Income Taxes - Balance Sheet Location (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosures [Line Items]
Deferred Tax Assets, Regulatory Assets and Liabilities, Accelerated Tax Depreciation	$ 919.8
Deferred Tax Assets, Regulatory Assets and Liabilities, Undetermined refund	91,900,000
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	0	$ 0	$ 117,185,000
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Revenue Change	51,200,000
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, effect on income tax expense	17,900,000
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Effect on Income Taxes due to revaluation	80,900,000
Subsidiaries [Member]
Income Tax Disclosures [Line Items]
Deferred Tax Assets, Regulatory Assets and Liabilities	(1,012,300,000)		71.5
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	0	0	36,328,000
ARAM [Member]
Income Tax Disclosures [Line Items]
Deferred Other Tax Expense (Benefit)	$ 27,600,000	$ 29,800,000
Income Tax Reg Treatment [Member] | Subsidiaries [Member]
Income Tax Disclosures [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, PTC revaluation			33,300,000
Income Tax Non-Reg Treatment [Member] | Subsidiaries [Member]
Income Tax Disclosures [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, unregulated portion			$ 3,000,000.0