Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($)
$ in Millions |
3 Months Ended |
9 Months Ended |
Sep. 30, 2016 |
Sep. 30, 2015 |
Sep. 30, 2016 |
Sep. 30, 2015 |
| DB Pension Plan [Member] |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
$ 10
|
$ 13
|
$ 31
|
$ 37
|
| Interest expense |
21
|
26
|
64
|
77
|
| Expected return on plan assets |
(36)
|
(35)
|
(110)
|
(104)
|
| Amortization of Net loss |
17
|
22
|
52
|
67
|
| Amortization of Prior service cost (credit) |
1
|
|
3
|
1
|
| Net periodic cost (credit) |
13
|
26
|
40
|
78
|
| DB Pension Plan [Member] | Consumers Energy Company [Member] |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
11
|
12
|
31
|
36
|
| Interest expense |
20
|
25
|
62
|
74
|
| Expected return on plan assets |
(35)
|
(34)
|
(107)
|
(101)
|
| Amortization of Net loss |
16
|
22
|
50
|
66
|
| Amortization of Prior service cost (credit) |
1
|
|
3
|
1
|
| Net periodic cost (credit) |
13
|
25
|
39
|
76
|
| OPEB [Member] |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
5
|
7
|
14
|
19
|
| Interest expense |
12
|
14
|
35
|
43
|
| Expected return on plan assets |
(22)
|
(23)
|
(65)
|
(68)
|
| Amortization of Net loss |
5
|
5
|
16
|
16
|
| Amortization of Prior service cost (credit) |
(10)
|
(10)
|
(31)
|
(31)
|
| Net periodic cost (credit) |
(10)
|
(7)
|
(31)
|
(21)
|
| OPEB [Member] | Consumers Energy Company [Member] |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
4
|
7
|
13
|
19
|
| Interest expense |
12
|
14
|
34
|
42
|
| Expected return on plan assets |
(20)
|
(22)
|
(60)
|
(65)
|
| Amortization of Net loss |
5
|
6
|
16
|
17
|
| Amortization of Prior service cost (credit) |
(10)
|
(10)
|
(30)
|
(30)
|
| Net periodic cost (credit) |
$ (9)
|
$ (5)
|
$ (27)
|
$ (17)
|