Conceptual Design of a data warehouse (declaring the grain)

Question

I am trying to design a data warehouse using this example scrubbed data. I need to declare the "grain". I know its the lowest level of detail in the fact table, but I am having a hard time determining it. Any help would be appreciated. (Sorry about the table column alignments - i tried my best to get them aligned)

TABLE: Banks

Bank_ID  Bank_Name  ABA_Code        Bank_City   Bank_State   Recip_Zip
1       Bank of NY  102938475    Albany        NY       12204
2       Bank of NJ  283746591    Englewood  NJ      07631
3       NY Federal  564738290    Albany     NY      12204
4       NJ Federal  123987475    Englewood  NJ      07631
5       NY Credit   654321789    Rochester  NY      14602

Banks consist of one or more accounts.

TABLE: Accounts

Acct_ID Acct_No          Bank_Id    Title
1            102938458  1   Subscriber Payment Acct
2            918273645  1   Provider Payment Acct
3            228374659  2   Zero Dollar Acct
4            374658291  2   BCB Provider Acct
5            234567891  3   BCB Subscriber Acct

Accounts are where the payment checks o electronic payments for drawn from.

TABLE: Recipients

Recip_ID   Recip_Type   Recip_No    Recip_Title   Recip_City    Recip_State   Recip_Zip
1         P          100001 Art Gains     Albany    NY       12204
2         S          220202 Mick Tean     Englewood NJ       07631
3         S          103003 Jake Adam     Binghamton    NY       13905
4         P          404004 Jane Doe         Rochester  NY       14602
5         P          977005 Jac Angel     Belvidere NJ       07823

Recipients (of a payment) can be Providers (P) or Subscribers (S)

TABLE: Payments

Payment_ID  Acct_ID Check_No    Check_Amt   Check_Date  Recip_No
1   1   102938475   6919.70 06/17/2011  100001
2   1   102938476   23.50   09/09/2011  220202
3   2   459287654   3.12    01/21/2012  404004
4   2   459287655   17345.01    07/07/2012  977005
5   3   102938476   300.47  02/11/2013  103003

Payments are master records and are the highest level of transaction. They consist of one or more claims.

TABLE: Claims

Claim_ID    Payment_Id  Claim_Amt   Claim_Date  Claim_Source
1   1   3459.85 06/01/2011  AA
2   1   3459.85 06/15/2011  AA
3   2   23.50   01/02/2012  AA
4   3   100.00  01/28/2012  BB
5   3   200.47  02/01/2013  BB

Claims consist of one or more Line items (services rendered by a provider)

TABLE: Claim Line Items

Item_ID Claim_ID    Start_Date  End_Date    Proc_Code   Proc_Desc   Billed_Amt
1   1   05/01/2011  05/01/2011  30.01   CRM NO  1230.25
2   1   05/01/2011  05/01/2011  30.02   ARB DD  1950.00
3   1   05/01/2011  05/01/2011  30.12   DSJ 01  279.60
4   2   12/15/2011  12/15/2011  99.98   LLL SS  729.95
5   2   12/15/2011  12/15/2011  99.99   IT2 GG  2729.90

Line Items are (services provided) the lowest transactional level detailing the procedure and cost.