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, 2011	$ 3,963.3	$ 2.3	$ 598.5	$ 3,362.5
Net income	546.3			546.3
Common stock cash dividends of $1.20, $1.445, and $1.56 per share for Year ended 2012, 2013 and 2014, 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
Net income	577.4			577.4
Common stock cash dividends of $1.20, $1.445, and $1.56 per share for Year ended 2012, 2013 and 2014, respectively	(328.9)			(328.9)
Exercise of stock options	48.5		48.5
Purchase of common stock	(223.4)		(223.4)
Tax benefit from share based compensation	18.1		18.1
Stock-based compensation and other	6.2		6.2
Ending - Balance at Dec. 31, 2013	4,233.0	2.3	349.7	3,881.0
Net income	588.3			588.3
Common stock cash dividends of $1.20, $1.445, and $1.56 per share for Year ended 2012, 2013 and 2014, respectively	(352.0)			(352.0)
Exercise of stock options	50.3		50.3
Purchase of common stock	(123.2)		(123.2)
Tax benefit from share based compensation	16.8		16.8
Stock-based compensation and other	6.5		6.5
Ending - Balance at Dec. 31, 2014	$ 4,419.7	$ 2.3	$ 300.1	$ 4,117.3