Consolidated Statements of Common Equity (USD $) In Millions	Total	Common Stock [Member]	Other Paid In Capital [Member]	Retained Earnings [Member]
Beginning - Balance at Dec. 31, 2009	$ 3,566.9	$ 2.3	$ 755.8	$ 2,808.8
Net income	456.5			456.5
Common stock cash dividends of $0.80, $1.04, and $1.20 per share for Year ended 2010, 2011 and 2012, respectively	(187.0)			(187.0)
Exercise of stock options	90.9		90.9
Purchase of common stock	(156.6)		(156.6)
Tax benefit from share based compensation	21.9		21.9
Stock-based compensation and other	9.5		9.5
Ending - Balance at Dec. 31, 2010	3,802.1	2.3	721.5	3,078.3
Net income	526.2			526.2
Common stock cash dividends of $0.80, $1.04, and $1.20 per share for Year ended 2010, 2011 and 2012, respectively	(242.0)			(242.0)
Exercise of stock options	54.4		54.4
Purchase of common stock	(193.9)		(193.9)
Tax benefit from share based compensation	11.9		11.9
Stock-based compensation and other	4.6		4.6
Ending - Balance at Dec. 31, 2011	3,963.3	2.3	598.5	3,362.5
Net income	546.3			546.3
Common stock cash dividends of $0.80, $1.04, and $1.20 per share for Year ended 2010, 2011 and 2012, respectively	(276.3)			(276.3)
Exercise of stock options	49.8		49.8
Purchase of common stock	(153.2)		(153.2)
Stock-based compensation and other	5.2		5.2
Ending - Balance at Dec. 31, 2012	$ 4,135.1	$ 2.3	$ 500.3	$ 3,632.5