GOODWILL AND OTHER INTANGIBLE ASSETS - GOODWILL (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Apr. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Changes in the gross carrying amount of goodwill and accumulated impairment losses
Balance at the beginning of the period, gross goodwill		$ 953.1	$ 949.3
Balance at the beginning of the period, accumulated impairment losses		(297.7)	(297.7)
Balance at the beginning of the period, net goodwill		655.4	651.6
Adjustment to ITF intellectual property		0	3.8
Balance at the end of the period, gross goodwill		953.1	953.1
Balance at the end of the period, accumulated impairment losses		(297.7)	(297.7)
Balance at the end of the period, net goodwill		655.4	655.4
Goodwill impairment loss	0
Natural Gas Utility
Changes in the gross carrying amount of goodwill and accumulated impairment losses
Balance at the beginning of the period, gross goodwill		933.5	933.5
Balance at the beginning of the period, accumulated impairment losses		(297.7)	(297.7)
Balance at the beginning of the period, net goodwill		635.8	635.8
Adjustment to ITF intellectual property		0	0
Balance at the end of the period, gross goodwill		933.5	933.5
Balance at the end of the period, accumulated impairment losses		(297.7)	(297.7)
Balance at the end of the period, net goodwill		635.8	635.8
Holding Company and Other
Changes in the gross carrying amount of goodwill and accumulated impairment losses
Balance at the beginning of the period, gross goodwill		19.6	15.8
Balance at the beginning of the period, accumulated impairment losses		0	0
Balance at the beginning of the period, net goodwill		19.6	15.8
Adjustment to ITF intellectual property		0	3.8
Balance at the end of the period, gross goodwill		19.6	19.6
Balance at the end of the period, accumulated impairment losses		0	0
Balance at the end of the period, net goodwill		$ 19.6	$ 19.6