STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Comprehensive income:
Net income	$ 263.1	$ 213.3	$ 159.2
Defined pension and other postretirement benefits
Net pension and retiree medical loss arising during the period, net of tax of $(0.1), $0 and $0, respectively	(0.2)	0.0	0.0
Reclassification of loss to net income, net of tax of $0	0.2	0.0	0.1
Derivative instruments:
Reclassification of loss to net income, net of tax of $0	0.0	0.1	0.0
Other comprehensive income	0.0	0.1	0.1
Comprehensive income	$ 263.1	$ 213.4	$ 159.3