'thoroughly-detailed' is a compound formed from the adverb 'thoroughly' and the adjective 'detailed'. 'Thoroughly' comes from Old English 'thurh' (via Middle English 'thurwe/through') where the element meant 'through' or 'completely'; 'detailed' derives from Old French 'detailler' (to cut in pieces; to set out in particulars).

'thorough' changed from Old English 'thurh'/'thuruh' (meaning 'through') into Middle English 'thorough' and later formed the adverb 'thoroughly'; 'detail' came into English from Old French 'detail'/'detailler' (from Vulgar Latin/Old French roots related to cutting), producing the adjective 'detailed' meaning 'set out in particulars'. The modern compound 'thoroughly-detailed' combines these to mean 'completely given in particulars'.

Initially, elements meant 'through' (for 'thorough') and 'to cut into pieces' or 'particulars' (for 'detail'); over time they evolved so that 'thoroughly' came to mean 'completely/carefully' and 'detailed' to mean 'containing many particulars', together producing the current sense of 'completely and precisely described'.