Income Taxes (Details) - USD ($)		1 Months Ended	3 Months Ended	12 Months Ended
		Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Federal Tax Reform [Abstract]
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		$ 1,500,000,000
Tax Cuts and Jobs Act, Incomplete Accounting, Provisional Income Tax Expense (Benefit)		19,000,000
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Net Income Reduction		5,000,000
Tax Audits [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Accrued										$ (1,600,000)	$ (1,200,000)		$ (900,000)	$ (2,000,000.0)
Interest Income (Expense) related to unrecognized tax benefits				$ (400,000)	$ (300,000)		$ 1,100,000
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions										14,800,000	11,600,000
Unrecognized tax benefit — Temporary tax positions										4,900,000	5,300,000
Total unrecognized tax benefit		18,100,000	$ 16,900,000	16,900,000	18,100,000		18,100,000		$ 60,800,000	19,700,000	16,900,000		18,100,000	$ 60,800,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1				16,900,000	18,100,000		60,800,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions				2,600,000	2,000,000.0		2,700,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions				(500,000)	(300,000)		(1,700,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions				700,000	600,000		5,700,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions				0	(1,100,000)		(49,400,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities				0	(2,400,000)		0
Balance at Dec. 31		18,100,000	16,900,000	$ 19,700,000	$ 16,900,000		$ 18,100,000		$ 60,800,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards										(16,300,000)	(12,700,000)
Net Deferred Tax Liability associated with the Unrecognized Tax Benefit Amounts and Related NOLs and Tax Credit Carryforwards										(11,300,000)	(7,300,000)
Decrease in Unrecognized Tax Benefits is Reasonably Possible										13,700,000
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				21.00%	21.00%	[1]	35.00%	[1]	35.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent				7.10%	7.10%	[1]	5.80%	[1]
Effective Income Tax Rate Reconciliation, Tax Credit, Percent				(11.80%)	(13.60%)	[1]	(11.40%)	[1]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent				0.00%	0.00%	[1]	2.70%	[1]
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items	[2]			(7.40%)	(8.80%)	[1]	(0.30%)	[1]
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent				(1.50%)	(1.10%)	[1]	(1.00%)	[1]
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits				0.50%	0.10%	[1]	(1.60%)	[1]
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent				0.10%	0.50%	[1]	(0.20%)	[1]
Effective Income Tax Rate Reconciliation, Percent				8.00%	5.20%	[1]	29.00%	[1]
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)				$ 80,700,000	$ (16,800,000)		$ 29,600,000
Current State and Local Tax Expense (Benefit)				7,900,000	5,200,000		14,700,000
Current Change In Unrecognized Tax Expense (Benefit)				(800,000)	(1,100,000)		(36,200,000)
Deferred Federal Income Tax Expense (Benefit)				(86,200,000)	(2,400,000)		121,600,000
Deferred State and Local Income Tax Expense (Benefit)				43,200,000	42,100,000		46,700,000
Deferred Change In Unrecognized Tax Expense (Benefit)				4,000,000.0	1,600,000		24,900,000
Deferred investment tax credits				(1,400,000)	(1,400,000)		(1,600,000)
Income taxes				47,400,000	27,200,000		199,700,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items				96,700,000	70,100,000		(1,176,400,000)
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities				(135,400,000)	(28,200,000)		1,369,900,000
Other Comprehensive Income (Loss), Tax				(300,000)	(600,000)		(300,000)
Deferred Income Tax Expense (Benefit)				$ (39,000,000.0)	$ 41,300,000		$ 193,200,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment										2,361,000,000.0	2,257,600,000	[3]
Deferred Tax Liabilities, Regulatory Assets										257,400,000	260,800,000	[3]
Deferred Tax Liabilities, Operating Lease Asset										169,600,000	0	[3]
Deferred Tax Liabilities, Compensation and Benefits, Employee Benefits										68,400,000	64,700,000	[3]
Deferred Tax Liabilities, Other										10,200,000	9,100,000	[3]
Deferred Tax Liabilities, Gross										2,866,600,000	2,592,200,000	[3]
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets Tax credit carryforward										526,500,000	467,300,000	[3]
Deferred Tax Assets Regulatory Liabilities										364,900,000	380,500,000	[3]
Deferred Tax Assets, Operating Lease Liabilities										169,600,000	0	[3]
Deferred Tax Assets, Operating Loss Carryforwards										10,400,000	17,900,000	[3]
Deferred Tax Assets Deferred Investment Tax Credits										5,900,000	6,400,000	[3]
Deferred Tax Assets, Valuation Allowance										(66,000,000.0)	(78,600,000)	[3]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits										37,500,000	38,600,000	[3]
Deferred Tax Assets Rate Refund										11,000,000.0	49,700,000	[3]
Deferred Tax Assets, Other										27,700,000	28,000,000.0	[3]
Deferred Tax Assets, Net of Valuation Allowance										1,087,500,000	909,800,000	[3]
Deferred Tax Liabilities, Net										1,779,100,000	1,682,400,000	[3]
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Tax Credit Carryforward, Amount										449,000,000.0	379,400,000
Potential Tax Adjustments			$ 0
State and Local Jurisdiction
Tax Audits [Abstract]
Operating Loss Carryforwards										133,000,000.0	221,200,000
Operating Loss Carryforwards, Valuation Allowance										(500,000)	(800,000)
Tax Credit Carryforward Net Of Federal Detriment	[4]									77,500,000	87,900,000
Valuation Allowance for Tax Credit Carryforward Net of Federal Benefit	[5]									(65,900,000)	(78,500,000)
Federal detriment										20,600,000	23,400,000
Federal Benefit										$ 17,500,000	$ 20,900,000
Plant Related Regulatory Liability [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit		1,100,000,000
Non-Plant Related Regulated Liability [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit		56,000,000
Non-Plant Related Regulatory Asset [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Asset, Provisional Income Tax Expense (Benefit)		$ 133,000,000

[1] ^(a)  Prior periods have been reclassified to conform to current year presentation.

[2] ^(b)  Regulatory differences for income tax primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit of excess deferred credits are offset by corresponding revenue reductions.

[3] Prior periods have been reclassified to conform to current year presentation

[4] State tax credit carryforwards are net of federal detriment of $20.6 million and $23.4 million as of Dec. 31, 2019 and 2018, respectively.

[5] Valuation allowances for state tax credit carryforwards were net of federal benefit of $17.5 million and $20.9 million as of Dec. 31, 2019 and 2018, respectively.