PROPERTY, PLANT, AND EQUIPMENT (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property, plant, and equipment
Less: Accumulated depreciation	$ 3,343.1	$ 3,221.0
Property, plant and equipment, net	6,859.8	6,206.2
Holding Company and Other | Nonutility operations
Property, plant, and equipment
Property, plant, and equipment, gross	144.6	131.1
Less: Accumulated depreciation	81.1	80.4
Net property, plant, and equipment excluding construction work in progress	63.5	50.7
Construction work in progress	73.9	38.0
Property, plant and equipment, net	137.4	88.7
Holding Company and Other | Nonregulated operations
Property, plant, and equipment
Property, plant, and equipment, gross	173.9	130.5
Less: Accumulated depreciation	39.5	30.5
Net property, plant, and equipment excluding construction work in progress	134.4	100.0
Construction work in progress	10.4	7.1
Property, plant and equipment, net	144.8	107.1
Holding Company and Other | Nonregulated operations | PDI
Property, plant, and equipment
Property, plant, and equipment, gross	140.2	109.8
Holding Company and Other | Nonregulated operations | Other nonregulated
Property, plant, and equipment
Property, plant, and equipment, gross	33.7	20.7
Regulated operations
Property, plant, and equipment
Property, plant, and equipment, gross	9,399.2	8,717.7
Less: Accumulated depreciation	3,185.9	3,073.2
Net property, plant, and equipment excluding construction work in progress	6,213.3	5,644.5
Construction work in progress	351.8	351.5
Plant to be retired, net	12.5	14.4
Property, plant and equipment, net	6,577.6	6,010.4
Regulated operations | Electric Utility
Property, plant, and equipment
Property, plant, and equipment, gross	3,587.4	3,289.2
Regulated operations | Natural Gas Utility
Property, plant, and equipment
Property, plant, and equipment, gross	$ 5,811.8	$ 5,428.5