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 income (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, 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	960.9
Net income (loss) attributable to noncontrolling interests	0.0
Other comprehensive income (loss)	(2.3)
Balance at Sep. 30, 2020	10,559.1	10,450.7	3.2	4,163.8	6,290.1	(6.4)	30.4	78.0
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
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) attributable to noncontrolling interests	0.2	0.0	0.0	0.0	0.0	0.0	0.0	0.2
Other comprehensive income (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
Statements of equity
Net income attributed to common shareholders	266.8	266.8	0.0	0.0	266.8	0.0	0.0	0.0
Net income (loss) attributable to noncontrolling interests	0.0
Other comprehensive income (loss)	0.9	0.9	0.0	0.0	0.0	0.9	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	3.0	3.0	0.0	3.0	0.0	0.0	0.0	0.0
Purchase of common stock	(6.4)	(6.4)	0.0	(6.4)	0.0	0.0	0.0	0.0
Distributions to noncontrolling interests	(0.6)	0.0	0.0	0.0	0.0	0.0	0.0	(0.6)
Stock-based compensation and other	2.2	2.2	0.0	2.2	0.0	0.0	0.0	0.0
Balance at Sep. 30, 2020	10,559.1	10,450.7	3.2	4,163.8	6,290.1	(6.4)	30.4	78.0
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 income (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
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	1,076.1
Net income (loss) attributable to noncontrolling interests	(2.3)
Other comprehensive income (loss)	3.2
Balance at Sep. 30, 2021	11,106.6	10,908.5	3.2	4,144.4	6,764.5	(3.6)	30.4	167.7
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
Statements of equity
Net income attributed to common shareholders	276.0	276.0	0.0	0.0	276.0	0.0	0.0	0.0
Net income (loss) attributable to noncontrolling interests	(0.6)	0.0	0.0	0.0	0.0	0.0	0.0	(0.6)
Other comprehensive income (loss)	1.1	1.1	0.0	0.0	0.0	1.1	0.0	0.0
Common stock dividends	(213.8)	(213.8)	0.0	0.0	(213.8)	0.0	0.0	0.0
Exercise of stock options	2.8	2.8	0.0	2.8	0.0	0.0	0.0	0.0
Purchase of common stock	(4.7)	(4.7)	0.0	(4.7)	0.0	0.0	0.0	0.0
Capital contributions from noncontrolling interest	0.5	0.0	0.0	0.0	0.0	0.0	0.0	0.5
Distributions to noncontrolling interests	(0.9)	0.0	0.0	0.0	0.0	0.0	0.0	(0.9)
Stock-based compensation and other	2.4	2.4	0.0	2.4	0.0	0.0	0.0	0.0
Balance at Jun. 30, 2021	11,030.4	10,830.9	3.2	4,144.1	6,688.2	(4.6)	30.4	169.1
Statements of equity
Net income attributed to common shareholders	290.0	290.0	0.0	0.0	290.0	0.0	0.0	0.0
Net income (loss) attributable to noncontrolling interests	(1.6)	0.0	0.0	0.0	0.0	0.0	0.0	(1.6)
Other comprehensive income (loss)	1.0	1.0	0.0	0.0	0.0	1.0	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	2.5	2.5	0.0	2.5	0.0	0.0	0.0	0.0
Purchase of common stock	(4.4)	(4.4)	0.0	(4.4)	0.0	0.0	0.0	0.0
Capital contributions from noncontrolling interest	2.2	0.0	0.0	0.0	0.0	0.0	0.0	2.2
Distributions to noncontrolling interests	(2.0)	0.0	0.0	0.0	0.0	0.0	0.0	(2.0)
Stock-based compensation and other	2.2	2.2	0.0	2.2	0.0	0.0	0.0	0.0
Balance at Sep. 30, 2021	$ 11,106.6	$ 10,908.5	$ 3.2	$ 4,144.4	$ 6,764.5	$ (3.6)	$ 30.4	$ 167.7