Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($)
$ in Millions |
Sep. 30, 2015 |
Dec. 31, 2014 |
| Debt Securities [Member] |
|
|
| Investment Securities [Line Items] |
|
|
| Held to maturity securities - Cost |
$ 12
|
$ 11
|
| Held to maturity securities - Unrealized Losses |
|
|
| Held to maturity securities - Fair Value |
$ 12
|
$ 11
|
| CMS Energy Common Stock [Member] | Consumers Energy Company [Member] |
|
|
| Investment Securities [Line Items] |
|
|
| Available for sale - Cost |
4
|
5
|
| Available for sale - Unrealized Gains |
$ 24
|
$ 33
|
| Available for sale - Unrealized Losses |
|
|
| Available for sale - Fair Value |
$ 28
|
$ 38
|
| DB SERP [Member] | Mutual Fund [Member] |
|
|
| Investment Securities [Line Items] |
|
|
| Available for sale - Cost |
154
|
129
|
| Available for sale - Unrealized Losses |
7
|
2
|
| Available for sale - Fair Value |
147
|
127
|
| DB SERP [Member] | Mutual Fund [Member] | Consumers Energy Company [Member] |
|
|
| Investment Securities [Line Items] |
|
|
| Available for sale - Cost |
109
|
92
|
| Available for sale - Unrealized Losses |
4
|
2
|
| Available for sale - Fair Value |
$ 105
|
$ 90
|