Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) MWh in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) MWh MW	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Square Butte PPA (expires December 2026) [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Output Entitlement	50.00%
Output Being Purchased (MW)	227.5
Square Butte [Member] | Square Butte PPA (expires December 2026) [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Total Debt Outstanding | $	$ 327.7
PPA Counterparty Annual Debt Service | $	45.0
Cost of Purchased Power | $	73.3	$ 77.8	$ 70.1
Pro Rata Share of PPA Counterparty Interest Expense | $	$ 9.6	$ 10.1	$ 10.5
Square Butte [Member] | Square Butte PPA (expires December 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 December 2026) [Member] | Minnkota Power PSA (expires December 2026) [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Output Entitlement	28.00%	28.00%	23.00%
Minnkota Power [Member] | Minnkota Power PPA (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 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 PPA Beginning June 2017 (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
Manitoba Hydro [Member] | Manitoba Hydro PPA Beginning June 2015 (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Manitoba Hydro [Member] | Manitoba Hydro PPA Beginning June 2017 (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires May 2035) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	250
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires 2040) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	133
Contract Term (Years)	20 years
Oliver Wind [Member] | Oliver Wind I PPA (expires December 2031) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Oliver Wind [Member] | Oliver Wind II PPA (expires December 2032) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	48
Shell Energy [Member] | Shell Energy PPA (expires December 2019) [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 PSA (expires April 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Sold (MW)	100
Basin [Member] | Basin PSA (expires June 2018) [Member]
Power Purchase Agreements [Line Items]
Output Being Sold (MW)	100
Silver Bay Power [Member] | Silver Bay Power Self-Generation [Member]
Power Purchase Agreements [Line Items]
Generating Unit Capacity (MW)	90
Silver Bay Power [Member] | Silver Bay Power PSA through 2031 (Years 2016-2019) [Member] | Minimum [Member]
Power Purchase Agreements [Line Items]
Output Being Sold (MW)	50