CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Comprehensive income:
Net income	$ 551.7	$ 494.1	$ 463.5
Pension and retiree medical benefits:
Net pension and retiree medical losses arising during the period, net of tax of $0, $0, and ($0.1), respectively	0.0	0.0	(0.2)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax	0.0	0.0	(0.2)
Derivative instruments:
Reclassification of losses to net income, net of tax of $0.4, $0.6, and $0.7, respectively	1.2	1.0	1.0
Total derivative instruments, net of tax	1.2	1.0	1.0
Other comprehensive income	1.2	1.0	0.8
Comprehensive income	$ 552.9	$ 495.1	$ 464.3