Income Taxes (Details)		3 Months Ended	12 Months Ended
		Sep. 30, 2015	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2012	Dec. 31, 2016 USD ($) $ / kWh	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Consolidated Appropriations Act of 2016 [Abstract]
Excise Tax Delay			2 years
Unrecognized Tax Benefits, Interest on Income Taxes Accrued							$ (1,100,000)	$ (400,000)	$ (200,000)	$ (100,000)
Interest Income (Expense) related to unrecognized tax benefits			$ (700,000)	$ (200,000)	$ (100,000)
Unrecognized Tax Benefits, Income Tax Penalties Accrued							0	0	0
Tax Increase Prevention Act of 2014 [Abstract]
Number of years bonus depreciation was extended					1 year
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions							2,900,000	2,400,000
Unrecognized tax benefit — Temporary tax positions							16,800,000	15,000,000
Total unrecognized tax benefit			17,400,000	11,900,000	$ 8,400,000		19,700,000	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			17,400,000	11,900,000	8,400,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions			2,700,000	4,500,000	3,700,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions			0	(1,500,000)	(700,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions			500,000	2,500,000	2,800,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions			(900,000)	0	(1,200,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities			0	0	(1,100,000)
Balance at Dec. 31			$ 19,700,000	$ 17,400,000	$ 11,900,000
Tax Benefits Associated With Nol And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards							(5,800,000)	(4,300,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible							11,000,000
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, State and Local Income Taxes, Percent			3.20%	3.20%	2.80%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent			(0.70%)	(0.70%)	(0.80%)
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items, Percent			(0.50%)	(0.30%)	(2.10%)
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits, Percent			0.00%	0.10%	(0.10%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent			0.10%	0.10%	0.10%
Effective Income Tax Rate Reconciliation, Percent			37.10%	37.40%	34.90%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)			$ 45,287,000	$ (1,166,000)	$ 9,550,000
Current State and Local Tax Expense (Benefit)			8,754,000	(727,000)	2,611,000
Current Change In Unrecognized Tax Expense (Benefit)			680,000	5,244,000	6,548,000
Deferred Federal Income Tax Expense (Benefit)			195,064,000	246,096,000	208,781,000
Deferred State and Local Income Tax Expense (Benefit)			27,216,000	36,450,000	26,196,000
Deferred Change In Unrecognized Tax Expense (Benefit)			(278,000)	(4,650,000)	(7,154,000)
Deferred investment tax credits			(2,805,000)	(2,807,000)	(2,941,000)
Income Tax Expense (Benefit)			273,918,000	278,440,000	243,591,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items			230,931,000	285,144,000	254,142,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities			(8,418,000)	(7,229,000)	(26,649,000)
Other Comprehensive Income (Loss), Tax			(511,000)	(19,000)	330,000
Deferred Income Tax Expense (Benefit)			$ 222,002,000	$ 277,896,000	$ 227,823,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment							2,967,162,000	2,772,043,000
Deferred Tax Liabilities, Compensation and Benefits, Employee Benefits							87,693,000	105,049,000
Deferred Tax Liabilities, Other							69,931,000	82,732,000
Deferred Tax Liabilities, Gross							3,124,786,000	2,959,824,000
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards							115,328,000	147,763,000
Deferred Tax Assets Tax credit carryforward							34,658,000	35,240,000
Deferred Tax Assets Regulatory Liabilities							19,635,000	17,201,000
Deferred Tax Assets Deferred Investment Tax Credits							11,653,000	12,718,000
Deferred Tax Assets Unbilled Revenue Fuel Costs							10,070,000	29,694,000
Deferred Tax Assets Rate Refund							7,221,000	23,352,000
Deferred Tax Assets, Other							37,092,000	35,658,000
Deferred Tax Assets, Net of Valuation Allowance							235,657,000	301,626,000
Deferred Tax Liabilities, Net							2,889,129,000	2,658,198,000
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Earliest year subject to examination			2009
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							260,000,000	328,000,000
Tax Credit Carryforward, Amount							25,000,000	24,000,000
Carryforward expiration date range, low			2021
Carryforward expiration date range, high			2036
State and Local Jurisdiction
Tax Audits [Abstract]
Earliest year subject to examination			2009
Operating Loss Carryforwards							684,000,000	684,000,000
Tax Credit Carryforward Net Of Federal Detriment	[1]						13,000,000	13,000,000
Valuation Allowance for Tax Credit Carryforward Net of Federal Benefit	[2]						(3,000,000)	(1,000,000)
Federal detriment							7,000,000	7,000,000
Federal Benefit							$ 2,000,000	$ 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%

[1] (a) State tax credit carryforwards are net of federal detriment of $7 million and $7 million as of Dec. 31, 2016 and 2015, respectively.

[2] (b) Valuation allowances for state tax credit carryforwards were net of federal benefit of $2 million and $1 million as of Dec. 31, 2016 and 2015, respectively.