Design a financial ledger service

Design a general-purpose financial ledger service that records ~20,000 postings/sec across millions of accounts, must be immutable and auditable, and must answer 'what is account X's balance as of timestamp T' for any historical T. Reads (balance lookups) outnumber writes ~10:1. Describe the data model, how balances are computed without scanning the whole journal, and how you keep current balances correct under high write concurrency.