CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
NET INCOME	$ 216.0	$ 211.6	$ 197.3
Pension & other benefits
Amounts arising during the year	(5.6)	(10.1)	1.2
Reclassifications to periodic cost	5.4	4.7	6.9
Deferrals to regulatory assets	0.2	5.3	(8.0)
Pension & other benefits costs, net of tax	0.0	(0.1)	0.1
OTHER COMPREHENSIVE INCOME (LOSS), NET OF TAX	0.0	(0.1)	0.1
TOTAL COMPREHENSIVE INCOME	$ 216.0	$ 211.5	$ 197.4