Income Taxes (Details) (USD $)	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2012
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		15.00%
New top tax rate for dividends		20.00%
Bonus depreciation rate	50.00%	50.00%
Number of years bonus depreciation was extended	1 year	1 year
Tax Audits [Abstract]
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	$ (1,300,000)	$ 0	$ 0
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions	1,500,000	1,200,000
Unrecognized tax benefit — Temporary tax positions	11,700,000	2,900,000
Total unrecognized tax benefit	13,200,000	4,100,000	3,900,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1	4,100,000	3,900,000	4,800,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions	8,600,000	1,600,000	1,100,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0	0	(1,600,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions	2,300,000	3,100,000	800,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions	(300,000)	(300,000)	(1,200,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	(200,000)	(4,200,000)	0
Balance at Dec. 31	13,200,000	4,100,000	3,900,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards	(4,800,000)	(2,400,000)
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	3.40%	2.00%	2.20%
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits	0.20%	0.70%	0.00%
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items	(1.60%)	(1.10%)	(0.40%)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent	(0.40%)	(0.40%)	(0.20%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.10%	(0.10%)	0.30%
Effective Income Tax Rate Reconciliation, Percent	36.70%	36.10%	36.90%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	(57,201,000)	14,947,000	6,549,000
Current State and Local Tax Expense (Benefit)	2,512,000	2,943,000	2,712,000
Current Change In Unrecognized Tax Expense (Benefit)	6,715,000	(263,000)	(824,000)
Deferred Federal Income Tax Expense (Benefit)	121,882,000	33,489,000	50,189,000
Deferred State and Local Income Tax Expense (Benefit)	8,025,000	1,754,000	3,069,000
Deferred Change In Unrecognized Tax Expense (Benefit)	(6,390,000)	1,232,000	861,000
Deferred investment tax credits	(341,000)	(341,000)	(327,000)
Income Tax Expense (Benefit)	75,202,000	53,761,000	62,229,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items	124,875,000	38,333,000	55,749,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities	(1,262,000)	(1,761,000)	(1,533,000)
Other Comprehensive Income (Loss), Tax	(96,000)	(97,000)	(97,000)
Deferred Income Tax Expense (Benefit)	123,517,000	36,475,000	54,119,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	842,847,000	705,416,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits	50,696,000	52,081,000
Deferred Tax Liabilities, Other	28,591,000	30,066,000
Deferred Tax Liabilities, Net	922,134,000	787,563,000
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards	71,956,000	61,330,000
Deferred Tax Assets Rate Refund	18,405,000	17,192,000
Deferred Tax Assets Unbilled Revenue Fuel Costs	10,866,000	13,316,000
Deferred Tax Assets Regulatory Liabilities	10,794,000	9,724,000
Deferred Tax Assets, Tax Deferred Expense, Deferred Fuel Costs	6,006,000	6,877,000
Deferred Tax Assets, Other	6,816,000	6,708,000
Deferred Tax Assets, Net of Valuation Allowance	124,843,000	115,147,000
Deferred Tax Assets, Net	797,291,000	672,416,000
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	192,400,000	168,700,000
Tax Credit Carryforward, Amount	2,100,000	1,700,000
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	$ 58,500,000	$ 23,600,000
Carryforward expiration date range, low	2016
Carryforward expiration date range, high	2034