Statement of Shareholders' Equity (USD $) In Millions, unless otherwise specified	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance at beginning of period at Dec. 31, 2008	$ 1,351.6	$ 659.1	$ 712.8	$ (20.3)
Balance at beginning of period (in shares) at Dec. 31, 2008		81.0
Comprehensive income:
Net income	133.1		133.1
Pension/OPEB funded status adjustment - net of $0.7, $0.2 and $0.4 million in tax	0.5			0.5
Cash flow hedges:
Comprehensive income of unconsolidated affiliates - net of $3.8, $0.2 and $8.9 million in tax	13.0			13.0
Total comprehensive income	146.6
Common stock:
Issuance: option exercises and dividend reinvestment plan	5.8	5.8
Issuance: option exercises and dividend reinvestment plan (in shares)		0.3
Dividends ($1.385, $1.365 and $1.345 per share)	(108.6)		(108.6)
Other	1.8	1.9	(0.1)
Other (in shares)		(0.2)
Balance at end of period at Dec. 31, 2009	1,397.2	666.8	737.2	(6.8)
Balance at end of period (in shares) at Dec. 31, 2009		81.1
Comprehensive income:
Net income	133.7		133.7
Pension/OPEB funded status adjustment - net of $0.7, $0.2 and $0.4 million in tax	(0.3)			(0.3)
Cash flow hedges:
unrealized gains (losses) - net of $1.5, $1.5 and $0.0 million in tax	2.5			2.5
reclassifications to net income- net of tax	(0.1)			(0.1)
Comprehensive income of unconsolidated affiliates - net of $3.8, $0.2 and $8.9 million in tax	0.3			0.3
Total comprehensive income	136.1
Common stock:
Issuance: option exercises and dividend reinvestment plan	14.0	14.0
Issuance: option exercises and dividend reinvestment plan (in shares)		0.6
Dividends ($1.385, $1.365 and $1.345 per share)	(110.8)		(110.8)
Other	2.4	2.6	(0.2)
Balance at end of period at Dec. 31, 2010	1,438.9	683.4	759.9	(4.4)
Balance at end of period (in shares) at Dec. 31, 2010	81.7	81.7
Comprehensive income:
Net income	141.6		141.6
Pension/OPEB funded status adjustment - net of $0.7, $0.2 and $0.4 million in tax	(1.0)			(1.0)
Cash flow hedges:
unrealized gains (losses) - net of $1.5, $1.5 and $0.0 million in tax	(2.1)			(2.1)
reclassifications to net income- net of tax	(0.3)			(0.3)
Comprehensive income of unconsolidated affiliates - net of $3.8, $0.2 and $8.9 million in tax	(5.5)			(5.5)
Total comprehensive income	132.7
Common stock:
Issuance: option exercises and dividend reinvestment plan	7.9	7.9
Issuance: option exercises and dividend reinvestment plan (in shares)		0.2
Dividends ($1.385, $1.365 and $1.345 per share)	(113.2)		(113.2)
Other	(0.8)	1.3	(2.1)
Balance at end of period at Dec. 31, 2011	$ 1,465.5	$ 692.6	$ 786.2	$ (13.3)
Balance at end of period (in shares) at Dec. 31, 2011	81.9	81.9