CONDENSED CONSOLIDATED STATEMENTS OF INCOME (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Statement [Abstract]
Utility revenues	$ 606.9	$ 582.3	$ 2,425.1	$ 2,116.9
Nonregulated revenues	522.8	345.4	1,498.8	898.3
Total revenues	1,129.7	927.7	3,923.9	3,015.2
Utility cost of fuel, natural gas, and purchased power	222.8	228.2	1,083.9	926.4
Nonregulated cost of sales	475.3	264.0	1,360.0	730.0
Operating and maintenance expense	282.3	240.3	866.1	748.6
Depreciation and amortization expense	69.6	62.9	196.0	187.6
Taxes other than income taxes	24.4	23.8	76.4	73.9
Operating income	55.3	108.5	341.5	348.7
Earnings from equity method investments	23.1	22.2	68.2	65.5
Miscellaneous income	12.1	3.1	23.3	7.2
Interest expense	33.1	29.9	91.0	90.0
Other income (expense)	2.1	(4.6)	0.5	(17.3)
Income before taxes	57.4	103.9	342.0	331.4
Provision for income taxes	18.0	29.6	124.3	106.6
Net income from continuing operations	39.4	74.3	217.7	224.8
Discontinued operations, net of tax	(0.6)	(8.0)	4.7	(9.2)
Net income	38.8	66.3	222.4	215.6
Preferred stock dividends of subsidiary	(0.7)	(0.7)	(2.3)	(2.3)
Noncontrolling interest in subsidiaries	0	0.1	0.1	0.1
Net income attributed to common shareholders	$ 38.1	$ 65.7	$ 220.2	$ 213.4
Average shares of common stock
Basic (in shares)	79.8	78.5	79.3	78.5
Diluted (in shares)	80.2	79.3	79.9	79.3
Earnings per common share (basic)
Net income from continuing operations (in dollars per share)	$ 0.49	$ 0.94	$ 2.72	$ 2.84
Discontinued operations, net of tax (in dollars per share)	$ (0.01)	$ (0.10)	$ 0.06	$ (0.12)
Earnings per common share (basic) (in dollars per share)	$ 0.48	$ 0.84	$ 2.78	$ 2.72
Earnings per common share (diluted)
Net income from continuing operations (in dollars per share)	$ 0.48	$ 0.93	$ 2.70	$ 2.81
Discontinued operations, net of tax (in dollars per share)	$ (0.01)	$ (0.10)	$ 0.06	$ (0.12)
Earnings per common share (diluted) (in dollars per share)	$ 0.47	$ 0.83	$ 2.76	$ 2.69
Dividends per common share declared (in dollars per share)	$ 0.68	$ 0.68	$ 2.04	$ 2.04