PROPERTY, PLANT, AND EQUIPMENT (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Property, plant and equipment
Less: Accumulated depreciation	$ 1,483.1	$ 1,336.0
Property, plant, and equipment, net	2,887.7	2,353.0
Utility operations
Property, plant and equipment
Property, plant, and equipment, gross	4,019.1	3,543.6
Less: Accumulated depreciation	1,436.8	1,327.2
Net property, plant, and equipment excluding construction work in progress	2,582.3	2,216.4
Construction work in progress	285.2	130.2
Plant to be retired, net	14.4	0
Property, plant, and equipment, net	2,881.9	2,346.6
Utility operations | Electric Utility
Property, plant and equipment
Property, plant, and equipment, gross	3,289.2	2,841.7
Utility operations | Natural Gas Utility
Property, plant and equipment
Property, plant, and equipment, gross	729.9	701.9
Nonutility operations | Other
Property, plant and equipment
Property, plant, and equipment, gross	15.2	15.2
Less: Accumulated depreciation	9.4	8.8
Property, plant, and equipment, net	$ 5.8	$ 6.4