Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($)
$ in Millions |
Sep. 30, 2016 |
Dec. 31, 2015 |
| Debt Securities [Member] |
|
|
| Investment Securities [Line Items] |
|
|
| Held to maturity securities - Cost |
$ 13
|
$ 11
|
| Held to maturity securities - Unrealized Gains |
1
|
|
| Held to maturity securities - Unrealized Losses |
|
|
| Held to maturity securities - Fair Value |
14
|
11
|
| CMS Energy Common Stock [Member] | Consumers Energy Company [Member] |
|
|
| Investment Securities [Line Items] |
|
|
| Available for sale - Cost |
4
|
4
|
| Available for sale - Unrealized Gains |
30
|
25
|
| Available for sale - Unrealized Losses |
|
|
| Available for sale - Fair Value |
34
|
29
|
| DB SERP [Member] | Mutual Fund [Member] |
|
|
| Investment Securities [Line Items] |
|
|
| Available for sale - Cost |
149
|
152
|
| Available for sale - Unrealized Gains |
|
|
| Available for sale - Unrealized Losses |
5
|
6
|
| Available for sale - Fair Value |
144
|
146
|
| DB SERP [Member] | Mutual Fund [Member] | Consumers Energy Company [Member] |
|
|
| Investment Securities [Line Items] |
|
|
| Available for sale - Cost |
107
|
108
|
| Available for sale - Unrealized Gains |
|
|
| Available for sale - Unrealized Losses |
4
|
4
|
| Available for sale - Fair Value |
$ 103
|
$ 104
|