v3.26.1
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
Jun. 02, 2017
Jan. 31, 2016
Jan. 31, 2018
Jan. 31, 2026
Jan. 31, 2017
Jan. 31, 2019
Jan. 31, 2018
Jan. 31, 2020
Jan. 31, 2019
Jan. 31, 2021
Jan. 31, 2020
Jan. 31, 2022
Jan. 31, 2021
Jan. 31, 2023
Jan. 31, 2022
Jan. 31, 2024
Jan. 31, 2023
Jan. 31, 2025
Jan. 31, 2024
Jan. 31, 2026
Jan. 31, 2025
Jan. 31, 2026
C000006477 [Member]                                            
Account Value [Line Items]                                            
Line Graph and Table Measure Name       C with CDSC (1% for 12 months)                                    
Accumulated Value   $ 10,000     $ 11,529   $ 12,034   $ 12,062   $ 13,077   $ 13,590   $ 13,554   $ 12,838   $ 13,778   $ 14,767 $ 15,781
C000006476 [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     11,565   12,073   12,064   13,118   13,594   13,556   12,835   13,777   14,767 15,781
C000006471 [Member]                                            
Account Value [Line Items]                                            
Line Graph and Table Measure Name       A with initial sales charge (4.25%)                                    
Accumulated Value   9,575     11,124   11,733   11,813   12,905   13,513   13,577   12,953   14,006   15,124 16,284
C000006484 [Member]                                            
Account Value [Line Items]                                            
Line Graph and Table Measure Name       A with initial sales charge (4.25%)                                    
Accumulated Value   9,575     11,125   11,806   11,865   13,002   13,504   13,545   12,924   14,109   15,386 16,594
C000006485 [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     11,532   12,146   12,116   13,199   13,606   13,525   12,811   13,881   15,050 16,082
C000006486 [Member]                                            
Account Value [Line Items]                                            
Line Graph and Table Measure Name       C with CDSC (1% for 12 months)                                    
Accumulated Value   10,000     11,554   12,150   12,118   13,203   13,611   13,528   12,807   13,904   15,050 16,083
C000006490 [Member]                                            
Account Value [Line Items]                                            
Line Graph and Table Measure Name       C with CDSC (1% for 12 months)                                    
Accumulated Value   10,000     10,037   10,549   10,804   11,880   12,144   12,080   11,005   11,209   11,883 12,211
C000006489 [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     10,059   10,598   10,881   11,995   12,293   12,259   11,196   11,432   12,151 12,519
C000006488 [Member]                                            
Account Value [Line Items]                                            
Line Graph and Table Measure Name       A with initial sales charge (4.25%)                                    
Accumulated Value   9,575     9,707   10,317   10,659   11,839   12,223   12,294   11,297   11,622   12,444 12,915
C000118060 [Member]                                            
Account Value [Line Items]                                            
Line Graph and Table Measure Name       MFS High Yield Pooled Portfolio                                    
Accumulated Value   10,000     11,777   12,522   12,755   14,067   14,773   14,978   14,402   15,764   17,165 18,648
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     10,145   10,363   10,597   11,619   12,167   11,806   10,819   11,046   11,274 12,047
Bloomberg Global Aggregate Index [Member]                                            
Account Value [Line Items]                                            
Line Graph and Table Measure Name       Bloomberg Global Aggregate Index                                    
Accumulated Value   10,000     10,235   10,999   10,902   11,620   12,418   11,694   10,327   10,425   10,451 11,346
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 $ 10,033   9,972 $ 10,360 10,323 $ 11,256 10,660 $ 11,708 11,582 $ 11,487 12,047 $ 11,114 11,820 $ 11,436 11,435 $ 11,674 11,767 $ 12,223 12,012 12,577
Bloomberg U S Corporate High Yield 2 Issuer Capped Index [Member]                                            
Account Value [Line Items]                                            
Line Graph and Table Measure Name       Bloomberg U.S. Corporate High-Yield 2% Issuer Capped Index                                    
Accumulated Value   10,000     12,077   12,874   13,097   14,329   15,386   15,701   14,882   16,263   17,837 19,211
ICE BofA Global High Yield Constrained Index USD Hedged [Member]                                            
Account Value [Line Items]                                            
Line Graph and Table Measure Name       ICE BofA Global High Yield - Constrained Index (USD Hedged)                                    
Accumulated Value   $ 10,000     $ 11,958   $ 12,814   $ 12,992   $ 14,362   $ 15,267   $ 15,316   $ 14,446   $ 15,781   $ 17,372 $ 18,745