Consolidated Statements of Common Equity (USD $) In Millions, unless otherwise specified	Total	Common Stock [Member]	Other Paid In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Stock Options Exercisable [Member]
Beginning - Balance at Dec. 31, 2008	$ 3,336.9	$ 2.3	$ 751.2	$ 2,584.2	$ (0.9)	$ 0.1
Net income	382.4			382.4
Other comprehensive income
Hedging, net	0.4				0.4
Comprehensive income	382.8	0	0	382.4	0.4	0
Common stock cash dividends of $0.675, $0.80 and $1.04 per share for Year ended 2009, 2010 and 2011, respectively	(157.8)			(157.8)
Exercise of stock options	17.0		17.0
Purchase of common stock	(29.6)		(29.6)
Tax benefit from share based compensation	6.3		6.3
Stock-based compensation and other	11.3		11.4	0		(0.1)
Ending - Balance at Dec. 31, 2009	3,566.9	2.3	756.3	2,808.8	(0.5)	0
Net income	456.5			456.5
Other comprehensive income
Hedging, net	0.4				0.4
Comprehensive income	456.9	0	0	456.5	0.4	0
Common stock cash dividends of $0.675, $0.80 and $1.04 per share for Year ended 2009, 2010 and 2011, 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	9.1		9.1
Ending - Balance at Dec. 31, 2010	3,802.1	2.3	721.6	3,078.3	(0.1)	0
Net income	526.2			526.2
Other comprehensive income
Hedging, net	0.2				0.2
Comprehensive income	526.4	0	0	526.2	0.2	0
Common stock cash dividends of $0.675, $0.80 and $1.04 per share for Year ended 2009, 2010 and 2011, 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	4.4		4.4
Ending - Balance at Dec. 31, 2011	$ 3,963.3	$ 2.3	$ 598.4	$ 3,362.5	$ 0.1	$ 0