Commitments and Contingencies (Details Textual 2) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other Commitments [Line Items]
Loss Contingency Insurance Policy Deductible for Each Line of Coverage Excluding Health		$ 1,000,000
Loss Contingency Health Insurance Deductible For Qualified Individuals		100,000
Outstanding Performance Bonds		912,000,000
Estimated Cost to Complete Bonded Projects		87,000,000
Insurance Claims [Member]
Other Commitments [Line Items]
Loss Contingency Insurance Expense		23,600,000	$ 21,000,000	$ 20,100,000
LE Myers Co [Member]
Other Commitments [Line Items]
Litigation Settlement, Amount		$ 230,000
Northern States Power Company Wisconsin Vs The City of Ashland Wisconsin [Member] | LE Myers Co [Member] | Minimum [Member]
Other Commitments [Line Items]
Period During Which Defendant Operated Manufactured Gas Plant	6 years
Northern States Power Company Wisconsin Vs The City of Ashland Wisconsin [Member] | LE Myers Co [Member] | Maximum [Member]
Other Commitments [Line Items]
Period During Which Defendant Operated Manufactured Gas Plant	12 years