Property, Plant, and Equipment (Details) $ in Millions	Jan. 01, 2017 USD ($) mile	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Less: Accumulated depreciation		$ 3,619.6	$ 3,461.9
Total property, plant and equipment		9,832.3	9,767.5
Subsequent event | UMERC Transfer
Transfer of Assets to UMERC
Miles of electric distribution lines transferred | mile	2,500
Net book value of property, plant, and equipment transferred	$ 83.0
Utility Operations
Property, Plant and Equipment [Line Items]
Property, plant and equipment		11,232.9	10,863.1
Less: Accumulated depreciation		3,606.9	3,447.2
Net		7,626.0	7,415.9
Construction work in progress		111.5	170.3
Total property, plant and equipment		7,737.5	7,586.2
Nonutility Operations
Property, Plant and Equipment [Line Items]
Property, plant and equipment		46.4	54.0
Less: Accumulated depreciation		12.7	14.7
Net		33.7	39.3
Construction work in progress		0.9	0.3
Total property, plant and equipment		34.6	39.6
Nonutility Operations | Capital Leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment		2,898.0	2,876.7
Less: Accumulated depreciation		837.8	735.0
Total property, plant and equipment		$ 2,060.2	$ 2,141.7