Commitments, Guarantees and Contingencies - North Dakota Wind Development (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013 kV Miles
DC Transmission Line from Square Butte [Member]
North Dakota Wind Development [Line Items]
Transmission Line Length (Miles)	465
Transmission Line Capacity (kV)	250
Bison Wind Energy Center [Member]
North Dakota Wind Development [Line Items]
Generating Capacity (MW)	292
Bison 4 [Member]
North Dakota Wind Development [Line Items]
Generating Capacity (MW)	205
Estimated Capital Expenditures	$ 345
Capital Cost Spent to Date	$ 7.7