package aws

import (
	"github.com/aws/aws-sdk-go/aws/arn"
	"github.com/hashicorp/terraform/helper/schema"
)

// See http://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/billing-getting-started.html#step-2
var billingAccountId = "386209384616"

func dataSourceAwsBillingServiceAccount() *schema.Resource {
	return &schema.Resource{
		Read: dataSourceAwsBillingServiceAccountRead,

		Schema: map[string]*schema.Schema{
			"arn": {
				Type:     schema.TypeString,
				Computed: true,
			},
		},
	}
}

func dataSourceAwsBillingServiceAccountRead(d *schema.ResourceData, meta interface{}) error {
	d.SetId(billingAccountId)
	arn := arn.ARN{
		Partition: meta.(*AWSClient).partition,
		Service:   "iam",
		AccountID: billingAccountId,
		Resource:  "root",
	}
	d.Set("arn", arn.String())

	return nil
}

package aws

import (
	"fmt"

	"github.com/hashicorp/terraform/helper/schema"
)

// See http://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/billing-getting-started.html#step-2
var billingAccountId = "386209384616"

func dataSourceAwsBillingServiceAccount() *schema.Resource {
	return &schema.Resource{
		Read: dataSourceAwsBillingServiceAccountRead,

		Schema: map[string]*schema.Schema{
			"arn": {
				Type:     schema.TypeString,
				Computed: true,
			},
		},
	}
}

func dataSourceAwsBillingServiceAccountRead(d *schema.ResourceData, meta interface{}) error {
	d.SetId(billingAccountId)
	d.Set("arn", fmt.Sprintf("arn:%s:iam::%s:root", meta.(*AWSClient).partition, billingAccountId))

	return nil
}

--- 
+++ 
@@ -1,8 +1,7 @@
 package aws
 
 import (
-	"fmt"
-
+	"github.com/aws/aws-sdk-go/aws/arn"
 	"github.com/hashicorp/terraform/helper/schema"
 )
 
@@ -24,7 +23,13 @@
 
 func dataSourceAwsBillingServiceAccountRead(d *schema.ResourceData, meta interface{}) error {
 	d.SetId(billingAccountId)
-	d.Set("arn", fmt.Sprintf("arn:%s:iam::%s:root", meta.(*AWSClient).partition, billingAccountId))
+	arn := arn.ARN{
+		Partition: meta.(*AWSClient).partition,
+		Service:   "iam",
+		AccountID: billingAccountId,
+		Resource:  "root",
+	}
+	d.Set("arn", arn.String())
 
 	return nil
 }
Use AWS ARN structure - aws_billing_service_account data source.
