Income Taxes (Details) (USD $)	3 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2010
Tax Increase Prevention Act of 2014 [Abstract]
Bonus depreciation rate		50.00%	50.00%
Number of years bonus depreciation was extended		1 year	1 year
American Taxpayer Relief Act of 2012 [Abstract]
Original top tax rate for dividends			20.00%
Current top tax rate for dividends			15.00%
Bonus depreciation rate		50.00%	50.00%
Number of years bonus depreciation was extended		1 year	1 year
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
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions		1,900,000	2,500,000
Unrecognized tax benefit — Temporary tax positions		10,000,000	5,900,000
Total unrecognized tax benefit		11,900,000	8,400,000	9,600,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1		8,400,000	9,600,000	11,400,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions		3,700,000	3,900,000	1,900,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions		(700,000)	0	(1,500,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions		2,800,000	3,300,000	2,000,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions		(1,200,000)	(900,000)	(4,200,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		(1,100,000)	(7,500,000)	0
Balance at Dec. 31		11,900,000	8,400,000	9,600,000
Tax Benefits Associated With Nol And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards		(3,900,000)	(7,000,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible		(1,000,000)
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		2.80%	3.00%	2.30%
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		(2.10%)	(1.40%)	(0.60%)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent		(0.80%)	(0.80%)	(0.60%)
Effective Income Tax Rate Reconciliation, Deduction, Medicare Prescription Drug Benefit, Percent		0.00%	0.00%	(2.50%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		0.10%	(0.30%)	0.00%
Effective Income Tax Rate Reconciliation, Percent		34.90%	35.60%	33.70%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)		9,550,000	(52,408,000)	176,354,000
Current State and Local Tax Expense (Benefit)		2,611,000	(7,252,000)	24,502,000
Current Change In Unrecognized Tax Expense (Benefit)		6,548,000	(2,918,000)	(3,447,000)
Deferred Federal Income Tax Expense (Benefit)		208,781,000	273,916,000	38,309,000
Deferred State and Local Income Tax Expense (Benefit)		26,196,000	38,243,000	(4,424,000)
Deferred Change In Unrecognized Tax Expense (Benefit)		(7,154,000)	4,094,000	4,207,000
Deferred investment tax credits		(2,941,000)	(2,935,000)	(2,957,000)
Income Tax Expense (Benefit)		243,591,000	250,740,000	232,544,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items		254,142,000	335,580,000	41,233,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities		(26,649,000)	(19,616,000)	(9,574,000)
Other Comprehensive Income (Loss), Tax		330,000	289,000	6,433,000
Deferred Income Tax Expense (Benefit)		227,823,000	316,253,000	38,092,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment		2,467,260,000	2,197,685,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits		110,556,000	100,003,000
Deferred Tax Liabilities, Other		140,080,000	132,436,000
Deferred Tax Liabilities, Net		2,717,896,000	2,430,124,000
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards		143,158,000	137,043,000
Deferred Tax Assets Unbilled Revenue Fuel Costs		57,654,000	49,171,000
Deferred Tax Assets Tax credit carryforward		34,493,000	30,643,000
Deferred Tax Assets Rate Refund		43,735,000	26,943,000
Deferred Tax Assets Deferred Investment Tax Credits		13,781,000	14,905,000
Deferred Tax Assets Regulatory Liabilities		14,549,000	14,893,000
Deferred Tax Assets, Other		37,472,000	37,614,000
Deferred Tax Assets, Net of Valuation Allowance		344,842,000	311,212,000
Deferred Tax Assets, Net		2,373,054,000	2,118,912,000
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Year(s) no longer subject to audit as statute of limitations has expired		2008
Earliest year subject to examination		2009
Year(s) under examination	2010 and 2011
Tax Adjustments, Settlements, and Unusual Provisions		12,000,000
Operating Loss Carryforwards		319,500,000	315,500,000
Tax Credit Carryforward, Amount		22,300,000	19,400,000
Carryforward expiration date range, low		2021
Carryforward expiration date range, high		2034
State and Local Jurisdiction
Tax Audits [Abstract]
Earliest year subject to examination		2009
Operating Loss Carryforwards		690,100,000	664,600,000
Tax Credit Carryforward Net Of Federal Detriment		$ 12,200,000	$ 11,200,000
Carryforward expiration date range, low		2017
Carryforward expiration date range, high		2033