Income Taxes (Details) (USD $)	3 Months Ended	12 Months Ended				1 Months Ended	3 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2010	Dec. 31, 2013 Internal Revenue Service (IRS)	Sep. 30, 2012 Internal Revenue Service (IRS)	Dec. 31, 2013 Internal Revenue Service (IRS)	Dec. 31, 2012 Internal Revenue Service (IRS)	Dec. 31, 2013 State and Local Jurisdiction	Dec. 31, 2013 State and Local Jurisdiction	Dec. 31, 2012 State and Local Jurisdiction
Prescription Drug Tax Benefit and Medicare Part D [Abstract]
Income tax expense (benefit) related to retiree prescription drug benefits	$ (17,000,000)
Number of applicable provisions addressing deductibility of retiree health care costs					1
Tax Audits [Abstract]
Year(s) no longer subject to audit as statute of limitations has expired								2008
Earliest year subject to examination								2009			2009
Year(s) under examination							2010 and 2011
Tax Adjustments, Settlements, and Unusual Provisions						10,000,000
Tax year(s) for which income tax examination has been completed										2006 through 2009
Operating Loss Carryforwards						315,500,000		315,500,000	264,700,000	664,600,000	664,600,000	595,100,000
Tax Credit Carryforward, Amount						19,400,000		19,400,000	16,200,000
Tax Credit Carryforward Net Of Federal Detriment										11,200,000	11,200,000	11,600,000
Carryforward expiration date range, low								2021			2019
Carryforward expiration date range, high								2033			2033
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions		2,500,000	1,300,000
Unrecognized tax benefit — Temporary tax positions		5,900,000	8,300,000
Total unrecognized tax benefit		8,400,000	9,600,000	11,400,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1		9,600,000	11,400,000	11,600,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions		3,900,000	1,900,000	3,400,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions		0	(1,500,000)	(800,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions		3,300,000	2,000,000	5,800,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions		(900,000)	(4,200,000)	(900,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		(7,500,000)	0	(7,700,000)
Balance at Dec. 31		8,400,000	9,600,000	11,400,000
Tax Benefits Associated With Nol And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards		(7,000,000)	(5,300,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible		(2,000,000)
Amounts accrued for penalties related to unrecognized tax benefits		0	0	0
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, State and Local Income Taxes, Percent		3.00%	2.30%	2.50%
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits		0.10%	0.10%	(0.10%)
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items		(1.40%)	(0.60%)	(0.20%)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent		(0.80%)	(0.60%)	(0.80%)
Effective Income Tax Rate Reconciliation, Deduction, Medicare Prescription Drug Benefit, Percent		0.00%	(2.50%)	(0.10%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(0.30%)	0.00%	0.20%
Effective Income Tax Rate Reconciliation, Percent		35.60%	33.70%	36.50%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)		(52,408,000)	176,354,000	1,889,000
Current State and Local Tax Expense (Benefit)		(7,252,000)	24,502,000	(796,000)
Current Change In Unrecognized Tax Expense (Benefit)		(2,918,000)	(3,447,000)	3,326,000
Deferred Federal Income Tax Expense (Benefit)		273,916,000	38,309,000	207,620,000
Deferred State and Local Income Tax Expense (Benefit)		38,243,000	(4,424,000)	22,994,000
Deferred Change In Unrecognized Tax Expense (Benefit)		4,094,000	4,207,000	(4,059,000)
Deferred investment tax credits		(2,935,000)	(2,957,000)	(2,613,000)
Income Tax Expense (Benefit)		250,740,000	232,544,000	228,361,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items		335,580,000	41,233,000	216,393,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities		(19,616,000)	(9,574,000)	(1,987,000)
Other Comprehensive Income (Loss), Tax		289,000	6,433,000	12,149,000
Deferred Income Tax Expense (Benefit)		316,253,000	38,092,000	226,555,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment		2,197,685,000	1,838,065,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits		100,003,000	99,286,000
Deferred Tax Liabilities, Other		132,436,000	128,453,000
Deferred Tax Liabilities, Net		2,430,124,000	2,065,804,000
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards		137,043,000	129,829,000
Deferred Tax Assets Unbilled Revenue Fuel Costs		49,171,000	54,353,000
Deferred Tax Assets Tax credit carryforward		30,643,000	27,752,000
Deferred Tax Assets Rate Refund		26,943,000	4,533,000
Deferred Tax Assets Deferred Investment Tax Credits		14,905,000	15,992,000
Deferred Tax Assets Regulatory Liabilities		14,893,000	14,164,000
Deferred Tax Assets, Other		37,614,000	35,849,000
Deferred Tax Assets, Net of Valuation Allowance		311,212,000	282,472,000
Deferred Tax Assets, Net		$ 2,118,912,000	$ 1,783,332,000