Leases - Lessor Leases Narrative (Details) - USD ($) $ in Millions		3 Months Ended
	Mar. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Power Sales Agreement
Lessor, Lease, Description [Line Items]
Leasing income		$ 48
Variable lease income		$ 34
Power Sales Agreement | Minimum
Lessor, Lease, Description [Line Items]
Operating leases remaining term	7 years
Power Sales Agreement | Maximum
Lessor, Lease, Description [Line Items]
Operating leases remaining term	24 years
Natural Gas Processing Plant | Consumers Energy Company
Lessor, Lease, Description [Line Items]
Direct financing lease term	20 years	20 years
CMS Energy Subsidiary | Natural Gas Transportation Agreement | Consumers Energy Company
Lessor, Lease, Description [Line Items]
Direct financing lease term			20 years