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 Interest
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, 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