Benefit Plans and Other Postretirement Benefits, Employees Represented by Local Labor Unions (Details)	Dec. 31, 2015 Employee
Employees Represented by Local Labor Unions Under Collective Bargaining Agreements Receiving Benefits [Abstract]
Approximate percent of employees receiving benefits who are represented by local labor unions under collective bargaining agreements (as a percent)	47.00%
NSP-Minnesota
Employees Represented by Local Labor Unions Under Collective Bargaining Agreements Receiving Benefits [Abstract]
Number of bargaining employees receiving benefits under several collective bargaining agreements	1,983
Number of nuclear operation bargaining employees receiving benefits under several collective bargaining-agreements	265
NSP-Wisconsin
Employees Represented by Local Labor Unions Under Collective Bargaining Agreements Receiving Benefits [Abstract]
Number of bargaining employees receiving benefits under several collective bargaining agreements	400
PSCo
Employees Represented by Local Labor Unions Under Collective Bargaining Agreements Receiving Benefits [Abstract]
Number of bargaining employees receiving benefits under several collective bargaining agreements	2,024
SPS
Employees Represented by Local Labor Unions Under Collective Bargaining Agreements Receiving Benefits [Abstract]
Number of bargaining employees receiving benefits under several collective bargaining agreements	842