Commitments, Guarantees and Contingencies - North Dakota Wind Development (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
North Dakota Wind Development [Line Items]
Capital Cost Spent to Date	$ 45.7	$ 27.8
Bison 1 Wind Project [Member]
North Dakota Wind Development [Line Items]
Transmission Line Length (Miles)	22
Transmission Line Capacity (kV)	230
Generating Unit Capacity (MW)	82
Total Project Cost	174.3
Bison 2 Wind Project [Member]
North Dakota Wind Development [Line Items]
Generating Unit Capacity (MW)	105
Expected Capital Cost	160.0
Capital Cost Spent to Date	129.2
Bison 3 Wind Project [Member]
North Dakota Wind Development [Line Items]
Generating Unit Capacity (MW)	105
Expected Capital Cost	160.0
Capital Cost Spent to Date	$ 131.0
DC Transmission Line from Square Butte [Member]
North Dakota Wind Development [Line Items]
Transmission Line Length (Miles)	465
Transmission Line Capacity (kV)	250