Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net income	$ 1,201.4	$ 1,134.7	$ 1,060.5
Defined benefit plans
Other comprehensive loss, net of tax	(2.7)	(1.5)	(5.5)
Comprehensive income	1,198.7	1,133.2	1,055.0
Preferred stock dividends of subsidiary	1.2	1.2	1.2
Comprehensive (income) loss attributed to noncontrolling interests	(0.3)	0.5	0.0
Comprehensive income attributed to common shareholders	1,197.2	1,132.5	1,053.8
Cash flow hedges
Derivatives accounted for as cash flow hedges
Net derivative loss, net of tax benefit of $1.6, $1.3, and $0.8, respectively	(4.3)	(3.5)	(2.1)
Reclassification of net (gain) loss to net income, net of tax	1.5	(0.8)	(1.2)
Cash flow hedges, net	(2.8)	(4.3)	(1.7)
Cash flow hedges | Cumulative Effect, Period of Adoption, Adjustment
Derivatives accounted for as cash flow hedges
Cumulative effect adjustment from adoption of ASU 2018-02	0.0	0.0	1.6
Defined benefit plans
Defined benefit plans
Pension and OPEB adjustments arising during the period, net of tax expense (benefit) of $(0.2), $1.0, and $(1.2), respectively	(0.5)	2.6	(3.1)
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	0.6	0.2	0.3
Defined benefit plans, net	0.1	2.8	(3.8)
Defined benefit plans | Cumulative Effect, Period of Adoption, Adjustment
Derivatives accounted for as cash flow hedges
Cumulative effect adjustment from adoption of ASU 2018-02	$ 0.0	$ 0.0	$ 1.0