CONDENSED CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Millions	Total	Total Common Shareholders' Equity	Common stock	Additional paid in capital	Retained earnings	Accumulated other comprehensive income	Preferred stock of subsidiary	Noncontrolling interests
Balance at Dec. 31, 2017	$ 9,491.8	$ 9,461.4	$ 3.2	$ 4,278.5	$ 5,176.8	$ 2.9	$ 30.4	$ 0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholders	390.1	390.1	0.0	0.0	390.1	0.0	0.0	0.0
Other comprehensive income (loss)	1.7	1.7	0.0	0.0	0.0	1.7	0.0	0.0
Common stock dividends	(174.2)	(174.2)	0.0	0.0	(174.2)	0.0	0.0	0.0
Exercise of stock options	2.1	2.1	0.0	2.1	0.0	0.0	0.0	0.0
Purchase of common stock	(15.8)	(15.8)	0.0	(15.8)	0.0	0.0	0.0	0.0
Stock-based compensation and other	2.5	2.5	0.0	2.5	0.0	0.0	0.0	0.0
Balance at Mar. 31, 2018	9,698.2	9,667.8	3.2	4,267.3	5,392.7	4.6	30.4	0.0
Balance at Dec. 31, 2017	9,491.8	9,461.4	3.2	4,278.5	5,176.8	2.9	30.4	0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholders	854.3
Net loss attributed to noncontrolling interests	0.0
Other comprehensive income (loss)	(0.5)
Capital contributions from noncontrolling interest	0.0
Balance at Sep. 30, 2018	9,842.7	9,775.3	3.2	4,261.6	5,508.1	2.4	30.4	37.0
Balance at Mar. 31, 2018	9,698.2	9,667.8	3.2	4,267.3	5,392.7	4.6	30.4	0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholders	231.0	231.0	0.0	0.0	231.0	0.0	0.0	0.0
Other comprehensive income (loss)	(2.1)	(2.1)	0.0	0.0	0.0	(2.1)	0.0	0.0
Common stock dividends	(174.5)	(174.5)	0.0	0.0	(174.5)	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	(4.0)	(4.0)	0.0	(4.0)	0.0	0.0	0.0	0.0
Stock-based compensation and other	4.6	4.6	0.0	4.7	(0.1)	0.0	0.0	0.0
Balance at Jun. 30, 2018	9,756.2	9,725.8	3.2	4,271.0	5,449.1	2.5	30.4	0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholders	233.2	233.2	0.0	0.0	233.2	0.0	0.0	0.0
Net loss attributed to noncontrolling interests	0.0
Other comprehensive income (loss)	(0.1)	(0.1)	0.0	0.0	0.0	(0.1)	0.0	0.0
Common stock dividends	(174.3)	(174.3)	0.0	0.0	(174.3)	0.0	0.0	0.0
Exercise of stock options	8.8	8.8	0.0	8.8	0.0	0.0	0.0	0.0
Purchase of common stock	(22.2)	(22.2)	0.0	(22.2)	0.0	0.0	0.0	0.0
Acquisition of a noncontrolling interest	37.0	0.0	0.0	0.0	0.0	0.0	0.0	37.0
Stock-based compensation and other	4.1	4.1	0.0	4.0	0.1	0.0	0.0	0.0
Balance at Sep. 30, 2018	9,842.7	9,775.3	3.2	4,261.6	5,508.1	2.4	30.4	37.0
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholders	420.1	420.1	0.0	0.0	420.1	0.0	0.0	0.0
Other comprehensive income (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
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholders	890.1
Net loss attributed to noncontrolling interests	(0.5)
Other comprehensive income (loss)	(4.5)
Capital contributions from noncontrolling interest	14.6
Balance at Sep. 30, 2019	10,186.4	10,051.0	3.2	4,185.0	5,869.9	(7.1)	30.4	105.0
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholders	235.7	235.7	0.0	0.0	235.7	0.0	0.0	0.0
Net loss attributed to noncontrolling interests	(0.2)	0.0	0.0	0.0	0.0	0.0	0.0	(0.2)
Other comprehensive income (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
Capital contributions from noncontrolling interest	5.5	0.0	0.0	0.0	0.0	0.0	0.0	5.5
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	2.9	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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholders	234.3	234.3	0.0	0.0	234.3	0.0	0.0	0.0
Net loss attributed to noncontrolling interests	(0.3)	0.0	0.0	0.0	0.0	0.0	0.0	(0.3)
Other comprehensive income (loss)	(0.5)	(0.5)	0.0	0.0	0.0	(0.5)	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	16.0	16.0	0.0	16.0	0.0	0.0	0.0	0.0
Purchase of common stock	(31.9)	(31.9)	0.0	(31.9)	0.0	0.0	0.0	0.0
Capital contributions from noncontrolling interest	4.3	0.0	0.0	0.0	0.0	0.0	0.0	4.3
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	2.9	3.0	0.0	3.0	0.0	0.0	0.0	(0.1)
Balance at Sep. 30, 2019	$ 10,186.4	$ 10,051.0	$ 3.2	$ 4,185.0	$ 5,869.9	$ (7.1)	$ 30.4	$ 105.0