Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) MWh Years MW	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Square Butte PPA (expires 2026) [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Output Entitlement	50.00%
Square Butte [Member] | Square Butte PPA (expires 2026) [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Debt Outstanding | $	$ 376.4
PPA Counterparty Annual Debt Service | $	45.0
Cost of Purchased Power | $	77.8	$ 70.1	$ 71.1
Pro Rata Share of PPA Counterparty Interest Expense | $	$ 10.1	$ 10.5	$ 10.5
Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Generating Unit Capacity (MW)	455
Minnkota Power [Member] | Square Butte PPA (expires 2026) [Member] | Minnkota Sales Agreement [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Output Entitlement	28.00%	23.00%
Minnkota Power [Member] | Minnkota Power PPA (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
NextEra Energy, Inc. [Member]
Power Purchase Agreements [Line Items]
Number of PPAs	2
NextEra Energy, Inc. [Member] | Oliver Wind I PPA (expires 2031) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Fixed Capacity Charges | $	$ 0.0
NextEra Energy, Inc. [Member] | Oliver Wind II PPA (expires 2032) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	48
Fixed Capacity Charges | $	$ 0.0
Manitoba Hydro [Member]
Power Purchase Agreements [Line Items]
Number of PPAs	5
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires April 2022) [Member] | Minimum [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MWh) | MWh	1,000,000
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires 2035) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	250
Term of Contract (Years) | Years	15
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires 2040) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	133
Term of Contract (Years) | Years	20
Manitoba Hydro [Member] | Manitoba Hydro PPA Beginning June 2017 (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Great River Energy [Member] | Great River Energy Capacity and Energy PPA (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Great River Energy [Member] | Great River Energy Capacity Only PPA Beginning June 2016 (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Great River Energy [Member] | Great River Energy Capacity Only PPA Beginning June 2017 (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
TransAlta [Member] | TransAlta Off-Peak Hours PPA (expires December 2019) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
TransAlta [Member] | TransAlta On-Peak Hours PPA (expire December 2019) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	100
Basin [Member] | Basin Power Sales Agreement (expires April 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Sold (MW)	100
Term of Contract (Years) | Years	10
Basin [Member] | Basin Power Sales Agreement (expires 2018) [Member]
Power Purchase Agreements [Line Items]
Output Being Sold (MW)	100
Term of Contract (Years) | Years	2