CONSOLIDATED STATEMENTS OF INCOME (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Operating revenues	$ 4,144.2	$ 3,485.5	$ 3,012.9
Cost of sales	2,133.0	1,598.7	1,349.1
Operating and maintenance expense	1,199.7	1,086.7	943.0
Depreciation and amortization expense	287.5	263.4	247.3
Taxes other than income taxes	97.0	97.2	94.0
Merger transaction costs	10.4	0	0
Gain on sale of UPPCO, net of transaction costs	(85.4)	0	0
Gain on abandonment of PDI's Winnebago Energy Center	(5.0)	0	0
Operating income	507.0	439.5	379.5
Earnings from equity method investments	88.3	91.5	87.2
Miscellaneous income	31.0	21.9	9.0
Interest expense	154.8	127.4	118.9
Other expense	(35.5)	(14.0)	(22.7)
Income before taxes	471.5	425.5	356.8
Provision for income taxes	193.4	158.0	117.9
Net income from continuing operations	278.1	267.5	238.9
Discontinued operations, net of tax	1.8	87.3	45.4
Net income	279.9	354.8	284.3
Preferred stock dividends of subsidiary	(3.1)	(3.1)	(3.1)
Noncontrolling interest in subsidiaries	0.1	0.1	0.2
Net income attributed to common shareholders	$ 276.9	$ 351.8	$ 281.4
Average shares of common stock
Basic (in shares)	80.2	79.5	78.6
Diluted (in shares)	80.7	80.1	79.3
Earnings per common share (basic)
Net income from continuing operations (in dollars per share)	$ 3.43	$ 3.33	$ 3.00
Discontinued operations, net of tax (in dollars per share)	$ 0.02	$ 1.10	$ 0.58
Earnings per common share (basic) (in dollars per share)	$ 3.45	$ 4.43	$ 3.58
Earnings per common share (diluted)
Net income from continuing operations (in dollars per share)	$ 3.41	$ 3.30	$ 2.98
Discontinued operations, net of tax (in dollars per share)	$ 0.02	$ 1.09	$ 0.57
Earnings per common share (diluted) (in dollars per share)	$ 3.43	$ 4.39	$ 3.55