v3.26.1
Shareholder Report, Average Annual Return (Details)
 12 Months Ended 60 Months Ended 104 Months Ended 120 Months Ended
Jan. 31, 2026
Jan. 31, 2026
Jan. 31, 2026
 [7]
Jan. 31, 2026
C000006477 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C with CDSC (1% for 12 months)      
Average Annual Return, Percent [1] 5.87% 3.03%   4.67%
C000006476 [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 [2] 2.87% 2.70%   4.67%
C000006471 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A with initial sales charge (4.25%)      
Average Annual Return, Percent 3.10% 2.90%   5.00%
C000006484 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A with initial sales charge (4.25%)      
Average Annual Return, Percent 3.27% 3.31%   5.20%
C000006485 [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] 2.86% 3.08%   4.87%
C000006486 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C with CDSC (1% for 12 months)      
Average Annual Return, Percent [4] 5.86% 3.39%   4.87%
C000006490 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C with CDSC (1% for 12 months)      
Average Annual Return, Percent [5] 1.77% 0.11%   2.02%
C000006489 [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 [6] (0.95%) 0.02%   2.27%
C000006488 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A with initial sales charge (4.25%)      
Average Annual Return, Percent (0.62%) 0.23%   2.59%
C000118060 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name MFS High Yield Pooled Portfolio      
Average Annual Return, Percent 8.64% 4.77%   6.43%
Without Sales Load [Member] | C000116932 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R6 without sales charge      
Average Annual Return, Percent 8.04% 4.17%   5.83%
Without Sales Load [Member] | C000006472 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R4 without sales charge      
Average Annual Return, Percent 7.93% 4.12%   5.75%
Without Sales Load [Member] | C000006483 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R3 without sales charge      
Average Annual Return, Percent 7.67% 3.80%   5.42%
Without Sales Load [Member] | C000006482 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R2 without sales charge      
Average Annual Return, Percent 7.41% 3.47%   5.12%
Without Sales Load [Member] | C000006480 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R1 without sales charge      
Average Annual Return, Percent 6.87% 3.03%   4.67%
Without Sales Load [Member] | C000006478 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name I without sales charge      
Average Annual Return, Percent 7.94% 4.05%   5.72%
Without Sales Load [Member] | C000006477 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C without sales charge      
Average Annual Return, Percent 6.87% 3.03%   4.67%
Without Sales Load [Member] | C000006476 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name B without sales charge      
Average Annual Return, Percent 6.87% 3.03%   4.67%
Without Sales Load [Member] | C000006471 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A without sales charge      
Average Annual Return, Percent 7.67% 3.80%   5.45%
Without Sales Load [Member] | C000006484 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A without sales charge      
Average Annual Return, Percent 7.85% 4.21%   5.65%
Without Sales Load [Member] | C000006485 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name B without sales charge      
Average Annual Return, Percent 6.86% 3.40%   4.87%
Without Sales Load [Member] | C000006486 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C without sales charge      
Average Annual Return, Percent 6.86% 3.39%   4.87%
Without Sales Load [Member] | C000006487 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name I without sales charge      
Average Annual Return, Percent 7.93% 4.43%   5.90%
Without Sales Load [Member] | C000064677 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R1 without sales charge      
Average Annual Return, Percent 7.05% 3.43%   4.85%
Without Sales Load [Member] | C000064678 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R2 without sales charge      
Average Annual Return, Percent 7.58% 3.95%   5.39%
Without Sales Load [Member] | C000064679 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R3 without sales charge      
Average Annual Return, Percent 7.84% 4.18%   5.71%
Without Sales Load [Member] | C000064680 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R4 without sales charge      
Average Annual Return, Percent 8.11% 4.48%   5.91%
Without Sales Load [Member] | C000064681 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R6 without sales charge      
Average Annual Return, Percent 8.23% 4.57%   6.02%
Without Sales Load [Member] | C000191232 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name R6 without sales charge      
Average Annual Return, Percent 3.86% 1.16% 2.93%  
Without Sales Load [Member] | C000103072 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name I without sales charge      
Average Annual Return, Percent 3.64% 1.08%   3.02%
Without Sales Load [Member] | C000006490 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name C without sales charge      
Average Annual Return, Percent 2.77% 0.11%   2.02%
Without Sales Load [Member] | C000006489 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name B without sales charge      
Average Annual Return, Percent 3.03% 0.36%   2.27%
Without Sales Load [Member] | C000006488 [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name A without sales charge      
Average Annual Return, Percent 3.79% 1.11%   3.04%
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 [8],[9] 6.85% (0.20%)   1.88%
Bloomberg Global Aggregate Index [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name Bloomberg Global Aggregate Index      
Average Annual Return, Percent [10],[11] 8.56% (1.79%)   1.27%
Bloomberg Municipal Bond Index [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name Bloomberg Municipal Bond Index      
Average Annual Return, Percent 4.70% [12],[13] 0.86% [12],[13] 2.34% [14] 2.32% [12],[13]
Bloomberg U S Corporate High Yield 2 Issuer Capped Index [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name Bloomberg U.S. Corporate High-Yield 2% Issuer Capped Index      
Average Annual Return, Percent [8],[9] 7.70% 4.54%   6.75%
ICE BofA Global High Yield Constrained Index USD Hedged [Member]        
Average Annual Return [Line Items]        
Line Graph and Table Measure Name ICE BofA Global High Yield - Constrained Index (USD Hedged)      
Average Annual Return, Percent [11] 7.91% [10] 4.19% [10]   6.49% [15]
[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]
For the period from the commencement of the class's investment operations, June 2, 2017 through January 31, 2026.
[8]
Source: FactSet Research Systems Inc.
[9] Source: FactSet Research Systems Inc.
[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.