Nuclear Obligations (Details) $ in Thousands		5 Months Ended	12 Months Ended
		May 31, 2014 $ / kWh	Dec. 31, 2016 USD ($) Canister $ / kWh		Dec. 31, 2015 USD ($)		Dec. 31, 2014 USD ($)
Public Utilities, General Disclosures [Line Items]
Fair Value Assumptions, Risk Free Interest Rate			3.25%		3.01%
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
Cumulative fuel disposal assessments paid to DOE							452,100
Regulatory Plant Decommissioning Recovery [Abstract]
Assumed annual escalation rate during plant removal activities			4.36%		4.36%
Assumed annual escalation rate during spent fuel management and site restoration activities			3.36%		3.36%
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 Funds			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 dollars)			3,012,342		$ 3,012,342
Effect of escalating costs (to 2016 and 2015 dollars, respectively, at 4.36/3.36 percent)			258,278		126,464
Estimated decommissioning cost obligation (in current dollars)			3,270,620		3,138,806
Effect of escalating costs to payment date (4.36/3.36 percent)			7,934,874		8,066,688
Estimated future decommissioning costs (undiscounted)			11,205,494		11,205,494
Effect of discounting obligation (using average risk-free interest rate of 3.25 percent and 3.01 percent for 2016 and 2015, respectively)			(7,068,362)		(6,891,392)
Discounted decommissioning cost obligation			4,137,132		4,314,102
Underfunding of external decommissioning fund compared to the discounted decommissioning obligation			2,276,370		2,589,952
Regulatory Basis to GAAP Basis Reconciliation [Abstract]
Differences in Discount Rate and Market Risk Premium			(1,043,655)		(1,275,438)
Operating and Maintenance Costs Not Included for GAAP			(844,155)		(897,640)
Asset Retirement Obligation			2,782,229		2,608,562		2,446,631
Annual Decommissioning Recorded As Depreciation Expense [Abstract]
Annual decommissioning recorded as depreciation expense: (a) (b)	[1],[2]		$ 20,372		6,862		7,138
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			$ 2,249,322		2,141,024		$ 2,037,947
Monticello [Member]
Fuel Disposal [Abstract]
Number of authorized canisters filled and placed In dry cask nuclear storage facility | Canister			16
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
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Funded Status of Nuclear Decommissioning Obligation [Abstract]
Assets Held in External Decommissioning Trust			$ 1,860,762	[3]	$ 1,724,150	[4]

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

[2] Decommissioning expense in 2016 includes Minnesota’s retail jurisdiction annual funding requirement of approximately $14 million. The 2014 and 2015 expense was offset by the DOE settlement refund.

[3] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $132.8 million of equity investments in unconsolidated subsidiaries and $98.3 million of miscellaneous investments.

[4] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $130.0 million of equity investments in unconsolidated subsidiaries and $48.9 million of miscellaneous investments.