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, 2018	$ 9,842.7	$ 9,788.9	$ 3.2	$ 4,250.1	$ 5,538.2	$ (2.6)	$ 30.4	$ 23.4
Statements of equity
Net income attributed to common shareholders	420.1	420.1	0.0	0.0	420.1	0.0	0.0	0.0
Other comprehensive loss	(1.4)	(1.4)	0.0	0.0	0.0	(1.4)	0.0	0.0
Common stock dividends	(186.2)	(186.2)	0.0	0.0	(186.2)	0.0	0.0	0.0
Exercise of stock options	32.6	32.6	0.0	32.6	0.0	0.0	0.0	0.0
Purchase of common stock	(70.7)	(70.7)	0.0	(70.7)	0.0	0.0	0.0	0.0
Acquisition of a noncontrolling interest	69.0	0.0	0.0	0.0	0.0	0.0	0.0	69.0
Non-cash capital contributions from noncontrolling interest	4.8	0.0	0.0	0.0	0.0	0.0	0.0	4.8
Stock-based compensation and other	1.2	1.2	0.0	1.2	0.0	0.0	0.0	0.0
Balance at Mar. 31, 2019	10,112.1	9,984.5	3.2	4,213.2	5,772.1	(4.0)	30.4	97.2
Balance at Dec. 31, 2018	9,842.7	9,788.9	3.2	4,250.1	5,538.2	(2.6)	30.4	23.4
Statements of equity
Net income attributed to common shareholders	655.8
Net income (loss) attributed to noncontrolling interests	(0.2)
Other comprehensive loss	(4.0)
Non-cash capital contributions from noncontrolling interest	10.2
Balance at Jun. 30, 2019	10,148.3	10,016.2	3.2	4,197.9	5,821.7	(6.6)	30.4	101.7
Balance at Mar. 31, 2019	10,112.1	9,984.5	3.2	4,213.2	5,772.1	(4.0)	30.4	97.2
Statements of equity
Net income attributed to common shareholders	235.7	235.7	0.0	0.0	235.7	0.0	0.0	0.0
Net income (loss) attributed to noncontrolling interests	(0.2)	0.0	0.0	0.0	0.0	0.0	0.0	(0.2)
Other comprehensive loss	(2.6)	(2.6)	0.0	0.0	0.0	(2.6)	0.0	0.0
Common stock dividends	(186.1)	(186.1)	0.0	0.0	(186.1)	0.0	0.0	0.0
Exercise of stock options	17.5	17.5	0.0	17.5	0.0	0.0	0.0	0.0
Purchase of common stock	(35.6)	(35.6)	0.0	(35.6)	0.0	0.0	0.0	0.0
Distributions to noncontrolling interests	(0.9)	0.0	0.0	0.0	0.0	0.0	0.0	(0.9)
Non-cash capital contributions from noncontrolling interest	5.5	0.0	0.0	0.0	0.0	0.0	0.0	5.5
Stock-based compensation and other	2.9	2.8	0.0	2.8	0.0	0.0	0.0	0.1
Balance at Jun. 30, 2019	10,148.3	10,016.2	3.2	4,197.9	5,821.7	(6.6)	30.4	101.7
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 income (loss) attributed to noncontrolling interests	(0.2)	0.0	0.0	0.0	0.0	0.0	0.0	(0.2)
Other comprehensive 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, 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	694.1
Net income (loss) attributed to noncontrolling interests	0.0
Other comprehensive loss	(3.2)
Non-cash capital contributions from noncontrolling interest	0.0
Balance at Jun. 30, 2020	10,492.7	10,383.7	3.2	4,165.0	6,222.8	(7.3)	30.4	78.6
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
Statements of equity
Net income attributed to common shareholders	241.6	241.6	0.0	0.0	241.6	0.0	0.0	0.0
Net income (loss) attributed to noncontrolling interests	0.2	0.0	0.0	0.0	0.0	0.0	0.0	0.2
Other comprehensive loss	(0.2)	(0.2)	0.0	0.0	0.0	(0.2)	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	4.3	4.3	0.0	4.3	0.0	0.0	0.0	0.0
Purchase of common stock	(9.9)	(9.9)	0.0	(9.9)	0.0	0.0	0.0	0.0
Purchase of additional ownership interest in Upstream from noncontrolling interest	(31.0)	0.0	0.0	0.0	0.0	0.0	0.0	(31.0)
Distributions to noncontrolling interests	(0.7)	0.0	0.0	0.0	0.0	0.0	0.0	(0.7)
Stock-based compensation and other	3.3	3.3	0.0	3.3	0.0	0.0	0.0	0.0
Balance at Jun. 30, 2020	$ 10,492.7	$ 10,383.7	$ 3.2	$ 4,165.0	$ 6,222.8	$ (7.3)	$ 30.4	$ 78.6