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Income Taxes - Additional information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes
Investment impairment		$ 52		$ 52
Income tax expense at U.S. federal statutory rate			21.00%	21.00%	21.00%
Net reduction in income tax expense due to Tax Cuts and Jobs Act					$ (12)
Equity in net losses of unconsolidated entities			$ 68	$ 55	41
Other federal tax credits			7	11	10
Nondeductible acquisition transaction costs			27	10
Reduction in provision for income taxes due to tax audit settlements			$ 10	2	40
Expected time of completion of IRS audits			15 months
Increase (decrease) to accruals and related deferred taxes			$ (3)	(22)	(52)
Reduction in provision for income taxes due to state net operating losses and credits			12	14	22
Excess tax benefits associated with equity-based compensation			27	25	17
Increase (decrease) in provision for income taxes due to impairments				15	1
Investments Qualifying for Federal Tax Credits [Member]
Income Taxes
Equity in net losses of unconsolidated entities			73	46	30
Income tax (expense) benefit, including tax credits, from equity method investment			87	96	57
Interest expense			11	$ 9	$ 3
Refined Coal Facility [Member]
Income Taxes
Equity method investments impairment charges	$ 7		$ 7

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to nondeductible impairment loss.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other tax credits.

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The increase (decrease) during the period in the amount due for taxes based on the reporting entity's earnings or attributable to the entity's income earning process (business presence) within a given jurisdiction.

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Amount of increase (decrease) to previously recorded tax expense. Includes, but is not limited to, significant settlements of income tax disputes, and unusual tax positions or infrequent actions taken by the entity, including tax assessment reversal, and IRS tax settlement.

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Income Taxes [Line Items]

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The amount by which the fair value of an investment is less than the carrying amount of that investment and the decline in fair value is deemed to be other than temporary.

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Maximum number of months expected for IRS audits to be completed.

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Reduction in provision for income taxes due to state net operating loss and credit.

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