Income Taxes - Changes in VA for Deferred Tax Assets (Detail) - Valuation Allowance for Deferred Tax Assets [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Income Tax [Line Items]
Balance at beginning of period		$ 1,307	$ 200	$ 335
Addition due to the Combination		39	836
2018 Federal benefit of state adjustment	[1]	(62)
Charged to costs and expenses		206	250	(32)
Charged to other accounts		(18)	21	(103)
Balance at end of period		$ 1,472	$ 1,307	$ 200

[1] The change in the 2018 Federal Benefit of State (FBOS) VA was included in the State tax, not in the VA. The change in the 2019 FBOS VA is included in the VA. Therefore, the 2018 FBOS VA was adjusted to reflect the adjusted beginning balance with FBOS.