Acquisition of Minnesota Limited, Inc. (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Acquisition of Minnesota Limited, Inc. [Abstract]
Acquisition date									March 31, 2011
Purchase price allocation [Abstract]
Business acquisition, net of cash acquired									$ 83,400,000	$ 0	$ 0
Number of years goodwill is deductible (which does not amortize pursuant to FASB guidance) for computing current income tax expense (in years)									15
Transaction costs associated with the acquisition included in other operating expenses during the period									652,200,000	538,400,000	514,000,000
Nonutility									870,200,000	567,400,000	494,300,000
Net income (loss)	46,600,000	35,300,000	15,100,000	44,600,000	45,400,000	16,400,000	8,700,000	63,200,000	141,600,000	133,700,000	133,100,000
Minnesota Limited, Inc. [Member]
Purchase price allocation [Abstract]
Working capital assets	21,500,000								21,500,000
Working capital liabilities	(6,700,000)								(6,700,000)
Net Working Capital	14,800,000								14,800,000
Property, plant and equipment	34,400,000								34,400,000
Identifiable intangible assets	19,100,000								19,100,000
Goodwill	20,300,000								20,300,000
Net assets acquired	88,600,000								88,600,000
Debt obligation assumed	(5,200,000)								(5,200,000)
Business acquisition, net of cash acquired									83,400,000
Transaction costs associated with the acquisition	600,000								600,000
Transaction costs associated with the acquisition included in other operating expenses during the period									200,000
Lease Obligation									83,333
Number of years the company is obligated to lease acquiree's corporate headquarters	10								10
Nonutility									116,500,000
Net income (loss)									9,400,000
Business Acquisition, Pro Forma Revenue									2,346,300,000	2,239,700,000	2,210,000,000
Business Acquisition, Pro Forma Net Income (Loss)									$ 141,400,000	$ 134,600,000	$ 138,100,000
Business Acquisition, Pro Forma Earnings Per Share, Basic									$ 1.73	$ 1.66	$ 1.71
Business Acquisition, Pro Forma Earnings Per Share, Diluted									$ 1.73	$ 1.66	$ 1.70