Consolidated Statements of Changes in Equity (USD $) In Millions, unless otherwise specified	Total	$25 Par Preferred Stock [Member]	$.01 Par Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interest [Member]	$25 Par Preferred Stock [Member]	$25 Par Preferred Stock [Member] Retained Earnings [Member]
Balance at Dec. 31, 2011	$ 3,058.7	$ 6.1	$ 1.1	$ 884.2	$ 1,875.3	$ (2.2)	$ 294.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	379.4				377.3		2.1
Other comprehensive income (loss)	(0.2)					(0.2)
Contributions from noncontrolling interest	7.8						7.8
Dividends on common stock	(85.9)				(85.9)
Dividends on preferred stock								(0.2)	(0.2)
Options exercised and stock subscribed, net of shares withheld for employee taxes	(1.1)			(1.1)
Excess tax benefit from share-based compensation	31.5			31.5
Share-based compensation	10.7			10.7
Balance at Dec. 31, 2012	3,400.7	6.1	1.1	925.3	2,166.5	(2.4)	304.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	353.3				351.4		1.9
Other comprehensive income (loss)	0.4					0.4
Dividends on common stock	(94.8)				(94.8)
Dividends on preferred stock								(0.2)	(0.2)
Options exercised and stock subscribed, net of shares withheld for employee taxes	(0.6)			(0.6)
Excess tax benefit from share-based compensation	4.2			4.2
Share-based compensation	13.6			13.6
Balance at Dec. 31, 2013	3,676.6	6.1	1.1	942.5	2,422.9	(2.0)	306.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	504.3				502.6		1.7
Other comprehensive income (loss)	(1.2)					(1.2)
Contributions from noncontrolling interest	0.9						0.9
Dividends on common stock	(123.6)				(123.6)
Dividends on preferred stock								(0.2)	(0.2)
Options exercised and stock subscribed, net of shares withheld for employee taxes	(2.0)			(2.0)
Excess tax benefit from share-based compensation	(0.7)			(0.7)
Share-based compensation	10.0			10.0
Balance at Dec. 31, 2014	$ 4,064.1	$ 6.1	$ 1.1	$ 949.8	$ 2,801.7	$ (3.2)	$ 308.6