Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended				9 Months Ended		12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Contingency [Line Items]
U.S. federal corporate statutory tax rate		21.00%	24.50%	35.00%	21.00%	24.50%		24.50%
Enacted tax rate					21.00%
Income tax expense related to deferred tax assets								$ 23,000,000
Tax Act, income tax expense			$ 9,000,000
Income tax liability related to Transition Tax	$ 0				$ 0
Income tax expense	(12,000,000)		$ 61,000,000		86,000,000	$ 132,000,000
Tax benefit due to reversal of deferred tax assets valuation allowance						$ 17,000,000
Maximum
Income Tax Contingency [Line Items]
Reasonably possible decrease in gross unrecognized tax benefits from ongoing audits and settlement	$ 2,000,000				$ 2,000,000
Forecast
Income Tax Contingency [Line Items]
U.S. federal corporate statutory tax rate							21.00%