CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Comprehensive income:
Net income	$ 542.6	$ 492.3	$ 490.1
Defined pension and other postretirement benefits:
Net pension and retiree medical (loss) gain arising during the period, net of tax of $(0.1), $0.3 and $(0.4), respectively	(0.4)	0.6	(0.5)
Reclassification of loss to net income, net of tax of $0.1, $0.1 and $0.1, respectively	0.2	0.2	0.1
Derivative instruments:
Net fair value increase, net of tax of $0	0.0	0.0	0.1
Reclassification of loss to net income, net of tax of $0.3, $0.3 and $0.6, respectively	0.8	0.7	0.9
Net fair value decrease, net of tax of $0	0.0	(0.1)	0.0
Other comprehensive income	0.6	1.4	0.6
Comprehensive income	$ 543.2	$ 493.7	$ 490.7