Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Federal statutory tax rate	21.00%	21.00%	35.00%
Asset expensing	100.00%
Federal statutory tax rate	4.50%	5.50%
Provision for income taxes	$ 77,000,000	$ 81,000,000	$ 197,000,000
Decrease in deferred income taxes offset to regulatory liability	$ 5,000,000
Deferred tax assets expiration date	2032 and 2037
General business credit	$ 175,000,000
Deferred tax general business credits expiration date	2027 and 2039
Investment tax credits	$ 163,000,000
Investment tax credit expiration date	2034 and 2039
Uncertain tax positions	$ 9,000,000	8,000,000	8,000,000	$ 7,000,000
Pre-tax charges (benefits)	0	0	0
Interest accrued	0	0	$ 0
Penalties	$ 0
Statutes of limitations	3 years
Income tax examination period	1 year
Federal [Member]
Income Taxes [Line Items]
Federal and Florida net operating losses (NOL's) carryforward	$ 340,000,000
Federal [Member] | R&D Tax Credits [Member]
Income Taxes [Line Items]
Uncertain tax positions	$ 9,000,000	$ 8,000,000
Florida [Member]
Income Taxes [Line Items]
Federal statutory tax rate	4.46%	5.50%
Provision for income taxes	$ (4,000,000)
Customer refund liability	3,000,000
Decrease in deferred income taxes offset to regulatory liability	5,000,000
Federal and Florida net operating losses (NOL's) carryforward	$ 274,000,000