CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Comprehensive income:
Net income	$ 204.5	$ 207.1	$ 444.8	$ 463.1
Pension and retiree medical benefits:
Net pension and retiree medical losses arising during the period, net of tax of $0, $(0.1), $0 and $(0.1), respectively	0.0	(0.2)	0.0	(0.2)
Amortization of losses included in net periodic benefit cost, net of tax of $0, $0.1, $0 and $0.1, respectively	0.0	0.2	0.0	0.2
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Reclassification Adjustment, before Tax	0.0	0.0	0.0	0.0
Derivative instruments:
Reclassification of losses to net income, net of tax of $0.1, $0.1, $0.3 and $0.3, respectively	0.3	0.3	0.9	0.9
Other comprehensive income	0.3	0.3	0.9	0.9
Comprehensive income	$ 204.8	$ 207.4	$ 445.7	$ 464.0