent 4.51% (0.28%)   1.84%
Without Sales Load [Member] | C000006722 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R2 without sales charge      
Average Annual Return, Percent 4.94% 0.21%   2.34%
Without Sales Load [Member] | C000006723 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R3 without sales charge      
Average Annual Return, Percent 5.20% 0.46%   2.60%
Without Sales Load [Member] | C000006712 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R4 without sales charge      
Average Annual Return, Percent 5.63% 0.74%   2.87%
Without Sales Load [Member] | C000070424 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R6 without sales charge      
Average Annual Return, Percent 5.65% 0.80%   2.94%
Without Sales Load [Member] | C000006745 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A without sales charge      
Average Annual Return, Percent 4.04% 2.50%   2.41%
Without Sales Load [Member] | C000006750 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name B without sales charge      
Average Annual Return, Percent 3.44% 1.77%   1.68%
Without Sales Load [Member] | C000006751 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C without sales charge      
Average Annual Return, Percent 3.16% 1.63%   1.57%
Without Sales Load [Member] | C000006744 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name I without sales charge      
Average Annual Return, Percent 4.37% 2.65%   2.58%
Without Sales Load [Member] | C000006753 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R1 without sales charge      
Average Annual Return, Percent 3.16% 1.60%   1.55%
Without Sales Load [Member] | C000006755 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R2 without sales charge      
Average Annual Return, Percent 3.96% 2.24%   2.18%
Without Sales Load [Member] | C000006756 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R3 without sales charge      
Average Annual Return, Percent 4.11% 2.40%   2.33%
Without Sales Load [Member] | C000006746 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R4 without sales charge      
Average Annual Return, Percent 4.20% 2.66%   2.61%
Bloomberg U S Aggregate Bond Index [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name Bloomberg U.S. Aggregate Bond Index      
Average Annual Return, Percent [10],[11] 4.06% 0.18%   1.67%
Bloomberg Municipal Bond Index [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name Bloomberg Municipal Bond Index      
Average Annual Return, Percent 6.34% [12],[13] 0.90% [12],[13] 2.20% [14] 2.20% [12],[13]
Bloomberg 1 3 Year U S Government Credit Bond Index [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name Bloomberg 1-3 Year U.S. Government/Credit Bond Index      
Average Annual Return, Percent [15] 3.42% 2.07%   2.03%
Bloomberg 1 9 Year Municipal Bond Index [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name Bloomberg 1-9 Year Municipal Bond Index      
Average Annual Return, Percent 4.91% [12],[13] 1.27% [12],[13] 1.85% [14] 1.81% [12],[13]
Bloomberg U S Credit Index [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name Bloomberg U.S. Credit Index      
Average Annual Return, Percent [10],[11] 5.25% 0.65%   2.62%
[1]
Assuming redemption at the end of the applicable period.
[2] Assuming redemption at the end of the applicable period.
[3]
Assuming redemption at the end of the applicable period.
[4] Assuming redemption at the end of the applicable period.
[5] Assuming redemption at the end of the applicable period.
[6]
Assuming redemption at the end of the applicable period.
[7]
Assuming redemption at the end of the applicable period.
[8]
Assuming redemption at the end of the applicable period.
[9]
For the period from the commencement of the class's investment operations, September 1, 2017 through April 30, 2026.
[10]
Source: FactSet Research Systems Inc.
[11] Source: FactSet Research Systems Inc.
[12]
Source: FactSet Research Systems Inc.
[13] Source: FactSet Research Systems Inc.
[14]
Source: FactSet Research Systems Inc.
[15]
Source: FactSet Research Systems Inc.