Income Taxes - Additional Information (Detail) - USD ($)		12 Months Ended
	Sep. 27, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Federal statutory tax rate		21.00%	35.00%	35.00%
Asset expensing		100.00%
Deferred tax assets		$ 275,000,000	$ 194,000,000
Deferred tax liabilities		1,074,000,000	1,019,000,000
Change in net deferred taxes offset to regulatory tax liability			$ 755,000,000
Asset expensing, uncertainty provisional percentage	100.00%		100.00%
Provision for income taxes		$ 81,000,000	$ 197,000,000	$ 152,000,000
Deferred tax assets expiration date		2033 and 2037
General business credit		$ 78,000,000
Deferred tax general business credits expiration date		2028 and 2038
Uncertain tax positions		$ 8,000,000	8,000,000	7,000,000	$ 0
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		8,000,000	$ 8,000,000
Florida [Member]
Income Taxes [Line Items]
Federal and Florida net operating losses (NOL's) carryforward		$ 274,000,000