Income Taxes (Details) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2012	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Income Tax Examination [Line Items]
Tax Cuts and Jobs Act of 2017, Corporate Federal Tax Rate	21.00%
Tax Cuts and Jobs Act of 2017, Net Operating Loss Deduction Limitation, Percent of Taxable income	80.00%
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit, Gross				$ 559,000,000
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit)				8,000,000
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Net Income Reduction				$ 2,000,000
Consolidated Appropriations Act of 2016 [Abstract]
Excise Tax Delay				2 years
Unrecognized Tax Benefits, Interest on Income Taxes Accrued											$ (900,000)		$ 0	$ (100,000)
Interest Income (Expense) related to unrecognized tax benefits				$ 1,400,000	$ (900,000)		$ 100,000
Unrecognized Tax Benefits, Interest Receivable (Payable) on Income Taxes Accrued										$ (500,000)	900,000		0
Unrecognized Tax Benefits, Income Tax Penalties Accrued										0	0		0
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions										2,300,000	4,500,000
Unrecognized tax benefit — Temporary tax positions										2,000,000	24,200,000
Total unrecognized tax benefit	$ 4,300,000			28,700,000	24,700,000		13,200,000			4,300,000	28,700,000		$ 24,700,000	$ 13,200,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1				28,700,000	24,700,000		13,200,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions				900,000	1,400,000		4,200,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions				(600,000)	0		(600,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions				1,300,000	3,900,000		9,000,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions				(19,900,000)	(1,300,000)		(1,100,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities				(6,100,000)	0		0
Balance at Dec. 31	$ 4,300,000			$ 4,300,000	$ 28,700,000		$ 24,700,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards										(5,900,000)	(5,900,000)
Decrease in Unrecognized Tax Benefits is Reasonably Possible										1,000,000
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent				35.00%	35.00%	[1]	35.00%	[1]
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent				0.90%	1.00%	[1]	1.00%	[1]
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Percent				(3.50%)	0.00%	[1]	0.00%	[1]
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits, Percent				(1.00%)	0.80%	[1]	0.50%	[1]
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items, Percent				(0.70%)	(0.50%)	[1]	(0.30%)	[1]
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent				(0.80%)	(1.00%)	[1]	(0.80%)	[1]
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent				0.20%	(0.20%)	[1]	1.70%	[1]
Effective Income Tax Rate Reconciliation, Percent				30.10%	35.10%	[1]	37.10%	[1]
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)				$ (20,858,000)	$ (40,853,000)		$ (1,327,000)
Current State and Local Tax Expense (Benefit)				(12,725,000)	(2,929,000)		2,448,000
Current Change In Unrecognized Tax Expense (Benefit)				(24,333,000)	3,126,000		11,281,000
Deferred Federal Income Tax Expense (Benefit)				89,934,000	116,404,000		67,640,000
Deferred State and Local Income Tax Expense (Benefit)				14,437,000	7,757,000		5,399,000
Deferred Change In Unrecognized Tax Expense (Benefit)				22,094,000	(1,178,000)		(10,203,000)
Deferred investment tax credits				(133,000)	(213,000)		(213,000)
Income Tax Expense (Benefit)				68,416,000	82,114,000		75,025,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items				(414,231,000)	128,393,000		63,453,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities				540,744,000	(5,416,000)		(780,000)
Other Comprehensive Income (Loss), Tax				(48,000)	6,000		163,000
Deferred Income Tax Expense (Benefit)				126,465,000	$ 122,983,000		$ 62,836,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment										659,165,000	1,034,675,000	[2]
Deferred Tax Liabilities, Regulatory Assets										47,519,000	14,811,000	[2]
Deferred Tax Liabilities, Compensation and Benefits, Employee Benefits										33,815,000	51,895,000	[2]
Deferred Tax Liabilities, Other										4,604,000	3,267,000	[2]
Deferred Tax Liabilities, Gross										745,103,000	1,104,648,000	[2]
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets Regulatory Liabilities										115,302,000	(13,167,000)	[2]
Deferred Tax Assets, Operating Loss Carryforwards										26,238,000	100,179,000	[2]
Deferred Tax Assets Unbilled Revenue Fuel Costs										10,448,000	10,226,000	[2]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits										5,769,000	9,656,000	[2]
Deferred Tax Assets, Tax Credit Carryforwards										5,178,000	3,738,000	[2]
Deferred Tax Assets, Other										7,262,000	4,879,000	[2]
Deferred Tax Assets, Net of Valuation Allowance										170,197,000	115,511,000	[2]
Deferred Tax Liabilities, Net										574,906,000	989,137,000	[2]
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Year(s) under examination			2012 and 2013						2010 and 2011
Tax years under examination, Concluded		2012 and 2013
Year of carryback claim under examination									2009
Potential Tax Adjustments				$ 14,000,000
Earliest year subject to examination				2009
Operating Loss Carryforwards										115,000,000	275,000,000
Tax Credit Carryforward, Amount										5,000,000	4,000,000
Carryforward expiration date range, low				2021
Carryforward expiration date range, high				2037
State and Local Jurisdiction
Tax Audits [Abstract]
Earliest year subject to examination				2009
Operating Loss Carryforwards										$ 40,000,000	$ 60,000,000
Carryforward expiration date range, low				2021
Carryforward expiration date range, high				2036
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; 2016 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent										100.00%
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; 2018 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent										60.00%
Consolidated Appropriations Act of 2016; 2019 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent										40.00%
Investment Tax Credit Rate, Percent										30.00%
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%
Plant Related Regulatory Liability [Member]
Income Tax Examination [Line Items]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit				$ 426,000,000
Non-Plant Related Regulated Liability [Member]
Income Tax Examination [Line Items]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit				28,000,000
Non-Plant Related Regulatory Asset [Member]
Income Tax Examination [Line Items]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Asset, Provisional Income Tax Expense (Benefit)				$ 45,000,000

[1] The prior periods included in this footnote have been reclassified to conform to current year presentation.

[2] The prior period included in this footnote has been reclassified to conform to current year presentation.