Nuclear Obligations (Details) $ in Thousands		5 Months Ended	12 Months Ended
		May. 31, 2014 $ / kWh	Dec. 31, 2015 USD ($) Canister $ / kWh	Dec. 31, 2014 USD ($)		Dec. 31, 2013 USD ($)
Public Utilities, General Disclosures [Line Items]
Risk free interest rate used to discount cost obligation			3.01%	2.82%
Fuel Disposal [Abstract]
Fuel disposal fee charge (in dollars per KWh) | $ / kWh		0.001	0
DOE fuel disposal assessments included in fuel expense			$ 0	$ 5,000		$ 10,000
Cumulative fuel disposal assessments paid to DOE			$ 452,100
Regulatory Plant Decommissioning Recovery [Abstract]
Assumed annual escalation rate during plant removal activities			4.36%	3.63%
Assumed annual escalation rate during spent fuel management and site restoration activities			3.36%	2.63%
Number of years approved for use in decommissioning scenario (in years)			60 years
Percentage of total obligation for decommissioning expected to be funded by external decommissioning trust fund			100.00%
Approved annual accrual for decommissioning costs			$ 14,000
Funded Status of Nuclear Decommissioning Obligation [Abstract]
Estimated decommissioning cost obligation from most recently approved study (in 2014 and 2011 dollars, respectively)			3,012,342	$ 2,694,079
Effect of escalating costs (to 2015 and 2014 dollars, respectively, at 4.36/3.36 percent and 3.63/2.63 percent, respectively)			126,464	289,907
Estimated decommissioning cost obligation (in current dollars)			3,138,806	2,983,986
Effect of escalating costs to payment date (4.36/3.36 percent and 3.63/2.63 percent, respectively)			8,066,688	5,597,302
Estimated future decommissioning costs (undiscounted)			11,205,494	8,581,288
Effect of discounting obligation (using average risk-free interest rate of 3.01 percent and 2.82 percent for 2015 and 2014, respectively)			(6,891,392)	(5,044,470)
Discounted decommissioning cost obligation			4,314,102	3,536,818
Underfunding of external decommissioning fund compared to the discounted decommissioning obligation			2,589,952	1,832,897
Regulatory Basis to GAAP Basis Reconciliation [Abstract]
Differences in Discount Rate and Market Risk Premium			(1,275,438)	(1,275,101)
Operating and Maintenance Costs Not Included for GAAP			(897,640)	(547,135)
Differences in Cost Studies			0	323,365
Asset Retirement Obligation			2,608,562	2,446,631	[1]	1,815,390
Annual Decommissioning Recorded As Depreciation Expense [Abstract]
Annual decommissioning recorded as depreciation expense: (a)	[2]		$ 6,862	7,138		6,402
Minimum
Regulatory Plant Decommissioning Recovery [Abstract]
Assumed after tax rate of return used to determine funding for external decommissioning trust fund			5.23%
Maximum
Regulatory Plant Decommissioning Recovery [Abstract]
Assumed after tax rate of return used to determine funding for external decommissioning trust fund			6.30%
Nuclear Plant [Member]
Regulatory Basis to GAAP Basis Reconciliation [Abstract]
Asset Retirement Obligation			$ 2,141,024	2,037,947	[1]	$ 1,628,298
Monticello [Member]
Fuel Disposal [Abstract]
Number of authorized canisters filled and placed In dry cask nuclear storage facility | Canister			15
Number of authorized canisters in dry cask nuclear storage facility | Canister			30
Prairie Island [Member]
Fuel Disposal [Abstract]
Number of authorized canisters filled and placed In dry cask nuclear storage facility | Canister			40
Number of authorized canisters in dry cask nuclear storage facility | Canister			64
Nuclear Decommissioning Fund
Funded Status of Nuclear Decommissioning Obligation [Abstract]
Assets Held in External Decommissioning Trust			$ 1,724,150	$ 1,703,921

[1] (b) There were no ARO liabilities settled during the year ended Dec. 31, 2014.

[2] Decommissioning expense does not include depreciation of the capitalized nuclear asset retirement costs.