Income Taxes (Details)	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2015 USD ($) $ / kWh	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	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%
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions					$ 2,400,000	$ 1,900,000
Unrecognized tax benefit — Temporary tax positions					15,000,000	10,000,000
Total unrecognized tax benefit	$ 11,900,000	$ 8,400,000	$ 9,600,000	$ 9,600,000	17,400,000	11,900,000	$ 8,400,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1	11,900,000	8,400,000	9,600,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions	4,500,000	3,700,000	3,900,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	(1,500,000)	(700,000)	0
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions	2,500,000	2,800,000	3,300,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions	0	(1,200,000)	(900,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	0	(1,100,000)	(7,500,000)
Balance at Dec. 31	$ 17,400,000	$ 11,900,000	$ 8,400,000	$ 9,600,000
Tax Benefits Associated With Nol And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards					(4,300,000)	(3,900,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible					11,000,000
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	3.20%	2.80%	3.00%
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits, Percent	0.10%	(0.10%)	0.10%
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items, Percent	(0.30%)	(2.10%)	(1.40%)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent	(0.70%)	(0.80%)	(0.80%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.10%	0.10%	(0.30%)
Effective Income Tax Rate Reconciliation, Percent	37.40%	34.90%	35.60%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	$ (1,166,000)	$ 9,550,000	$ (52,408,000)
Current State and Local Tax Expense (Benefit)	(727,000)	2,611,000	(7,252,000)
Current Change In Unrecognized Tax Expense (Benefit)	5,244,000	6,548,000	(2,918,000)
Deferred Federal Income Tax Expense (Benefit)	246,096,000	208,781,000	273,916,000
Deferred State and Local Income Tax Expense (Benefit)	36,450,000	26,196,000	38,243,000
Deferred Change In Unrecognized Tax Expense (Benefit)	(4,650,000)	(7,154,000)	4,094,000
Deferred investment tax credits	(2,807,000)	(2,941,000)	(2,935,000)
Income Tax Expense (Benefit)	278,440,000	243,591,000	250,740,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items	285,144,000	254,142,000	335,580,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities	(7,229,000)	(26,649,000)	(19,616,000)
Other Comprehensive Income (Loss), Tax	(19,000)	330,000	289,000
Deferred Income Tax Expense (Benefit)	$ 277,896,000	$ 227,823,000	$ 316,253,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment					2,772,043,000	2,467,260,000
Deferred Tax Liabilities, Compensation and Benefits, Employee Benefits					105,049,000	110,556,000
Deferred Tax Liabilities, Other					101,219,000	140,080,000
Deferred Tax Liabilities, Net					2,658,198,000	2,373,054,000
Deferred Tax Liabilities, Gross					2,978,311,000	2,717,896,000
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards					147,763,000	143,158,000
Deferred Tax Assets Unbilled Revenue Fuel Costs					48,181,000	57,654,000
Deferred Tax Assets Rate Refund					23,352,000	43,735,000
Deferred Tax Assets Tax credit carryforward					35,240,000	34,493,000
Deferred Tax Assets Regulatory Liabilities					17,201,000	14,549,000
Deferred Tax Assets Deferred Investment Tax Credits					12,718,000	13,781,000
Deferred Tax Assets, Other					35,658,000	37,472,000
Deferred Tax Assets, Net of Valuation Allowance					320,113,000	344,842,000
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
Operating Loss Carryforwards					328,000,000	320,000,000
Tax Credit Carryforward, Amount					24,000,000	22,000,000
Carryforward expiration date range, low	2021
Carryforward expiration date range, high	2035
State and Local Jurisdiction
Tax Audits [Abstract]
Earliest year subject to examination	2009
Operating Loss Carryforwards					684,000,000	690,000,000
Tax Credit Carryforward Net Of Federal Detriment					13,000,000	12,000,000
Valuation Allowance for Tax Credit Carryforward Net of Federal Benefit					(1,000,000)	0
Federal detriment					7,000,000	$ 7,000,000
Federal Benefit					$ 1,000,000
Carryforward expiration date range, low	2017
Carryforward expiration date range, high	2033
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; 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%
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%
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%
American Taxpayer Relief Act of 2012; 2013 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent							50.00%
American Taxpayer Relief Act of 2012; 2014 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent						50.00%