Benefit Plans and Other Postretirement Benefits, Multiemployer Plans (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) Employer	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Multiemployer Plans [Abstract]
Number of employers that must be exceeded during a given period in order for certain union workers to participate in multiemployer plans | Employer	1
Multiemployer Pension Plans
Multiemployer Plans [Abstract]
Multiemployer contributions	$ 18,167	$ 20,410	$ 23,645
Multiemployer Pension Plans | NSP-Minnesota
Multiemployer Plans [Abstract]
Average number of NSP-Minnesota union employees covered by the multiemployer pension plan	900	1000
Multiemployer contributions	$ 17,223	$ 20,254	23,515
Multiemployer Pension Plans | NSP-Wisconsin
Multiemployer Plans [Abstract]
Multiemployer contributions	944	156	130
Multiemployer Postretirement Benefit Plans
Multiemployer Plans [Abstract]
Multiemployer contributions	135	273	390
Multiemployer Postretirement Benefit Plans | NSP-Minnesota
Multiemployer Plans [Abstract]
Multiemployer contributions	$ 135	$ 273	$ 390