Leases - Assets and Liabilities of Lessee (Details) $ in Millions	Sep. 30, 2019 USD ($)
Operating leases
Right-of-use assets	$ 49
Lease liabilities
Current lease liabilities	9
Noncurrent lease liabilities	39
Finance lease liability	69
Finance leases
Right-of-use assets	74
Lease liabilities
Current lease liabilities	7
Non-current lease liabilities	$ 62
Weighted-average remaining lease term (in years)
Operating leases	16 years
Finance leases	12 years
Weighted-average discount rate
Operating leases	3.70%
Finance leases	1.90%
Consumers Energy Company
Operating leases
Right-of-use assets	$ 41
Lease liabilities
Current lease liabilities	8
Noncurrent lease liabilities	33
Finance lease liability	69
Finance leases
Right-of-use assets	74
Lease liabilities
Current lease liabilities	7
Non-current lease liabilities	$ 62
Weighted-average remaining lease term (in years)
Operating leases	14 years
Finance leases	12 years
Weighted-average discount rate
Operating leases	3.70%
Finance leases	1.90%
Related Party Lease
Lease liabilities
Finance lease liability	$ 25
Lease liabilities
Current lease liabilities	1
Related Party Lease | Consumers Energy Company
Lease liabilities
Finance lease liability	25
Lease liabilities
Current lease liabilities	$ 1