How do I file taxes with an offshore brokerage that does not issue a 1099 B form (I have a document with all trade details but not a specific form)?

each trade is supposed to be enter on form 8949 separately and  coded in box 1 as C for short term or F as long term.  your alternative is to just enter totals, complete IRS form 8453 with the box referring to 8949 checked.   

here's a link to the form  read the instructions

https://www.irs.gov/pub/irs-pdf/f8453.pdf