Retrive balance on managed account

Problem

When I list transaction history on a managed account, I see a list of transaction data. In each of the transaction object I see the value of the "source" attr that start with "py". The transaction object looks like this: `{ "id": "txn_17bBwe2eZvKYlo2Cuwcyi9or", "object": "balance_transaction", "amount": 400, "available_on": 1455235200, "created": 1454687788, "currency": "usd", "description": "Charge for test@example.com", "fee": 42, "fee_details": [ ], "net": 358, "source": "py_17bBwe2eZvKYlo2Crk3VGEG8", "sourced_transfers": { "object": "list", "data": [ [code block] }, "status": "pending", "type": "charge" }` I want to retrieve the detail from the value come from "source", how can I do that? I tried retrieve from Charge,Balance but no luck.