Income Taxes (Details)		1 Months Ended			3 Months Ended	12 Months Ended
		Aug. 31, 2016	Jun. 30, 2016	Feb. 29, 2016	Sep. 30, 2015	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2016 USD ($) $ / kWh	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Consolidated Appropriations Act of 2016 [Abstract]
Excise Tax Delay						2 years
Tax Increase Prevention Act of 2014 [Abstract]
Number of years bonus depreciation was extended								1 year
Unrecognized Tax Benefits, Interest on Income Taxes Accrued											$ (3,400,000)	$ (100,000)	$ (300,000)	$ (600,000)
Unrecognized Tax Benefits, Interest on Income Taxes Accrued						$ (3,300,000)	$ 200,000	$ 300,000
Unrecognized Tax Benefits, Income Tax Penalties Accrued												0	0
Federal Tax Loss Carryback Claims [Abstract]
Number Of Years Of Tax Loss Carryback Period						two
Tax Adjustments, Settlements, and Unusual Provisions							5,000,000	17,000,000	$ 12,000,000	$ 15,000,000
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions											29,600,000	25,800,000
Unrecognized tax benefit — Temporary tax positions											104,100,000	94,900,000
Total unrecognized tax benefit						$ 120,700,000	66,500,000	41,200,000	41,200,000		133,700,000	120,700,000	$ 66,500,000	$ 41,200,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1						120,700,000	66,500,000	41,200,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions						8,200,000	27,100,000	28,700,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions						(300,000)	(4,500,000)	(2,000,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions						9,800,000	34,800,000	16,000,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions						(4,700,000)	(2,900,000)	(6,000,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities						0	(300,000)	(9,600,000)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations						0	0	(1,800,000)
Balance at Dec. 31						$ 133,700,000	$ 120,700,000	$ 66,500,000	$ 41,200,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards											(43,800,000)	(36,700,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible											61,000,000
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent						35.00%	35.00%	35.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent						(4.20%)	(2.70%)	(2.60%)
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items, Percent						(0.50%)	(1.00%)	(1.30%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent						4.20%	4.10%	4.00%
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits, Percent						0.20%	0.60%	0.20%
Effective Income Tax Reconciliation, Net operating loss carryback, Percent						0.00%	(0.30%)	(0.90%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent						(0.60%)	(0.20%)	(0.50%)
Effective Income Tax Rate Reconciliation, Percent						34.10%	35.50%	33.90%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)						$ (2,809,000)	$ (36,129,000)	$ (73,160,000)
Current State and Local Tax Expense (Benefit)						(3,345,000)	2,324,000	9,225,000
Current Change In Unrecognized Tax Expense (Benefit)						5,924,000	45,933,000	23,915,000
Deferred Federal Income Tax Expense (Benefit)						476,439,000	480,078,000	505,236,000
Deferred State and Local Income Tax Expense (Benefit)						112,308,000	92,132,000	84,787,000
Deferred Change In Unrecognized Tax Expense (Benefit)						(2,097,000)	(36,342,000)	(20,645,000)
Deferred investment tax credits						(5,203,000)	(5,277,000)	(5,543,000)
Income Tax Expense (Benefit)						581,217,000	542,719,000	523,815,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items						630,877,000	546,664,000	616,934,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities						(44,638,000)	(11,810,000)	(48,674,000)
Other Comprehensive Income (Loss), Tax						415,000	1,013,000	1,117,000
Deferred income tax expense (benefit), other						(4,000)	1,000	1,000
Deferred Income Tax Expense (Benefit)						586,650,000	$ 535,868,000	$ 569,378,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment											7,696,833,000	7,119,023,000
Deferred Tax Liabilities, Regulatory Assets											313,034,000	308,130,000
Deferred Tax Liabilities, Other											186,007,000	229,005,000
Deferred Tax Liabilities, Gross											8,195,874,000	7,656,158,000
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards											753,851,000	851,242,000
Deferred Tax Assets Tax credit carryforward											497,518,000	404,738,000
Deferred Tax Assets Rate Refund											32,810,000	50,441,000
Deferred Tax Assets Environmental Remediation											30,288,000	38,663,000
Deferred Tax Assets Regulatory Liabilities											28,249,000	36,257,000
Deferred Tax Assets Deferred Investment Tax Credits											27,436,000	29,650,000
Deferred Tax Assets Unbilled Revenue Fuel Costs											11,387,000	57,220,000
Deferred Tax Assets, Valuation Allowance											(57,515,000)	(27,679,000)
Deferred Tax Assets, Other											87,531,000	62,184,000
Deferred Tax Assets, Net of Valuation Allowance											1,411,555,000	1,502,716,000
Deferred Tax Liabilities, Net											6,784,319,000	6,153,442,000
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Year(s) under examination					2012 and 2013					2010 and 2011
Year of carryback claim under examination										2009
Potential Tax Adjustments						$ 14,000,000
Earliest year subject to examination						2009
Operating Loss Carryforwards											1,916,000,000	2,153,000,000
Tax Credit Carryforward, Amount											424,000,000	360,000,000
Carryforward expiration date range, low						2021
Carryforward expiration date range, high						2036
Colorado
Tax Audits [Abstract]
Year(s) under examination						None
Earliest year subject to examination						2009
Minnesota
Tax Audits [Abstract]
Year(s) under examination			2010 through 2014			None
Earliest year subject to examination						2009
Texas
Tax Audits [Abstract]
Year(s) under examination				2009 and 2010		None
Earliest year subject to examination						2009
Wisconsin
Tax Audits [Abstract]
Year(s) under examination		2012 and 2013
Earliest year subject to examination						2012
State and Local Jurisdiction
Tax Audits [Abstract]
Operating Loss Carryforwards											1,949,000,000	2,124,000,000
Operating Loss Carryforwards, Valuation Allowance											(59,000,000)	(65,000,000)
Tax Credit Carryforward Net Of Federal Detriment	[1]										74,000,000	45,000,000
Valuation Allowance for Tax Credit Carryforward Net of Federal Benefit	[2]										(54,000,000)	(24,000,000)
Federal detriment											40,000,000	24,000,000
Federal Benefit											$ 29,000,000	$ 13,000,000
Carryforward expiration date range, low						2017
Carryforward expiration date range, high						2035
Consolidated Appropriations Act of 2016; 2015, 2016, 2017 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent											50.00%
Consolidated Appropriations Act of 2016; 2018 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent											40.00%
Production Tax Credit Rate, Percent											60.00%
Consolidated Appropriations Act of 2016; 2019 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent											30.00%
Production Tax Credit Rate, Percent											40.00%
Investment Tax Credit Rate, Percent											30.00%
Consolidated Appropriations Act of 2016; 2020 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent											26.00%
Consolidated Appropriations Act of 2016; 2021 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent											22.00%
Consolidated Appropriations Act of 2016; After 2021 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent											10.00%
Consolidated Appropriations Act of 2016; 2015 Impact [Member]
Tax Audits [Abstract]
Federal detriment											$ 3,500,000
Federal Benefit											4,900,000
Consolidated Appropriations Act of 2016 [Abstract]
Other Deductions and Charges											1,200,000,000
Federal R&E Benefit											$ 6,800,000
Consolidated Appropriations Act of 2016; 2016 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent											100.00%
Production Tax Credit per KWh | $ / kWh											0.023
Consolidated Appropriations Act of 2016; 2017 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent											80.00%
Tax Increase Prevention Act of 2014; 2014 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent													50.00%
Tax Increase Prevention Act of 2014; 2015 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent											50.00%

[1] State tax credit carryforwards are net of federal detriment of $40 million and $24 million as of Dec. 31, 2016 and 2015, respectively.

[2] Valuation allowances for state tax credit carryforwards were net of federal benefit of $29 million and $13 million as of Dec. 31, 2016 and 2015, respectively.