Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income	$ 1,060.5	$ 1,204.9	$ 940.2
Other comprehensive loss, net of tax	(5.5)	0.0	(1.7)
Comprehensive income	1,055.0	1,204.9	938.5
Preferred stock dividends of subsidiary	1.2	1.2	1.2
Comprehensive income attributed to common shareholders	1,053.8	1,203.7	937.3
Cash flow hedges
Other comprehensive (loss) income, net of tax
Cumulative effect adjustment from adoption of ASU 2018-02	1.6	0.0	0.0
Derivatives accounted for as cash flow hedges
Net derivative losses, net of tax	(2.1)	0.0	0.0
Reclassification of net gains to net income, net of tax	(1.2)	(1.3)	(1.3)
Cash flow hedges, net	(1.7)	(1.3)	(1.3)
Defined benefit plans
Other comprehensive (loss) income, net of tax
Cumulative effect adjustment from adoption of ASU 2018-02	(1.0)	0.0	0.0
Defined benefit plans
Pension and OPEB adjustments arising during the period, net of tax of $(1.2), $0.6, and $0.1, respectively	(3.1)	0.9	(0.8)
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	0.3	0.4	0.4
Defined benefit plans, net	$ (3.8)	$ 1.3	$ (0.4)