Commitments and Contingencies (Details) (USD $)	6 Months Ended
	Jun. 30, 2014
Recorded Unconditional Purchase Obligation Payment Schedule [Abstract]
2014	$ 259,000,000
2015	528,000,000
2016	454,000,000
2017	384,000,000
2018	376,000,000
After 2018	174,000,000
Loss Contingency Pending Natural Gas Measurement Lawsuits Number	1
Site Contingency Remaining Minnesota Sites	7
Site Contingency Sites Without Liability	2
Site Contingency Completed Remediation	2
LegalEnvironmentalAndOtherRegulatoryMattersLegalMattersGasMarketManipulationCasesAbstract
Accrual for Environmental Loss Contingencies	13,000,000
Annual amount included in rates to fund normal on-going remediation costs	285,000
Amount collected from insurance companies to be used for future environmental remediation	6,000,000
Guarantee of Affiliate Senior Notes	51,000,000
Midstream Partnership [Member]
LegalEnvironmentalAndOtherRegulatoryMattersLegalMattersGasMarketManipulationCasesAbstract
Guarantee of Affiliate Senior Notes	1,100,000,000
Maximum [Member]
LegalEnvironmentalAndOtherRegulatoryMattersLegalMattersGasMarketManipulationCasesAbstract
Maximum estimated remediation costs for the Minnesota sites	41,000,000
Minimum years of remediation for the Minnesota sites (in years)	50
Minimum [Member]
LegalEnvironmentalAndOtherRegulatoryMattersLegalMattersGasMarketManipulationCasesAbstract
Maximum estimated remediation costs for the Minnesota sites	$ 6,000,000
Minimum years of remediation for the Minnesota sites (in years)	30