Notes Receivable (Changes of Allowance for Loan Losses) (Details) (EnerBank USA [Member], USD $) In Millions | 3 Months Ended | 9 Months Ended |
|---|---|---|
Sep. 30, 2011 | Sep. 30, 2011 | |
EnerBank USA [Member] | ||
| Allowance for loan losses, at beginning of period | $ 5 | $ 5 |
| Charge-offs | (1) | (4) |
| Recoveries | 1 | |
| Provision for loan losses | 1 | 3 |
| Allowance for loan losses, at end of period | $ 5 | $ 5 |