CONDENSED CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Millions	Total	Total common shareholders' equity	Common stock	Additional paid in capital	Retained earnings	Accumulated other comprehensive loss	Preferred stock of subsidiary	Noncontrolling interests
Balance at Dec. 31, 2019	$ 10,254.6	$ 10,113.4	$ 3.2	$ 4,186.6	$ 5,927.7	$ (4.1)	$ 30.4	$ 110.8
Statements of equity
Net income attributed to common shareholders	452.5	452.5	0.0	0.0	452.5	0.0	0.0	0.0
Net loss attributable to noncontrolling interests	(0.2)	0.0	0.0	0.0	0.0	0.0	0.0	(0.2)
Other comprehensive income (loss)	(3.0)	(3.0)	0.0	0.0	0.0	(3.0)	0.0	0.0
Common stock dividends	(199.5)	(199.5)	0.0	0.0	(199.5)	0.0	0.0	0.0
Exercise of stock options	16.0	16.0	0.0	16.0	0.0	0.0	0.0	0.0
Purchase of common stock	(40.4)	(40.4)	0.0	(40.4)	0.0	0.0	0.0	0.0
Distributions to noncontrolling interests	(0.5)	0.0	0.0	0.0	0.0	0.0	0.0	(0.5)
Stock-based compensation and other	5.1	5.1	0.0	5.1	0.0	0.0	0.0	0.0
Balance at Mar. 31, 2020	10,484.6	10,344.1	3.2	4,167.3	6,180.7	(7.1)	30.4	110.1
Balance at Dec. 31, 2020	10,662.5	10,469.7	3.2	4,143.7	6,329.6	(6.8)	30.4	162.4
Statements of equity
Net income attributed to common shareholders	510.1	510.1	0.0	0.0	510.1	0.0	0.0	0.0
Net loss attributable to noncontrolling interests	(0.1)	0.0	0.0	0.0	0.0	0.0	0.0	(0.1)
Other comprehensive income (loss)	1.1	1.1	0.0	0.0	0.0	1.1	0.0	0.0
Common stock dividends	(213.7)	(213.7)	0.0	0.0	(213.7)	0.0	0.0	0.0
Exercise of stock options	1.2	1.2	0.0	1.2	0.0	0.0	0.0	0.0
Purchase of common stock	(6.6)	(6.6)	0.0	(6.6)	0.0	0.0	0.0	0.0
Acquisition of a noncontrolling interest	6.2	0.0	0.0	0.0	0.0	0.0	0.0	6.2
Capital contributions from noncontrolling interest	2.0	0.0	0.0	0.0	0.0	0.0	0.0	2.0
Distributions to noncontrolling interests	(0.4)	0.0	0.0	0.0	0.0	0.0	0.0	(0.4)
Stock-based compensation and other	5.3	5.3	0.0	5.3	0.0	0.0	0.0	0.0
Balance at Mar. 31, 2021	$ 10,967.6	$ 10,767.1	$ 3.2	$ 4,143.6	$ 6,626.0	$ (5.7)	$ 30.4	$ 170.1