Commitments, Guarantees and Contingencies - North Dakota Wind Development (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
North Dakota Wind Development [Line Items]
Capital Cost Spent to Date	$ 39.6	$ 27.8
Bison 1 Wind Project [Member]
North Dakota Wind Development [Line Items]
Number of Phases	2
Generating Unit Capacity (MW)	82
Transmission Line Capacity (kV)	230
Total Project Cost	173.8
Transmission Line Length (Miles)	22
Bison 1 Wind Project, Land Restoration and Upgrades [Member]
North Dakota Wind Development [Line Items]
Expected Capital Cost	3.6
Capital Cost Spent to Date	2.6
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	123.5
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	$ 100.7
DC Transmission Line from Square Butte [Member]
North Dakota Wind Development [Line Items]
Transmission Line Capacity (kV)	250
Transmission Line Length (Miles)	465