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	621.1
Other comprehensive income (loss)	(0.4)
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
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
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	655.8
Other comprehensive income (loss)	(4.0)
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
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
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	5.4	0.0	0.0	0.0	0.0	0.0	0.0	5.4
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	2.8	2.8	0.0	2.8	0.0	0.0	0.0	0.0
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