v3.26.1
Investment in Master Trust - Investments and other assets and changes in net assets (Details) (Imported) - U.S. Group Savings Plan - USD ($)
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Investments at contract value    
Net assets available for benefits $ 9,331,538,538 $ 8,494,721,918
Net investment income 1,355,244,496 993,427,567
Transfer out (see Note1) 184,536,749  
Net Assets:    
Beginning of year 8,494,721,918 7,800,386,450
End of year 9,331,538,538 8,494,721,918
Sanofi U.S. Group Savings Master Trust    
Investments    
Total investments at fair value 8,808,891,943 7,969,065,149
Investments at contract value    
Stable value fund 487,868,228 489,852,592
Net assets available for benefits 9,296,760,171 8,458,917,741
Net Appreciation in the value of investments 1,341,677,244 978,908,389
Dividend income 1,689,256 1,860,132
Interest 16,171,561 16,180,920
Net investment income 1,359,538,061 996,949,441
Transfer out (see Note1) (521,695,631) (305,936,717)
Increase in Net Assets 837,842,430 691,012,724
Net Assets:    
Beginning of year 8,458,917,741 7,767,905,017
End of year 9,296,760,171 8,458,917,741
Sanofi U.S. Group Savings Master Trust | Self-directed brokerage accounts    
Investments    
Total investments at fair value 195,671,531 166,382,492
Sanofi U.S. Group Savings Master Trust | Common collective trust funds    
Investments    
Total investments at fair value 8,321,132,717 7,524,576,465
Sanofi U.S. Group Savings Master Trust | Domestic equities    
Investments    
Total investments at fair value 196,628,547 181,509,806
Sanofi U.S. Group Savings Master Trust | Fixed income securities    
Investments    
Total investments at fair value 58,038,372 54,099,426
Sanofi U.S. Group Savings Master Trust | Common stock    
Investments    
Total investments at fair value 37,420,776 42,496,960
Sanofi U.S. Group Savings Plan's Interest in Sanofi U.S. Group Savings Master Trust    
Investments    
Total investments at fair value 8,778,889,295 7,941,107,998
Investments at contract value    
Stable value fund 486,208,714 487,292,544
Net assets available for benefits 9,265,098,009 8,428,400,542
Net Appreciation in the value of investments 1,337,457,183 975,490,001
Dividend income 1,670,761 1,841,210
Interest 16,116,552 16,096,356
Net investment income 1,355,244,496 993,427,567
Transfer out (see Note1) (518,547,029) (303,062,094)
Increase in Net Assets 836,697,467 690,365,473
Net Assets:    
Beginning of year 8,428,400,542 7,738,035,069
End of year 9,265,098,009 8,428,400,542
Sanofi U.S. Group Savings Plan's Interest in Sanofi U.S. Group Savings Master Trust | Self-directed brokerage accounts    
Investments    
Total investments at fair value 195,671,531 166,382,492
Sanofi U.S. Group Savings Plan's Interest in Sanofi U.S. Group Savings Master Trust | Common collective trust funds    
Investments    
Total investments at fair value 8,293,849,028 7,499,066,608
Sanofi U.S. Group Savings Plan's Interest in Sanofi U.S. Group Savings Master Trust | Domestic equities    
Investments    
Total investments at fair value 194,752,347 179,849,867
Sanofi U.S. Group Savings Plan's Interest in Sanofi U.S. Group Savings Master Trust | Fixed income securities    
Investments    
Total investments at fair value 57,671,979 53,800,531
Sanofi U.S. Group Savings Plan's Interest in Sanofi U.S. Group Savings Master Trust | Common stock    
Investments    
Total investments at fair value $ 36,944,410 $ 42,008,500