Consolidated Statements of Changes in Equity - USD ($) $ in Millions	Total	$25 Par Preferred Stock [Member]	$25 Par Preferred Stock [Member]	$.01 Par Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Retained Earnings [Member] $25 Par Preferred Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Non-controlling Interest [Member]
Balance at Dec. 31, 2017	$ 4,865.4		$ 6.1	$ 1.0	$ 943.3	$ 3,611.4		$ (12.9)	$ 316.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification due to adoption of ASU 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income	0.0					0.7		(0.7)
Net income	629.4					627.4			2.0
Other comprehensive income (loss)	2.7							2.7
Contributions from noncontrolling interest	1.2								1.2
Dividends on common stock	(146.7)				0.0	(146.7)
Dividends on preferred stock		$ (0.2)					$ (0.2)
Share repurchases	(243.5)		(0.4)	0.0	(21.1)	(222.0)
Options exercised and stock subscribed, net of shares withheld for employee taxes	4.0			0.0	4.0
Share-based compensation	20.4				20.4
Balance at Dec. 31, 2018	5,132.7		5.7	1.0	946.6	3,870.6		(10.9)	319.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	540.8					538.9			1.9
Other comprehensive income (loss)	(18.2)							(18.2)
Contributions from noncontrolling interest	1.8								1.8
Dividends on common stock	(146.5)				0.0	(146.5)
Dividends on preferred stock		(0.2)					(0.2)
Share repurchases	(710.0)		(0.1)	0.0	(48.4)	(661.5)
Forward contract for accelerated share repurchases	(82.5)				(82.5)
Options exercised and stock subscribed, net of shares withheld for employee taxes	4.1			0.0	4.1
Share-based compensation	23.9				23.9
Balance at Dec. 31, 2019	4,745.9		5.6	1.0	843.7	3,601.3		(29.1)	323.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	619.1					617.0			2.1
Other comprehensive income (loss)	29.5							29.5
Contributions from noncontrolling interest	0.9								0.9
Dividends on common stock	(153.7)				0.0	(153.7)
Dividends on preferred stock		(0.2)					$ (0.2)
Share repurchases	(896.4)	$ (0.2)	(0.2)	(0.1)	(51.3)	(844.8)
Forward contract for accelerated share repurchases	(75.0)				(75.0)
Settlement of forward contract for accelerated share repurchases	82.5				82.5
Options exercised and stock subscribed, net of shares withheld for employee taxes	6.2			0.0	6.2
Share-based compensation	24.8				24.8
Balance at Dec. 31, 2020	$ 4,383.6		$ 5.4	$ 0.9	$ 830.9	$ 3,219.6		$ 0.4	$ 326.4