CONSOLIDATED STATEMENTS OF INCOME (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Statement [Abstract]
Utility revenues	$ 3,425.6	$ 2,959.5	$ 3,294.5
Nonregulated revenues	2,209.0	1,252.9	1,391.4
Total revenues	5,634.6	4,212.4	4,685.9
Utility cost of fuel, natural gas, and purchased power	1,570.4	1,326.3	1,635.3
Nonregulated cost of sales	1,937.2	1,040.2	1,274.2
Operating and maintenance expense	1,192.3	1,031.3	1,024.8
Depreciation and amortization expense	266.6	250.7	247.7
Taxes other than income taxes	100.4	96.4	97.1
Operating income	567.7	467.5	406.8
Earnings from equity method investments	91.5	87.2	79.4
Miscellaneous income	29.8	9.3	5.3
Interest expense	128.2	120.2	128.2
Other expense	(6.9)	(23.7)	(43.5)
Income before taxes	560.8	443.8	363.3
Provision for income taxes	210.8	149.8	133.3
Net income from continuing operations	350.0	294.0	230.0
Discontinued operations, net of tax	4.8	(9.7)	0.5
Net income	354.8	284.3	230.5
Preferred stock dividends of subsidiary	(3.1)	(3.1)	(3.1)
Noncontrolling interest in subsidiaries	0.1	0.2	0
Net income attributed to common shareholders	$ 351.8	$ 281.4	$ 227.4
Average shares of common stock
Basic (in shares)	79.5	78.6	78.6
Diluted (in shares)	80.1	79.3	79.1
Earnings (loss) per common share (basic)
Net income from continuing operations (in dollars per share)	$ 4.37	$ 3.70	$ 2.89
Discontinued operations, net of tax (in dollars per share)	$ 0.06	$ (0.12)	$ 0.00
Earnings per common share (basic) (in dollars per share)	$ 4.43	$ 3.58	$ 2.89
Earnings (loss) per common share (diluted)
Net income from continuing operations (in dollars per share)	$ 4.33	$ 3.67	$ 2.87
Discontinued operations, net of tax (in dollars per share)	$ 0.06	$ (0.12)	$ 0.00
Earnings per common share (diluted) (in dollars per share)	$ 4.39	$ 3.55	$ 2.87