diff --git a/checkout_sdk/common/enums.py b/checkout_sdk/common/enums.py
index 025dd12..b3b96af 100644
--- a/checkout_sdk/common/enums.py
+++ b/checkout_sdk/common/enums.py
@@ -550,6 +550,8 @@ class AccountHolderIdentificationType(str, Enum):
 
 
 class DocumentType(str, Enum):
+    """Identity documents. The document type accepted when verifying a bank account is a
+    separate enum, InstrumentDocumentType, because the API keeps the two apart."""
     PASSPORT = 'passport'
     NATIONAL_IDENTITY_CARD = 'national_identity_card'
     DRIVING_LICENSE = 'driving_license'
@@ -558,6 +560,13 @@ class DocumentType(str, Enum):
     ELECTORAL_ID = 'electoral_id'
 
 
+class InstrumentDocumentType(str, Enum):
+    """The document type accepted by the legal document that verifies a bank account when
+    creating a payment instrument. The API accepts only bank_statement here, which is also the
+    default, and rejects the identity document types in DocumentType."""
+    BANK_STATEMENT = 'bank_statement'
+
+
 class AccountChangeIndicatorType(str, Enum):
     THIS_TRANSACTION = "this_transaction"
     LESS_THAN_THIRTY_DAYS = "less_than_thirty_days"
diff --git a/tests/accounts/instrument_document_type_test.py b/tests/accounts/instrument_document_type_test.py
new file mode 100644
index 0000000..05cea4e
--- /dev/null
+++ b/tests/accounts/instrument_document_type_test.py
@@ -0,0 +1,19 @@
+from checkout_sdk.common.enums import DocumentType, InstrumentDocumentType
+
+
+def test_should_expose_bank_statement_for_instrument_documents():
+    assert InstrumentDocumentType.BANK_STATEMENT == 'bank_statement'
+    assert InstrumentDocumentType.BANK_STATEMENT.value == 'bank_statement'
+
+
+def test_should_be_the_only_accepted_instrument_document_type():
+    # The spec declares a single-value enum here, so anything else is a caller error rather
+    # than a value we forgot to add.
+    assert [d.value for d in InstrumentDocumentType] == ['bank_statement']
+
+
+def test_should_keep_bank_statement_out_of_the_identity_document_type():
+    # bank_statement belongs to the instrument document enum, not the identity one. The API
+    # keeps them separate, and this is what stops the two being merged the next time someone
+    # reports the value as missing from DocumentType.
+    assert 'bank_statement' not in [d.value for d in DocumentType]