Benefit Plans and Other Postretirement Benefits, Multiemployer Plans (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) Employer	Dec. 31, 2015 USD ($)	Dec. 31, 2014 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	$ 14,550	$ 18,167	$ 20,410
Multiemployer Pension Plans | NSP-Minnesota
Multiemployer Plans [Abstract]
Average number of NSP-Minnesota union employees covered by the multiemployer pension plan	700
Multiemployer contributions	$ 13,843	17,223	20,254
Multiemployer Pension Plans | NSP-Wisconsin
Multiemployer Plans [Abstract]
Multiemployer contributions	707	944	156
Multiemployer Postretirement Benefit Plans
Multiemployer Plans [Abstract]
Multiemployer contributions	86	135	273
Multiemployer Postretirement Benefit Plans | NSP-Minnesota
Multiemployer Plans [Abstract]
Multiemployer contributions	$ 86	$ 135	$ 273