Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Percentage of taxable income not distributed for any year elected as REIT status that subject to income tax			100.00%
Provision for federal, state and foreign income tax expense	$ 5	$ 9	$ 6	$ 28
Income tax federal statutory rate	21.00%	35.00%	21.00%	35.00%

Amount of deferred income tax expense (benefit) pertaining to income (loss) from continuing operations.

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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration